<P> Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions . They are prokaryotic and thus belong to the kingdom Monera, and they uniquely belong to the domain of archaea . They are common in wetlands, where they are responsible for marsh gas, and in the digestive tracts of animals such as ruminants and humans, where they are responsible for the methane content of belching in ruminants and flatulence in humans . In marine sediments the biological production of methane, also termed methanogenesis, is generally confined to where sulfates are depleted, below the top layers . Moreover, the methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments . 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 . </P> <P> Methanogens are coccoid (spherical shaped) 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 . They are mostly anaerobic organisms that cannot function under aerobic conditions, but recently a species (Candidatus Methanothrix paradoxum) has been identified that can function in aerobic conditions . They are very sensitive to the presence of oxygen even at trace level . Usually, they cannot sustain oxygen stress for a prolonged time . However, Methanosarcina barkeri is exceptional in possessing a superoxide dismutase (SOD) enzyme, and may survive longer than the others in the presence of O. Some methanogens, called hydrogenotrophic, use carbon dioxide (CO) as a source of carbon, and hydrogen as a reducing agent . </P>

Where would you expect to find members of domain archaea classified as methanogens
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