Any of various archaea that produce methane; they include such genera as Methanobacillus and Methanothrix. Methanogens are obligate anaerobes found in oxygen-deficient environments, such as marshes, swamps, sludge (formed during sewage treatment), and the digestive systems of ruminants. Mostly they obtain their energy by using carbon dioxide to oxidize hydrogen, with the production of methane:

Formate, methanol, or acetate may also be used as substrates by certain types. Methanogenic bacteria are important in the production of biogas and are responsible for the ‘marsh gas’ emitted in swamps. They are also major inhabitants of the anaerobic guts of ruminants, such as cattle and sheep, and of termites. Hence they are responsible for the methane produced by grazing ruminants, which is a significant contributor to greenhouse gas emissions.