Methanogen


Methanogen
Methanopyrus kandleri

Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria. 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 belching in ruminants and flatulence in humans.[1] In marine sediments biomethanation is generally confined to where sulfates are depleted, below the top layers.[2] 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 its carbon and energy requirements.

Contents

Physical description

Methanogens are usually coccoid (spherical) 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. Methanogens are also anaerobic.

Although methanogens cannot function under aerobic conditions they can sustain oxygen stresses for a prolonged time.[3] Methanosarcina barkeri is exceptional in possessing a superoxide dismutase (SOD) enzyme, and may survive longer than the others.[4]

Some methanogens, 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.

Methanogens lack peptidoglycan, a polymer that is found in the cell walls of the Bacteria but not 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.

Methanogens and (extreme) environments

Although most marine biogenic methane is the result of carbon dioxide (CO2) reduction, a small amount is derived from acetate (CH3COO-).[5] 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.

Methanogens play the vital ecological 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 electron acceptors other than CO2 (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 are key agents of remineralization of organic carbon in continental margin sediments and other aquatic sediments with high rates of sedimentation and high sediment organic matter. Under the correct conditions of pressure and temperature, biogenic methane can accumulate in massive deposits of methane clathrates,[6] which account for a significant fraction of organic carbon in continental margin sediments and represent a key reservoir of a potent greenhouse gas.[7]

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 reproduce at temperatures of 15 to 100 degrees Celsius. They are known to be the most common 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.[8]

Another study[9] 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 United States, 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.[10]

Some scientists have proposed that the presence of methane in the Martian atmosphere may be indicative of native methanogens on that planet.[11]

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.[12] Most methanogens are autotrophic producers, but those that oxidize CH3COO- are classed as Chemotroph instead.

Strains of methanogens

  • Methanobacterium bryantii
  • Methanobacterium formicum
  • Methanobrevibacter arboriphilicus
  • Methanobrevibacter gottschalkii
  • Methanobrevibacter ruminantium
  • Methanobrevibacter smithii
  • Methanocalculus chunghsingensis
  • Methanococcoides burtonii
  • Methanococcus aeolicus
  • Methanococcus deltae
  • Methanococcus jannaschii
  • Methanococcus maripaludis
  • Methanococcus vannielii
  • Methanocorpusculum labreanum
  • Methanoculleus bourgensis (Methanogenium olentangyi & Methanogenium bourgense)
  • Methanoculleus marisnigri
  • Methanofollis liminatans
  • Methanogenium cariaci
  • Methanogenium frigidum
  • Methanogenium organophilum
  • Methanogenium wolfei
  • Methanomicrobium mobile
  • Methanopyrus kandleri
  • Methanoregula boonei
  • Methanosaeta concilii
  • Methanosaeta thermophila
  • Methanosarcina acetivorans
  • Methanosarcina barkeri
  • Methanosarcina mazei
  • Methanosphaera stadtmanae
  • Methanospirillium hungatei
  • Methanothermobacter defluvii (Methanobacterium defluvii)
  • Methanothermobacter thermautotrophicus (Methanobacterium thermoautotrophicum)
  • Methanothermobacter thermoflexus (Methanobacterium thermoflexum)
  • Methanothermobacter wolfei (Methanobacterium wolfei)
  • Methanothrix sochngenii

See also

References

  1. ^ Joseph W. Lengeler (1999). Biology of the Prokaryotes. Stuttgart: Thieme. p. 796. ISBN 0632053577. 
  2. ^ J.K. Kristjansson, et al. (1982). "Different Ks values for hydrogen of methanogenic bacteria and sulfate-reducing bacteria: an explanation for the apparent inhibition of methanogenesis by sulfate". Arch. Microbiol. 131: 278–282. doi:10.1007/BF00405893. 
  3. ^ 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
  4. ^ http://spacecenter.uark.edu/JillJabstract.doc
  5. ^ M.J. Whiticar, et al. (1986). "Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation — isotope evidence". Geochim. Cosmochim. Acta 50: 393–709. doi:10.1016/0016-7037(86)90346-7. 
  6. ^ Kvenvolden, K. (1995). "A review of the geochemistry of methane in natural gas hydrate". Organic Geochemistry 23 (11-12): 997–1008. doi:10.1016/0146-6380(96)00002-2. 
  7. ^ Milkov, Alexei V (2004). "Global estimates of hydrate-bound gas in marine sediments: how much is really out there?". Earth-Science Reviews 66 (3-4): 183–197. Bibcode 2004ESRv...66..183M. doi:10.1016/j.earscirev.2003.11.002. 
  8. ^ Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0507601102)
  9. ^ Icarus (vol. 178, p. 277)cs:Methanogen
  10. ^ Extreme bugs back idea of life on Mars
  11. ^ Crater Critters: Where Mars Microbes Might Lurk
  12. ^ Thauer, R. K. and Shima, S. (2006). "Biogeochemistry: Methane and microbes". Nature 440 (7086): 878–879. doi:10.1038/440878a. PMID 16612369. 

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Look at other dictionaries:

  • methanogen — n. A type of archaebacteria found in anaerobic environments such as animal intestinal tracts or sediments or sewage, and capable of producing {methane}; a source of natural gas. [WordNet 1.5] …   The Collaborative International Dictionary of English

  • methanogen — [mə than′ə jən, mə than′əjen΄] n. an anaerobic microorganism that lives on carbon dioxide and hydrogen and gives off methane, found in swamp sediment, cows stomachs, etc. methanogenic [mə than′əjen′ik] adj …   English World dictionary

  • Methanogen — Methangärung schematisch Methanbildung ist die mikrobielle Bildung von Methan (CH4). Sie stellt die letzte Stufe des anaeroben Abbaus von biogenen Stoffen unter anoxischen Bedingungen dar. In dieser obligat anaerob ablaufenden Phase wird… …   Deutsch Wikipedia

  • methanogen — methanogenic, adj. /me than euh jeuhn, jen /, n. any of a diverse group of widely distributed archaebacteria that occur in anaerobic environments, as the intestinal tracts of animals, freshwater and marine sediments, and sewage, and are capable… …   Universalium

  • methanogen — noun any of various archaebacteria capable of methanogenesis …   Wiktionary

  • methanogen — Any methane producing bacterium of the family Methanobacteriaceae. * * * me·than·o·gen mə than ə .jen n any of various anaerobic microorganisms that produce methane from carbon dioxide and hydrogen, are classified as bacteria (as of the family… …   Medical dictionary

  • methanogen — [ mɛθənə(ʊ)dʒ(ə)n, mɛ θanə(ʊ) ] noun Biology a methane producing microorganism. Derivatives methanogenesis noun methanogenic adjective …   English new terms dictionary

  • methanogen — me·than·o·gen …   English syllables

  • methanogen — meth•an•o•gen [[t]mɛˈθæn ə dʒən, ˌdʒɛn[/t]] n. mcr any of a group of archaebacteria that occur in diverse anaerobic environments and are capable of producing methane from a limited number of chemical sources, as carbon dioxide and hydrogen •… …   From formal English to slang

  • methanogen — noun archaebacteria found in anaerobic environments such as animal intestinal tracts or sediments or sewage and capable of producing methane; a source of natural gas • Hypernyms: ↑archaebacteria, ↑archaebacterium, ↑archaeobacteria,… …   Useful english dictionary


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