Methanogenesis or biomethanation is the formation of methane coupled to energy conservation by microbes known as methanogens. It is the fourth and final stage of anaerobic digestion. Organisms capable of producing methane for energy conservation 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 anoxic environments, it is the final step in the decomposition of biomass. Methanogenesis is responsible for significant amounts of natural gas accumulations, the remainder being thermogenic.
Biochemistry
Methanogenesis in microbes is a form of anaerobic respiration. 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 two best described pathways involve the use of acetic acid (acetoclastic) or inorganic carbon dioxide (hydrogenotrophic) as terminal electron acceptors:
CO2 + 4 H2 → CH4 + 2 H2O CH3COOH → CH4 + CO2 During anaerobic respiration of carbohydrates, H2 and acetate are formed in a ratio of 2:1 or lower, so H2 contributes only c. 33% to methanogenesis, with acetate contributing the greater proportion. In some circumstances, for instance in the rumen, where acetate is largely absorbed into the bloodstream of the host, the contribution of H2 to methanogenesis is greater. 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, tetramethylammonium, dimethyl sulfide, and methanethiol. The catabolism of the methyl compounds is mediated by methyl transferases to give methyl coenzyme M.
Proposed mechanism The biochemistry of methanogenesis involves the following coenzymes and cofactors: F420, coenzyme B, coenzyme M, methanofuran, and methanopterin. The mechanism for the conversion of CH3–S bond into methane involves a ternary complex of the enzyme, with the substituents forming a structure α2β2γ2. Within the complex, 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 CH4. 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).
Reverse methanogenesis Some organisms can oxidize methane, functionally reversing the process of methanogenesis, also referred to as the anaerobic oxidation of methane (AOM). Organisms performing AOM have been found in multiple marine and freshwater environments including methane seeps, hydrothermal vents, coastal sediments and sulfate-methane transition zones. These organisms may accomplish reverse methanogenesis using a nickel-containing protein similar to methyl-coenzyme M reductase used by methanogenic archaea. Reverse methanogenesis occurs according to the reaction:
SO2−4 + CH4 → HCO−3 + HS− + H2O
Importance in carbon cycle Methanogenesis is the final step in the anaerobic 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
Enteric fermentation occurs in the gut of some animals, especially ruminants. In the rumen, anaerobic organisms, including methanogens, digest cellulose into forms nutritious to the animal. Without these microorganisms, animals such as cattle would not be able to consume grasses. 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. In this way, ruminants contribute about 25% of anthropogenic methane emissions. One method of methane production control in ruminants is by feeding them 3-nitrooxypropanol.
In humans Some humans produce flatus that contains methane. In one study of the feces of nine adults, five of the samples contained archaea capable of producing methane. 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.
In plants Many experiments have suggested that leaf tissues of living plants emit methane. A study done in 2006 estimated that global vegetation released between 60 and 240 million tonnes of methane yearly, corresponding to 40% of annual methane emissions. In a follow up study done in 2009, plants grown in carbon-13 enriched environments were observed to not emit significant amounts of methane. 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. Such an instance can be seen in seasonally flooded parts of the Amazon Rainforest, where trees in said areas pumped 200 times the normal amount of methane out from each tree, accounting for around 40 million tonnes of methane emitted per year.
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