Methanogenesis
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Methanogenesis or biomethanation is the formation of methane by microbes. This is an important and widespread form of microbial metabolism. In most environments, it is the final step in the decomposition of biomass. Recently, it has been demonstrated that leaf tissues of living plants emit methane [1]. Although the mechanism by which such methane production occurs is, as yet, unknown, the implications are far-reaching; this is an example of methanogenesis occurring in non-microbes, presumably under aerobic conditions. Since not much is known about the pathways employed in plants in producing methane, the rest of this article focuses on the well-studied production of methane by microbes.
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[edit] Biochemistry of methanogenesis
Methanogenesis is a form of anaerobic respiration[2]. Methanogens do not use oxygen to breathe; in fact, oxygen inhibits the growth of methanogens. The terminal electron acceptor in methanogenesis is not oxygen, but carbon. The carbon can occur in a small number of organic compounds, all with low molecular weights. The two best described pathways involve the use of carbon dioxide and acetic acid as terminal electron acceptors:
CO2 + 4 H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2
However, methanogenesis has been shown to use carbon from other small organic compounds, such as formic acid, methanol, methylamines, dimethyl sulfide, and methanethiol.
The biochemistry of methanogenesis is relatively complex, involving the following coenzymes and cofactors: F430, coenzyme B, coenzyme M, methanofuran, and methanopterin.
[edit] Occurrence of methanogens
Organisms capable of methogensis are called methanogens. Microbes performing methanogenesis have no nucleus or membrane-bound organelles (i.e. they are prokaryotes). Methanogens are considered to be a very old group of organisms, being members of the archaebacteria, also known as archaea (depending on what taxonomic system is being used).
Methanogens cannot exist in the presence of oxygen, so they are only found in environments in which the oxygen has been depleted. Most commonly these are environments experiencing the decay of organic matter, such as wetland soils, the digestive tracts of animals, and aquatic sediments. Methanogenesis also occurs in areas where oxygen and decaying organic matter are both absent, such as the terrestrial deep subsurface, deep-sea hydrothermal vents, and oil reservoirs.
[edit] Importance in carbon cycle
Methanogenesis is the final step in the decay of organic matter. During the decay process, electron acceptors (such as oxygen, ferric iron, sulfate, nitrate, and manganese) become depleted, while hydrogen (H2) and carbon dioxide accumulate. Light organics produced by 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.
Methanogenesis is useful to humanity. Through methanogenesis, organic waste can be converted to useful methane "biogas." Methanogenesis occurs in the guts of humans and other animals. While methanogenesis is not believed to be necessary for human digestion, it is required for the nutrition of ruminant animals, such as cattle and goats. In the rumen (known incorrectly as the "second stomach" possessed by some animals), anaerobic organisms (including methanogens) digest cellulose into forms usable by the animal. Without the microbes of the rumen, cattle cannot survive without being fed a special diet.
[edit] Reverse methanogenesis
Methanogens can also utilize methane as a substrate in conjunction with the reduction of sulfate and nitrate.[3]
[edit] References
- ^ Kepler F; et al. (2006). "Methane emissions from terrestrial plants under aerobic conditions.". Nature. 439: 187–191.
- ^ Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377-2406.
- ^ Thauer, R. K. and Shima, S., "Biogeochemistry: Methane and microbes", Nature, 2006, 440, 878-879
