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Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a domain distinct from bacteria. 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. Not to be confused with methanotrophs which rather consume methane for their carbon and energy requirements. ==Physical description== 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 anaerobic organisms and cannot function under 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 O2.〔http://spacecenter.uark.edu/JillJabstract.doc〕 Some methanogens, called hydrogenotrophic, use carbon dioxide (CO2) as a source of carbon, and hydrogen as a reducing agent. The reduction of carbon dioxide into methane in the presence of hydrogen can be expressed as follows: :CO2 + 4 H2 → CH4 + 2H2O Some of the CO2 is reacted with the hydrogen to produce methane, which creates an electrochemical gradient across cell 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 in those of 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「methanogen」の詳細全文を読む スポンサード リンク
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