Methane formation and oxidation by prokaryotes

The review deals with systematization and generalization of new information concerning the phylogenetic and functional diversity of prokaryotes involved in the methane cycle. Methane is mostly produced by methanogenic archaea, which are responsible for the terminal stage of organic matter decomposition in a number of anoxic ecotopes. Although phylogeny, physiology, and biochemistry of methanogens have been extensively studied, important discoveries were made recently. Thus, members of deep phylogenetic lineages within the Euryarchaeota phylum (Methanomassiliicoccales, Candidatus Methanofastidiosa, Methanonatronarchaeia) and even outside it (Ca. Verstraetearchaeota and Ca. Bathyarchaeota) were reported to carry out methyl-reducing methanogenesis. Moreover, evidence was obtained on aerobic methane production by marine heterotrophic bacteria, which demethylate polysaccharide esters of methylphosphonic acid. Methanotrophic microorganisms oxidize methane both aerobically and anaerobically, decreasing significantly the release of this greenhouse gas into the atmosphere. In the presence of oxygen methane is oxidized by methanotrophic members of Alpha- and Gammaproteobacteria, as well as by Verrucomicrobia. Methanotrophic gammaproteobacteria have been recently revealed in hypoxic and even anoxic environments, where they probably oxidize methane either in a trophic consortium with oxygenic phototrophs and/or methylotrophs or using electron acceptors other than oxygen. Anaerobic methane oxidation has been known for a long time. Sulfat- and nitrate-dependent anaerobic methane oxidation carried out by the ANME archaea via reverse methanogenesis are the best studied processes. While metal-dependent anaerobic methane oxidation is considered possible, the mechanisms and agents responsible for this process have not been reliably identified. Intracellular oxygen production during nitrite-dependent anaerobic methane oxidation was shown for bacteria Ca. Methylomirabilis oxyfera. These findings stimulate interest in the processes and microorganisms of the methane cycle.