Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria. They are common in wetlands, where they are responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they are responsible for the methane content of belching in ruminants and flatulence in some humans. In marine sediments biomethanation is generally confined to where sulfates are depleted, below the top layers. Others are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of the Earth's crust, kilometers below the surface.
Methanogens are usually coccoid (spherical) or bacilli (rod shaped). There are over 50 described species of methanogens, which do not form a monophyletic group, although all methanogens belong to Archaea. Methanogens are also anaerobic. Although methanogens cannot function under aerobic conditions they can sustain oxygen stresses for a prolonged time. Methanosarcina barkeri is exceptional in possessing a superoxide dismutase (SOD) enzyme, and may survive longer than the others. Some methanogens, called hydrogenotrophic, use carbon dioxide (CO2) as a source of carbon, and hydrogen as a reducing agent. Some of the CO2 is reacted with the hydrogen to produce methane, which produces an electrochemical gradient across a membrane, used to generate ATP through chemiosmosis. In contrast, plants and algae use water as their reducing agent. Methanogens lack peptidoglycan, a polymer that is found in the cell walls of the Bacteria but not Archaea. Some methanogens have a cell wall that is composed of pseudopeptidoglycan. Other methanogens do not, but have at least one paracrystalline array (S-layer) made up of proteins that fit together like a jigsaw puzzle.