Anaerobic oxidation of methane (AOM) is a microbial process occurring mainly in anoxic marine sediments. During AOM methane is oxidized with sulfate as the terminal electron acceptor: CH4 + SO42- → HCO3- + HS- + H2O
According to current knowledge, AOM is mediated by a syntrophic consortium [1] of methanotrophic archaea and sulfate-reducing bacteria. They often form small aggregates or sometimes voluminous mats. The archaeal partner is abbreviated ANME, which stands for "anaerobic methanotroph". ANME's are very closely related to methanogenic archaea and recent investigations suggest that AOM is an enzymatic reversal of methanogenesis. It is still poorly understood how the syntrophic partners interact and which intermediates are exchanged between the archaeal and bacterial cell. The research on AOM is hindered by the fact that the responsible organisms have not been isolated. This is because these organisms show very slow growth rates with a minimum doubling time of a few months. Countless isolation efforts have not been able to isolate one of the anaerobic methanotrophs, a possible explanation can be that the ANME archaea and the SRB have an obligate syntrophic interaction and can therefore not be isolated individually.
In benthic marine areas with strong methane releases from fossil reservoirs (e.g. at cold seeps, mud volcanoes or gas hydrate deposits) AOM can be so high that chemosynthetic organisms like filamentous sulfur bacteria (see Beggiatoa) or animals (clams, tube worms) with symbiont sulfide-oxidizing bacteria can thrive on the large amounts of hydrogen sulfide that are produced during AOM. The production of bicarbonate from AOM can result in the precipitation of calcium carbonate or so-called authigenic carbonates.
AOM is considered to be a very important process reducing the emission of the greenhouse gas methane 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 consortia of archaea and bacteria are also able to oxidize methane with nitrate instead of sulfate.[1][2] Recent findings also suggest that nitrite reduction can be coupled to methane oxidation by a single bacterial species without the need for an archaeal partner[2]. The organisms involved in the nitrate reducing process are not directly related to the ANME organisms found in marine sediments where methane oxidation is coupled to sulfate reduction.
In the literature two different abbreviations of anaerobic oxidation of methane can be found: AOM (anaerobic oxidation of methane) and AMO (anaerobic methane oxidation). AMO was considered to be grammatically incorrect, because it implies that anaerobic methane is oxidized.