Methanogenesis orbiomethanation is the formation ofmethane coupled to energy conservation bymicrobes known asmethanogens. It is the fourth and final stage ofanaerobic digestion. Organisms capable of producing methane for energy conservation have been identified only from thedomainArchaea, a groupphylogenetically distinct from botheukaryotes andbacteria, although many live in close association with anaerobic bacteria. The production of methane is an important and widespread form of microbialmetabolism. Inanoxic environments, it is the final step in the decomposition ofbiomass. Methanogenesis is responsible for significant amounts ofnatural gas accumulations, the remainder being thermogenic.[1][2][3]
Methanogenesis in microbes is a form ofanaerobic respiration.[4] Methanogens do not use oxygen to respire; in fact, oxygen inhibits the growth of methanogens. The terminalelectron acceptor in methanogenesis is not oxygen, but carbon. The two best described pathways involve the use ofacetic acid (acetoclastic) or inorganiccarbon dioxide (hydrogenotrophic) as terminal electron acceptors:
During anaerobic respiration of carbohydrates, H2 and acetate are formed in a ratio of 2:1 or lower, so H2 contributes onlyc. 33% to methanogenesis, with acetate contributing the greater proportion. In some circumstances, for instance in therumen, where acetate is largely absorbed into the bloodstream of the host, the contribution of H2 to methanogenesis is greater.[5]
However, depending on pH and temperature, methanogenesis has been shown to use carbon from other small organic compounds, such asformic acid (formate),methanol,methylamines,tetramethylammonium,dimethyl sulfide, andmethanethiol. The catabolism of the methyl compounds is mediated by methyl transferases to give methyl coenzyme M.[4]
The biochemistry of methanogenesis involves the following coenzymes and cofactors:F420,coenzyme B,coenzyme M,methanofuran, andmethanopterin.
The mechanism for the conversion ofCH
3–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 thecofactor F430.[6] One proposed mechanism invokes electron transfer from Ni(I) (to give Ni(II)), which initiates formation ofCH
4. Coupling of the coenzyme Mthiyl radical (RS.) with HS coenzyme B releases a proton and re-reduces Ni(II) by one electron, regenerating Ni(I).[7]
Some organisms can oxidize methane, functionally reversing the process of methanogenesis, also referred to as theanaerobic 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.[8] These organisms may accomplishreverse methanogenesis using a nickel-containing protein similar tomethyl-coenzyme M reductase used by methanogenic archaea.[9] Reverse methanogenesis occurs according to the reaction:
Methanogenesis is the final step in the anaerobic decay of organic matter. During the decay process,electron acceptors (such asoxygen,ferriciron,sulfate, andnitrate) become depleted, whilehydrogen (H2) andcarbon dioxide accumulate. Light organics produced byfermentation 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.
Enteric fermentation occurs in the gut of some animals, especially ruminants. In therumen, 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 bybelching (eructation). The average cow emits around 250 liters of methane per day.[11] In this way, ruminants contribute about 25% of anthropogenicmethane emissions. One method of methane production control in ruminants is by feeding them3-nitrooxypropanol.[12]
Some humans produceflatus that contains methane. In one study of thefeces of nine adults, five of the samples containedarchaea capable of producing methane.[13] Similar results are found in samples of gas obtained from within therectum.
Even among humans whose flatus does contain methane, the amount is in the range of 10% or less of the total amount of gas.[14]
Many experiments have suggested thatleaf tissues of living plants emit methane.[15] 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.[16] In a follow up study done in 2009, plants grown in carbon-13 enriched environments were observed to not emit significant amounts of methane.[17] 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.[18] 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.[19]
Methanogens are observed in anoxic soil 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.[20] Methanogenesis is not strictly limited to anoxic ecosystems such as peats and bogs; damp mineral soils can also contain high methane levels between the microscopic spaces of decaying organic matter.[21] As a result, the process of methanogenesis is common in rice fields and wetlands as these areas are flooded fields and are a natural methane source.[22] The production of methane in flooded soils like these requires microbes that prefer low oxygen levels. Aerating methanogenic soils increases levels of sulfates and nitrates, nutrients that reduce the production of methane.[23] For methanogenesis to continue, nitrate and sulfate levels will need to decrease. In a separate study conducted in remote arctic soils, higher amounts of methanogens had a direct correlation with increased potential methane production.[24] Methanogenesis in upland soils is often localized withinanoxic microsites, even when the bulk soil remains aerobic.
Methanogens are a notable part of the microbial communities in continental and marinedeep biosphere.[25][26][27]
Approximately one third ofmethanogens which have been described arise from marine origins, a majority being from the cladeEuryarchaeota.[28] In the marine environment, methanogenic microorganisms compete for resources withsulfate-reducers.[28] As a result of this, sulfate-depleted areas of high organic matter loading and sediments are areas of methanogen predominance.[28] The anaerobic nature of sediments allow for methanogenic activity and flourishing of methanogenic communities, making marine sediments an important habitat for methane generating microbial communities. A major compound which methanogens consume to generate methane isacetate, which composes two thirds of global methane production.[28] Another compound which contributes to marine sediment methanogenesis iscarbon monoxide, which is oxidized intocarbon dioxide, before undergoing a series of reactions to produce energy asmethane is released from the microbe.[28] This compound is considered non-competitive with sulfate-reducers, allowing for free use by methanogens. In examination of the microorganismM. acetivorans, methane synthesis pathways retain similarities with freshwater taxa, however proteins distinct to the marine sediment microbes are found which operate on the methanogenic pathway.[28] The estimated annual release of methane from the ocean into the atmosphere is approximately 0.7-14 billion kg CH4 per year.[28] Despite the requirement of anoxic conditions for main methanogenic processes, supersaturation of methane in surface ocean waters creates the “marine methane paradox”, which leads to the release of methane into the atmosphere from the ocean.[28][29][30][31]
Recent studies seek to explain this paradox by examining the possibility of methane synthesis in the surface ocean despite oxic conditions. Oxic sources of methane were discovered in microbial communities starved of phosphorus in surface oceans,[30] where thecatabolism of the compoundmethyl-phosphonic acid (Mpn) has been found to co-produce methane in oxic ocean waters, providing a potential explanation to the paradox.[29][30][31]N. maritimus, a widespread archaeon in the ocean, was found to contain pathways for the synthesis of methyl-phosphonic acid within these oxic ocean waters.[30] The production of this compound from surrounding materials allows for methanogenesis via breakdown by surrounding bacteria and microbes. Furthermore, the prevalence of Mpn synthesis is consistent with abundance of Mpn reducing taxa such asPelagibacter,[30] The linkage between the producers of Mpn and the degraders of the compound lead to the production of methane. In microbes which reduce methyl-phosphonic acids, C-P lyase proteins have been found to be crucial to this reduction process[29][30][4], which acts as a source of phosphorus for the microbes as well as releasing methane. Mutants which disrupted Mpn degradation pathways were found to also show degradation of methanogenesis, confirming the link between the breakdown of methyl-phosphonic acid compounds and the production of methane within oxic ocean environments. Upregulation of transport andhydrolysis ofphosphonate compounds within bacteria was found to occur in phosphate limitation,[30] further illustrating the use of these compounds for necessary metabolic activity. The presence of this Mpn synthesis-degradation within the oxic conditions of the surface ocean explain the supersaturation of methane which caused the “marine methane paradox”, providing evidence for methanogenesis outside of the anoxic conditions which are necessary for the usual methanogenic pathways.
Methanogenesis can also be beneficially exploited, to treatorganic waste, to produce useful compounds, and the methane can be collected and used asbiogas, a fuel.[32] It is the primary pathway whereby most organic matter disposed of vialandfill is broken down.[33] Some biogas plants use methanogenesis to combine the CO2 with hydrogen to create more methane.[34]
Methane is an importantgreenhouse gas with aglobal warming potential 25 times greater than carbon dioxide (averaged over 100 years).[35] Methanogenesis inlivestock and the decay of organic material contributes to global warming.
The presence of atmospheric methane has a role in the scientific search forextra-terrestrial life. The justification is that on an astronomical timescale, methane in the atmosphere of an Earth-like celestial body will quickly dissipate, and that its presence on such a planet or moon therefore indicates that something is replenishing it. If methane is detected (by using aspectrometer for example) this may indicate that life is, or recently was, present.This was debated[36] when methane was discovered in the Martian atmosphere by M.J. Mumma of NASA's Goddard Flight Center, and verified by theMars Express Orbiter (2004)[37] and inTitan's atmosphere by theHuygens probe (2005).[38] This debate was furthered with the discovery of 'transient', 'spikes of methane' on Mars by theCuriosity Rover.[39]
It is argued thatatmospheric methane can come from volcanoes or other fissures in the planet's crust and that without anisotopic signature, the origin or source may be difficult to identify.[40][41]
On 13 April 2017, NASA confirmed that the dive of theCassini orbiter spacecraft on 28 October 2015 discovered anEnceladus plume which has all the ingredients for methanogenesis-based life forms to feed on. Previous results, published in March 2015, suggested hot water is interacting with rock beneath the sea of Enceladus; the new finding supported that conclusion, and add that the rock appears to be reacting chemically. From these observations scientists have determined that nearly 98 percent of the gas in the plume is water, about 1 percent is hydrogen, and the rest is a mixture of other molecules including carbon dioxide, methane and ammonia.[42]