Microbial communities in oil-contaminated seawater
Introduction
In the event of an oil spill, we could assume that the oil would be naturally dispersed and degraded within a few years. Past surveys have indicated that natural removal of the oil in marine sediments is slow and oil deposits persist for many years. Bioremediation offers one available option for an oil spill response; however, the use of bioremediation remains a controversial issue, as its effectiveness varies with different oil spills. Very little is known about the biological processes involved in the clean-up of contaminated marine environments, although the addition of nitrogen and phosphorus nutrients has been shown to accelerate the speed of biodegradation.
We summarize in this review some of the important advances in microbial ecology that have enabled the identification of some microbial populations effective in the degradation of hydrocarbons in natural environments. We also emphasize that we have little or no understanding of the vast majority of marine bacteria that remain uncultured, and more efforts should be made to improve current methods for isolating oil-degrading or oil-emulsifying bacteria, not only for assessing the fate and effects of the spilled oil, but also for isolating novel bacteria that would be useful for the petroleum industry [1•].
Section snippets
Diversity of marine microbial communities
Since the pioneering work on marine bacteria by ZoBell [2], many bacterial strains have been isolated from coastal and oceanic environments; these bacteria, including the genera Pseudomonas, Vibrio and Flavobacterium, have been considered to be representative of marine bacteria. It has been recognized that surveys of microbial diversity without cultivation are necessary, however, as most of the microscopically detected microbial cells in seawater cannot be cultivated in conventional media [3].
Marine bacteria capable of degrading petroleum hydrocarbons
Petroleum constituents are classified into four fractions: saturates, aromatics, resins and asphaltenes. Each of these fractions contains a large number of compounds. Saturates are hydrocarbons containing no double bond and are further classified according to their chemical structures into alkanes (paraffins) and cycloalkanes (naphthenes). They are the major constituents of crude oil. Aromatic hydrocarbons have one or more aromatic rings with or without alkyl substitution(s). In contrast to the
Petroleum hydrocarbon biodegradation by Cyanobacteria
The release into the sea of as much as eight million barrels of crude oil during the Persian Gulf War of 1991 brought serious environmental damage to the region. Consequently, most of the intertidal cyanobacterial mats were severely affected. Some months after, however, Cyanobacteria started to grow on the top of oil deposited within the intertidal zone [41]. Complex microbial communities can be established in laminated cyanobacterial mats according to the gradients of light and redox
Anaerobic petroleum biodegradation
Recent investigations have demonstrated that several classes of petroleum hydrocarbons, including alkanes [46] and mono- and polycyclic aromatic compounds [47], were degraded in the absence of oxygen, with nitrate, ferrous iron or sulfate as an electron acceptor, or under conditions of methanogenesis.
The sulfate-reducing bacterium, strain Hxd3, which is capable of oxidizing alkanes to CO2 under sulfate-reducing conditions, has been isolated from soil. This strain was closely related to the
Response of a bacterial community to oil pollution
Hydrocarbon-degrading microorganisms usually exist in very low abundance in marine environments. Pollution by petroleum hydrocarbons, however, may stimulate the growth of such organisms and cause changes in the structure of microbial communities in the contaminated area. Identification of the key organisms that play roles in pollutant biodegradation is important for understanding, evaluating and developing in situ bioremediation strategies. For this reason, many efforts have been made to
Conclusions and future prospects
As already described, the development of molecular methods for detecting specific microbial populations without cultivation has improved our understanding of microbial community structures in natural environments and their transition in responding to changes in the environmental conditions. However, the assignment of specific metabolic activities in situ to particular microorganisms is not yet an easy task. Indeed, many recent efforts to understand environmental microbiology have been aimed at
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
The authors were supported by grants from the New Energy and Industrial Technology Development Organization (NEDO).
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