Methanogenesis

Methanogenesis or biomethanation is the formation of methane by microbes known as methanogens. Organisms capable of producing methane have been identified only from the 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.

Biochemistry of methanogenesis

Cycle for methanogenesis, showing intermediates.

Methanogenesis in microbes is a form of anaerobic respiration.^[1] Methanogens do not use oxygen to respire; 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, 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, electron acceptors (such as oxygen, ferric iron, sulfate, and nitrate) 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.

Natural occurrence

In ruminants

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

Enteric fermentation occurs in the guts of some animals, especially ruminants. In the rumen, anaerobic organisms, including methanogens, digest cellulose into forms usable by the animal. Without these microorganisms, animals such as cattle would not be able to consume grass. The useful products of methanogenesis are absorbed by the gut, but methane is released from the animal mainly by belching (eructation). The average cow emits around 250 liters of methane per day.^[4]

In humans

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.

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]

In plants

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]

Underground

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]

Role in global warming

Atmospheric methane 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.

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.^[11] It is the primary pathway whereby most organic matter disposed of via landfill is broken down.^[12]

Methanogenesis and extra-terrestrial life

The presence of atmospheric methane has a role in the scientific search for extra-terrestrial life. 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^[13] 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)^[14] and in Titan's atmosphere by the Huygens probe (2005).^[15] This debate was furthered with the discovery of 'transient', 'spikes of methane' on mars by the Curiosity Rover.^[16]

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.^[17][18]

See also

• Aerobic methane production
• Anaerobic digestion
• Electromethanogenesis
• Hydrogen cycle
• Mootral
• Anaerobic oxidation of methane
• Methanotroph

References