Living organisms are classified into three Domains of Bacteria, Archaea or Eucarya based on biochemical evidences by Woese et al. (1990). On this basis, all methanogens turn out to be archaea, not bacteria. Their cell membrane is characterized by grycerolipids of isoprenoids with ether-bonds, and some derivatives of these constituents are recognized as biomarkers of archaeal origin.
Methanogens are autotrophic so that they can chemically assimilate inorganic carbon. CO2-reduction and decarboxylation of acetate are the two main reactions of methane generation, and unique coenzymes work on both. Marine environment is suitable for hydrogenotrophic C02-reductive methanogens, which live in symbiosis with H2-generative and other bacteria. On the other hand, the freshwater environment is favored by acetotrophic archaea. These two pathways are distinguished by the carbon and hydrogen isotopic compositions of accumulated methane.
