The evolution of oligotrophy - Wiley Online Library

14 downloads 11823 Views 173KB Size Report
*Corresponding Author Email: [email protected] ... The evolution of oligotrophy: implications for the breeding of crop plants for low input agricultural ...
Annals of Applied Biology (2005), 146:261–280

261

The evolution of oligotrophy: implications for the breeding of crop plants for low input agricultural systems J A RAVEN1*, M ANDREWS2 and A QUIGG3 1

Division of Environmental and Applied Biology, University of Dundee at SCRI, Invergowrie, Dundee DD2 5DA, UK 2 School of Sciences, University of Sunderland, Sunderland SR1 3SD, UK 3 Department of Marine Biology, Texas A & M University, Galveston, TX 77551, USA Summary

Oligotrophy, the obligate or facultative capacity to live in low-nutrient habitats, has played a major role in the evolution of photosynthetic organisms. ● Energy/carbon deficiency: evolution of photosynthesis about 3.5 Gyr (billion years) ago, then use of H2O as electron donor, and accumulation of O2 from about 2.3 Gyr ago. ● Deficiency in combined N: evolution of biological N2 fixation about 2.0–2.3 Gyr ago. ● Deficiency in soluble relative to particulate organic C: evolution of phagotrophy in eukaryotes, opening the way to endosymbiotic origin of photosynthesis in eukaryotes. ● Deficiency of P and Fe resulting from oxygenation: evolution of mechanisms increasing access to P and Fe. ● Deficiency of H2O for land plants gaining C from the atmosphere: evolution of homoiohydry following origin of significant land flora from 0.5 Gyr ago. ● Deficiency of CO2 resulting from increased weathering by land plants: evolution of large leaves. ● Increased competition for resources among land plants: evolution of mechanisms economizing in use of soil-derived resources, and increasing ability to acquire resources. Economising on resource use in photosynthetic organisms is subject to a number of constraints. There are very limited possibilities for reducing the use of N in proteins with a given catalytic function, but greater possibilities using substitution of an analogous protein with that function. The same applies to Fe. Possibilities for economising on the use of P are very limited if the growth rate is to be maintained: the marine cyanobacterium Prochlorococcus is a good example of restricted P requirement. H2O use can be constrained by C4 and, especially, CAM photosynthesis. A possible role of the study of oligotrophy in the context of sustainable, low-input agriculture includes modified agricultural practice to minimise losses of resources. Information on oligotrophy and its evolution can also be used to inform the alteration of crop plants by genetic modification related to resource acquisition (e.g. associative, or nodule-based, symbiotic diazotrophy) and the economy of resource use (e.g. partial or complete conversion of a C3 crop to a C4 crop which could economise in the use of N and/or H2O). The attempts to convert C3 to C4 plants have not thus far been fully successful, and the advantages of conversion to C4 are being increasingly offset by the effect of increasing atmospheric CO2 on C3 plants. However, more success has been achieved with selection of the most appropriate diazotrophic symbionts for crop plants in particular environments. Key words: Evolution, iron, nitrogen, oligotrophy, phosphorus, photosynthesis, plant breeding, symbiosis, sustainable agricultural systems Introduction It is possible that there are lessons to be learned for low-input agriculture from the study of photosynthetic organisms which normally grow in habitats in which the rate of accumulation of biomass is restricted by the supply of one or more resources (‘stress-toleraters’ of Grime, 2002). Such habitats are termed oligotrophic, and the organisms which can grow in such habitats are termed oligotrophs. The concept of oligotrophy originated with the Swedish limnologist Einar Naumann (1919) who suggested *Corresponding Author Email: [email protected] © 2005 Association of Applied Biologists

a classification of lakes according to their trophic status, with an oligotrophic lake characterised as having low production associated with low nitrogen and phosphorus availability (Naumann, 1929; Carlson & Simpson, 1996). The terms oligotrophic and oligotrophy are widely used in limnology and in oceanography, but are less commonly used in studies of terrestrial (agro)ecosystems. The paper considers oligotrophy in photosynthetic organisms from two points of view. Firstly we consider the role that resource limitation has had in the evolution of photosynthetic organisms on Earth,

J A RAVEN ET AL.

262

highlighting the restrictions on the biochemistry of organisms resulting from the intrinsic chemistry of the elements, and their availability during evolution. Many major evolutionary innovations can plausibly be considered as responses to a shortage of one or more resources. Secondly, we discuss the implications of the evolutionary response to resource limitation on the breeding of crop plants for sustainable low input agricultural systems. Resource Shortage as a Factor in the Macroevolution of Photosynthetic Organisms The natural selection of the elements ‘The natural selection of the elements’ is the title of a paper by Williams (1981, and elaborated on by Williams & Fráusta da Silva, 1996) which sets out the roles of two factors in determining the use of particular elements in biology. One is the chemical properties of the elements in relation to natural selection at the origin of life in terms of their roles, e.g. in catalysis and structures. The other factor is the availability of the elements at the time that the catalysis of the structure evolved. Intimately involved in consideration of the roles of the chemical elements is the energy source for the growth and maintenance of the organism, so suggesting the evolutionary role of resource shortage in the evolution of photosynthetic organisms; this requires consideration of the trophic modes of early organisms. H2O is a resource whose availability is especially problematic for land plants. Aside from energy and H2O, the emphasis in the discussion is on C, N, P and Fe as evolutionary, ecologically and agriculturally very important nutrient elements. The origin of photosynthesis The first organisms on Earth were probably chemolithotrophic (i.e. gaining energy from inorganic chemical reactions, and nutrient requirements from

inorganic compounds) or, less plausible, chemoorganotrophic (i.e. gaining energy from organic chemical reactions, and nutrient requirements from organic or, sometimes, inorganic compounds) (Wolstencoft & Raven, 2002; Raven & Skene, 2003). However, the energy substrates required for chemolithotrophy or chemo-organotrophy are finite, and are only slowly replaced once used (Wolstencroft & Raven, 2002; Raven & Skene, 2003). Solar energy provides a continuing energy source for photolithotrophic organisms, i.e. those that gain energy from electromagnetic radiation, and their nutrient requirements from inorganic compounds. When photolithotrophy ultimately involved the use of H2O as an essentially inexhaustible source of reductant in converting CO2 into organic compounds, the potential for energy flow though living organisms on the early Earth increased by several orders of magnitude (Wolstencroft & Raven, 2002; Raven & Skene, 2003). This scenario casts restricted supplies of the substrates for the chemical energisation of life as a major factor in natural selection which favoured photosynthetic organisms. As to the timing of the evolution of ‘plant-type’ (i.e. H2O as reductant) photosynthesis, earlier suggestions of cyanobacteria from 3.5 Gyr (Gyr = billion years) ago are now in doubt (Brasier et al., 2002; cf. Schopf et al., 2002). Molecular fossils suggest that cyanobacteria, and presumably oxygenic photosynthesis, occurred at least 2.7 Gyr ago (Summons et al., 1999). The net accumulation of O2 began at least 2.3 Gyr ago (Bekker et al., 2004), with consequences for the availability of P and Fe which will be explored later. O2 -evolving photosynthesis is monophyletic, and its core involves a complex set of reactions (Blankenship, 2002) which only had a limited means of responding to subsequent environmental changes (increasing O2 and decreasing CO2) without additional reaction sequences (Table 1).

Table 1. Time scale of some major evolutionary and environmental changes: marine photolithotrophs Time Not later than 2.7 Gyr ago

Evolutionary Event Origins of O2-evolving photosynthesis

Reference Summons et al. (1999)

Not later that 2.3 Gyr ago

Net accumulation of O2, with decreased availability of P and Fe Bjerrum & Canfield (2002); Bekker et al. (2004)

2.7 - 2.0 Gyr ago

Origin of diazotrophy as the requirement for combined N increased and the availability of combined N from lightning decreased

Navarro-Gonzalez et al.(2001); Boucher et al. (2003); Hessler et al. (2004)

Up to 2.7 Gyr ago (molecular) 2.1 Gyr ago (structural fossil)

Origin of eukaryotes and of phagotrophy

Brockes et al. (1999); Knoll (2003)

Up to 2.1 Gyr ago (probable alga), Origin of photosynthetic eukaryotes from macro-fossil about 1.4 Gyr ago (red alga) evidence

Knoll (2003)

About 1.4 Gyr ago

Raven (1987, 1998)

Origin of eukaryotic vacuolation, increased surface area intercepting photosynthetically active radiation, and nutrients, per unit cytoplasm, and storage of resources

Evolution of oligotrophy

The origin of diazotrophy The increased potential for production of reduced carbon compounds by photosynthesis from ~ 2.7 Gyr might well have been temporally associated with a decrease in the quantity of combined N (as NO) produced by lightning from N2 and CO2 as a result of the lowering of atmospheric CO2 levels (NavarroGonzález et al., 2001; Hessler et al., 2004) (Table 1). Although this slower rate of NO production may have been short-lived in geological terms (~ 100 Myr: Myr = million years), it could have been a selective force for biological N2 fixation in a world rich in organic C relative to combined N (Falkowski, 1997). It is important to point out that there is no direct chemical fossil evidence for diazotrophy 2.0–2.7 Gyr ago, although molecular genetic evidence suggests that the nitrogenase reductase-nitrogenase complex is monophyletic (Fig. 4 of Boucher et al., 2003) and of early origin. The evolution of nitrogenase has involved diversification of metal requirements. The ancestral nitrogenases had Fe and Mo as their transition metal cofactors; the apomorphic condition is that Mo is replaced by V or by Fe. The evolutionary significance of this substitution is presumably related to lack of Mo in particular environments. Anbar & Knoll (2002) discuss the possible shortage of combined N, as judged from natural abundance 15 N/14N ratios in sedimentary deposits, during the ‘sulfidic ocean’ phase (see Poulton et al., 2004) in the Proterozoic. However, the possibility that different N isotope discrimination when N2 fixation involves a V nitrogenase (Rowell et al., 1998) should also be considered in interpreting the N isotope ratio data. All nitrogenases are very O2-sensitive, and a range of additional components evolved which allowed diazotrophy to occur in the presence of external O2. The most microscopically obvious are the heterocysts of such filamentous cyanobacteria as Anabaena and Nostoc; here nitrogenase is confined to the heterocysts which lack photosystem II and hence O2 evolution, with organic C supplied from adjacent photosynthetic cells, with the reduced N produced in the heterocysts supplied to the adjacent cells. The origin of eukaryotes: phagotrophy and endosymbiosis Molecular markers for eukaryotes date back to 2.7 Gyr ago (Brockes et al., 1999; Knoll, 2003), although it is not certain that these markers necessarily meant that eukaryote features such as a nucleus, mitochondria and phagotrophy were all in place at that time (Maynard-Smith & Szathmáry, 1995). Evidence for body fossils of eukaryotes possibly goes back to ~ 2.1 Gyr, but with more certain evidence from 1.2–1.8 Gyr as multicellular red algae (Kasting & Catling, 2003; Knoll, 2003) (Table 1). It is possible that a selective factor favouring phagotrophy is the

263

increased biomass and productivity of cyanobacterial primary producers which initially would only have been attacked by viruses and parasitic prokaryotes. Phagotrophy, a trophic mode only practised by certain eukaryotes, would have been an important new link in food webs. While this is not per se an aspect of oligotrophy as a source of evolutionary innovation, competition among saprotrophic prokaryotes for the dissolved organic C resulting from viral lysis and bacterial parasitism could have been a selective force for phagotrophy as a means of accessing primary producer biomass without the intervention of parasites. The acquisition of mitochondria by endosymbiosis following phagotrophy (Knoll, 2003) of a proteobacterium would have allowed the phagotrophic, chemo-organotrophic eukaryote to make more energetically effective use of ingested material than was the case for the host organism before the acquisition of mitochondria. Ex hypothesis the host phagotroph lacked the oxidative phosphorylation pathway, and thus had a relatively low yield of ATP per unit organic C substrate metabolised. The proteobacterial endosymbiont contributed the oxidative phosphorylation pathway and thus increased the yield of ATP per unit organic C oxidised. More effective phagotrophic feeding on cyanobacteria could have caused at least local resource limitation for the phagotrophs, with a consequent selective advantage for phagotrophs which had photosynthetic (cyanobacterial) symbionts enabling them to survive between infrequent particulate meals. Genetic integration of the endosymbiont to yield a plastid-containing eukaryote would have produced, initially, an organism with the potential for phagotrophy and photosynthesis, i.e. the phagotrophic rather than the saprotrophic variant of mixotrophy. Most extant photosynthetic eukaryotes have lost their capacity for phagotrophy, although a significant number of unicellular and colonial microalgae have retained phagotrophy, e.g. many chrysophytes, cryptophytes, dinoflagellates and euglenoids (Raven, 1997). All of the photosynthetic algae listed as retaining phagotrophy obtained their plastids by secondary endosymbiosis, i.e. ingestion of a green or a red eukaryotic alga by a phagotrophic chemo-organotroph, with subsequent genetic integration (Armbrust et al., 2004; Falkowski et al., 2004). The particle-ingesting mixotrophs that have retained a high photosynthetic capacity seem to use particle ingestion mainly as a means of supplying nutrients such as P and Fe rather than organic C and energy (Raven, 1997). Cavalier-Smith (2005) has suggested that the origin of eukaryotes with cell walls, found now in most fungi, oomycetes, many algae and plants, acted as a constraint on the increase of genome size, as seen when comparing

264

J A RAVEN ET AL.

genome sizes among protists with and without cell walls. Cavalier-Smith’s argument is that DNA accounts for a significant fraction of cell P (other than stored P) in organisms with large quantities of DNA per unit biomass, and that phagotrophy and mixotrophy are more reliable sources of P for wall-less organisms than is the uptake of H2PO4–/ HPO42– by saprotrophs (most fungi; oomycetes) and photolithotrophs (most algae; plants). Some photosynthetically competent flowering plants have evolved extracellular phagotrophy, analogous to what occurs in the gut of arthropods and chordates, again related largely to the supply of nutrients such as P and Fe. The early competitive advantage of such eukaryotic photolithotrophs may have lain in their greater morphogenetic potential than was the case for cyanobacteria, with the possibility of additional means of restricting the activities of grazers. The cyanobacterial immediate ancestor of plastids had a genome size of ~ 8000 genes, i.e. at the upper end of the range known for extant cyanobacteria (Martin et al., 2002). This endosymbiosis followed by genetic integration introduced many genes other than those related to photosynthesis, and very likely including those for N2 fixation (Martin et al., 2002). Many of these genes were retained after transfer to the eukaryote nucleus (e.g. those responsible for cellulose synthesis); those for diazotrophy were lost for as yet incompletely understood reasons (Martin et al., 2002; Douglas & Raven, 2003). Some transfer of genes from the genomes of the eukaryotic endosymbionts to the nucleus of the chemoorganotrophic host has also occurred in secondary endosymbioses (Armbrust et al., 2004). Lateral gene transfer not obviously related to endosymbiosis also occurred early in the evolution of photosynthetic eukaryotes. Thus, red algae and the organisms which obtained plastids from them by secondary endosymbiosis of red algal cells have a form ID Rubisco (ribulose bisphosphate carboxylase-oxygenase) rather than the Form IB Rubisco of the cyanobacterial endosymbiont which was the ancestor of all extant plastids and which occurs in green algae and higher plants, as well as in organisms that have obtained their plastids by secondary endosymbiosis of green algal cells (Falkowski & Raven, 1997; Falkowski et al., 2004). However, the possibility that the Form ID Rubisco came from the proteobacterial endosymbiont which was the ancestor of mitochondria has not been completely discounted. A further example of lateral gene transfer involving Rubisco is the occurrence of a Form II Rubisco in peridinin-containing dinoflagellate algae (Falkowski & Raven, 1997; Falkowski et al., 2004). As is discussed later in this paper, such lateral gene transfers have now been achieved by molecular genetic transformation (e.g.

Whitney & Andrews, 2001; Whitney et al., 2001). A final comment about the role of symbiosis in the evolution of extant photosynthetic organisms concerns symbioses today in which the photosynthetic component is not genetically integrated to a large extent with the chemo-organotroph even when the photobiont only seems to occur in symbiosis in nature. In some cases at least, the photobiont can be grown axenically. For aquatic organisms today, the symbioses involve a cyanobacterium or alga in symbiosis with either a phagotrophic (e.g. sponges, cnidarians, ascidians) or a saprotrophic (an ascomycete fungus in the case of the few aquatic lichens such as Lichina) chemo-organotroph. These symbioses are widely considered to facilitate nutrient recycling between trophic levels in nutrientpoor habitats (Douglas, 1994). We shall see that symbioses are very significant in the nutrition of terrestrial photolithotrophs, with oligotrophy presumably being a major selective force in the evolution of the symbioses. Oxygenation, oxidation and availability of Fe and P Increasing, albeit not linearly, global oxygen levels from ~ 2.3 Gyr ago would have oxidized much of the Fe in the illuminated part of the ocean from the ferrous to the ferric state, rendering this essential trace element less available than was formerly the case (see Anbar & Knoll, 2002). The corresponding oxidation of the land surface would have acted to sequester some of the P released as soluble phosphate by rock weathering. This would have had implications for the P flux to the ocean, compounding the sequestration of marine P by the greater abundance of Fe oxides prior to 1.9 Gyr ago (Bjerrum & Canfield, 2002) (Table 1). When substantial primary productivity became established on land perhaps 1 Gyr ago, and certainly 500 Myr ago, photolithotrophs had a restricted supply of both P and Fe. Restrictions on P and Fe supply had implications for the availability of combined N: in today’s oceans, diazotrophy is generally more restricted by the supply of Fe and of P than is primary productivity using combined N (Falkowski & Raven, 1997; Sañudo-Wilhelmy et al., 2001; Mills et al., 2004). It is of interest that Fe deficiency in cyanobacteria and in algae generally does not cause a significant change in the C:N ratio of the organisms, regardless of whether they are cyanobacterial diazotrophs or eukaryotes using combined N (references in Cooke et al., 2004). This suggests that a deficiency in intracellular N occurs to the same relative extent in the pathways of C assimilation and of N assimilation. The O2 sensitivity of diazotrophy is another reason why combined N supply might be restricted in a more oxygenated world (but see Anbar & Knoll, 2002; cf. Poulton et al., 2004; Kah et al., 2004).

Evolution of oligotrophy

Vacuolation of cells could have been a response to resource deprivation: this phenomenon at once provides storage capacity for any temporary excess of resources over what can be used immediately in a temporally variable environment, and at least as importantly increases the surface area per unit of active cytoplasm for light interception and nutrient uptake (Raven, 1987, 1998). Tozzi et al. (2004) suggest that intracellular storage, e.g. in vacuoles, by introducing a delay between depletion of extracellular nutrients and nutrient limitation of growth, is important in growth of organisms in environments with temporally varying nutrient inputs such as in a turbulent upper mixed layer in the ocean. They further argue that this contributes to the success of vacuolate diatoms relative to nonvacuolate phytoplankton such as coccolithophores. Nutrients can be stored in cyanobacteria and algae without large vacuoles, e.g. N in organic form as cyanophycin in cyanobacteria, P as polyphosphate bodies in cyanobacteria and eukaryotic algae, and Fe as (phyto)ferritin (although it is not certain if ferritin is commonly found in algae: Armbrust et al., 2004; cf. Thiel, 2004) or some analogous complex (Nagasaka et al., 2003). Embryophytes, in which most cells are vacuolate, can also store nutrients in the cytoplasm, e.g. N as specific storage proteins, or by a generalised increase in protein N, P as phytic acid, and Fe as phytoferritin (Marschner, 1995; Epstein & Bloom, 2005). However, there are two possible advantages of vacuolation. One possible advantage is that there may be a greater storage capacity for nutrients per unit of cytoplasm than with other storage modes, although more quantitative comparisons are needed to establish the extent to which this is true. Certainly the storage of N as NO3– requires that this ion is in a vacuole (Raven, 1987, 1998). A related possible advantage in light-limited environments is that N can be stored in the vacuole as NO3– so that the energy used in NO3– assimilation is only required when the N is used in growth rather than over a shorter time period when external NO3– is available (Raven, 1987, 1998). Vacuolation is mainly a eukaryotic phenomenon, but also occurs in certain large-celled bacteria (Schulz & Jørgensen, 2001). Oligotrophy in relation to increased biodiversity and energy flow in the oceans Martin (1996, 2003) suggests, on the basis of the fossil record and of palaeoenvironmental proxies, that nutrient availability to the marine habitat shows a general increase through the Phanerozoic, i.e. the last 540 Myr. This apparent decrease in the extent of oligotrophy in the ocean can be in part related to the increase in terrestrial biomes with increased weathering and biological activity with increased fluvial inputs of nutrients to the ocean (Martin, 1996, 2003; Katz et al., 2005). However, it should

265

be noted that there are still very significant areas of oligotrophic ocean today (Falkowski & Raven, 1997; Karl & Tien, 1997; Wu et al., 2000), despite the additional tendency toward eutrophication due to man’s input of, for example, additional atmospheric combined N even into parts of the ocean distant from land (Paerl, 1985: Cornell et al., 1995). Life on land: H2O The growth of plants on land today, with absorption of light and of CO2 from the atmosphere, typically involves the loss of more than 10 times as much H2O in transpiration as is used in photosynthesis and other metabolic processes plus what is required in the hydration of the organism (Raven, 1977). The higher CO2 content of the atmosphere during the early evolution of higher plants 500–400 Myr ago meant that less H2O would have been lost during growth, but H2O would still have been a limiting resource for plants. Some successful land plants (e.g. bryophytes) have little control over H 2 O loss (poikilohydry) and are generally desiccation tolerant in the vegetative state (Raven, 2002; Raven & Edwards, 2004). However, the dominant plants over most of the land surface today are desiccation intolerant in the vegetative stage, and survive in the hydrated state in the face of irregular rainfall. This involves mechanisms which permit H 2 O vapour loss during photosynthesis when soil H2O is available, and restrict H2O loss (at the expense of limiting photosynthesis) when the evaporative potential of the atmosphere is high relative to H2O supply from the soil. This homoiohydric state involves cuticle, stomata, intercellular gas spaces, xylem or a functional analogue, and roots or their equivalents (Raven, 1984, 1993, 1996, 1997, 2002; Edwards et al., 1996; Raven & Edwards, 2001, 2004; Hetherington & Woodward, 2003). This evolution and retention of a complex system was (and is) contingent on the absence of an assured continual supply of soil H2O (Table 2). Life on land: soil-derived nutrients We have seen that the oxygenated atmosphere imposes restrictions on the availability of combined N by diazotrophy (although providing some combined N from lightning), P and Fe. The evolution of the root system of vascular plants involved a number of innovations which can be related to nutrient acquisition, although anchorage and H2O acquisition were, and are, also clearly important functions of roots (Raven & Edwards, 2001). Symbiosis with arbuscular mycorrhizal fungi, facilitating the acquisition of P and other elements, preceded the evolution of roots sensu stricto (Raven & Edwards, 2001). Such innovations as symbiotic diazotrophy and ectomycorrhizas related to N acquisition, and cluster roots and dauciform roots related to P

J A RAVEN ET AL.

266

acquisition, postdated the evolution of roots sensu stricto (Lamont, 1974, 2003; Smith & Read, 1997; Raven & Edwards, 2001; Grime, 2002; Skene, 2003; Epstein & Bloom, 2005) (Table 2). Symbiotic diazotrophy increases combined N supply to plants in habitats with low combined N availability, at the expense of an increased requirement for energy, H2O, Fe and Mo (Raven, 1988; Sañudo-Wilhelmy et al., 2001; Kustka et al., 2003; Andrews et al., 2004; Raven et al., 2004b). Ectomycorrhizas are, inter alia, able to mobilise otherwise unavailable complex organic N sources by the secretion of extracellular enzymes (Smith & Read, 1997). The net primary assimilation of sources of reduced combined N, e.g. NH4+ and organic N, and attendant biosyntheses producing biomass, involve the production of excess H+. The same is true for symbiotic diazotrophy. In vascular land plants the disposal of the excess H+ involves excretion to the rooting medium. This is usually achieved by restricting assimilation of N2 and of reduced N to the below-ground parts of the plants (Raven & Smith, 1976). Where shoots are involved in this N assimilation, e.g. in the assimilation of atmospherically derived NH3 or NH4+ or N2 fixation in stem nodules, xylem and phloem transport processes are involved in effectively moving the excess H+ to the roots (Raven & Smith, 1976). NO3– assimilation can, by contrast, occur in shoots as well as in roots, since the excess OH– can be neutralised by organic acid synthesis (Raven & Smith, 1976). Andrews (1986) points out that there are genotypic differences among closely related organisms adapted to different temperatures in the location of NO3– assimilation, with more NO3– assimilation in the shoot in the coldintolerant genotype. This is an example of the other observed, and theoretically predicted, effects of the location of NO3– assimilation on plant performance (Andrews et al., 2004). The non-symbiotic adaptations related to P acquisition include a root and root hair system which exploits a large volume of soil, with active H+ efflux,

organic acid efflux, and surface and/or extracellular reductants helping to release H2PO4–/HPO42– from insoluble Ca phosphates and ferric oxide complexes (Marschner, 1995; Zhu & Lynch, 2004). There are also surface phosphatases which release inorganic phosphate from organic P in the soil (see Phoenix et al., 2004). As well as exploiting a large volume of soil, some non-symbiotic means of accessing P from insoluble complexes in the soil use a dense array of determinate root in the form of cluster roots and dauciform roots which can more thoroughly change the soil P chemistry in a limited volume (Lamont, 1974, 2003; Tinker & Nye, 2000; Grime, 2002; Epstein & Bloom, 2005). Acidic soils limit P supply by interaction with Al species: means by which plants access and acquire P in these conditions is discussed by Kochian et al. (2004). Non-symbiotic means of accessing Fe include siderophores in grasses, and H+ active efflux, organic acid efflux, and surface reductases and/or extracellular reductants in all other vascular plants that have been investigated (Marschner, 1995; Epstein & Bloom, 2005). Life on land: CO2 The earliest vascular plants had either small leaves, in lycophytes or no recognisable leaves at all, as in the rhyniophytes that were probably closely related to the ancestors of the euphyllophytes. The euphyllophytes comprise all extant vascular plants other than the lycophytes, i.e. ferns, psilophytes, horsetails and seed plants. Photosynthesis in these early euphyllophytes almost 400 Myr ago involved the homoiohydric apparatus in more of less erect cylindrical axes. The first large laminar leaves in the euphyllophytes are found in fossils from ~ 360 Myr ago, i.e. over 50 Myr after the first vascular plants are known from the fossil record (Table 2).This evolutionary delay is attributed by Beerling et al. (2001) and Osborne et al. (2004) to problems of thermoregulation in a hot-house world with high CO2 levels before enhanced weathering by plants with deep roots reduced the atmospheric CO2 level. The high CO2 of the Upper Silurian

Table 2. Time scale of major evolutionary and environmental changes: terrestrial photolithotrophs Time Evolutionary Event Not later than 470 Myr ago Origin of embryophytes as desiccation-tolerant organisms at poikilohydric bryophyte level

Reference Raven & Edwards (2004)

Not later than 420 Myr ago Origin of vascular (or near-vascular) embryophytes with potential Raven & Edwards (2004) for homoiohydry, possessing xylem (or other endohydric conducting system), cuticle, gas spaces, stomata; leaves on lycophytes Not later than 400 Myr ago Origin of roots

Raven & Edwards (2001, 2004)

400-360 Myr ago

Beerling et al. (2001); Osborne et al. (2004)

Origin of euphyllophyte leaves; increase in leaf size with decreasing CO2 during the Devonian

Not later than 365 Myr ago Origin of seeds: gymnosperms

Marshall & Hemsley (2003); Gerrienne et al. (2004)

Not later than 365 Myr ago Origin of fruits, flowers: angiosperms

Friis et al. (2003)

Evolution of oligotrophy

and Lower Devonian was correlated with a low stomatal density, which meant decreased dissipation in the latent heat of evaporation of absorbed solar radiation which had not been stored as the products of photosynthesis. Convective heat loss would not be adequate to prevent potentially lethal overheating in broad leaves (Beerling et al., 2001; Osborne et al., 2004). With the drawdown of atmospheric CO2 and increasing stomatal density in the Upper Devonian, the potential for evaporative cooling increased, and the broader euphyllophyte leaves found in both progymnosperms and pteridosperms would not have led to potentially lethal overheating (Beerling et al., 2001; Osborne et al., 2004). Osborne et al. (2004) point out that the euphyllophyte leaf evolved by the Lower Devonian about 390 Myr ago (Hao et al., 2003), but that the production of wide euphyllophyte leaves did not occur until the Upper Devonian some 360 Myr ago. The great genotypic range of leaf form in extant vascular plants can, to a significant extent, be related to the radiation, temperature and water supply environments during evolution of the organism, with implications for low-input agriculture (see Jones, 1992). Another outcome of lowered CO 2 levels in the atmosphere, with near-extant or even higher levels of O2, was the evolution of biochemical and physiological variants of photosynthetic C acquisition which overcame some of the constraints imposed by the kinetics of Rubisco which restricted the rate of net photosynthesis relative to the evaporative loss of H2O (Keeley & Rundell, 2003). The kinetic constraints comprise apparent evolutionary limits on high substrate-saturated rates of carboxylation per active site of the enzyme combined with high affinities for CO2 and large selectivity for CO2 over the competing alternate substrate O2. The variants on the basic photosynthetic metabolism with C3 physiology are related to the additional occurrence of CCMs (carbon concentrating mechanisms). In land plants these are the C4 and CAM (crassulacean acid metabolism) variants of photosynthesis which have a higher affinity for CO2 than does C3 photosynthesis. C4 photosynthesis generally involves the co-operation of two cell types. Rubisco and the photosynthetic carbon reduction cycle, and enzymes releasing CO2 from a C4 dicarboxylic acid, occur in bundle sheath cells with very restricted access to CO2 from the intercellular gas spaces. The mesophyll cells, with good diffusive access to CO2 from intercellular gas spaces, contain PEPc (phosphoenolpyruvate carboxylase), carbonic anhydrase as a means of converting CO2 into HCO3–, the inorganic C species used by PEPc, and some other enzymes of the C3-C4 carboxylic acid cycle. C4 dicarboxylates move to bundle sheath cells, and C3 monocarboxylates move to the mesophyll cells, via plasmodesmata. A minority of C4 plants on land,

267

and all of the few characterised from aquatic habitats, have all of the reactions in a single cell (Edwards et al., 2004). The lower evaporative cooling in extant C4 and CAM plants than in C3 plants means that these plants operate at a higher leaf temperature than do corresponding C3 plants. However, the habitat over the last ~30 Myr for which these organisms have existed on land means that the suggested restrictions on leaf breadth in the Lower Devonian do not pertain to our cooler world. CCMs have probably occurred for much longer in aquatic habitats (Raven et al., 2002b) where evaporative H2O loss would not be a problem, but the much lower potential for diffusive supply of inorganic C to the surface of photolithotrophs under water is a significant constraint on photosynthesis when inorganic C concentrations are relatively low. CCMs today are more widespread in aquatic than terrestrial habitats, and it is highly likely that CCMs evolved in aquatic organisms in some earlier lowCO2 episode than the decline in CO2 which parallels the evolution of CCMs in higher land plants over the last ~30 Myr (Raven et al., 2002b; Keeley & Rundell, 2003). Life on land: Resource use in relation to reproductive strategies The annual habit, in which vegetative individuals of semelparous (i.e. reproducing only once) plants live less than one year, probably evolved among vascular embryophytes only in the last 120 Myr in leptosporangiate ferns and flowering plants (Raven, 1986). There are important resource allocation implications of the semelparous habit, with predictions from models that the optimal strategy (Rosen, 1967) is a temporal division of resource acquisition from subsequent reproduction (Cohen, 1966). Such a temporal division involves use of the resource relocation mechanisms used in the deciduous and the seed habits after the switch from resource acquisition to reproduction (Raven et al., 2002a). Nutrient Availability and the Growth of Photolithotrophs Ecological stoichiometry: general considerations ‘Ecological stoichiometry’ (Sterner & Elser, 2002) is an aspect of macroecology which considers how the ratio of elements available to, and within, organisms relates to the functioning of the organisms. Sterner & Elser (2002) and Elser et al. (2003) review evidence as to the relationship of growth rate to elemental content for the major nutrients N (essential for nucleic acids, nucleotide cofactors and proteins in all organisms) and P (essential for nucleic acids and nucleotide cofactors in all organisms). For many chemo-organotrophs the faster-growing organisms

268

J A RAVEN ET AL.

have a greater P quota in their biomass (Sterner & Elser, 2002). This finding is to be expected in experiments on a given organism when growth rate is determined by the supply of P supply, or by the supply of any resources other than N, when ontogenetic changes of growth rates are examined, and when phylogenetic differences in maximum specific growth rate are considered within a clade. Work reviewed and synthesised by Sterner & Elser (2002) relates the P content of the organism to the rRNA content: rRNA is a, and usually the, major P-containing structure and catalyst in organisms. The argument runs that, since faster growth requires a more rapid synthesis of protein, and the rate of protein synthesis in vivo is commonly limited by the amount of rRNA (Waldron & Lacroute, 1975; Bonven & Gullov, 1979), a higher specific growth rate requires a higher rRNA (and hence P) content. The higher rRNA content involves a lower protein:rRNA ratio, and protein per unit biomass, in organisms growing at the higher specific growth rates. The implication here is that slower-growing organisms, other than those in which the growth rate is limited by N supply, have an excess of proteins over what is required for the catalytic roles associated with a low specific growth rate. The absence of as widespread a correlation of specific growth rate with the content of rRNA and of P in photosynthetic organisms as in chemoorganotrophs (Healey & Hendzel, 1975; Sterner & Elser, 2002; Raven et al., 2004a) was related to the possibility of P storage under high-P growth conditions. However, many data on photolithotrophs also show the absence of a clear correlation of specific growth rate with rRNA content, and P storage cannot be invoked here (Raven et al., 2002a, 2004a). Furthermore, P storage is not confined to photolithotrophs; it is also common in saprotrophic (bacteria; fungi; obligately chemo-organotrophic algae) chemo-organotrophs which provided much of the early data on the correlation of specific growth rate with rRNA content and P content (McMurrough & Rose, 1967; Wehr & Parks, 1969; Aiba et al., 1973; Poynton, 1973; Sebastian et al., 1973; Sturani et al., 1973; Alberghina et al., 1975; Kief &Warner, 1981; Falkowski, 2000; Geider & La Roche, 2002; Sterner & Elser, 2002). Vrede et al. (2004; see also Anderson et al., 2004) have examined more data on elemental stoichiometries in organisms, including photolithotrophs, in relation to the specific growth rates as a function of the content of nucleic acids and proteins. This has provided some rationalisation of the absence of a relationship between specific growth rate and P content. An alternative, and theoretically superior, approach is that of Ågren (2004) who models the relationship of growth rate to protein (mainly correlated with

N) content and to RNA (mainly correlated with P) content. From biochemical considerations, the ratio of N:C should increase linearly with growth rate while P:C should increase quadratically. This is because the rate of catalysis by proteins that is required to support N-limited growth is directly proportional to the specific growth rate, while the rate of synthesis of proteins, i.e. the catalytic activity of the ribosome complement, involves a quadratic relationship of growth rate to P content. The general model of Ågren (2004) shows an increase in the protein:rRNA (N:P) ratio with increasing growth rate up to a certain growth rate, with a decline in protein:rRNA with further increase in specific growth rate up to the maximum rate. Ågren (2004) then compares the modeled relationship with measurements in the literature on the specific growth rate of the freshwater green alga Selenastrum minutum and seedlings of the tree Betula pendula as a function of organismal C, N and P content when growth rate is limited by the supply of N or that of P. This analysis shows that the observed relationship of the organismal N:P ratio to specific growth rate is generally above the predicted ‘hump-back’ relationship for P limited growth, and below the predicted relationship for N limited growth, with convergence on the predicted value at the highest specific growth rates. The model of Ågren (2004) shows that the decreasing N:P ratio in biomass with increasing growth rate in many chemoorganotrophs does not apply to all of the modeled relationship between specific growth rate and N:P ratio, regardless of their trophic mode, and that the ‘deviations’ of photolithotrophs can be explained in general terms by the model. Ågren (2004) admits that his model does not necessarily give the best fit to the experimental data, but rightly maintains that the model is useful in view of its firm mechanistic basis and its explanatory powers. Klausmeier et al. (2004a,b) have also modeled N:P, and protein: rRNA, in relation to growth rate in phytoplankton (cf. Tyrrell, 1999; Flynn, 2002; see also Leonardos & Geider, 2004). For the other two elements we consider, C and Fe, there is a great contrast in the genetic and acclimatory variations in their content in biomass. There is very little possibility of variation of C in dry biomass, and especially in ash-free dry biomass, even with the greatest variation in content of the major biochemical components, i.e. protein, nucleic acids, carbohydrates, lipid (Geider & La Roche, 2002). However, there is a much greater possibility of variation in Fe content in biomass, based on observation and modeling (Raven, 1988, 1990; Sañudo-Wilhelmy et al., 2001; Kustka et al., 2003; Strzepek & Harrison, 2004).

Evolution of oligotrophy

Nutrient limitations on photosynthetic growth in the ocean For marine phytoplankton the Redfield Ratio (Redfield, 1958) of the major elements C, N and P of C106:N16:P1 (by atoms) is a spatially and temporally averaged elemental ratio of these organisms (Falkowski, 2000; Geider & La Roche, 2002). Comparison of this ratio with the elemental ratios in algal cultures whose growth rate had been controlled by variations in the supply of available N and P in the growth medium led Goldman et al. (1979) to conclude that marine phytoplankton were, in general, growing at close to their maximum specific growth rate. The averaged content of C, N and P for a range (originally 15 species, and ultimately 25) of marine phytoplankton organisms cultured under nutrient-replete conditions is close to that of the Redfield Ratio (Quigg et al., 2003; Ho et al., 2003; Falkowski et al., 2004), especially with respect to the N:P ratio. However, for the picoplanktonic cyanobacteria of the oligotrophic ocean, the C:P and N:P ratios in nutrient replete culture were significantly higher than the Redfield Ratio (Bertilsson et al., 2003; Heldal et al., 2003; cf. Klausmeier et al.,2004a,b). The significance of this ‘deviation’ for the ecology of the strains of Prochlorococcus marinus and of Synechococcus sp. is considered below. Sterner & Elser (2002) and Hedin (2004) point out that the N:P ratios in embryophytic land plants are usually higher than the Redfield ratio, and the C:N and C:P ratios are considerably higher; this is presumably related to organic structural material containing minimal N and P (Raven, 2000; Raven et al., 2004a) and also, in part, to the extracellular bound P which is more significant in phytoplankton than in land plants (Sañudo-Wilhelmy et al., 2004). Following the work of Redfield, geochemists embraced the view that P was the element limiting primary productivity in the ocean, and that any temporary limitation by N could be overcome by the biological fixation of N2 from the ‘infinite’ pool in the ocean and, especially, the atmosphere (Falkowski & Raven, 1997; Tyrell, 1999; Falkowski, 2000). That this is not generally the case is shown by comparison of the average of a large number of measurements of the NO3–:HPO42– ratios in the surface ocean with the Redfield Ratio (Falkowski & Raven, 1997). The ratio of the nutrients is slightly lower than the Redfield Ratio, i.e. 15.0 for the Atlantic Ocean, 14.8 for the Pacific Ocean, and 14.3 for the Indian Ocean. The present ocean is, over most of its area, N-limited, so that there is presumably a limitation on diazotrophy relative to the algebraic sum of other combined N inputs from rivers and the atmosphere and combined N losses by sedimentation and denitrification. The limitation of diazotrophy in the surface ocean by the supply of some nutrient which is required in

269

larger amounts by cells fixing N2 than by other cells (Sañudo-Wilhelmy et al., 2001; Kustka et al., 2003) has now been shown in many places. In general, the limiting nutrient is Fe (supplied mainly by dust input), although in the eastern Equatorial Atlantic, Fe and P co-limit, while in the south-western North Atlantic the limitation on diazotrophy is by P and (with deep mixing) light (Sañudo-Wilhelmy et al., 2001; Mills et al., 2004). In other parts of the ocean, the so-called high nutrient-low chlorophyll (HNLC) areas, there are low primary production rates despite the presence of relatively high NO3– and HPO42– concentrations. These HNLC areas, the north-eastern Subarctic Pacific, the eastern Equatorial Pacific, and the Southern Ocean, are limited by Fe, in this case not for N2 fixation, but for primary productivity using combined N (Martin & Fitzwater, 1988; Martin et al., 1990; Falkowski & Raven, 1997; Watson et al., 2000). There are also parts of the ocean in which primary production is limited, at least intermittently, by P supply (Karl et al., 1993; Karl & Tien, 1997; Cullen, 1999; Wu et al., 2000; Benitez-Nelson & Karl, 2002; Ridare & Guieu, 2002; Bjorkman & Karl. 2003; Heldal et al., 2003; Krom et al., 2004). The very low cellular P quota of picoplanktonic cyanobacteria from the oligotrophic ocean (Bertilsson et al., 2003; Heldal et al., 2003) could be viewed as an adaptation to a very low P availability. Bertilsson et al. (2003) point out that the P content of these exceptionally small cells is close to the theoretical lower limit, with ~ 50% of the cell quota of P in DNA despite the very small genomes of these organisms (Moore et al., 1998; Bryant, 2003; Dufresne et al., 2003; Palenik et al., 2003; Rocap et al., 2003; Lindell et al., 2004; Millard et al., 2004; Cavalier-Smith, 2005; Gregory, 2005; Knight et al., 2005). They also have relatively low rRNA contents, although work in which their rRNA content has been measured as a function of growth rate has not yielded as consistent a pattern as for many chemo-organotrophs (references in Raven et al., 2004a). The high C:P ratio in these cells has been suggested by Heldal et al. (2003) to relate to a ‘low-cost’ (in terms of P) means of increasing the P-specific rate of uptake of P by the cells by slightly increasing the cell volume per unit P. This resource acquisition strategy contrasts with that of oligotrophic chemo-organotrophic (saprotrophic) bacteria in which C and energy may well be limiting, and where the cell volume is maintained by a dilute (low C per unit volume) cytoplasm (Koch, 1997; Button et al., 1998; Button & Robertson, 2001). A further means of increasing the surface area per unit of limiting resource in the cytoplasm is vacuolation (Raven, 1987), an option which is not open to picoplankton (Raven, 1987, 1998, 1999; Schulz & Jørgensen, 2001). Fe is the limiting nutrient for high-nutrient, low

270

J A RAVEN ET AL.

chlorophyll parts of the ocean (Martin & Fitzwater, 1988; Martin et al., 1990; Moore et al., 2002). Mesoscale Fe enrichment in these areas increases the biomass of primary producers and, to an even greater extent, primary productivity. Fe addition does not act equally on all members of the primary producer community by increasing the populations of all species in the same ratios as they occur in the natural low-Fe environment; rather, there is a greater increase in large diatoms than in smaller phytoplankton organisms (e.g. Watson et al., 2000; Boyd, 2002; Boyd et al., 2004). The increase in abundance of large diatoms with Fe addition relates to increased diffusive supply of Fe to the cell surface of the larger cells (Sunda & Huntsman, 1997; Raven, 1988, 1999; Volker & Wolf-Gladrow, 1999, 2000) rather than to a deficiency in the range of Fe uptake mechanisms at the plasmalemma level in diatoms (Armbrust et al., 2004). Nutrient limitations on photosynthetic growth on land On land, as with the ocean, we concentrate on the major nutrients N and P, and look for large-scale patterns. Hedin (2004) summarises data from Reich & Oleksyn (2004), Kerkhoff et al. (unpublished) and McGroddy et al. (2004) for the N:P ratio in leaves of trees as a function of latitude. There is a decrease in the (atomic) N:P ratio with latitude from 45–50 at the equator to 12–30 at 70o. Reich & Oleksyn (2004) show that their results are consistent with the proposal that colder climates favour high leaf N:C and P:C ratios, as this partly offsets the effects of cold on metabolic rate. The results are also consistent with variations in the N and P supply with latitude. The data and analyses do not permit distinction between the relative importance of these two suggestions. As indicated above, the N:P ratio in leaves of land plants is generally higher than the Redfield Ratio of 16:1. However, the N:P data for phytoplankton are for cells which are photosynthesising and growing, while the data for higher plants comes from mature leaves which are photosynthesising but no longer growing. As pointed out by Raven et al. (2002a, 2004a) this means that the minimum provision of rRNA in phytoplankton cells involves protein synthesis for growth and maintenance, while in mature leaves the rRNA is only needed for maintenance-related protein synthesis, e.g. in replacing photodamaged D1 protein in the reaction centre of photosystem two. Part of the explanation for the higher N:P ratio in mature leaves than in phytoplankton cells could relate to the lower requirements for rRNA in the mature leaves. The data on Betula pendula analysed by Ågren (2004) involved whole-plant N and P. A further interesting finding (McGroddy et al., 2004; Hedin, 2004) is the latitudinal changes in the N:P ratio of abscised leaves. While the N:P ratio in

abscised leaves of temperate forests is similar to that of mature leaves before preparation for abscission, the just-abscised leaves in tropical forests have N: P ratios up to twice that of functional leaves on the tree. This means that tropical trees resorb relatively more P than N from their leaves prior to abscission, while temperate trees resorb the two elements in approximately the same proportion as they occur in the leaves. McGroddy et al. (2004) speculate that there is an upper limit on the fraction of total leaf N that can be resorbed prior to abscission in temperate trees which appear to be generally N-limited. Continuing with the theme of N or P limitation of forests, Wardle et al. (2004) examined N and P limitation in forests in six sites around the world as a function of the time for which they had been undisturbed since they became established after, variously, glacial retreat, lava flow, dune formation, fire, or uplift of marine sediments. The decline, as decrease in m2 of tree basal area per hectare of habitat of undisturbed forests is correlated with a decrease in P availability relative to that of N. Birks & Birks (2004) suggest that increasing P limitation could underlie the declining phases (oligocratic, telocratic) of forests after the phases (protocratic, mesocratic) of increasing biomass during the ‘typical’ interglacial episodes in the Pleistocene glacial-interglacial cycles. Nutrient use efficiency in plant growth in terms of how effectively already-acquired nutrients are used in agriculture is commonly expressed as the rate at which biomass is gained per unit of the nutrient already present in the plant (nitrogen productivity in the usage of Ågren, 2004). Ecology, which deals more frequently with longer-lived plants, also uses the mean residence time of the nutrient element in the plant: this, multiplied by the nutrient productivity, yields a redefined nutrient use efficiency (Berendse & Aerts, 1987). There are trade-offs between the two components of nutrient use efficiency in the sense of Berendse & Aerts (Silla & Escudero, 2004) which could be significant in low-input agriculture. One aspect of nutrient availability on land today which differs from that of much of history of terrestrial biota is the anthropogenic input of atmospheric combined N (Raven & Yin, 1998; NEGTAP, 2001). This anthropogenic N comes as NHy (NH3 + NH4+) from intensive animal husbandry, biomass burning and low-temperature combustion of coal, and as NOx (NO + NO2 + NO3–) from internal combustion engines and gas turbines (NEGTAP, 2001). These inputs are localised for NHy, with deposition mainly within tens to hundreds of km of the source. NOx travels up to thousands of kilometres from the source, but dispersal is not global as is the case for anthropogenic CO2. There is no analogous anthropogenic atmospheric input of P. Accordingly, much intensive agriculture is partly underwritten

Evolution of oligotrophy

by anthropogenic combined N inputs from the atmosphere. Molecular biology and elemental budgets Recent work (Baudouin-Cornu et al., 2001, 2004; Bragg & Hyder, 2004) shows intriguing relationships between genome base composition and the use of nutrient elements in producing nucleic acids and proteins. Baudouin-Cornu et al. (2001) found that S-assimilating enzymes of the saprotrophic chemo-organotrophs Escherichia coli and Saccharomyces cerevisiae were depleted in S relative to the average protein in these cells, and the C-assimilating enzymes were depleted in C. A less marked, but still statistically significant, similar trend was found for the N content of N-assimilating enzymes of Saccharomyces; there was doubt as to the enzymes involved in N assimilation in Escherichia so no analysis was undertaken. This means that the shortage of one of these three elements causes less restriction on producing the enzymes required to assimilate that element than to the production of the suite of other enzymes in the cell. The basis for this work was that of Mazel & Marlière (1989) who showed that the cyanobacterium Calothrix had the potential to make low-S versions of its most abundant protein, the apoprotein of the photosynthetic antenna pigment phycocyanin rather than the normal apoprotein, when grown in a low-S environment. Further work (Baudouin-Cornu et al., 2004) showed that the C content of the proteome (C atoms per protein) showed considerable variation among species, both for bulk protein and for orthologs. This work showed a strong correlation between a low C content in proteins (i.e. more amino acids with few C atoms in their side-chains, glycine and alanine, relative to asparagine, lysine and isoleucine ) and a high fraction of guanosine plus cytosine (G* + C*) in the genome. There is a weak positive correlation between G* + C* content of the genome and the N content of proteins, and no correlation between G* + C* in the genome and the S content of proteins. Using different methods, Bragg & Hyder (2004) independently showed the positive correlation between high G* + C* in the genome and a low C and high N content in the proteome. They also point out the correlation between the N:C ratio of individual amino acids and their codons. Applying these correlations to the three fully sequenced genomes of strains of Prochlorococcus marinus, MED4 has a G* +C* of 30.8%, SS120 has 36.4% and MIT 9313 has 50.7%. These are in the low to mid range of G* + C* values, and correspond to high to mid-value C:N values in the proteome, in general agreement with the elemental composition measurements for whole cells (Bertilsson et al., 2003; Heldal et al., 2003). It must be acknowledged

271

that the C:N in the proteome is not the major determinant of the C:N ratio of the cells and the fraction of C in the dry matter, granted the range of values of C:N ratios in the protein amino acids and the observed range of amino acid compositions of the components of the proteome. The values depend more on such factors as the ratio of protein to N-free structural components and compounds storing C and energy, and whether the C and energy is stored as polysaccharide or as lipid (Geider & La Roche, 2002). Further determinants of the C:N ratio in organisms are the nature of the light-harvesting complexes in photosynthesis and of compatible solutes, free radical scavengers, defence compounds and UV screens (Raven, 2000; Raven et al., 2004a). Furthermore, it is possible that the content of G* + C* in the genome may also be selected for by the extent of exposure to UV-B radiation in the normal environment of the organism, since one of the damaging results of the absorption of UV-B radiation is the formation of thymidine dimers. Clearly, fewer such dimers will be formed if the genome has a lower A* + T* (adenine plus thymidine), i.e. a higher G* + C*, as a result of chronic UV-B exposure, all other things being equal (Hess et al., 2001). However the evidence from the different genotypes of Prochlorococcus does not support the view that UV-B flux in the natural environment is an important selective factor for the A* + T* content of the genome (Hess et al., 2001). Evolution of Oligotrophy: Implications for Low Input Agricultural Systems Background We have seen that resource deprivation was a very significant factor in the evolutionary origin of photosynthetic organisms, and of many important functional and structural changes during the evolution of these organisms. How can these effects which have occurred over at least the last 2.7 Gyr be related to the development of low input agricultural systems? It must be acknowledged that early agriculture was, perforce, low-input, and that much agriculture in developing countries is low-input. Furthermore, present-day agriculture uses low input concepts. One example is the use of diazotrophic legumes in crop rotations, providing combined N for themselves and for subsequent crops. Another example is the use of C4 arable and pasture grasses, with their higher water use efficiency of growth, in semi-arid areas. A third example is the use of species with cluster roots in relation to P acquisition: examples are some species of Lupinus which have cluster roots as well as N2-fixing root nodules, and the proteaceous shade tree Grevillea in tropical agriculture and the fruit (nut) tree Macadamia of tropical and sub-

272

J A RAVEN ET AL.

tropical sylviculture. These species are often used outside their normal range, and there is considerable potential to further extend their range, e.g. Andrews et al. (2001a). We consider in turn the resources C (and H2O and photosynthetically active radiation), N, P and Fe. Carbon, H2O and photosynthetically active radiation The first point that we make here is that it is worth considering non-photosynthetic organisms as genetic resources for plant breeding. Perhaps the most obvious chemo-organotrophic sources are common symbionts such as arbuscular mycorrhizas; a point that will be followed up under the consideration of P and Fe. More taxonomically restricted in their distribution are N2-fixing root nodules with their chemo-organotrophic rhizobia; these will be followed up when N nutrition is considered. Increasing or maintaining the rate of photosynthesis despite less input of H2O, N, P or Fe might be achieved by genetic modification. Perhaps the most frequently suggested means of reducing H2O use per unit of photosynthetic C gain by up to 30% in a C3 crop is the introduction of some or all components of a C4 pathway of photosynthesis (Matsuoka et al., 2001; cf. Brown, 1999). Such a transformation could also economise on N or Fe requirements to achieve a given rate of photosynthesis per unit biomass (Raven, 1990; Brown, 1999). Thus far such transformations have not been successfully performed. This should not be seen as criticism of the scientists involved, since the C4 involves a number of enzymes which, although also found in C3 plants, are expressed at a greater level, usually as a different isoform (Sage & Monson, 1999; Keeley & Rundell, 2003). Furthermore, the pathway usually involves the co-operation of two cell types with differential gene expression (Sage & Monson, 1999; Keeley & Rundell, 2003). C4 metabolism has evolved independently at least 31 times in flowering plants (Kellog, 1999; Keeley & Rundell, 2003), so it is not a very ‘difficult’ pathway in terms of natural selection. Furthermore, new hope may be found for the introduction of C4 photosynthesis into C3 crops in the increasing range of organisms which are known to have C4 photosynthesis involving only a single cell type (Edwards et al., 2004) rather than when the component reactions are divided between two cell types. The organisms with this mechanism include a number of species in the terrestrial flowering plant family Chenopodiaceae, the submerged aquatic flowering plant Hydrilla (Hydrocharitaceae), and, among algae, the green macroalga Udotea flabellum and, possibly, Thalassiosira pseudonana (Edwards et al., 2004). A factor in determining the essentiality of the conversion of C3 crops to C4 metabolism is the increasing CO2 level in the atmosphere which,

by acclimation of the plants, can achieve a mixture of one or more of increased CO2 fixation rate, less H2O lost in transpiration per unit C fixed, and less N required in the plant to achieve a given rate of photosynthesis (Long et al., 2004). Less radical transgenic possibilities discussed by Raven et al. (2004a) involve the introduction of genes from non-plant photosynthetic organisms into C3 vascular plants. Miyagawa et al. (2001) expressed the cyanobacterial gene for the bifunctional fructose1,6-bisphosphate-1-phosphatase/sedoheptulose-1,7bisphosphate-1-phosphatase in Nicotiana plastids. Despite the presence of separate enzymes for the two activities in embryophytes, the transformed plants showed increased photosynthetic and growth rates, at least under optimal conditions (Miyagawa et al., 2001). In the context of low-input agriculture, it is not known how the transformed Nicotiana performs with restricted water or N availability. More immediately associated with increased resource use efficiency in C3 photosynthesis by crops is the replacement of the native Form IB Rubisco by algal Form ID Rubiscos with higher CO2/O2 selectivities, e.g. from red algae or diatoms. Such a substitution could increase the photosynthetic rate per unit nitrogen over the entire range of CO2 and photosynthetically active radiation (Whitney et al., 2001). However, while Nicotiana transformed with a diatom Rubisco produces a transcript of the gene, the translation products are not assembled into a functional enzyme (Whitney et al., 2001). Expression of the Form II Rubisco from the α-proteobacterium Rhodospirillum rubrum in Nicotiana only supported the photosynthesis and growth of the plant at high CO2 concentrations, due to the very low CO2/O2 selectivity of the bacterial enzyme (Whitney & Andrews, 2001). A further potential means of economizing on N in photosynthesis is by altering the N allocated to light-harvesting pigment-protein complexes while maintaining the photon absorption capacity. However, higher plants have some of the most Neconomical light harvesting structures known, with much lower ratios of apoprotein to chlorophyll and carotenoid chromophores than characterize the phycobiliproteins of most cyanobacteria, and glaucocystophyte and rhodophyte algae (Raven et al., 2004a). Another objective for breeding is manipulation of leaf size, morphology and orientation which can determine the fraction of incident photosynthetically active radiation which is absorbed and the allocation of this energy not incorporated into biomass during photosynthesis between the latent heat of evaporation during transpiration and loss as sensible heat (Jones, 1992). The data assembled by Raven et al. (2004b) show that, phenotypically, nutrient deficiency decreases the dry weight gain per unit water transpired, i.e. a

Evolution of oligotrophy

lower water use efficiency than in nutrient-sufficient organisms. We do not know of attempts to increase the water use efficiency of plants growing with a decreased nutrient availability. Nitrogen Dealing first with N acquisition, much effort has been expended on increasing the role of biological N2 fixation in the nutrition of non-nodulated crop plants. Associative N2 fixation, i.e. diazotrophic Bacteria (never, as far as is known, Archaea) on or, more commonly, within plants but with little or no change in plant structure, has been reported from a variety of land plants (e.g. Hurek et al., 2002). Bringing about an increased contribution of associative diazotrophy to the N supply to crops has been a significant preoccupation of those interested in diazotrophy in the context of agriculture (McCully, 2001; Lodewyckx et al., 2002; Andrews et al., 2003). Andrews et al. (2003) point out that low applications of N-fertilisers in Brazil over the last century could have selected crop varieties which can especially benefit from (associative) N2 fixation (Baldani et al., 2002). Potential inputs of combined N of up to 100 kg N ha–1 are possible for sugar cane (Saccharum spp.) and for wetland rice (Oryza sativa) (Urquiaga et al., 1992; Boddey et al., 2001; James et al., 2004). However, at the moment little of agricultural significance has been achieved in this area. While endophytic diazotrophic bacteria may have potential as inoculants for specific crops, substantial experimentation is required before this can be adequately assessed. Engineering of nodule-based (rhizobial) diazotrophy into non-leguminous crops is even more of a challenge than increasing the role of associative N2 fixation, although the potential gain is much higher, i.e up to 200 kg combined N ha–1. Legume nodules have complex interacting developmental, structural, biochemical and physiological characteristics related inter alia to protection of nitrogenase from O2 inhibition and to supplying organic C from, and removing organic N to, the host plant, and maintaining H2O balance (Raven et al., 1989). Legume root nodules clearly involve a substantial number of host genes, probably a greater number than are involved in C4 photosynthesis. Certainly C4 photosynthesis has a greater phylogenetic range at higher taxonomic levels in the angiosperms than do rhizobial, or actinorrhizal, diazotrophic symbioses involving nodules (Kellog, 1999). It is clear that engineering nodulated symbiotic diazotrophy in non-legume crop plants is some way off. More immediately relevant to the short to medium term of modifying crop plants in the context of lowinput agriculture is the possibility of capitalizing on the occurrence of rhizobial symbioses with legumes in a wide range of environments. It would thus be

273

possible that rhizobia can be selected for particular agricultural environments, and there has been some success here (Cummings & Andrews, 2003). An example is growth of Phaseolus vulgaris on the Cerrados which comprise 25% of the land area of Brazil. The Cerrados is subjected to temperatures in excess of 40ºC, with a limited water supply and acidified soils with associated Al toxicity. P. vulgaris is grown on 1.2 million hectares of Cerrados land, and rhizobial inoculants tolerant of these conditions are required. Commercial inoculants such as Rhizobium leguminosarum bv. Phaseoli (SEMIA 4064) lose their ability to fix N2 under Cerrados conditions (Hungria & Vargas, 2000). A successful innoculant for P .vulgaris in the Cerrados is R. tropici PRF81, isolated from Cerrados soil and recommended as a P. vulgaris innoculant in Brazil since 1998 (Hungria et al., 2000). Yield increases of up to 900 kg ha–1, compared to noninoculated controls, equivalent to the addition of 30 kg combined N ha–1, were found in a 2 year trial of PRF81 applied to P. vulgaris in the Cerrados. Five other strains of indiginous rhizobia were as effective as currently recommended P. vulgaris inoculants, including PRF81, in nodulating, and improving yields of, P.vulgaris (Mostasso et al., 2002). Similar results have been obtained for Vigna unguiculata in Ghana; here inoculating with rhizobia isolated from Ghanaian soil gave yields comparable to those of plants fertilised with 70 kg combined N ha–1 (Fening & Danso, 2002) The acquisition of combined N by crops is also susceptible to modification, with sensitivity analysis indicating roots morphology rather than transporters as the target for improving combined N acquisition in low-input agriculture (Tinker & Nye, 2000; Andrews et al., 2004; Epstein & Bloom, 2005). ‘Improved’ here means an increased fraction of supplied fertiliser N which is used by the plant, with correspondingly less lost from the system as gaseous NHy or NOx, or as NO3– in through-flow. Even after breeding programmes aimed at maximising transfer of N to grass caryopses, 25% of N remains in vegetative tissues of high yielding wheat cultivars, and 100 kg N ha–1 remains in unharvested vegetative tissue of barley crops (Andrews et al., 2004). For cereals, selection for high GS1 (glutamine synthetase 1) in senescing leaves and, in some cases, high NADHGOGAT (NADH-linked glutamine-oxoglutarate aminotransferase, or glutamate synthase) in developing grains can help maximise the transfer of plant N to grains, and increase spikelet weight and N content (Andrews et al., 2004).The shortage of combined N (or of P, S, Fe or H2O) increases the fraction of plant biomass allocated to the root system (Andrews et al., 1999, 2001b). The discussion of N acquisition in terrestrial vascular plants under ‘Evolution’ showed that

274

J A RAVEN ET AL.

there were genetic differences in the location of nitrate assimilation in relation to the optimal growth conditions of a genotype. Manipulation, e.g. by genetic transformation, of the site of nitrate assimilation could be used to increase crop yield under specific conditions of temperature and of water availability (Andrews et al., 2004). Turning to N use within the plant, C4 metabolism, and the effects of increasing atmospheric CO 2 concentrations on plants with C3 metabolism, have already been mentioned as means by which crop plants can economise on N (as the major soluble protein Rubisco) within the plant in producing biomass. A second major protein component of leaves is the light-harvesting complexes of photosynthesis; we have already seen that there is no possibility of significantly reducing N use by modification of light-harvesting machinery. Raven et al. (2004a) have shown that most of the obvious means of reducing the N costs of non-protein, nonnucleic acid components of land plants have been pre-empted by evolution. Thus, cell walls consist largely of N-free polysaccharides and lignin, UV-B screens and free radical scavengers are also largely N-free, as are many of the compatible solutes which are important in crops derived from halophytes, or which are subjected to continuing water deficits (Raven et al., 2004a). One aspect of N nutrition that is also relevant to P acquisition is the involvement of N in the chitinous walls of mycorrhizal fungi. The chitin monomer has a C:N ratio of 8, while that of cellulose and other plant cell wall polysaccharides is infinity. For the arbuscular mycorrhizas, more generally thought to be associated with P rather than with N limitation (Smith & Read, 1997) this may not be too great a problem. However, for the various kinds of ectomycorrhizas, where the general view is that they are more associated with N than with P deficiency (Smith & Read, 1997), the N-containing polysaccharide in the cell walls of the fungal partner is an additional N requirement for the N-limited symbiosis. There does not seem to be any obvious solution to this additional N cost of the N-acquiring mechanism in low-input forestry with ectomycorrhizal trees such as members of the Pinaceae, and many members of the genus Eucalyptus sensu lato (often with arbuscular mycorrhizas as well, particularly when young), or for horticultural crops such as Vaccinium spp. (blueberry) with ericoid mycorrhizas. Phosphorus Land plants have a range of characteristics associated with improved P acquisition. These include symbioses (mainly arbuscular mycorrhizas) and non-symbiotic features, e.g. cluster roots, dauciform roots, surface phosphatases, and secretion of organic acids, H+ and reductants from roots with

no special morphological features. The symbiotic and the non-symbiotic features may overlap, e.g. secretion of organic acids (among other solutes) by roots infected by arbuscular mycorrhizas, especially under conditions of limited P supply (Jones et al., 2004; Usha et al., 2004). The resource costs of producing and operating these symbioses, structures and secretions include P itself, but also energy, C (and hence H2O loss in transpiration) and N, including N in the chitin of mycorrhizas. The resource costs other than those of P would not have major effects on growth if P was the major growthlimiting resource. It is not immediately clear how improved P acquisition can be achieved in crops by, for example, genetic modification until we have more understanding of the costs and benefits of the various symbiotic and non-symbiotic modes of P acquisition. We have already seen that there are few if any means of economizing on P use which do not compromise growth rate (Raven et al., 2004a). However, the use of mycorrhizal inoculants can consistently give greater crop growth rates after particular horticultural treatments such as some fungicides and soil solarisation (Wininger et al., 2003). Iron Fe deficiency in horticulture is commonly overcome by foliar application of chelated Fe. It is not clear if either the reductant-based or the siderophorebased mechanisms of Fe acquisition in higher plants could be improved by genetic modification in the context of low-input agriculture. Reducing the Fe requirement of the growth of O2-evolving organisms by replacing Fe-requiring catalysts with analogues which do not require Fe could perhaps achieve savings of 10% (Raven et al., 1999) in, for example, algae in the ‘red’ line of evolution, and many algae in the ‘green’ line of evolution, which have cytochrome c6 rather than plastocyanin, and CuZn superoxide dismutase rather than the Fe or Mn forms of this enzyme. However, in higher plants and their charophycean algal immediate ancestors some of these possibilities of saving on Fe requirements have already been achieved in evolution, e.g. the occurrence of plastocyanin rather than cytochrome c6 as the electron donor to photosystem I, and the occurrence of Cu-Zn superoxide dismutase as the major form of this enzyme in the plastids and cytosol (Raven et al., 1999). Larger savings in Fe can be achieved by manipulation of the ratio of the Fecontaining protein complexes in photosynthesis so as to maintain the rate of photosynthesis while reducing the content of photosystem I, the complex with the greatest Fe content (Raven et al., 1999). This has been achieved naturally in the diatom Thalassiosira oceanica, although apparently with a reduced ability to adjust photosynthesis to short-term environmental

Evolution of oligotrophy

changes (Strzepek & Harrison, 2004). Conclusions Several major evolutionary innovations in photosynthetic organisms can be related to the limited supply of resources. Progress is being made towards a mechanistic basis for variations in growth rate with the content of major nutrients in photosynthetic organisms. There are correlations between the C and N content of the proteome and the N content, and guanosine plus cytosine content of the genome. Both plesiomorphic and apomorphic characters can be related to oligotrophy. Knowledge of the evolution of oligotrophy will help to guide the breeding of plants for low input, sustainable agricultural systems using both conventional plant breeding and methods involving genetic transformation. Acknowledgements J A Raven acknowledges support from the Natural Environment Research Council UK for work on resource acquisition in diatoms. A Quigg acknowledges support from the Texas Institute of Oceanography. References Ågren G I. 2004. The C:N:P stoichiometry of autotrophs – theory and observations. Ecology Letters 7:185–191. Aiba S, Humphrey A E, Millis N F. 1973. Biochemical Engineering, 2nd edn. New York: Academic Press. Alberghina F A M, Sturani E, Goehlke J R. 1975. Levels and rates of synthesis of ribosomal ribonucleic acid, transfer ribonucleic acid, and protein in Neurospora crassa in different steady states of growth. Journal of Biological Chemistry 250:4381–4388. Anbar A D, Knoll A H. 2002. Proterozoic ocean chemistry and evolution: A bioinorganic bridge? Science 297:1137–1142. Anderson T R, Boersma M, Raubenheimer D. 2004. Stoichiometry: linking elements to biochemicals. Ecology 85:1193–1202. Andrews M. 1986. The partitioning of nitrate assimilation between root and shoot of higher plants. Plant, Cell and Environment 9:511–519. Andrews M, Sprent J I, Raven J A, Eady P E. 1999. Relationships between shoot to root ratio, growth and leaf soluble protein concentration of Pisum sativum, Phaseolus vulgaris and Triticum aestivum under different nutrient deficiencies. Plant, Cell and Environment 22:949–958. Andrews M, McKenzie D A, Joyce A, Andrews M E. 2001a. The potential of lentil (Lens culinaris) as a grain legume in the UK: an assessment based on a crop growth model. Annals of Applied Biology 139:293–300. Andrews M, Raven J A, Sprent J I. 2001b. Environmental effects on dry matter partitioning between shoot and root of crop plants: relations with growth and shoot protein concentration. Annals of Applied Biology 138:57–68. Andrews M, James E K, Cummings S P, Zavalin A A, Vinogarova L V, McKenzie B. 2003. Use of nitrogen fixing bacteria inoculants as a substitute for nitrogen fertilizer for dryland graminaceous crops: Progress made, mechanisms of action and future potential. Symbiosis 35:209–229.

275

Andrews M, Lea P J, Raven J A, Lindsey K. 2004. Can genetic manipulation of plant nitrogen assimilation enzymes result in increased crop yield and greater N-use efficiency? An assessment. Annals of Applied Biology 145:25–40. Armbrust E V, Berges J A, Bowler C, Green B R, Martinez D, Putnam N H, Zhou S, Allen A E, Apt K E, Bechner M, Brzezinski M A, Chaal B K, Chiovitti A, Davis A K, Demarest M S, Detter J C, Glavina T, Goodstein D, Hadi M Z, Hellsten U, Hildebrand M, Jenkins B D, Jurka J, Kapitonov V V, Kröger N, Lau W W Y, Lane, T W, Larimer F W, Lippmeier J C, Lucas S, Medina M, Montsat A, Obornik M, Parker M S, Palenik B, Pazour G J, Richardson P M, Rynearson T A, Saito M A, Schwartz D C, Thamatrakoln K, Valentin K, Vardi A, Wilkerson F P, Rokhsar D S. 2004. The genome of the diatom Thalassiosira pseudonana: ecology, evolution, and metabolism. Science 306:79–86. Baldani J I, Reis V M, Baldani V L D, Goi S R, Döbereiner J. 2002. A brief story of nitrogen fixation in sugarcane – reasons for success in Brazil. Functional Plant Biology 29:417–423. Baudouin-Cornu P, Surdin-Kerjan Y, Marlière P, Thomas D. 2001. Molecular evolution of protein atomic composition. Science 293:297–300. Baudouin-Cornu P, Schuerer K, Marlière P, Thomas D. 2004. Intimate evolution of proteins. Proteome atomic content correlates with genome base composition. Journal of Biological Chemistry 279:5421–5428. Beerling D J, Osborne C P, Chaloner W G. 2001. Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the Late Palaeozoic era. Nature 410:352–354. Bekker A, Holland H D, Wang P-L, Rumble D III, Stein H J, Hannah J L, Coetzee L L, Beukes N J. 2004. Dating the rise in atmospheric oxygen. Nature 427:117–120. Benitez-Nelson C R, Karl D M. 2002. Phosphorus cycling in the North Pacific Subtropical Gyre using 32P and 33P. Limnology and Oceanography 47:762–770. Berendse F, Aerts R. 1987. Nitrogen use efficiency: a biologically meaningful definition? Functional Ecology 1:293–296. Bertilsson S, Berglund O, Karl D M, Chisholm S W. 2003. Elemental composition of marine Prochlorococcus and Synechococcus: implications for the ecological stoichiometry of the sea. Limnology and Oceanography 48:1721–1731. Birks H J B, Birks H H. 2004. The rise and fall of forests. Science 305:484–485. Bjerrum C J, Canfield D E. 2002. Ocean productivity before about 1.9 Gyr ago limited by phosphorus adsorption onto iron oxides. Nature 417:159–162. Bjorkman K M, Karl D M. 2003. Bioavailability of dissolved organic phosphorus in the euphotic zone at station ALOHA, North Pacific Subtropical Gyre. Limnology and Oceanography 48:1049–1057. Blankenship R E. 2002. Molecular Mechanisms of Photosynthesis. Malden, Massachusetts: Blackwell Science. Boddey R M, Polidoro J C, Resende A S, Alves B J R, Urquiaga S. 2001. Use of the 15N natural abundance technique for the quantitation of the contribution of N2 fixation to sugar cane and other grasses. Australian Journal of Plant Physiology 28:889–895. Bonven B, Gullov K. 1979. Peptide chain elongation rate and ribosomal activity in Saccharomyces cerevisae as a function of the growth rate. Molecular and General Genetics 170:225–230. Boucher Y, Douady C J, Papke R T, Walsh D A, Boudreau M E R, Nesbø C L, Case R J, Doolittle W F. 2003. Lateral gene transfer and the origins of prokaryotic groups. Annual Review of Genetics 37:283–328. Boyd P W. 2002. The role of iron in the biogeochemistry of the

276

J A RAVEN ET AL.

Southern Ocean and equatorial Pacific: a comparison of in situ iron enrichments. Deep-Sea Research Part II – Topical Studies in Oceanography 49:1803–1821. Boyd P W, Law C S, Wong C S, Nojiri Y, Tsuda A, Levasseur M, Takeda S, Rivkin R, Harrison P J, Strzepek R, Gower J, McKay R M, Abraham E, Arychuk M, Barwell-Clarke J, Crawford W, Crawford D, Hale M, Harada K, Johnson K, Kiyosawa H, Kudo I, Marchetti A, Miller W, Needoba J, Nishioka J, Ogawa H, Page J, Robert M, Saito H, Sastri A, Sherry N, Soutar T, Sutherland N, Taira Y, Whitney F, Wong S-K E, Yoshimura T. 2004. The decline and fate of an iron-induced subarctic phytoplankton bloom. Nature 428:549–553. Bragg J G, Hyder C L 2004. Nitrogen versus carbon use in prokaryotic genomes and proteomes. Proceedings of the Royal Society of London Series B (Supplement) 271: S374–S377. Brasier M D, Green O W, Jephcoat A P, Kleppe A K, Van Kranendonk M J, Lindsay J F, Steele A, Grassineau N V. 2002. Questioning the evidence for Earth’s earliest fossils. Nature 416:76–81. Brockes J L, Logan G A, Buick R, Summons R E. 1999. Archaean molecular fossils and the early rise of eukaryotes. Science 285:1033–1036. Brown R H. 1999. Agronomic implications of C4 photosynthesis. In C4 Plant Biology, pp. 473–507. Eds R F Sage and R K Monson. San Diego: Academic Press. Bryant D A. 2003. The beauty of small things revealed. Proceedings of the National Academy of Sciences of the USA 100:9647–9649. Button D K, Robertson B R. 2001. Determination of DNA content of aquatic bacteria by flow cytometry. Applied and Environmental Microbiology 67:1636–1645. Button D K, Robertson B R, Lepp P W, Schmidt T M. 1998. A small, dilute-cytoplasm, high-affinity, novel bacterium isolated by extinction culture and having kinetic constants compatible with growth at ambient concentrations of dissolved nutrients in seawater. Applied and Environmental Microbiology 64:4467–4476. Carlson R E, Simpson J. 1996. A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society. 96 pp. Cavalier-Smith T. 2005. Economy, speed and size matter: evolutionary forces driving nuclear genome miniaturization and expansion. Annals of Botany 95:147–175. Cohen D. 1966. Optimising reproduction in a randomly varying environment. Journal of Theoretical Biology 12:119–129. Cooke R R M, Hurd C L, Lord J M, Peake B M, Raven J A, Rees T A V. 2004. Iron and zinc content of Hormosira banksii in New Zealand. New Zealand Journal of Marine and Freshwater Research 38:73–85. Cornell S, Rendall A, Kickells T. 1995. Atmospheric inputs of dissolved organic nitrogen to the oceans. Nature 376:243–246. Cullen J J. 1999. Iron, nitrogen and phosphorus in the ocean. Nature 402:372. Cummings S P, Andrews M. 2003. Use of specific N2 fixing genotypes as crop inoculants: progress made and potential for stressful soil environments. In Advances in Ecological Sciences 19: Ecological and Sustainable Development IV, pp. 755–765. Eds E Tiezzi, C A Brebbia and J L Usó. Southampton: WIT Press. Douglas A E. 1994. Symbiotic Interactions. Oxford: Oxford University Press. Douglas A E, Raven J A. 2003. Genomes at the interface between bacteria and organelles. Philosophical Transactions of the Royal Society of London Series B 358:5–18. Dufresne A, Salanoubat M, Partensky F, Artiguenave F, Axmann I M, Barbe V, Duprat S, Galperin M Y, Koonin E V, Le Gall F, Makarova K S, Ostrowski M, Oztas S,

Robert C, Rogozin I B, Scanlan D J, Tandeau de Marsac N, Weissenbach J, Wincker P, Wolf Y I, Hess W R. 2003. Genome sequence of the cyanobacterium Prochlorococcus marinus SS120, a nearly minimal oxyphototrophic genome. Proceedings of the National Academy of Sciences of the USA 100:10020–10025. Edwards D, Abbott G D, Raven J A. 1996. Cuticles in early land plants: a palaeoecological evaluation. In Plant Cuticles, pp 1–31. Ed. G Kierstens. Oxford: Bios Scientific Publishers. Edwards G E, Franceschi V R, Voznesenskaya E V. 2004. Single-cell C4 photosynthesis versus the dual-cell (Kranz) paradigm. Annual Review of Plant Biology 55:173–196. Elser J J, Acharya K, Kyle M, Cotner J, Makino W, Markow T, Watts T, Hobbie S, Fagan W, Schade J, Hood J, Sterner R W. 2003. Growth rate – stoichiometry couplings in diverse biota. Ecology Letters 6:936–943. Epstein E, Bloom A J. 2005. Mineral Nutrition of Plants: Principles and Perspectives, Second Edn. Sunderland, Massachusetts: Sinauer Associates, Inc. Falkowski P G. 1997. Evolution of the nitrogen cycle and its influence on the biological sequestration of CO2 in the ocean. Nature 387:272–275. Falkowski P G. 2000. Rationalising elemental ratios in unicellular algae. Journal of Phycology 36:3–6. Falkowski P G, Raven J A. 1997. Aquatic Photosynthesis. Malden, Massachusetts: Blackwell Science. Falkowski P G, Katz M E, Knoll A H, Quigg A, Raven J A, Schofield O, Taylor F J R. 2004. The evolution of modern eukaryotic phytoplankton. Science 305:354–360. Fening J O, Danso S K A. 2002. Variation in symbiotic effectiveness of cowpea bradyrhizobia indigenous to Ghanaian soils. Applied Soil Ecology 21:23–29. Flynn K J. 2002. How critical is the critical N:P ratio? Journal of Phycology 38:961–970. Friis E M, Doyle J A, Endress P K, Leng Q. 2003. Archaefructus – angiosperm precursor or specialized early angiosperm? Trends in Plant Science 8:369–373. Geider R J, La Roche J. 2002. Redfield revisited: variability of C:N:P in marine microalgae and its biochemical basis. European Journal of Phycology 37:1–17. Gerrienne P, Mayer-Berthaud B, Fairon-Demaret M, Streel M, Steemans P. 2004. Runcaria, a Middle Devonian seed plant precursor. Science 306:856–858. Goldman J C, McCarthy J J, Peavey D G. 1979. Growth rate influence on the chemical composition of phytoplankton in oceanic waters. Nature 299:210–215. Gregory T R. 2005. The C-value enigma in plants and animals: a review of parallels and an appeal for partnership. Annals of Botany 95:133–146. Grime J P. 2002. Plant Strategies, Vegetation Processes and Ecosystem Properties. Second Edn. Chichester: John Wiley & Sons. Hao S-G, Beck C B, Wang D-M. 2003. Structure of the earliest leaves: Adaptations to high concentrations of atmospheric CO2. International Journal of Plant Sciences 164:71–75. Healey F P, Hendzell L L. 1975. Effect of phosphorus deficiency on two algae growing in chemostats. Journal of Phycology 11:303–309. Hedin L O. 2004. Global organization of terrestrial plant– nutrient interactions. Proceedings of the National Academy of Sciences of the USA 101:10849–10850. Heldal M, Scanlan D J, Norland S, Thingstad F, Mann N H. 2003. Elemental composition of single cells of various strains of marine Prochlorococcus and Synechococcus using X-ray microanalysis. Limnology and Oceanography 48:1732–1743. Hess W R, Rocap G, Ting C S, Larimer F, Stilwagen S, Lamerdin J, Chisholm S W. 2001. The photosynthetic apparatus of Prochlorococcus: Insights through comparative

Evolution of oligotrophy

genomics. Photosynthesis Research 70:53–71. Hessler A M, Lowe D R, Jones R L, Bird D K. 2004. A lower limit for atmospheric carbon dioxide levels 3.2 billion years ago. Nature 428:736–738. Hetherington A M, Woodward F I. 2003. The role of stomata in sensing and driving environmental change. Nature 424:901–908. Ho T-Y, Quigg A, Finkel Z V, Milligan A J, Wyman K, Falkowski P G, Morel F M M. 2003. The elemental composition of some marine phytoplankton. Journal of Phycology 39:1145–1159. Hungria M, Vargas M A T. 2000. Environmental factors affecting N2 fixation in grain legumes in the tropics, with an emphasis on Brazil. Field Crops Research 65:151–164. Hungria M, Andrade D S, Chueire L M O, Probanza A, Guttierez-Mañero F J, Megías M. 2000. Isolation and characterization of new, efficient and competitive bean (Phaseolus vulgaris L.) rhizobia from Brazil. Soil Biology and Biochemistry 32:1515–1528. Hurek T, Handley L L, Reinhold-Hurek B, Piche Y. 2002. Azoarcus grass endophytes contribute fixed nitrogen to the plant in an unculturable state. Molecular Plant-Microbe Interactions 15:233–342. James E K, Gyaneshwar P, Olivares L, Andrews M. 2004. N2 fixation by non-legumes: The potential of associative and endophytic N2 fixation in agricultural systems. Aspects of Applied Biology 72, Advances in Applied Biology, pp. 125– 131. Warwick, UK: The Association of Applied Biologists. Jones D L, Hodge A, Kuzyakov Y. 2004. Plant and mycorrhizal regulation of rhizodeposition. New Phytologist 163:459– 480. Jones H G. 1992. Plants and Microclimate. A Quantitative Approach to Environmental Plant Physiology, 2nd edn. Cambridge: Cambridge University Press. Kah L C, Lyons T W, Frank T D. 2004. Low marine sulfate and protracted oxygenation in the Proterozoic. Nature 431:834–838. Karl D M, Tien G. 1997. Temporal variability in dissolved phosphorus concentrations in the Subtropical North Pacific Ocean. Marine Chemistry 56:77–96. Karl D M, Tien G, Dore J, Winn C D. 1993. Total dissolved nitrogen and phosphorus concentrations at the United States FGOFS Station ALOHA; Redfield Reconciliation. Marine Chemistry 46:203–208. Kasting J F, Catling D. 2003. Evolution of a habitable planet. Annual Review of Astronomy and Astrophysics 41:429–463. Katz M E, Finkel Z V, Grzebyk D, Knoll A H, Falkowski P G. 2005. Evolutionary trajectories and biogeochemical impacts of marine eukaryotic phytoplankton. Annual Review of Ecology, Evolution and Systematics 35:523–556. Keeley J E, Rundel P W. 2003. Evolution of CAM and C4 carbon concentrating mechanisms. International Journal of Plant Physiology 164:S55–S77. Kellog E A. 1999. Phylogenetic aspects of the evolution of C4 photosynthesis. In C4 Plant Biology, pp. 411–444. Eds R F Sage and R K Monson. San Diego:Academic Press. Kief D R, Warner J R. 1981. Co-ordinate control of synthesis of ribosomal ribonucleic acid and protein during a nutritional shift-up in Saccharomyces cerevisae. Molecular Cell Biology 1:1007–1015. Klausmeier C A, Litchman E, Daufresne T, Levin S A. 2004a. Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton. Nature 429:171–174. Klausmeier C A, Litchman E, Levin S A. 2004b. Phytoplankton growth and stoichiometry under multiple nutrient limitation. Limnology and Oceanography 49:1463–1470. Knight C A, Mollinari A N A, Petrov S A. 2005. The large genome constraint hypothesis: evolution, ecology and phenotype. Annals of Botany 95:177–190.

277

Knoll A H. 2003. Life on a Young Planet. The First Three Billion Years of Evolution on Earth. Princeton: Princeton University Press. Koch A L. 1997. Microbial physiology and ecology of slow growth, Microbiology and Molecular Biology Reviews 61:305–318. Kochian L V, Hoekenga O A, Piňeros M A. 2004. How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorus efficiency. Annual Review of Plant Biology 55:459–493. Krom M D, Herut B, Mantoura R F C. 2004. Nutrient budget for the Eastern Mediterranean: Implications for phosphorus limitation. Limnology and Oceanography 49:2105–2114. Kustka A, Sañudo-Wilhelmy S, Carpenter E J, Capone D G, Raven J A. 2003. A revised estimate of the iron use efficiency of nitrogen fixation, with special reference to the marine cyanobacterium Trichodesmium spp. (Cyanophyta). Journal of Phycology 39:12–25. Lamont B B. 1974. The biology of dauciform roots in the sedge Cyathochaete avenacea. New Phytologist 73:167–174. Lamont B B. 2003. Structure, ecology and function of root clusters – a review. Plant and Soil 248:1–19. Leonardos N, Geider R J. 2004. Responses of elemental and biochemical composition of Chaetoceros muelleri to growth under varying light and nitrate:phosphate supply ratios and their influence on critical N:P. Limnology and Oceanography 49:2105–2404. Lindell D, Sullivan M B, Johnson Z I, Tolonen A C, Rohwer F, Chisholm S W. 2004. Transfer of photosynthesis genes to and from Prochlorococcus viruses. Proceedings of the National Academy of Sciences of the USA 101:11013–11018. Lodewyckx C, Vangronsveld J, Porteous F, Moore E R B, Taghavi S, Mezgeay M, van der Lelie D. 2002. Endophytic bacteria and their potential applications. Critical Reviews in Plant Sciences 21:583–606. Long S P, Ainsworth E A, Rogers A, Ort D R. 2004. Rising atmospheric carbon dioxide: plants FACE the future. Annual Review of Plant Biology 55:591–628. Marschner H. 1995. Mineral Nutrition of Higher Plants, Second Edn. London: Academic Press. Marshall J E A, Hemsley A R. 2003. A Mid Devonian seedmegaspore from East Greenland and the origin of seed plants. Palaeontology 46:647–670. Martin J H, Fitzwater S E. 1988. Iron deficiency limits phytoplankton growth in the North-East Pacific Subarctic. Nature 331:341–343. Martin J H, Fitzwater S E, Gordon R M. 1990. Iron deficiency limits phytoplankton growth in Antarctic waters. Global Biogeochemical Cycles 4:5–13. Martin R E. 1996. Secular increase in nutrient levels through the Phanerozoic: Implications for productivity, biomass, and diversity of the marine biosphere. Palaios 11:209–219. Martin R E. 2003. The fossil record of biodiversity: nutrients, productivity, habitat area and differential preservation. Lethaia 36:179–193. Martin W, Rujan T, Richly E, Hansen A, Cornelsen S, Lins T, Leister D, Stoebe B, Hasegawa M, Penny D. 2002. Evolutionary analysis of Arabidopsis, cyanobacterial and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus. Proceedings of the National Academy of Sciences of the USA 99:12246– 12251. Matsuoka M, Furbank R T, Fukayama H, Miyao M. 2001. Molecular engineering of C4 photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 52:297–314. Maynard-Smith J, Szathmáry E. 1995. The Major Transitions in Evolution. Oxford: Oxford University Press. Mazel D, Marlière P. 1989. Adaptive eradication of methionine and cysteine from cyanobacterial light-harvesting proteins.

278

J A RAVEN ET AL.

Nature 341:245–248. McGroddy M E, Daufresne T, Hedin L O. 2004. Scaling of C:N:P stoichiometry in forests worldwide: implications of terrestrial Redfield-type ratios. Ecology 85:2390–2401. McCully M E. 2001. Niches for bacterial endophytes in crop plants: a plant biologists view. Australian Journal of Plant Physiology 28:983–990. McMurrough I, Rose A H. 1967. Effect of growth rate and substrate limitation on the composition and structure of the cell wall of Saccharomyces cerevisae. Biochemical Journal 105:189–203. Millard A, Clokie M R J, Shub D A, Mann N H. 2004. Genetic organization of the psbAD region in phages infecting marine Synechococcus strains. Proceedings of the National Academy of Sciences of the USA 101:11007–11012. Mills M M, Ridame C, Davey M, La Roche J, Geider R J. 2004. Iron and phosphorus co-limitation in the eastern tropical North Atlantic. Nature 429:292–294. Miyagawa Y, Tamoi M, Shigeoka S. 2001. Overexpression of a cyanobacterial fructose-1,6-/sedoheptulose-1,7bisphosphatase in tobacco enhances photosynthesis and growth. Nature Biotechnology 19:965–969. Moore J K, Doney S C, Glover D M, Fung I Y. 2002. Iron cycling and nutrient-limitation patterns in the surface waters of the world ocean. Deep-Sea Research II – Topical Studies in Oceanography 49:463–507. Moore L R, Rocap GM, Chisholm S W. 1998. Physiology and molecular phylogeny of coexisting Prochlorococcus ecotypes. Nature 393:464–467. Mostasso L, Mostasso F L, Dias B G, Vargas M A T, Hungria M. 2002. Selection of bean (Phaseolus vulgaris L.) rhizobial strains for the Brazilian Cerrados. Field Crops Research 65:121–132. Nagasaka S, Nishizawa N K, Watanabe T, Mori S, Yoshimura E. 2003. Evidence that electron-dense bodies in Cyanidium caldarium have an iron-storage role. Biometals 16:465–470. Naumann E. 1919. Nagra synpunkter angaende limnoplanktons okologi med sarskild hansym till fytoplankton. Svensk Botanisk Tidskrift 13:129–163. Naumann E. 1929. The scope and chief problems of regional limnology. International Revue gesamt Hydrobiologie 21:423–446. Navarro-González R, McKay C P, Mvondo D N. 2001. A possible nitrogen crisis for Archaean life due to reduced nitrogen fixation by lightning, Nature 412:61–64. NEGTAP. 2001. Transboundary Air Pollution: Acidification, Eutrophication and Ground-Level Ozone in the UK. London: The Stationery Office, with Defra. Osborne C P, Beerling D J, Lomax B H, Chaloner W G. 2004. Biophysical constraints on the origin of leaves inferred from the fossil record. Proceedings of the National Academy of Sciences of the USA 101:10360–10362. Paerl H W. 1985. Enhancement of marine primary productivity by nitrogen-enriched acid rain. Nature 315:747–749. Palenik B, Brahamsha B, Larimer F W, Land M, Hauser L, Chain P, Lamerdin J, Regala W, Allen E E, McCarren J, Paulsen I, Dufresne A, Partensky F, Webb E A, Waterbury J. 2003. The genome of a motile marine Synechococcus. Nature 424:1037–1042. Phoenix G K, Booth R E, Leake J R, Read D J, Grime J P, Lee J A. 2004. Simulated pollutant nitrogen deposition increases P demand and enhances root-surface phosphatase activities of three plant functional types in calcareous grassland. New Phytologist 161:279–289. Poulton S W, Fralick P W, Canfield D E. 2004. The transition to a sulphidic ocean ~ 1.84 billion years ago. Nature 431:173–177. Poynton R O. 1973. Effect of growth rate on the macromolecular composition of Prototheca zopfii, a colorless alga which

divides by multiple fission. Journal of Bacteriology 113:203–211. Quigg A, Finkel Z V, Irwin A J, Rosenthal Y, Ho T-Y, Reinfelder J R, Schofield O, Morel F M M, Falkowski P G. 2003. The evolutionary inheritance of elemental stoichiometry in marine phytoplankton. Nature 425:291–294. Raven J A. 1977. The evolution of vascular land plants in relation to supracellular transport processes. Advances in Botanical Research 5:153–219. Raven J A. 1984. Physiological correlates of the morphology of early vascular plants. Botanical Journal of the Linnean Society 88:105–126. Raven J A. 1986. Evolution of plant life forms. In On the Economy of Plant Form and Function pp. 421–492. Ed. T Givinsh. New York: Cambridge University Press. Raven J A. 1987. The role of vacuoles. New Phytologist 106:357–422. Raven J A. 1988. The iron and molybdenum use efficiencies of plant growth with different energy, carbon and nitrogen sources. New Phytologist 109:279–287. Raven J A. 1990. Predictions of Mn and Fe use efficiencies of phototrophic growth as a function of light availability for growth and C assimilation pathways. New Phytologist 116:1–18. Raven J A. 1993. The evolution of vascular land plants in relation to quantitative functioning of dead water-conducting cells and stomata. Biological Reviews 68:337–363. Raven J A. 1996. Into the voids: The distribution, function, development and maintenance of gas spaces in plants. Annals of Botany 78:137–142. Raven J A. 1997. Phagotrophy in phototrophs. Limnology and Oceanography 42:198–205. Raven J A. 1998. Small is beautiful. The picophytoplankton. Functional Ecology 12:503–513. Raven J A. 1999. Picophytoplankton. Progress in Phycological Research 13:33–106. Raven J A. 2000. Land plant biochemistry. Philosophical Transactions of the Royal Society of London Series B 355:833–846. Raven J A. 2002. Selection pressures on stomatal evolution. New Phytologist 153:371–386. Raven J A, Edwards D. 2001. Roots: evolutionary origin and biogeochemical significance. Journal of Experimental Botany 52:381–401. Raven J A, Edwards D. 2004. Physiological ecology of lower embryophytes: adaptations to the terrestrial environment. In The Evolution of Plant Physiology. From Whole Plants to Ecosystems, pp. 17–41. Eds A R Hemsley and I Poole. Linnean Society Symposium Series 21. London: Elsevier Academic Press. Raven J A, Skene K R. 2003. Chemistry of the oceans: the environment of early life. In Evolution on Planet Earth: the Impact of the Physical Environment, pp. 55–64. Eds L Rothschild and A Lister. London: Academic Press. Raven J A, Smith F A. 1976. Nitrogen assimilation and transport in vascular plants in relation to intracellular pH regulation. New Phytologist 76:415–431. Raven J A, Yin Z-H. 1998. The past, present and future of nitrogenous compounds in the atmosphere and their interactions with plants. New Phytologist 139:205–219. Raven J A, Sprent J I, McInroy S G, Hay G T. 1989. Water balance of N2-fixing root nodules: can phloem and xylem transport explain it? Plant, Cell and Environment 12:683–688. Raven J A, Evans M C W, Korb R E. 1999. The role of trace metals in photosynthetic electron transport in O2-evolving organisms. Photosynthesis Research 60:111–149. Raven J A, Handley L L, Andrews M. 2002a. Optimizing carbon-nitrogen budgets: perspectives for crop improvement. In Photosynthetic Nitrogen Metabolism and Associated

Evolution of oligotrophy

Carbon Metabolism, pp. 265–274. Eds C H Foyer and G Noctor. Dordrecht: Kluwer. Raven J A, Johnston A M, Kübler J E, Korb R, McInroy S G, Handley L L, Scrimgeour C M, Walker D I, Beardall J, Clayton M N, Vanderklift M, Fredriksen S, Dunton K H. 2002b. Seaweeds in cold seas: evolution and carbon acquisition, Annals of Botany 90:525–536. Raven J A, Handley L L, Andrews M. 2004a. Global aspects of C/N interactions determining plant-environment interactions. Journal of Experimental Botany 55:11–25. Raven J A, Handley L L, Wollenweber B. 2004b. Plant nutrition and water use efficiency. In Water Use Efficiency in Plant Biology, pp. 162–188. Ed. M Bacon. Oxford: Blackwell Science. Redfield A C. 1958. The biological control of chemical factors in the environment. American Scientist 46:205–221. Reich P B, Oleksyn J. 2004. Global patterns of plant leaf N and P in relation to temperature and latitude. Proceedings of the National Academy of Sciences of the USA 101:11001– 11006. Ridare C, Guieu C. 2002. Saharan input of phosphate to the oligotrophic water of the open water Mediterranean Sea. Limnology and Oceanography 47:856–869. Rocap G, Larimer F W, Lamerdin J, Malfatti S, Chain P, Ahlgren N A, Arellano A, Coleman M, Hauser L, Hess W R, Johnson Z I, Land M, Lindell D, Post A F, Regala W, Shah M, Shaw S L, Steglich C, Sullivan M B, Ting C S, Tononen A, Webb E A, Zinser E R, Chisholm S W. 2003. Genome divergence in two Prochlorococcus ecotypes reflects oceanic niche differentiation. Nature 424:1042–1047. Rosen R. 1967. Optimality Principles in Biology. London: Butterworths. Rowell P, James W, Smith W L, Handley L L, Scrimgeour C M. 1998. 15N discrimination in molybdenum- and vanadiumgrown N 2-fixing Anabaena variabilis and Azotobacter vinelandii. Soil Biology and Biochemistry 30:2177–2180. Sage R F, Monson R K. (Eds). 1999. C4 Plant Biology. San Diego: Academic Press. Sañudo-Wilhelmy S A, Kustka A B, Gobler C J, Hutchins D A, Yang M, Lwiza K, Burns J, Capone D G, Raven J A, Carpenter E J. 2001. Phosphorus limitation of nitrogen fixation by Trichodesmium in the central Atlantic Ocean. Nature 411:66–69. Sañudo-Wilhelmy S A, Tovar-Sanchez A, Fu F-X, Capone D G. Carpenter E J, Hutchins D A. 2004. The impact of surface-adsorbed phosphorus on phytoplankton Redfield stoichiometry. Nature 432:897–901. Schopf J W, Kudryavtsev A B, Agresti D G, Wdowiak T J, Czaja A D. 2002. Laser-Raman imagery of Earth’s earliest fossils. Nature 416:73–76. Schulz H N, Jørgensen B B. 2001. Big Bacteria. Annual Review of Microbiology 55:105–137. Sebastian J, Mian F, Halvorsen H O. 1973. Effect of growth rate on the level of the DNA-dependent RNA polymerases in Saccharomyces cerevisae. Journal of Molecular Biology 79:159–162. Silla F, Escudero A. 2004. Nitrogen-use efficiency: tradeoffs between N productivity and mean residence time at organ, plant and population levels. Functional Ecology 18:511–522. Skene K R. 2003. The evolution of physiology and development in the cluster root: teaching an old dog new tricks? Plant and Soil 248:21–30. Smith S E, Read D J. 1997. Mycorrhizal Symbiosis, 2nd edn. London: Academic Press. Sterner R W, Elser J J. 2002. Ecological Stoichiometry. Princeton: Princeton University Press. Strzepek R F, Harrison P J. 2004. Photosynthetic architecture differs in coastal and oceanic diatoms. Nature 431:689– 692.

279

Sturani E, Magnani F, Alberghina F A M. 1973. Inhibition of ribosomal RNA synthesis during a shift-down transition of growth in Neurospora crassa. Biochimica et Biophysica Acta 319:165–173. Summons R E, Jahnke L L, Hope J M, Logan G A. 1999. 2Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400:554–557. Sunda W G, Huntsman S A. 1997. Interrelated influence of iron, light and cell size on marine picophytoplankton growth. Nature 390:389–392. Thiel E C. 2004. Iron, ferritin, and nutrition. Annual Review of Nutrition 24:327–343. Tinker P B, Nye P H. 2000. Solute Movement in the Rhizosphere, 2nd edn. Oxford: Oxford University Press. Tozzi S, Schofield O, Falkowski P. 2004. Historical climate change and ocean turbulence as selective agents for two key phytoplankton functional groups. Marine Ecology Progress Series 274:123–132. Tyrrell T. 1999. The relative influence of nitrogen and phosphorus on oceanic primary production. Nature 400:525–531. Urquiaga S, Cruz K H S, Boddey R M. 1992. Contribution of nitrogen fixation to sugar-cane: nitrogen-15 and nitrogenbalance estimates. Soil Science Society of America Journal 56:105–114. Usha K, Saxena A, Singh B. 2004. Rhizosphere dynamics influenced by arbuscular mycorrhizal fungus (Glomus deserticola) and related changes in leaf nutrient status and yield of Kinnow mandarin {King (Citrus nobilis) × Willow Leaf (Citrus deliciosa)}. Australian Journal of Agricultural Research 55:571–576. Volker C, Wolf-Gladrow D A. 1999. Physical limits on iron uptake mediated by siderophores or surface reductases. Marine Chemistry 65:222–244. Volker C, Wolf-Gladrow D A. 2000. Numerical models of iron uptake by phytoplankton cells. Journal of Plant Nutrition 23:1657–1668. Vrede T, Dobberfuhl D R, Kooijman S A L M, Elser J J. 2004. Fundamental connections among organism C:N:P stoichiometry, macromolecular composition, and growth. Ecology 85:1217–1229. Waldron C, Lacroute F. 1975. Effect of growth rate on the amounts of ribosomal and transfer ribonucleic acids in yeast. Journal of Bacteriology 122:855–865. Wardle D A, Walker L R, Bardgett R D. 2004. Ecosystem properties and forest decline in contrasting long-term chronosequences. Science 305:509–513. Watson A J, Bakker D C E, Ridgwell A J, Boyd P W, Law C S. 2000. Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2. Nature 407:730–733. Wehr C T, Parks L W. 1969. Macromolecular synthesis in Saccharomyces cerevisae in different growth media. Journal of Bacteriology 98:458–466. Whitney S M, Andrews T J. 2001. Plastome-encoded bacterial ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) supports photosynthesis and growth in tobacco. Proceedings of the National Academy of Sciences of the USA 98:14738–14743. Whitney S M, Baldet P, Hudson G S, Andrews T J. 2001. Form I Rubiscos from non-green algae are expressed abundantly but not assembled in tobacco chloroplasts. The Plant Journal 26:535–547. Williams R J P. 1981. The natural selection of the elements. Proceedings of the Royal Society of London series B 213:361–397. Williams R J P, Fráusta da Silva J J R. 1996. The Natural Selection of the Chemical Elements. The Environment and Life’s Chemistry. Oxford: Clarendon Press.

280

J A RAVEN ET AL.

Wininger S, Gadkar V, Gamliel A, Skutelsky Y, Rabinowich E, Manor H, Kapulnik Y. 2003. Response of chive (Allium schoenoprasum) to AM fungal application following soil solarization under field conditions. Symbiosis 35:117–128. Wolstencroft R D, Raven J A. 2002. Photosynthesis: likelihood of occurrence and possibility of detection on Earth-like planets. Icarus 157:535–548.

Wu J, Sunda W, Boyle E A, Karl D M. 2000. Phosphate depletion in the western North Atlantic Ocean. Science 289:759–762. Zhu J, Lynch J P. 2004. The contribution of lateral branching to phosphorus acquisition efficiency in maize (Zea mays) seedlings. Functional Plant Biology 31:949–958.

(Revised version accepted 11 March 2005; Received 18 October 2004)