Methanotroph

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Methanotrophs are bacteria that are able to grow using methane as their only source of carbon and energy. They can grow aerobically or anaerobically and require single-carbon compounds to survive. Under aerobic conditions, they combine oxygen and methane to form formaldehyde, which is then incorporated into organic compounds. They also characteristically have a system of internal membranes within which methane oxidation occurs. Methanotrophs occur mostly in soils, and are especially common near environments where methane is produced. Their habitats include oceans, mud, marshes, underground environments, soils, rice paddies and landfills. They are of special interest to researchers studying global warming.

Gratuitous detoxification of some environmental contaminants such as chlorinated hydrocarbons by methanotrophs have made them attractive models for such bioremediation processes. Equally methane is a potential greenhouse gas far more potent that carbon dioxide. Therefore methanotrophs play a major role in the reduction of the release of methane into the atmosphere from environments such as rice paddies, landfills, bogs and swamps where methane production is relatively high.

Differences in the method of formaldehyde fixation and membrane structure divide the methanotrophs into two groups, called type I and type II. These are respectively classified as the Methylococcaceae and Methylocystaceae. Although both are included among the Proteobacteria, RNA trees place them in different subgroups, so they are not closely related. However, they appear closely related to groups that oxidize ammonia; these may also oxidize methane, although they cannot use it as a carbon source.

Methanotrophy is a special case of methylotrophy, using single-carbon compounds that are less oxidized than carbon dioxide. Most methylotrophs, however, can also make use of multi-carbon compounds which differentiates them from methanotrophs that are usually fastidious methane and methanol oxidizers.

Investigations in marine environments revealed that methane can be oxidized anaerobically by consortia of methane oxidizing archaea and sulfate-reducing bacteria. Anaerobic oxidation of methane (AOM) mainly occurs in anoxic marine sediments. The exact mechanism of methane oxidation under anaerobic conditions is still a topic of debate but the most widely accepted theory is that the archaea use the reversed methanogenesis pathway to produce carbon dioxide and another, unknown substance. This unknown intermediate is then used by the sulfate-reducing bacteria to be able to gain energy from the reduction of sulfate to hydrogen sulfide. The anaerobic methanothrophs are not related to the known aerobic methanotrophs; the closest cultured relative to the anaerobic methanotrophs are the methanogens in the order of Methanosarcinales.