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Microbial Components Marta Estrada and Dolors Vaqué
Abstract
This chapter presents an overview of the diversity, distribution and ecology of major groups of microbial plankton in the Mediterranean Sea, including phytoplankton, viruses, heterotrophic prokaryotes and flagellates, and ciliates. Some protists with hard structures like diatoms, thecate dinoflagellates, coccolithophorids and tintinnids have been relatively well studied from a morphological point of view, but in general microbial diversity is poorly known, in particular with respect to prokaryotes and the smallest eukaryotes. This situation is rapidly changing, in a large part due to the incorporation of molecular techniques. The general oligotrophy of the Mediterranean, which increases from west to east, is reflected in a strong contribution of the picoplankton and the microbial food web. However, a variety of nutrient-enrichment mechanisms, including winter mixing, mesoscale hydrographic structures and land runoff, which operate at various spatio-temporal scales, may enhance primary production and result in the intermittent dominance of diatoms and the herbivorous food web. During the stratification period, a deep chlorophyll maximum is a general feature throughout the basin and plays a substantial role in the fertility of the Mediterranean. Keywords
“Phytoplankton mandala” • Bacteriophages • Blooming taxa • Chemotaxonomic and molecular techniques • Chlorophyll maximum, Colonial diatoms • Cryptophytes • Cyanobacterial genera • Dinophyceae • Eutrophication • Free-living pelagic bacterial cells • Global change • Heterotrophic bacteria • Large-scale phytoplankton community • Long- term trends • Mediterranean photic zone • Microbial diversity • Microbial DNA • Microbial eukaryotes • Microbial food web • Microbial plankton • Nano-and picoplanktonic flagellates • Pelagophyceae • Phytoplankton biomass • Phytoplankton blooms • Pigment chemotaxonomic approach • Plankton ecosystem dynamics • Plankton • Planktonic ciliates • Planktonic community • Prasinophyceae • Primary production • Prymnesiophytes • Seasonal succession of phytoplankton • Silicoflagellates • The mediterranean • Toxic benthic dinoflagellate • Viral community • Viral effects on bacterial biomass and production • Viral infection • Viruses • Winter phytoplankton bloom
M. Estrada (*) • D. Vaqué Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Barcelona, Spain e-mail:
[email protected] S. Goffredo and Z. Dubinsky (eds.), The Mediterranean Sea: Its history and present challenges, DOI 10.1007/978-94-007-6704-1_6, © Springer Science+Business Media Dordrecht 2014
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Introduction In the Mediterranean Sea, as in other marine ecosystems, microbial components of the plankton are major agents in biogeochemical cycles and account for most of the biodiversity. In spite of their importance, many groups of marine microbes are still poorly known, although the situation is rapidly changing thanks to the introduction of molecular techniques. The structure and dynamics of Mediterranean plankton has been considered in a number of collective volumes such as Margalef (1985), Moraitou-Apostolopoulou and Kiortsis (1985) and Minas and Nival (1988). Recently, Siokou-Frangou et al. (2010) published an updated review of studies conducted during the last 25 years in offshore epipelagic waters and Coll et al. (2010) presented a synthetic account of Mediterranean marine biodiversity. This chapter addresses the biodiversity and ecology of the major microbial groups in the Mediterranean, as part of the ecological background for a book on history and present challenges. In the following section (“Main groups and trophic roles of microbial plankton”), we present a brief account of the taxonomic affiliation and functional roles of microbial groups. The main fertilization mechanisms in the Mediterranean and the general patterns of temporal distribution and vertical organization of phytoplankton biomass are considered in sections “Fertilization mechanisms in the Mediterranean” and “Overview of diversity of the main microbial groups”. The next section (“Phytoplankton”), devoted to phytoplankton, presents an overview of the taxonomic diversity of the main groups, a brief review of harmful algal events in the Mediterranean and some insights obtained from the study of long-term series. Section “Viruses and heterotrophic microbes” deals with the distribution and diversity of viruses, heterotrophic prokaryotes and flagellates, and ciliates. Finally, section “Microbial and classical food webs. The example of the Catalan Sea” deals with the structure of the planktonic food webs and describes a case study in the NW Mediterranean.
ain Groups and Trophic Roles M of Microbial Plankton Until about three decades ago, the classical view of the planktonic food web featured phytoplankton of sizes above a few micrometers as primary producers, microzooplankton such as ciliates, as herbivores, and zooplankton (mainly copepods), as a bridge between the former trophic levels and fishes. Phytoplankton, roughly defined as the autotrophic component of the planktonic food web, was considered as mostly composed of eukaryotic microalgae. However, many exceptions had long been recognized, as some groups classically
M. Estrada and D. Vaqué
considered as phytoplankton, like the dinoflagellates, comprise heterotrophic taxa that prey on other organisms, and many autotrophic forms are also capable of phagotrophy and osmotrophy. Phototrophic organisms of sizes below a few micrometers had been described, but the available techniques did not allow their reliable enumeration. The occurrence of bacteria, considered mainly as decomposers, was known, but the methods used at the time, mostly based on culture techniques derived from medical microbiology, allowed only the detection of a small proportion of them. This general picture changed around the mid 1970s with the incorporation of DNA staining and epifluorescence microscopy, which allowed a reliable observation and counting of prokaryotic and eukaryotic cells 2 μm and >20 μm) and “microplankton” (>20 μm and
