Methanogenesis or biomethanation is the formation of methane by microbes known as methanogens. Organisms capable of producing methane have been identified only from the kingdom 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.
Recently, it has been demonstrated that leaf tissues of living plants emit methane. 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. Most of what is known about methanogenesis comes from microbial studies.
Biochemistry of methanogenesis
Methanogenesis in microbes is a form of anaerobic respiration. 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 (formate), 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.
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 occurs in the guts of humans and other animals, especially ruminants. In the rumen, anaerobic organisms including methanogens digest cellulose into forms usable by the animal, without them, livestock such as cattle would not be able to graze grass. The useful products of methanogenesis are absorbed by the gut, but the methane is released from the animal mainly by belching (eructation). The average cow emits around 600 litres of methane per day.
Role in global warming
Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential 21 times greater than carbon dioxide (averaged over 100 years), 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.
Methanogenesis can also be beneficially exploited, to treat organic waste, to produce useful compounds, and the methane can be collected and used as biogas, a fuel.
Methanogenesis and Extra-Terrestrial life
The presence of atmospheric methane has a role in the scientific search for extra-terrestrial life. The argument being that methane in the atmosphere will eventually dissipate, unless something is replenishing it. This something could then be the decomposition of organic matter (Methanogenesis). So, if it can be detected (by using a spectrometer for example) then that means there is, or relatively recently was, life present. This was debated when methane was discovered in the Martian atmosphere by (among others) the Mars Express Orbiter (2004) and in Titan's atmosphere by the Huygens probe(2005). 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 it is difficult to say what exactly was the origin.