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Methanogens are Archaea that produce methane as a metabolic by-product. 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 flatulence. Others are extremophiles, found in environments such as hot springs and undersea vents.
Methanogens are anaerobic. Some, called hydrotrophic, use carbon dioxide as a source of carbon and hydrogen as a reducing agent. Some of the carbon dioxide reacts with the hydrogen to produce methane, and in the process an electric potential is maintained across the cell membrane, which is used to generate ATP. In contrast, plants and algae use water as their reducing agent. Other methanogens are acetotrophic, breaking down acetate to produce carbon dioxide and methane.
There are over 50 species of methanogens, divided into three classes - the Methanobacteria, Methanococci, and Methanopyri. All are included among the Euryarchaeota. They are paraphyletic, and there is some reason to suppose the Methanococci are paraphyletic as well.
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