5.9F: Anoxic Hydrocarbon Oxidation

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Learning Objectives

Describe the process of anoxic hydrocarbon oxidation in regards to marine environments

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. The majority of hydrocarbons occur naturally in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen. The combustion of hydrocarbons is the primary energy source for current civilizations.

Crude oil contains aromatic compounds that are toxic to most forms of life. Their release into the environment by human spills and natural seepages can have detrimental effects. Marine environments are especially vulnerable. Despite its toxicity, a considerable fraction of crude oil entering marine systems is eliminated by the hydrocarbon-degrading activities of microbial communities. Although it was once thought that hydrocarbon compounds could only be degraded in the presence of oxygen, the discovery of anaerobic hydrocarbon-degrading bacteria and pathways show that the anaerobic degradation of hydrocarbons occurs naturally.

Figure: **Contaminated soil**: Microbes may be used to degrade toxic hydrocarbons in anaerobic environments.

The facultative denitrifying proteobacteria Aromatoleum aromaticum strain EbN1 was the first to be determined as an anaerobic hydrocarbon degrader, using toluene or ethylbenzene as substrates. Some sulfate-reducing bacteria can reduce hydrocarbons such as benzene, toluene, ethylbenzene, and xylene, and have been used to clean up contaminated soils. The genome of the iron-reducing and hydrocarbon degrading species Geobacter metallireducens was recently determined.

Anaerobic oxidation of methane (AOM) is a microbial process that occurs in anoxic marine sediments. During this process, the hydrocarbon methane is oxidized with sulfate as the terminal electron acceptor: CH₄ + SO₄^2- → HCO^3- + HS^– + H₂O. It is believed that AOM is mediated by a syntrophic aggregation of methanotrophic archaea and sulfate-reducing bacteria, although the exact mechanisms of this syntrophic relationship are still poorly understood. AOM is considered to be a very important process in reducing the emission of methane (a greenhouse gas) from the ocean into the atmosphere. It is estimated that almost 90% of all the methane that arises from marine sediments is oxidized anaerobically by this process. Recent investigations have shown that some syntrophic pairings are able to oxidize methane with nitrate instead of sulfate.

Key Points

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon.
The majority of hydrocarbons occur naturally in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen. The combustion of hydrocarbons is the primary energy source for current civilizations.
Anaerobic oxidation of methane (AOM) is a microbial process that occurs in anoxic marine sediments. AOM is considered to be a very important process, reducing the emission of methane (a greenhouse gas) from the ocean into the atmosphere by up to 90%.

Key Terms

methanotrophic: The ability to metabolize methane as an only source of carbon and energy.
syntrophic: When one species lives off the products of another species.
anoxic: Lacking oxygen.

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