Most crude oils contain hydrocarbons ranging in size from methane to molecules with hundreds of carbons. When crude oils reach the surface environment, they are biodegraded, at least under aerobic conditions.
Some examples of hydrocarbons found in crude oil
Numerous bacterial and fungal genera have species able to degrade hydrocarbons aerobically. The hallmark of an oil-degrading organism is its ability to insert oxygen atoms into the hydrocarbon. Once a hydrocarbon possesses a carboxylate or alcohol functionality it is almost invariably a readily degradable compound.
Initial steps in the biodegradation of linear and cyclic alkanes:
Initial steps in the aerobic degradation of naphthalene, as a representative multiringed aromatic, and toluene. The different initial steps of toluene degradation are examples of the diversity found in different organisms:
Despite theories that oil biodegradation was an exclusively aerobic process, some hydrocarbons are oxidized by bacteria under anaerobic conditions, where the oxygen is probably coming from water. Limited hydrocarbon biodegradation has no been shown under sulfate-, nitrate-, carbon dioxide- and ferric iron-reducing conditions. For example, it is not uncommon for slow biodegradation of larger hydrocarbon molecules to occur in the anaerobic environment that exists in lake sediments. However, anaerobic biodegradation is so slow compared to aerobic biodegradation and photo oxidation (the degradation of hydrocarbons by sunlight and oxygen), that most bioremediation focuses on utilizing aerobic processes combined with abundance of sun and oxygen, rather than anaerobic processes.
Proposed initial steps in the anaerobic biodegradation of toluene in different organisms:
SOURCES: Prince