Methanogen
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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 [[ruminant]]s and [[humans]], where they are responsible for the methane content of [[flatulence]]<ref>
{{cite book
|Author = Joseph W. Lengeler
| year = 1999
| title = Biology of the Prokaryotes
| isbn = 0632053577
| page = 796 }}
</ref> . In [[marine (ocean)|marine]] [[sediment]]s [[biomethanation]] is generally confined to where [[sulfate]]s are depleted, below the top layers.<ref>{{cite journal
|author=J.K. Kristjansson, ''et al.''
|year=1982
|title=Different Ks values for hydrogen of methanogenic bacteria and sulfate-reducing bacteria: an explanation for the apparent inhibition of methanogenesis by sulfate
|journal=Arch. Microbiol.
|volume=131
|pages=278–282
|doi=10.1007/BF00405893
}}</ref> Others are [[extremophile]]s, found in environments such as [[hot spring]]s and submarine [[hydrothermal vents]] as well as in the "solid" rock of the earth's crust, kilometers below the surface. Methanogens are usually coccoid or rod shaped. There are over 50 described species of methanogens, which is not [[monophyletic]] group, although all methanogens belong to [[Euryarchaeota]].
Methanogens are [[Anaerobic organism|anaerobic]]. Although methanogens cannot function under aerobic conditions they can sustain oxygen stresses for prolonged times<ref>V. Peters, R. Conrad, 1995, Methanogenic and other strictly anaerobic bacteria in desert soil and other oxic sois, Applied and Environmental Microbiology, 61: 1673-1676</ref>. An exception is ''[[Methanosarcina barkeri]]'', which contains a [[superoxide dismutase]] (SOD) enzyme and may survive longer.<ref>http://spacecenter.uark.edu/JillJabstract.doc</ref> Some, called hydrogenotrophic, use [[carbon dioxide]] (CO<sub>2</sub>) as a source of carbon, and [[hydrogen]] as a reducing agent. Some of the CO<sub>2</sub> is reacted with the hydrogen to produce methane, which produces an [[electrochemical gradient]] across a membrane, used to generate [[Adenosine_triphosphate|ATP]] through [[chemiosmosis]]. In contrast, plants and algae use water as their reducing agent.
Methanogens do not have a polymer that is found in cell walls of prokaryotes. Some methanogens have a cell wall that is composed of pseudomurein. Other methanogens that don't have a pseudomurein have at least one paracrystalline array (S-layer) which is made up of proteins that fit together like a puzzle.
Although most marine biogenic methane is the result of CO<sub>2</sub> reduction, a small amount is derived from [[acetate]] (CH<sub>3</sub>COO<sup>-</sup>).<ref>
{{cite journal
|author=M.J. Whiticar, ''et al.''
|year=1986
|title=Biogenic methane formation in marine and freshwater environments: CO<sub>2</sub> reduction vs. acetate fermentation — isotope evidence
|journal=Geochim. Cosmochim. Acta
|volume=50
|pages=393–709
|doi=10.1016/0016-7037(86)90346-7
}}</ref> Archaea that [[catabolize]] this for energy are referred to as ''acetotrophic'' or ''aceticlastic''. Methylotrophic archaea utilize methylated compounds such as [[methylamine]]s, [[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 acceptor]]s (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 can still reproduce from temperatures of 15 to 100 degrees Celsius.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.<ref>Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0507601102)</ref>
Another study<ref>Icarus (vol. 178, p. 277)cs:Methanogen</ref> 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.<ref>[http://www.newscientistspace.com/article/dn8428-extreme-bugs-back-idea-of-life-on-mars.html Extreme bugs back idea of life on Mars]</ref>
Some scientists have proposed that the presence of methane in the [[Mars|Martian]] atmosphere may be indicative of native methanogens on that planet.<ref>[http://www.space.com/scienceastronomy/051220_science_tuesday.html Crater Critters: Where Mars Microbes Might Lurk]</ref>
Closely related to the [[methanogen]]s are the anaerobic methane oxidizers, which utilize methane as a substrate in conjunction with the reduction of sulfate and nitrate.<ref>Thauer, R. K. and Shima, S., "Biogeochemistry: Methane and microbes", Nature, 2006, 440, 878-879</ref>
Methanogens are [[autotrophic]] producers.
==Examples==
*''[[Methanobacterium formicum]]''
*''[[Methanobacterium bryantii]]''
*''[[Methanobacterium thermo-autotrophicum]]''
*''[[Methanogenium cariaci]]''
*''[[Methanogenium frigidum ]]''
*''[[Methanothrix sochngenii]]''
==References==
<references/>
[[Category:Anaerobic digestion]]
[[Category:Archaea]]
[[Category:Environmental microbiology]]
[[Category:Microbiology]]
[[cs:Methanogen]]
[[es:Metanógeno]]
[[fr:Archéobactérie méthanogène]]
[[ja:メタン菌]]