Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University,. Baton Rouge ... Louisiana, sediment using '4C-labeled hydrocarbons. ..... College Program, a part of the National Sea GrantCollege.
APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Aug. 1980, p. 365-369 0099-2240/80/08-0365/05$02.00/0
Vol. 40, No. 2
Effect of Estuarine Sediment pH and Oxidation-Reduction Potential on Microbial Hydrocarbon Degradation GORDON A. HAMBRICK III,* RONALD D. DELAUNE, AND W. H. PATRICK, JR. Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State University, Baton Rouge, Louisiana 70803
Microbial mineralization rates of two petroleum hydrocarbons, as affected by pH and oxidation-reduction potential, were determined in a Barataria Bay, Louisiana, sediment using '4C-labeled hydrocarbons. Hydrocarbon mineralization rates were inferred from the activity of respired 14CO2. Sediment pH and oxidation-reduction potential were important factors in governing the population of hydrocarbon-degrading microorganisms in the sediment and subsequent mineralization rates. Highest mineralization rates occurred at pH 8.0, and the lowest occurred at pH 5.0. At all pH levels mineralization decreased with decreasing oxidation-reduction potential (i.e., increasing sediment anaerobiosis). Generally, mineralization rates for octadecane were greater than those for naphthalene. Aerobic microorganisms in the oxidized sediment were more capable of degrading hydrocarbons than anaerobic microorganisms in reduced sediment of the same pH. One of the major fates of released petroleum hydrocarbons in the coastal environment is their incorporation into bottom sediments (10). ZoBell and Prokop (19) found that the oil content of Barataria Bay (La.) sediments varied significantly between oil-impacted areas, which had oil contents greater than 1.0%, and unimpacted areas which had 0.01 to 0.1% oil content. Microbial degradation of oil appears to be the major process through which petroleum hydrocarbons are removed from the sediment environment (16). Hydrocarbons utilized by microorganisms are eventually mineralized to C02 and water. Intermediates of hydrocarbon mineralization are, in general, fatty acids from alkanes and hydroxy derivatives from aromatics (8). The persistence and biodegradability of crude oil is highly dependent on its composition and the molecular configuration of its hydrocarbon components (1, 17). The rate of microbial degradation decreases from normal alkanes to isoalkanes and cycloalkanes to aromatics (3). The ecological significance of the biodegradability of hydrocarbons is the persistence in the environment of those hydrocarbons, which are more slowly degraded and which are often the more toxic components of petroleum. Also affecting microbial oil degradation are environmental factors, such as oxygen concentration, extent of oil dispersion, temperature, salinity, water turbulence, organic matter concentration, hydrocarbon concentration, microbial predation, and nutrient availability (2, 18). Estuarine and marsh sediments are more or 365
less continuously submerged. Therefore sediment interstitial spaces are saturated with water. Oxygen diffuses slowly through water and is exhausted by microorganisms in the sediment within the first few millimeters of the sediment surface. Below the aerobic sediment surface, facultative anaerobic and true anaerobic microorganisms use the oxidized forms present in the sediment as electron acceptors, thereby decreasing the redox potential of the sediment. The result is the development of a two-layer system consisting of an oxidized or aerobic surface layer and an underlying reduced or anaerobic layer
(5).
Several researchers have reported that the extent of hydrocarbon degradation quickly decreases with increased sediment depth (3, 7, 14). The oxygen required for more rapid hydrocarbon degradation is not met in anaerobic sediments. ZoBell and Prokop (19) found that, under anaerobic conditions, a mixed microflora brought about the reduction of significant quantities of sulfate along with the gradual degradation of oil. They concluded that sulfates are the principal H+ acceptors in hydrocarbon biodegradation in anaerobic sediments. The sulfates are reduced to sulfides, which are usually precipitated as metal sulfides or released as hydrogen sulfide gas. Oxidation-reduction potential, hereafter referred to as redox potential, can be used as an indicator of the degree of oxidation of sediments. Submerged sediments display a range of redox potentials from +700 mV, which indicates highly
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oxidized sediment, to -300 mV, which indicates highly reduced sediment (5). Intense reduction in submerged sediments tend to buffer sediments close to pH 7.0 (13). The objective of this study was to determine and compare the relative rates of mineralization in estuarine sediment of two "C-labeled petroleum hydrocarbons, n-[1-"C]octadecane and [1(4, 5, 8)-_4C]naphthalene as affected by different pH levels and redox potentials. 14C-labeled hydrocarbons have been successfully used by other investigators in quantifying hydrocarbon degradation by determining the activity of recovered "4CO2 (4, 9).
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MATERIALS AND METHODS The study consisted of three sets of sediment-water suspensions, which were incubated at one of three pH levels: pH 5.0, pH 6.5, or pH 8.0. Each set consisted of four pairs of suspensions that were incubated at one of four redox potentials:
