Editing Methanogenesis

{{short description|Formation of methane by microbes}}
{{Use dmy dates|date=August 2020}}
'''Methanogenesis''' or '''biomethanation''' is the formation of [[methane]] coupled to energy conservation by [[microbe]]s known as [[methanogen]]s. 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 (biology)|domain]] [[Archaea]], a group [[Phylogenetics|phylogenetically]] distinct from both [[eukaryote]]s 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 [[Hypoxia (environmental)|anoxic]] environments, it is the final step in the decomposition of [[biomass (ecology)|biomass]].  Methanogenesis is responsible for significant amounts of [[natural gas]] accumulations, the remainder being thermogenic.<ref>{{cite journal | author = Katz B.| title = Microbial processes and natural gas accumulations | journal = The Open Geology Journal | year = 2011 | volume = 5| issue = 1 | pages = 75&ndash;83|doi=10.2174/1874262901105010075| bibcode = 2011OGJ.....5...75J | doi-access = free }}</ref><ref>{{cite journal|title=The origin, source, and cycling of methane in deep crystalline rock biosphere|year=2015|journal=Front. Microbiol.|doi=10.3389/fmicb.2015.00725|author=Kietäväinen and Purkamo|pmc=4505394|pmid=26236303|volume=6|page=725|doi-access=free}}</ref><ref>{{cite journal|title=Indications for an active petroleum system in the Laptev Sea, NE Siberia/publication/227744258_Indications_for_an_active_petroleum_system_in_the_Laptev_Sea_NE_Siberia|year=2005|journal=Journal of Petroleum Geology|doi=10.1111/j.1747-5457.2005.tb00088.x|author=Cramer and Franke|volume=28|issue=4|pages=369–384|bibcode=2005JPetG..28..369C|doi-access=|s2cid=129445357 }}</ref>

==Biochemistry==
[[File:Methanogenesis cycle.png|thumb|320px|Cycle for methanogenesis, showing intermediates.]]

Methanogenesis in microbes is a form of [[anaerobic respiration]].<ref name=RT>{{cite journal|author=Thauer, R. K.|title=Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson|journal=Microbiology|year=1998|volume=144|pages=2377–2406|doi=10.1099/00221287-144-9-2377|pmid=9782487|doi-access=free}}</ref> Methanogens do not use oxygen to respire; in fact, oxygen inhibits the growth of methanogens.  The terminal [[Oxidizing agent#Electron acceptor|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:
:CO<sub>2</sub> + 4 H<sub>2</sub> → [[Methane|CH<sub>4</sub>]] + 2 H<sub>2</sub>O

:CH<sub>3</sub>COOH → CH<sub>4</sub> + CO<sub>2</sub>

During anaerobic respiration of carbohydrates, H<sub>2</sub> and acetate are formed in a ratio of 2:1 or lower, so H<sub>2</sub> contributes only {{circa|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 H<sub>2</sub> to methanogenesis is greater.<ref>{{Cite journal|last=Conrad|first=Rolf|date=1999|title=Contribution of hydrogen to methane production and control of hydrogen concentrations in methanogenic soils and sediments|journal=FEMS Microbiology Ecology|volume=28|issue=3|pages=193–202|doi=10.1016/s0168-6496(98)00086-5|doi-access=free|bibcode=1999FEMME..28..193C }}</ref>

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.<ref name="RT" />

===Proposed mechanism===
The biochemistry of methanogenesis involves the following coenzymes and cofactors: [[Coenzyme F420|F420]], [[coenzyme B]], [[coenzyme M]], [[methanofuran]], and [[methanopterin]].

The mechanism for the conversion of {{chem|CH|3|–S}} bond into methane involves a ternary complex of the enzyme, with the substituents forming a structure α<sub>2</sub>β<sub>2</sub>γ<sub>2</sub>. Within the complex, methyl coenzyme M and coenzyme B fit into a channel terminated by the axial site on nickel of the [[cofactor F430]].<ref>{{cite journal |last1=Cedervall |first1=Peder |title=Structural Insight into Methyl-Coenzyme M Reductase Chemistry Using Coenzyme B Analogues |journal=Biochemistry |date=22 July 2010 |volume=49 |issue=35 |pages=7683–7693 |doi=10.1021/bi100458d |pmid=20707311 |pmc=3098740 }}</ref>  One proposed mechanism invokes electron transfer from Ni(I) (to give Ni(II)), which initiates formation of {{chem|CH|4}}.  Coupling of the coenzyme M [[thiyl radical]] (RS<sup>.</sup>) with HS coenzyme B releases a proton and re-reduces Ni(II) by one electron, regenerating Ni(I).<ref>{{cite journal  |vauthors=Finazzo C, Harmer J, Bauer C, etal |title=Coenzyme B induced coordination of coenzyme M via its thiol group to Ni(I) of F<sub>430</sub> in active methyl-coenzyme M reductase |journal=J. Am. Chem. Soc. |volume=125 |issue=17 |pages=4988–9 |date=April 2003 |pmid=12708843 |doi=10.1021/ja0344314 |bibcode=2003JAChS.125.4988F }}</ref>

===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.<ref>{{Cite journal|last1=Ruff|first1=S. Emil|last2=Biddle|first2=Jennifer F.|last3=Teske|first3=Andreas P.|last4=Knittel|first4=Katrin|last5=Boetius|first5=Antje|last6=Ramette|first6=Alban|date=2015-03-31|title=Global dispersion and local diversification of the methane seep microbiome|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=112|issue=13|pages=4015–4020|doi=10.1073/pnas.1421865112|issn=1091-6490|pmc=4386351|pmid=25775520|bibcode=2015PNAS..112.4015R|doi-access=free}}</ref> These organisms may accomplish '''reverse methanogenesis''' using a nickel-containing protein similar to [[Coenzyme-B sulfoethylthiotransferase|methyl-coenzyme M reductase]] used by methanogenic archaea.<ref>{{cite journal |doi=10.1155/2017/1654237 |title=Reverse Methanogenesis and Respiration in Methanotrophic Archaea |date=2017 |last1=Timmers |first1=Peer H. A. |last2=Welte |first2=Cornelia U. |last3=Koehorst |first3=Jasper J. |last4=Plugge |first4=Caroline M. |last5=Jetten |first5=Mike S. M. |last6=Stams |first6=Alfons J. M. |journal=Archaea |volume=2017 |pages=1–22 |doi-access=free |pmid=28154498 |pmc=5244752 |hdl=1822/47121 |hdl-access=free }}</ref> Reverse methanogenesis occurs according to the reaction:
: {{chem|SO|4|2−}} +  CH<sub>4</sub>   →   {{chem|HCO|3|−}}  + HS<sup>−</sup> + H<sub>2</sub>O<ref>{{cite journal  |vauthors=Krüger M, Meyerdierks A, Glöckner FO, etal |title=A conspicuous nickel protein in microbial mats that oxidize methane anaerobically |journal=Nature |volume=426 |issue=6968 |pages=878–81 |date=December 2003 |pmid=14685246 |doi=10.1038/nature02207 |bibcode = 2003Natur.426..878K |s2cid=4383740 }}</ref>

===Importance in carbon cycle===
Methanogenesis is the final step in the anaerobic decay of organic matter.  During the decay process, [[Oxidizing agent#Electron acceptor|electron acceptors]] (such as [[oxygen]], [[ferric]] [[iron]], [[sulfate]], and [[nitrate]]) become depleted, while [[hydrogen]] (H<sub>2</sub>) and [[carbon dioxide]] accumulate.  Light organics produced by [[fermentation (biochemistry)|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===
[[File:CSIRO ScienceImage 1898 Testing Sheep for Methane Production.jpg|thumb|Testing Australian sheep for exhaled methane production (2001), [[CSIRO]]]]
[[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.<ref>[http://www.abc.net.au/ra/innovations/stories/s1159618.htm ''Radio Australia'': "Innovations – Methane In Agriculture."] 15 August 2004. Retrieved 28 August 2007.</ref> 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|3-nitrooxypropanol]].<ref>{{cite journal | last1 = Hristov | first1 = A. N. | display-authors = etal | year = 2015 | title = An inhibitor persistently decreased enteric methane emission from dairy cows with no negative effect on milk production | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 112 | issue = 34 | pages = 10663–10668 | doi = 10.1073/pnas.1504124112 | pmid = 26229078 | pmc = 4553761 | bibcode = 2015PNAS..11210663H | doi-access = free }}</ref>

===In humans===
Some humans produce [[flatulence|flatus]] that contains methane.  In one study of the [[feces]] of nine adults, five of the samples contained [[archaea]] capable of producing methane.<ref>{{cite journal| author=Miller TL|author2=Wolin MJ |author3=de Macario EC |author4=Macario AJ | title=Isolation of Methanobrevibacter smithii from human feces| journal=Appl Environ Microbiol| year=1982| volume=43| pages=227–32 |pmid=6798932 |pmc=241804 |url=| issue=1|doi=10.1128/aem.43.1.227-232.1982 |bibcode=1982ApEnM..43..227M }}</ref> 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.<ref>{{cite encyclopedia
  | title =Human Digestive System
  | encyclopedia =Encyclopædia Britannica
  | url =http://www.britannica.com/eb/article-45361/human-digestive-system#294193.hook
  | access-date = 2007-08-22 }}</ref>

===In plants===
Many experiments have suggested that [[leaf]] tissues of living plants emit methane.<ref>{{cite journal  |vauthors=Kepler F, etal | title = ''Methane emissions from terrestrial plants under aerobic conditions'' | journal = Nature | year = 2006 | volume = 439 | issue = 7073 | pmid = 16407949| pages = 187&ndash;191 | doi=  10.1038/nature04420 | bibcode=2006Natur.439..187K| s2cid = 2870347 }}</ref> 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.<ref>{{Cite journal |last=Schiermeier |first=Quirin |date=2006-01-01 |title=Methane finding baffles scientists |url=https://www.nature.com/articles/439128a?utm |journal=Nature |language=en |volume=439 |issue=7073 |pages=128–128 |doi=10.1038/439128a |issn=1476-4687}}</ref> In a follow up study done in 2009, plants grown in carbon-13 enriched environments were observed to not emit significant amounts of methane.<ref>{{Cite journal |last=Hopkin |first=Michael |date=2007-05-01 |title=Missing gas saps plant theory |url=https://www.nature.com/articles/447011a?utm |journal=Nature |language=en |volume=447 |issue=7140 |pages=11–11 |doi=10.1038/447011a |issn=1476-4687}}</ref> 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.<ref>{{Cite web | url=http://sciencenow.sciencemag.org/cgi/content/full/2009/114/1 | title=News| date=30 October 2014}}</ref> 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.<ref>{{Cite web |title=Scientists Zero in on Trees as a Surprisingly Large Source of Methane |url=https://e360.yale.edu/features/scientists-probe-the-surprising-role-of-trees-in-methane-emissions |access-date=2025-05-01 |website=Yale e360 |language=en-US}}</ref>

===In soils===
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.<ref>{{cite journal|author1=Le Mer, J.|author2=Roger, P.|title=Production, oxidation, Emission and Consumption of Methane by Soils: A Review|journal=European Journal of Soil Biology|year=2001|volume=37|issue=1 |pages=25–50|doi=10.1016/S1164-5563(01)01067-6|bibcode=2001EJSB...37...25L |s2cid=62815957 }}</ref> 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.<ref>{{Cite web |date=2025-02-02 |title=Emission of methane from plants - PMC |url=https://web.archive.org/web/20250202173345/https://pmc.ncbi.nlm.nih.gov/articles/PMC2660970/ |access-date=2025-05-01 |website=web.archive.org}}</ref> 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 sink.<ref>{{Cite journal |last=Aliyev |first=Z. H. |date=2020-12-07 |title=Review on Methanogenesis and its Role |url=https://irispublishers.com/wjass/fulltext/review-on-methanogenesis-and-its-role.ID.000632.php |journal=World Journal of Agriculture and Soil Science |language=English |volume=6 |issue=2 |pages=1–7}}</ref> 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.<ref>{{Cite web |date=2025-02-25 |title=Methane Production in Soil Environments—Anaerobic Biogeochemistry and Microbial Life between Flooding and Desiccation |url=https://web.archive.org/web/20250225063228/https://www.mdpi.com/2076-2607/8/6/881 |access-date=2025-05-01 |website=web.archive.org}}</ref> 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.<ref>{{Cite journal |last=Wagner |first=Robert |last2=Zona |first2=Donatella |last3=Oechel |first3=Walter |last4=Lipson |first4=David |date=2017 |title=Microbial community structure and soil pH correspond to methane production in Arctic Alaska soils |url=https://enviromicro-journals.onlinelibrary.wiley.com/doi/10.1111/1462-2920.13854?utm= |journal=Environmental Microbiology |language=en |volume=19 |issue=8 |pages=3398–3410 |doi=10.1111/1462-2920.13854 |issn=1462-2920|url-access=subscription }}</ref>

=== In Earth's crust ===
Methanogens are a notable part of the microbial communities in continental and marine [[deep biosphere]].<ref>{{Cite journal|last=Kotelnikova|first=Svetlana|date=October 2002|title=Microbial production and oxidation of methane in deep subsurface|journal=Earth-Science Reviews|language=en|volume=58|issue=3–4|pages=367–395|doi=10.1016/S0012-8252(01)00082-4|bibcode=2002ESRv...58..367K}}</ref><ref>{{Cite journal|last1=Purkamo|first1=Lotta|last2=Bomberg|first2=Malin|last3=Kietäväinen|first3=Riikka|last4=Salavirta|first4=Heikki|last5=Nyyssönen|first5=Mari|last6=Nuppunen-Puputti|first6=Maija|last7=Ahonen|first7=Lasse|last8=Kukkonen|first8=Ilmo|last9=Itävaara|first9=Merja|date=2016-05-30|title=Microbial co-occurrence patterns in deep Precambrian bedrock fracture fluids|journal=Biogeosciences|language=en|volume=13|issue=10|pages=3091–3108|doi=10.5194/bg-13-3091-2016|issn=1726-4189|bibcode=2016BGeo...13.3091P|doi-access=free|hdl=10023/10226|hdl-access=free}}</ref><ref>{{Cite journal|last1=Newberry|first1=Carole J.|last2=Webster|first2=Gordon|last3=Cragg|first3=Barry A.|last4=Parkes|first4=R. John|last5=Weightman|first5=Andrew J.|last6=Fry|first6=John C.|date=2004|title=Diversity of prokaryotes and methanogenesis in deep subsurface sediments from the Nankai Trough, Ocean Drilling Program Leg 190|journal=Environmental Microbiology|language=en|volume=6|issue=3|pages=274–287|doi=10.1111/j.1462-2920.2004.00568.x|pmid=14871211|bibcode=2004EnvMi...6..274N |s2cid=15644142 |issn=1462-2920|url=http://orca.cf.ac.uk/7547/1/Newberry_et_al_EM_2004.pdf}}</ref>

=== In Marine Environments ===
Approximately one third of [[methanogen]]s which have been described arise from marine origins, a majority being from the clade ''[[Euryarchaeota]]''.<ref name=":0">{{Cite journal |last=Ferry |first=James G. |last2=Lessner |first2=Daniel J. |date=March 2008 |title=Methanogenesis in Marine Sediments |url=https://nyaspubs.onlinelibrary.wiley.com/doi/10.1196/annals.1419.007 |journal=Annals of the New York Academy of Sciences |language=en |volume=1125 |issue=1 |pages=147–157 |doi=10.1196/annals.1419.007 |issn=0077-8923|url-access=subscription }}</ref> In the marine environment, methanogenic microorganisms compete for resources with [[Sulfate-reducing microorganism|sulfate-reducers]].<ref name=":0" /> As a result of this, sulfate-depleted areas of high organic matter loading and sediments are areas of methanogen predominance.<ref name=":0" /> 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 is [[acetate]], which composes two thirds of global methane production.<ref name=":0" /> Another compound which contributes to marine sediment methanogenesis is [[carbon monoxide]], which is oxidized into [[carbon dioxide]], before undergoing a series of reactions to produce energy as [[methane]] is released from the microbe.<ref name=":0" /> This compound is considered non-competitive with sulfate-reducers, allowing for free use by methanogens. In examination of the microorganism ''M. 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.<ref name=":0" /> The estimated annual release of methane from the ocean into the atmosphere is approximately 0.7-14x10<sup>9</sup> kg CH<sub>4</sub> per year.<ref name=":0" /> 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.<ref name=":0" /><ref name=":1">{{Cite journal |last=Repeta |first=Daniel J. |last2=Ferrón |first2=Sara |last3=Sosa |first3=Oscar A. |last4=Johnson |first4=Carl G. |last5=Repeta |first5=Lucas D. |last6=Acker |first6=Marianne |last7=DeLong |first7=Edward F. |last8=Karl |first8=David M. |date=December 2016 |title=Marine methane paradox explained by bacterial degradation of dissolved organic matter |url=https://www.nature.com/articles/ngeo2837 |journal=Nature Geoscience |language=en |volume=9 |issue=12 |pages=884–887 |doi=10.1038/ngeo2837 |issn=1752-0894|url-access=subscription }}</ref><ref name=":2">{{Cite journal |last=Metcalf |first=William W. |last2=Griffin |first2=Benjamin M. |last3=Cicchillo |first3=Robert M. |last4=Gao |first4=Jiangtao |last5=Janga |first5=Sarath Chandra |last6=Cooke |first6=Heather A. |last7=Circello |first7=Benjamin T. |last8=Evans |first8=Bradley S. |last9=Martens-Habbena |first9=Willm |last10=Stahl |first10=David A. |last11=van der Donk |first11=Wilfred A. |date=2012-08-31 |title=Synthesis of Methylphosphonic Acid by Marine Microbes: A Source for Methane in the Aerobic Ocean |url=https://www.science.org/doi/10.1126/science.1219875 |journal=Science |language=en |volume=337 |issue=6098 |pages=1104–1107 |doi=10.1126/science.1219875 |issn=0036-8075 |pmc=3466329 |pmid=22936780}}</ref><ref name=":3">{{Cite journal |last=Carini |first=Paul |last2=White |first2=Angelicque E. |last3=Campbell |first3=Emily O. |last4=Giovannoni |first4=Stephen J. |date=2014-07-07 |title=Methane production by phosphate-starved SAR11 chemoheterotrophic marine bacteria |url=https://www.nature.com/articles/ncomms5346 |journal=Nature Communications |language=en |volume=5 |issue=1 |doi=10.1038/ncomms5346 |issn=2041-1723}}</ref>

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,<ref name=":2" /> where the [[catabolism]] of the compound [[Methylphosphonic acid|methyl-phosphonic acid]] (Mpn) has been found to co-produce methane in oxic ocean waters, providing a potential explanation to the paradox.<ref name=":1" /><ref name=":2" /><ref name=":3" /> ''[[Nitrosopumilus maritimus|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.<ref name=":2" /> 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 as ''[[Candidatus Pelagibacter communis|Pelagibacter]]'',<ref name=":2" /> 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 <ref name=":1" /><ref name=":2" /><sup>[4]</sup>, 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 and [[hydrolysis]] of [[phosphonate]] compounds within bacteria was found to occur in phosphate limitation,<ref name=":2" /> 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.

==Industry==
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.<ref>{{cite web|url=http://articles.economictimes.indiatimes.com/2015-04-07/news/60902488_1_wet-waste-waste-management-household-waste |archive-url=https://web.archive.org/web/20150715015907/http://articles.economictimes.indiatimes.com/2015-04-07/news/60902488_1_wet-waste-waste-management-household-waste |url-status=dead |archive-date=15 July 2015 |title=After Freedom Park, waste to light up Gandhinagar in Bengaluru|last=Nair|first=Athira |work=The Economic Times|date=14 July 2015}}</ref> It is the primary pathway whereby most organic matter disposed of via [[landfill]] is broken down.<ref>[http://users.ox.ac.uk/~ayoung/landfill.html DoE Report CWM039A+B/92] Young, A. (1992)</ref> Some biogas plants use methanogenesis to combine the {{CO2}} with hydrogen to create more methane.<ref>{{cite web |title=Nature Energy and Andel inaugurate power-to-gas facility in Denmark |url=https://www.bioenergy-news.com/news/nature-energy-and-andel-inaugurate-power-to-gas-facility-in-denmark/ |website=Bioenergy Insight Magazine |date=6 November 2023}}</ref>

==Role in global warming==
{{main|Atmospheric methane}}
Methane is an important [[greenhouse gas]] with a [[global warming potential]] 25 times greater than carbon dioxide (averaged over 100 years).<ref>{{cite book |url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/tssts-2-5.html |contribution=Global Warming Potentials |title=Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 |year=2007 |access-date=2012-05-24 |archive-url=https://web.archive.org/web/20130615124842/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/tssts-2-5.html |archive-date=15 June 2013 |url-status=dead }}</ref> Methanogenesis in [[livestock]] and the decay of organic material contributes to global warming.

==Extra-terrestrial life==
{{See also|Methane#Extraterrestrial methane}}
The presence of atmospheric methane has a role in the scientific search for [[extra-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 a [[spectrometer]] for example) this may indicate that life is, or recently was, present.
This was debated<ref>BBC article about methane as sign of life http://news.bbc.co.uk/2/hi/science/nature/4295475.stm</ref> 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)<ref>European Space Agency, Methane in Martian Atmosphere http://www.esa.int/esaMI/Mars_Express/SEMZ0B57ESD_0.html</ref> and in [[Titan (moon)|Titan]]'s atmosphere by the [[Huygens probe]] (2005).<ref>Space.Com article about methane on Huygens http://www.space.com/scienceastronomy/ap_huygens_update_050127.html</ref> This debate was furthered with the discovery of 'transient', 'spikes of methane' on Mars by the [[Curiosity Rover]].<ref>{{Cite news | url=https://www.telegraph.co.uk/news/science/space/11297326/Life-on-Mars-Nasa-finds-first-hint-of-alien-life.html |title = Life on Mars: NASA finds first hint of alien life|newspaper = The Telegraph|date = 15 March 2016|last1 = Knapton|first1 = Sarah}}</ref>

It is 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.<ref>New Scientist article about atmospheric methane https://www.newscientist.com/article.ns?id=dn7059</ref><ref>[https://web.archive.org/web/20041010151606/http://news.nationalgeographic.com/news/2004/10/1007_041007_mars_methane.html National Geographic Article about methane as sign of life]</ref>

On 13 April 2017, NASA confirmed that the dive of the [[Cassini–Huygens|Cassini orbiter]] spacecraft on 28 October 2015 discovered an [[Enceladus]] 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.<ref>{{Cite news|url=https://www.nasa.gov/press-release/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system|title=NASA Missions Provide New Insights into 'Ocean Worlds'|last=Northon|first=Karen|date=2017-04-13|work=NASA|access-date=2017-04-13}}</ref>

==See also==
*[[Aerobic methane production]]
*[[Anaerobic digestion]]
*[[Anaerobic oxidation of methane]]
*[[Electromethanogenesis]]
*[[Hydrogen cycle]]
*[[Methanotroph]]
*[[Mootral]]

== References ==
{{Reflist|30em}}

[[Category:Methanogenesis| ]]