Methanotrophs can be either
bacteria (aerobic and anerobic) or
archaea (anaerobic only). Which methanotroph species is present is mainly determined by the availability of
electron acceptors. Many types of methane oxidizing bacteria (MOB) are known with subclassifications being formed based on their different methods of formaldehyde fixation. There are several subgroups among the methanotrophic archaea. Methanotrophs have been historically classified broadly into three types which are defined by physiology, mechanism of methane metabolism, and morphology: Type I, II and X. Generally, Type I methanotrophs tend to dominate in cold, anaerobic environments, meaning there is limited oxygen availability, which often have high methane concentrations. However, at a high enough salinity, Type II will dominate even in cold temperatures. Type II methanotrophs tend to be more tolerant of stress and dominate in methane limited environments and acidic pHs. As methanotroph research expands, there is less of a clear line between Type I and II methanotrophs, so familial or species classifications are more useful for grouping these organisms as seen in Table 1. Research to understand why certain methanotrophs favor certain conditions and assimilation pathways is ongoing and relevant to predicting methanotroph responses to climate change.
Aerobic Under aerobic conditions, methanotrophs combine
oxygen and
methane to form
formaldehyde, which is then incorporated into organic compounds via the serine pathway or the ribulose monophosphate (RuMP) pathway, and carbon dioxide, which is released. Type I and type X methanotrophs are part of the
Gammaproteobacteria and they use the RuMP pathway to assimilate carbon. Type II methanotrophs are part of the
Alphaproteobacteria and use the serine pathway of carbon assimilation. They also characteristically have a system of internal membranes within which methane
oxidation occurs. Methanotrophs in
Gammaproteobacteria are known from the family
Methylococcaceae. Methanotrophs from
Alphaproteobacteria are found in families
Methylocystaceae and
Beijerinckiaceae. Aerobic methanotrophs are also known from the
Methylacidiphilaceae (phylum
Verrucomicrobiota). In contrast to
Gammaproteobacteria and
Alphaproteobacteria, methanotrophs in the phylum
Verrucomicrobiota are
mixotrophs. In 2021 a bacterial
bin from the phylum
Gemmatimonadota called "
Candidatus Methylotropicum kingii" showing aerobic methanotrophy was discovered thus suggesting methanotrophy to be present in the four bacterial phyla. In some cases, aerobic methane oxidation can take place in anoxic environments. "
Candidatus Methylomirabilis oxyfera" belongs to the
phylum NC10 bacteria, and can catalyze nitrite reduction through an "intra-aerobic" pathway, in which internally produced oxygen is used to oxidise methane. In clear water lakes, methanotrophs can live in the anoxic water column, but receive oxygen from
photosynthetic organisms, which they then directly consume to oxidize methane. No aerobic methanotrophic
archaea are known.
Anaerobic Under anoxic conditions, methanotrophs use different
electron acceptors for methane oxidation. This can happen in
anoxic habitats such as marine or lake
sediments,
oxygen minimum zones, anoxic water columns, rice paddies and soils. Some specific methanotrophs can reduce nitrate, nitrite, iron, sulfate, or manganese ions and couple that to methane oxidation without syntrophic partner. Investigations in marine environments revealed that methane can be oxidized anaerobically by consortia of methane oxidizing
archaea and
sulfate-reducing bacteria. This type of
anaerobic oxidation of methane (AOM) mainly occurs in anoxic marine sediments. The exact mechanism 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 intermediate, which is then used by the sulfate-reducing bacteria to gain energy from the reduction of sulfate to
hydrogen sulfide and water. The anaerobic methanotrophs are not related to the known aerobic methanotrophs; the closest cultured relatives to the anaerobic methanotrophs are the
methanogens in the
order Methanosarcinales.
Special species Methylococcus capsulatus is used to produce animal feed from natural gas. In 2010 a new bacterium
Candidatus Methylomirabilis oxyfera from the
phylum NC10 was identified that can couple the
anaerobic oxidation of methane to nitrite reduction without the need for a
syntrophic partner. Based on studies of Ettwig et al., Depending upon environmental conditions, these methanotrophs can also produce biomolecules during the wastewater treatment process that are useful for a wide range of applications. For example, methanotrophs undergoing glycolysis produce exopolysaccharides (EPS) which can be extracted and used in medicine. A well-known EPS is hyaluronic acid which is used widely in cosmetics and wound care. == Taxonomy ==