Which group of microorganisms produce methane?

The group of microorganisms that produce methane is called methanogens. Methanogens are anaerobic archaea that make methane as part of their energy metabolism, a process known as methanogenesis. They are not bacteria, although they were once often classified as methane-producing bacteria before archaea were recognized as a separate domain.

Methanogens live where oxygen is absent, including wetlands, lake and ocean sediments, landfills, sewage-treatment systems, rice paddies, and the digestive tracts of animals. In ruminants such as cattle, they consume products released by other microbes during digestion. Methanobrevibacter smithii is a prominent methanogen in the human gut.

Different methanogens use different starting materials. Many combine hydrogen with carbon dioxide to form methane, while others use acetate, methanol, or methylamines. Their activity is important in anaerobic ecosystems and in biogas production, but the methane they release is also a significant greenhouse-gas source.

Phototrophs capture light, thermophiles prefer high temperatures, and halophiles prefer salty environments; none of those labels specifically means methane production.