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'Methanogens' are archaea that produce methane as a metabolic byproduct in anoxic conditions. 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 flatulence. In marine sediments biomethanation is generally confined to where sulfates are depleted, below the top layers.^[1] 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. There are over 50 described species of methanogens, which are paraphyletic and all included among the Euryarchaeota.
Methanogens are anaerobic. Most methanogens are rapidly killed by the presence of oxygen. An exception is ''Methanosarcina barkeri'', which contains a superoxide dismutase (SOD) enzyme and may survive longer.^[2] Some, 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.
Although most marine biogenic methane is the result of CO₂ reduction, a small amount is derived from acetate (CH₃COO^-).^[3] Archaea that catabolize this for energy are referred to as ''acetotrophic'' or ''aceticlastic''. ''Methylotrophic'' archaea utilize methylated compounds such as methylamines, methanol, and methanethiol as well.
Ecologically, methanogens play the vital role in anaerobic environments of removing excess hydrogen and fermentation products that have been produced by other forms of anaerobic respiration. Methanogens typically thrive in environments in which all other electron acceptors (such as oxygen, nitrate, sulfate, and trivalent iron) have been depleted. In the deep rock they obtain their hydrogen from the thermal and radioactive breakdown of water.
Methanogens have been found in several extreme environments on Earth - buried under kilometres of ice in Greenland and living in hot, dry desert soil. They are known to be the most common prokaryotes archaebacteria in deep subteranean habitats.
Live microbes making methane were found in a glacial ice core sample retrieved from three kilometres under Greenland by researchers from the University of California, Berkeley, US.^[4]
Another study^[5] has also discovered methanogens in a harsh environment on Earth. Researchers studied dozens of soil and vapour samples from five different desert environments in Utah, Idaho and California in the US, and in Canada and Chile. Of these, five soil samples and three vapour samples from the vicinity of the Mars Desert Research Station in Utah were found to have signs of viable methanogens.^[6]
Some scientists have proposed that the presence of methane in the Martian atmosphere may be indicative of native methanogens on that planet.^[7]
Closely related to the methanogens are the anaerobic methane oxidizers, which utilize methane as a substrate in conjunction with the reduction of sulfate and nitrate.^[8]

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References

References

1.
Different Ks values for hydrogen of methanogenic bacteria and sulfate-reducing bacteria: an explanation for the apparent inhibition of methanogenesis by sulfate, J.K. Kristjansson, ''et al.'', , , Arch. Microbiol., 1982
2. http://spacecenter.uark.edu/JillJabstract.doc
3.
Biogenic methane formation in marine and freshwater environments: CO₂ reduction vs. acetate fermentation — isotope evidence, M.J. Whiticar, ''et al.'', , , Geochim. Cosmochim. Acta, 1986
4. Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0507601102)
5. Icarus (vol. 178, p. 277)cs:Methanogen
6. Extreme bugs back idea of life on Mars
7. Crater Critters: Where Mars Microbes Might Lurk
8. Thauer, R. K. and Shima, S., "Biogeochemistry: Methane and microbes", Nature, 2006, 440, 878-879