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Methanogenesis

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Title: Methanogenesis  
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Subject: Methane, Acidogenesis, Featured article candidates/Coal/archive1, Digestate, Cofactor F430
Collection: Anaerobic Digestion, Biodegradable Waste Management, Biodegradation, Hydrogen Biology, Sewerage
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Methanogenesis

Methanogenesis or biomethanation is the formation of domain Archaea, a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbial metabolism. In most environments, it is the final step in the decomposition of biomass.

Contents

  • Biochemistry of methanogenesis 1
    • Proposed mechanism of methanogenesis 1.1
    • Reverse methanogenesis 1.2
    • Importance in carbon cycle 1.3
  • Natural occurrence 2
    • In ruminants 2.1
    • In humans 2.2
    • In plants 2.3
    • Underground 2.4
  • Role in global warming 3
  • Methanogenesis and extra-terrestrial life 4
  • See also 5
  • References 6

Biochemistry of methanogenesis

Methanogenesis in microbes is a form of carbon dioxide and acetic acid as terminal electron acceptors:

CO2 + 4 H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2

However, depending on pH and temperature, methanogenesis has been shown to use carbon from other small organic compounds, such as formic acid (formate), methanol, methylamines, dimethyl sulfide, and methanethiol.

Proposed mechanism of methanogenesis

The biochemistry of methanogenesis is relatively complex, involving the following coenzymes and cofactors: F420, coenzyme B, coenzyme M, methanofuran, and methanopterin.

The mechanism for the conversion of CH
3
–S
bond into methane involves a ternary complex of methyl coenzyme M and coenzyme B fit into a channel terminated by the axial site on nickel of the cofactor F430. One proposed mechanism invokes electron transfer from Ni(I) (to give Ni(II)), which initiates formation of CH
4
. Coupling of the coenzyme M thiyl radical (RS.) with HS coenzyme B releases a proton and re-reduces Ni(II) by one-electron, regenerating Ni(I).[2]

Reverse methanogenesis

In reverse methanogenesis, methane reacts with the disulfide of coenzyme M to give coenzyme M and its methyl derivative:

RS-SR + CH4 → RS-H + RS-CH3

Organisms that promote this remarkable reaction contain 7% by weight of F430.[3]

Importance in carbon cycle

Methanogenesis is the final step in the decay of organic matter. During the decay process, fermentation also accumulate. During advanced stages of organic decay, all electron acceptors become depleted except carbon dioxide. Carbon dioxide is a product of most catabolic processes, so it is not depleted like other potential electron acceptors.

Only methanogenesis and fermentation can occur in the absence of electron acceptors other than carbon. Fermentation only allows the breakdown of larger organic compounds, and produces small organic compounds. Methanogenesis effectively removes the semi-final products of decay: hydrogen, small organics, and carbon dioxide. Without methanogenesis, a great deal of carbon (in the form of fermentation products) would accumulate in anaerobic environments.

Natural occurrence

In ruminants

Testing Australian sheep for exhaled methane production (2001), CSIRO

Methanogenesis occurs in the guts of humans and other animals, especially ruminants. In the

  1. ^ Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377-2406.
  2. ^
  3. ^
  4. ^ : "Innovations – Methane In Agriculture."Radio Australia 15 August 2004. Retrieved 28 August 2007.
  5. ^
  6. ^
  7. ^
  8. ^ http://sciencenow.sciencemag.org/cgi/content/full/2009/114/1
  9. ^
  10. ^
  11. ^
  12. ^ DoE Report CWM039A+B/92 Young, A. (1992)
  13. ^ Scientific Search for Extra-Terrestrial life http://articles/Extraterrestrial_life#Scientific_search_for_extraterrestrial_life
  14. ^ BBC article about methane as sign of life http://news.bbc.co.uk/2/hi/science/nature/4295475.stm
  15. ^ European Space Agency, Methane in Martian Atmosphere http://www.esa.int/esaMI/Mars_Express/SEMZ0B57ESD_0.html
  16. ^ Space.Com article about methane on Huygens http://www.space.com/scienceastronomy/ap_huygens_update_050127.html
  17. ^ http://www.telegraph.co.uk/news/science/space/11297326/Life-on-Mars-Nasa-finds-first-hint-of-alien-life.html
  18. ^ New Scientist article about atmospheric methane http://www.newscientist.com/article.ns?id=dn7059
  19. ^ National Geographic Article about methane as sign of life http://news.nationalgeographic.com/news/2004/10/1007_041007_mars_methane.html

References

See also

It is also argued that atmospheric methane can come from volcanoes or other fissures in the planet's crust and that without an isotopic signature, the origin or source may be difficult to identify.[18][19]

The presence of atmospheric methane has a role in the scientific search for extra-terrestrial life.[13] The justification is that methane in the atmosphere will eventually dissipate, unless something is replenishing it. If methane is detected (by using a spectrometer for example) this may indicate that life is, or recently was, present. This was debated[14] when methane was discovered in the Martian atmosphere by M.J. Mumma of NASA's Goddard Flight Center, and verified by the Mars Express Orbiter (2004)[15] and in Titan's atmosphere by the Huygens probe (2005).[16] This debate was furthered with the discovery of 'transient', 'spikes of methane' on mars by the Curiosity Rover.[17]

Methanogenesis and extra-terrestrial life

Methanogenesis can also be beneficially exploited, to treat organic waste, to produce useful compounds, and the methane can be collected and used as landfill is broken down.[12]

Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential 25 times greater than carbon dioxide (averaged over 100 years),[10] and methanogenesis in livestock and the decay of organic material is thus a considerable contributor to global warming. It may not be a net contributor in the sense that it works on organic material which used up atmospheric carbon dioxide when it was created, but its overall effect is to convert the carbon dioxide into methane which is a much more potent greenhouse gas.

Role in global warming

Methanogens are observed in anoxic underground environments, contributing to the degradation of organic matter. This organic matter may be placed by humans through landfill, buried as sediment on the bottom of lakes or oceans as sediments, and as residual organic matter from sediments that have formed into sedimentary rocks. Methanogenesis is responsible for a significant fraction of natural gas accumulations.[9]

Underground

Many experiments have suggested that leaf tissues of living plants emit methane.[7] Other research has indicated that the plants are not actually generating methane; they are just absorbing methane from the soil and then emitting it through their leaf tissues.[8]

In plants

Even among humans whose flatus does contain methane, the amount is in the range of 10% or less of the total amount of gas.[6]

Some humans produce flatus that contains methane. In one study of the feces of nine adults, only five of the samples contained archaea capable of producing methane.[5] Similar results are found in samples of gas obtained from within the rectum.

In humans

[4]

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