Editing Methanogen (section)

==Physical description==

Methanogens are usually cocci (spherical) or rods (cylindrical) in shape, but long filaments (''[[Methanobrevibacter filiformis]]'', ''Methanospirillum hungatei'') and curved forms (''[[Methanobrevibacter curvatus]]'', ''[[Methanobrevibacter cuticularis]]'') also occur. There are over 150 described species of methanogens,<ref>{{Cite web |title=Leibniz Institute DSMZ: Welcome to the Leibniz Institute DSMZ |url=https://www.dsmz.de/ |access-date=2024-07-17 |website=www.dsmz.de |language=en-US}}</ref> which do not form a [[monophyletic]] group in the phylum [[Euryarchaeota]] (see Taxonomy). They are exclusively [[anaerobic organism]]s that cannot function under aerobic conditions due to the extreme oxygen sensitivity of methanogenesis enzymes and FeS clusters involved in ATP production. However, the degree of oxygen sensitivity varies, as methanogenesis has often been detected in temporarily oxygenated environments such as rice paddy soil,<ref>{{Cite journal |last1=Angel |first1=Roey |last2=Matthies |first2=Diethart |last3=Conrad |first3=Ralf |date=2011-05-31 |title=Activation of Methanogenesis in Arid Biological Soil Crusts Despite the Presence of Oxygen |journal=PLOS ONE |volume=6 |issue=5 |pages=e20453 |doi=10.1371/journal.pone.0020453 |doi-access=free |pmid=21655270 |pmc=3105065 |bibcode=2011PLoSO...620453A |issn=1932-6203}}</ref><ref>{{Cite journal |last1=Angel |first1=Roey |last2=Claus |first2=Peter |last3=Conrad |first3=Ralf |date=April 2012 |title=Methanogenic archaea are globally ubiquitous in aerated soils and become active under wet anoxic conditions |journal=The ISME Journal |language=en |volume=6 |issue=4 |pages=847–862 |doi=10.1038/ismej.2011.141 |issn=1751-7362 |pmc=3309352 |pmid=22071343|bibcode=2012ISMEJ...6..847A }}</ref><ref>{{Cite journal |last=Conrad |first=Ralf |date=2020-06-11 |title=Methane Production in Soil Environments—Anaerobic Biogeochemistry and Microbial Life between Flooding and Desiccation |journal=Microorganisms |volume=8 |issue=6 |pages=881 |doi=10.3390/microorganisms8060881 |doi-access=free |pmid=32545191 |pmc=7357154 |issn=2076-2607}}</ref> and various molecular mechanisms potentially involved in oxygen and [[reactive oxygen species]] (ROS) detoxification have been proposed.<ref>{{Cite journal |last1=Lyu |first1=Zhe |last2=Lu |first2=Yahai |date=2017-11-14 |title=Metabolic shift at the class level sheds light on adaptation of methanogens to oxidative environments |url=http://dx.doi.org/10.1038/ismej.2017.173 |journal=The ISME Journal |volume=12 |issue=2 |pages=411–423 |doi=10.1038/ismej.2017.173 |pmid=29135970 |pmc=5776455 |bibcode=2018ISMEJ..12..411L |issn=1751-7362}}</ref> For instance, a recently identified species ''Candidatus'' Methanothrix paradoxum common in wetlands and soil can function in anoxic microsites within aerobic environments<ref>{{Cite journal |last1=Angle |first1=Jordan C. |last2=Morin |first2=Timothy H. |last3=Solden |first3=Lindsey M. |last4=Narrowe |first4=Adrienne B. |last5=Smith |first5=Garrett J. |last6=Borton |first6=Mikayla A. |last7=Rey-Sanchez |first7=Camilo |last8=Daly |first8=Rebecca A. |last9=Mirfenderesgi |first9=Golnazalsdat |last10=Hoyt |first10=David W. |last11=Riley |first11=William J. |last12=Miller |first12=Christopher S. |last13=Bohrer |first13=Gil |last14=Wrighton |first14=Kelly C. |date=2017-11-16 |title=Methanogenesis in oxygenated soils is a substantial fraction of wetland methane emissions |url=http://dx.doi.org/10.1038/s41467-017-01753-4 |journal=Nature Communications |volume=8 |issue=1 |page=1567 |doi=10.1038/s41467-017-01753-4 |pmid=29146959 |pmc=5691036 |bibcode=2017NatCo...8.1567A |issn=2041-1723}}</ref> but it is sensitive to the presence of [[oxygen]] even at trace level and cannot usually sustain oxygen stress for a prolonged time. However, ''[[Methanosarcina barkeri]]'' from a sister family Methanosarcinaceae is exceptional in possessing a [[superoxide dismutase]] (SOD) [[enzyme]], and may survive longer than the others in the presence of O<sub>2</sub>.<ref name=":0">{{cite journal |last1=Peters |first1=V. |last2=Conrad |first2=R. |date=1995 |title=Methanogenic and other strictly anaerobic bacteria in desert soil and other oxic sois |journal=Applied and Environmental Microbiology |volume=61 |issue=4 |pages=1673–1676|doi=10.1128/AEM.61.4.1673-1676.1995 |pmid=16535011 |pmc=1388429 |bibcode=1995ApEnM..61.1673P }}</ref>

As is the case for other archaea, methanogens lack [[peptidoglycan]], a polymer that is found in the [[cell wall]]s of [[bacteria]].<ref>{{Cite journal |last1=van Wolferen |first1=Marleen |last2=Pulschen |first2=Andre Arashiro |last3=Baum |first3=Buzz |last4=Gribaldo |first4=Simonetta |last5=Albers |first5=Sonja-Verena |date=2022-10-17 |title=The cell biology of archaea |journal=Nature Microbiology |language=en |volume=7 |issue=11 |pages=1744–1755 |doi=10.1038/s41564-022-01215-8 |issn=2058-5276 |pmc=7613921 |pmid=36253512}}</ref> Instead, some methanogens have a cell wall formed by [[pseudopeptidoglycan]] (also known as [[pseudomurein]]). Other methanogens have a [[paracrystalline]] protein array (S-layer) that fits together like a [[jigsaw puzzle]].<ref name=":2">{{Cite book|title=Bergey's Manual of Systematics of Archaea and Bacteria|pages = 1–8|last=Boone|first=David R.|date=2015|publisher=John Wiley & Sons, Ltd|isbn=9781118960608|language=en|doi=10.1002/9781118960608.gbm00495|chapter = Methanobacterium}}</ref> In some lineages there are less common types of cell envelope such as the proteinaceous sheath of ''Methanospirillum'' or the methanochondroitin of ''Methanosarcina'' aggregated cells.<ref>{{Cite journal |last1=Albers |first1=Sonja-Verena |last2=Meyer |first2=Benjamin H. |date=June 2011 |title=The archaeal cell envelope |url=https://www.nature.com/articles/nrmicro2576 |journal=Nature Reviews Microbiology |language=en |volume=9 |issue=6 |pages=414–426 |doi=10.1038/nrmicro2576 |pmid=21572458 |issn=1740-1526|url-access=subscription }}</ref>