ANAEROBIC MICROBIAL DEGRADATION OF HYDROCARBONS: ORGANISMS AND REACTIONS

Hydrocarbons represent an abundant class of organic compounds on a global scale. The natural accumulations of hydrocarbons (e.g. oil reservoirs or gas hydrates) or oil contaminated sites (e.g. ground water aquifers) are often characterized by the absence of molecular oxygen. Thus mineralization of hydrocarbons under anoxic conditions plays an important role in such environments. Key to the understanding of anaerobic microbial hydrocarbon oxidation was the isolation of pure bacterial cultures with such degradative capacities and the biochemical studies of the pathways involved.

The microbial degradation of hydrocarbons in the presence of oxygen has been known for many years. In all studied cases, molecular oxygen is used as highly reactive co-substrate in initial reactions catalyzed by enzymes termed oxygenases. Due to this essential role of molecular oxygen doubts about the feasibility of an anaerobic degradation of hydrocarbons have long prevailed. However, 10 years ago first reports on the isolation of anaerobic bacteria that could completely oxidize toluene to CO₂ were published. In the meantime a variety of denitrifying, sulfate- and iron-reducing and phototrophic bacteria were isolated that could also anaerobically degrade toluene and other alkylbenzenes. In addition, also anaerobic oxidation of n-alkanes was demonstrated under conditions of denitrification, sulfate-reduction and methanogenesis. Subsequent biochemical studies revealed that novel reactions were involved in the initial activation of hydrocarbons in the absence of oxygen. Toluene is activated in a radical reaction by addition of its methyl group to the double bond of fumarate, resulting in the formation of benzylsuccinate as the first intermediate. The same reaction principle has only recently been demonstrated for the anaerobic degradation of n-alkane . In contrast, ethylbenzene is activated at the methylene group by a dehydrogenase reaction yielding (S)-1-phenylethanol as the first intermediate.