View source for Anaerobic organism

{{Short description|Organism not requiring oxygen for its growth}}
{{Lead rewrite|date=October 2025|reason=This introductory summary does not sufficiently rely on the key definition of anaerobic metabolism while also providing non-essential information.}}
[[File:Spinoloricus.png|thumb|upright|100px|''[[Spinoloricus cinziae]]'', a metazoan that metabolises with hydrogen, lacking [[mitochondria]] and instead using [[hydrogenosomes]]]]

An '''anaerobic organism''' or '''anaerobe''' is any [[organism]] that does not require [[oxygen|molecular oxygen]] for its growth. It may react negatively or even die in the presence of free oxygen. Anaerobic organisms do not use oxygen as a terminal [[electron acceptor]] in their respiration process to produce energy, but a less powerful [[oxidizing agent]], such as nitrate, ferric ion, Mn(IV), sulfate or bicarbonate anions. In contrast, an [[aerobic organism]] (aerobe) is an organism that requires a sufficiently oxygenated environment to respire, produce its energy, and thrive. Because the anaerobic energy production was the first mechanism to be used by living microorganisms in their [[evolution]] and is much less efficient than the aerobic pathway, anaerobes are practically, ''de facto'', always [[unicellular organism]]s (e.g. [[bacteria]] and [[archaea]] ([[prokaryote]]s),<ref name="Levinson-2010">{{cite book |author=Levinson, W. |title=Review of Medical Microbiology and Immunology| publisher = McGraw-Hill |year=2010 |edition=11th |pages=91–93| isbn = 978-0-07-174268-9}}</ref> or [[protozoa]]ns ([[eukaryote]]s).<ref name="Upcroft-2001">{{cite journal|vauthors=Upcroft P, Upcroft JA | title=Drug Targets and Mechanisms of Resistance in | pages= 150–164 | pmc=88967 | doi=10.1128/CMR.14.1.150-164.2001 | pmid=11148007 | volume=14 | issue=1 | date=January 2001 | journal=Clin. Microbiol. Rev.}}</ref> However, a minuscule [[multicellular organism]], with an exceptionally rare metabolism and surviving in a [[Hypersaline lake|hypersaline brine pool]] in the darkness of the bottom of the [[Mediterranean Sea]], has been recently discovered. Meanwhile, it remains a scientific curiosity, as the much higher energy requirements of most multicellular organisms cannot be met by anaerobic respiration.<ref name="Kristensen-2010">{{cite journal |display-authors=4 |author=Danovaro R |author2=Dell'anno A |author3=Pusceddu A|author4=Gambi C |author5=Heiner I|author6=Kristensen RM |title=The first metazoa living in permanently anoxic conditions |journal=BMC Biology |volume=8 |issue=1 |page=30 |date=April 2010 |pmid=20370908 |pmc=2907586 |doi=10.1186/1741-7007-8-30 |doi-access=free }}</ref> Most [[Fungus|fungi]] (eukaryotes) are obligate [[Aerobic organism|aerobes]], requiring oxygen to survive and grow; however, some species, such as the [[Chytridiomycota]] that reside in the [[rumen]] of cattle, are obligate anaerobes; for these species, anaerobic respiration is used because oxygen would disrupt their metabolism or kill them.{{Citation needed|date=October 2025}} The deep [[seafloor]] and its underlying [[Compaction (geology)|unconsolidated]] [[sediment]]s ranks among the largest potential habitats for anaerobic microorganisms on [[Earth]]. Moreover, [[chemotroph#Chemoautotroph|chemoautotroph]] microbes also thrive around [[hydrothermal vent]]s, discharging [[Hot spring|hot water]] on the [[ocean]] [[seabed]] near [[mid-ocean ridge]]s, where anaerobic conditions prevail. These microbes produce energy in the absence of [[sunlight]] or oxygen through a process called [[anaerobic respiration]], whereby inorganic compounds and ions such as protons ({{Chem2|H+|auto=yes}}),<ref>{{Cite journal |last1=Sapra |first1=Rajat |last2=Bagramyan |first2=Karine |last3=Adams |first3=Michael W. W. |date=2003-06-24 |title=A simple energy-conserving system: proton reduction coupled to proton translocation |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=100 |issue=13 |pages=7545–7550 |doi=10.1073/pnas.1331436100 |doi-access=free |issn=0027-8424 |pmc=164623 |pmid=12792025 |bibcode=2003PNAS..100.7545S }}</ref> elemental [[sulfur]] and its derivatives ({{Chem2|[[sulfate|SO4(2-)]], [[thiosulfate|S2O3(2-)]]}}), or [[ferric]] ions, are reduced to drive [[oxidative phosphorylation]].

== First recorded observation ==
In his 14 June 1680 letter to the [[Royal Society]], [[Antonie van Leeuwenhoek]] described an experiment he carried out by filling two identical glass tubes about halfway with crushed pepper powder, to which some clean rain water was added. Van Leeuwenhoek sealed one of the glass tubes with a flame and left the other open. Several days later, he discovered in the open glass tube 'a great many very little animalcules, of divers sort having its own particular motion.' Not expecting to see any life in the sealed glass tube, Van Leeuwenhoek saw to his surprise 'a kind of living animalcules that were round and bigger than the biggest sort that I have said were in the other water.' The conditions in the sealed tube had become quite anaerobic due to the consumption of oxygen by aerobic microorganisms.<ref name="Gest-2004">Gest, Howard. (2004) ''The discovery of microorganisms by Robert Hooke and Antoni van Leeuwenhoek, Fellows of the Royal Society'', in: 'The Royal Society May 2004 Volume: 58 Issue: 2: pp. 12.</ref>

In 1913, [[Martinus Beijerinck]] repeated Van Leeuwenhoek's experiment and identified ''[[Clostridium butyricum]]'' as a prominent anaerobic bacterium in the sealed pepper infusion tube liquid. Beijerinck commented:

{{blockquote|We thus come to the remarkable conclusion that, beyond doubt, Van Leeuwenhoek in his experiment with the fully closed tube had cultivated and seen genuine anaerobic bacteria, which would happen again only after 200 years, namely about 1862 by Pasteur. That Leeuwenhoek, one hundred years before the discovery of oxygen and the composition of air, was not aware of the meaning of his observations is understandable. But the fact that in the closed tube he observed an increased gas pressure caused by fermentative bacteria and, in addition, saw the bacteria, prove, in any case, that he not only was a good observer but also was able to design an experiment from which a conclusion could be drawn.<ref name="Gest-2004"/>}}

== Classifications ==
[[Image:Ana.png|300px|thumb|right|Aerobic and anaerobic [[bacteria]] can be differentiated by culturing them in test tubes of [[thioglycolate broth]]: {{ordered list| [[Obligate aerobe]]s need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen concentration is highest. | [[Obligate anaerobe]]s are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest. | [[Facultative anaerobes]] can grow with or without oxygen because they can metabolize energy aerobically or anaerobically. They gather mostly at the top because aerobic respiration generates more [[adenosine triphosphate]] (ATP) than either fermentation or anaerobic respiration. | [[Microaerophile]]s need oxygen because they cannot ferment or respire anaerobically. However, they are poisoned by high concentrations of oxygen. They gather in the upper part of the test tube but not the very top. | Aerotolerant organisms do not require oxygen as they metabolize energy anaerobically. Unlike obligate anaerobes, however, they are not poisoned by [[oxygen]]. They are evenly distributed throughout the test tube.}} ]]

For practical purposes, there are three categories of anaerobe:
* '''[[Obligate anaerobe]]s''', which are harmed by the presence of oxygen.<ref name="Prescott-1996">{{cite book |vauthors=Prescott LM, Harley JP, Klein DA |title=Microbiology |publisher = Wm. C. Brown Publishers |year=1996 |edition=3rd |pages=130–131| isbn = 978-0-697-29390-9}}</ref><ref name="Brooks-2007">{{cite book |vauthors=Brooks GF, Carroll KC, Butel JS, Morse SA |title=Jawetz, Melnick & Adelberg's Medical Microbiology |publisher = McGraw Hill |year=2007 |edition=24th |pages=307–312| isbn = 978-0-07-128735-7 }}</ref> Two examples of obligate anaerobes are ''Clostridium botulinum'' and the bacteria which live near hydrothermal vents on the deep-sea ocean floor.
* '''[[Aerotolerant|Aerotolerant organism]]s''', which cannot use oxygen for growth, but tolerate its presence.<ref name="Hogg-2005">{{cite book |author=Hogg, S. |title=Essential Microbiology| publisher = Wiley |year=2005 |edition=1st |pages=99–100| isbn = 978-0-471-49754-7}}</ref>
* '''[[Facultative anaerobic organism|Facultative anaerobe]]s''', which can grow without oxygen but use oxygen if it is present.<ref name="Hogg-2005"/>

However, this classification has been questioned after recent research showed that human "obligate anaerobes" (such as ''Finegoldia magna'' or the methanogenic archaea ''[[Methanobrevibacter smithii]]'') can be grown in aerobic atmosphere if the culture medium is supplemented with antioxidants such as [[ascorbic acid]], [[glutathione]] and [[uric acid]].<ref>{{Cite journal|last1=La Scola|first1=B.|last2=Khelaifia|first2=S.|last3=Lagier|first3=J.-C.|last4=Raoult|first4=D.|date=2014|title=Aerobic culture of anaerobic bacteria using antioxidants: a preliminary report|journal=European Journal of Clinical Microbiology & Infectious Diseases|volume=33|issue=10|pages=1781–1783|doi=10.1007/s10096-014-2137-4|pmid=24820294|s2cid=16682688|issn=0934-9723}}</ref><ref>{{Cite journal|last1=Dione|first1=N.|last2=Khelaifia|first2=S.|last3=La Scola|first3=B.|last4=Lagier|first4=J.C.|last5=Raoult|first5=D.|date=2016|title=A quasi-universal medium to break the aerobic/anaerobic bacterial culture dichotomy in clinical microbiology|journal=Clinical Microbiology and Infection|volume=22|issue=1|pages=53–58|doi=10.1016/j.cmi.2015.10.032|pmid=26577141|doi-access=free}}</ref><ref>{{Cite journal|last1=Khelaifia|first1=S.|last2=Lagier|first2=J.-C.|last3=Nkamga|first3=V. D.|last4=Guilhot|first4=E.|last5=Drancourt|first5=M.|last6=Raoult|first6=D.|date=2016|title=Aerobic culture of methanogenic archaea without an external source of hydrogen|journal=European Journal of Clinical Microbiology & Infectious Diseases|volume=35|issue=6|pages=985–991|doi=10.1007/s10096-016-2627-7|pmid=27010812|s2cid=17258102|issn=0934-9723}}</ref><ref>{{Cite journal|last1=Traore|first1=S.I.|last2=Khelaifia|first2=S.|last3=Armstrong|first3=N.|last4=Lagier|first4=J.C.|last5=Raoult|first5=D.|date=2019|title=Isolation and culture of Methanobrevibacter smithii by co-culture with hydrogen-producing bacteria on agar plates|journal=Clinical Microbiology and Infection|volume=25|issue=12|pages=1561.e1–1561.e5|doi=10.1016/j.cmi.2019.04.008|pmid=30986553|doi-access=free}}</ref>

== Energy metabolism ==
Some obligate anaerobes use [[fermentation (biochemistry)|fermentation]], while others use [[anaerobic respiration]].<ref>{{cite book |last=Pommerville |first=Jeffrey |date=2010 |title=Alcamo's Fundamentals of Microbiology |publisher=Jones and Bartlett Publishers |page=177 |isbn=978-1-4496-5582-2}}</ref> Aerotolerant organisms are strictly fermentative.<ref>{{cite book |last1=Slonim |first1=Anthony |last2=Pollack |first2=Murray |date=2006 |title=Pediatric Critical Care Medicine |publisher=Lippincott Williams & Wilkins |page=130 |isbn=978-0-7817-9469-5}}</ref> In the presence of oxygen, facultative anaerobes use [[aerobic respiration]].<ref name="Hogg-2005" /> In the absence of [[oxygen]], some facultative anaerobes use [[fermentation]], while others may use anaerobic respiration.<ref name="Hogg-2005" />

=== Fermentation ===
There are many anaerobic fermentative reactions.

Fermentative anaerobic organisms typically use the lactic acid fermentation pathway:

{{block indent|[[glucose|C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>]] + 2 [[Adenosine diphosphate|ADP]] + 2 phosphate → 2 [[lactic acid]] + 2 [[Adenosine triphosphate|ATP]] + 2 H<sub>2</sub>O}}

The energy released in this reaction (without ADP and phosphate) is approximately 150 [[Kilojoule per mole|kJ per mol]], which is conserved in generating two ATP from ADP per [[glucose]]. This is only 5% of the energy per sugar molecule that the typical aerobic reaction generates.

Plants and fungi (e.g., yeasts) in general use alcohol (ethanol) fermentation when oxygen becomes limiting:

{{block indent|C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> ([[glucose]]) + 2 ADP + 2 phosphate → 2 C<sub>2</sub>H<sub>5</sub>OH + 2 CO<sub>2</sub>↑ + 2 ATP + 2 H<sub>2</sub>O}}

The energy released is about 180 kJ/mol, which is conserved when 2 ATP are generated from 2 ADP per glucose.

Anaerobic bacteria and [[archaea]] use these and many other fermentative pathways, e.g., [[propionic acid]] fermentation,<ref>{{Cite journal |last1=Piwowarek |first1=Kamil |last2=Lipińska |first2=Edyta |last3=Hać-Szymańczuk |first3=Elżbieta |last4=Kieliszek |first4=Marek |last5=Ścibisz |first5=Iwona |date=January 2018 |title=Propionibacterium spp.—source of propionic acid, vitamin B12, and other metabolites important for the industry |journal=Applied Microbiology and Biotechnology |language=en |volume=102 |issue=2 |pages=515–538 |doi=10.1007/s00253-017-8616-7 |issn=0175-7598 |pmc=5756557 |pmid=29167919}}</ref> [[butyric acid]] fermentation,<ref>{{Cite journal |last1=Seedorf |first1=Henning |last2=Fricke |first2=W. Florian |last3=Veith |first3=Birgit |last4=Brüggemann |first4=Holger |last5=Liesegang |first5=Heiko |last6=Strittmatter |first6=Axel |last7=Miethke |first7=Marcus |last8=Buckel |first8=Wolfgang |last9=Hinderberger |first9=Julia |last10=Li |first10=Fuli |last11=Hagemeier |first11=Christoph |last12=Thauer |first12=Rudolf K. |last13=Gottschalk |first13=Gerhard |date=2008-02-12 |title=The genome of Clostridium kluyveri, a strict anaerobe with unique metabolic features |journal=Proceedings of the National Academy of Sciences |language=en |volume=105 |issue=6 |pages=2128–2133 |doi=10.1073/pnas.0711093105 |issn=0027-8424 |pmc=2542871 |pmid=18218779 |bibcode=2008PNAS..105.2128S |doi-access=free }}</ref> solvent fermentation, [[mixed acid fermentation]], [[butanediol fermentation]], [[Stickland fermentation]], [[acetogenesis]], or [[methanogenesis]].{{citation needed|date=February 2022}}

=== CrP hydrolysis ===
[[Creatine]], an organic compound found in animals, provides a way for ATP to be utilized in the muscle. The phosphorylation of creatine allows for the storage of readily available phosphate that can be supplied to the muscles.<ref>{{Cite journal |last1=Sahlin |first1=Kent |last2=Harris |first2=Roger C. |date=2011-05-01 |title=The creatine kinase reaction: a simple reaction with functional complexity |journal=Amino Acids |language=en |volume=40 |issue=5 |pages=1363–1367 |doi=10.1007/s00726-011-0856-8 |pmid=21394603 |s2cid=12877062 |issn=1438-2199}}</ref>

{{block indent|creatine + ATP ⇌ phosphocreatine + ADP + H<sup>+</sup>}}

The reaction is reversible as well, allowing cellular ATP levels to be maintained during anoxic conditions.<ref>{{Citation |last1=Wang |first1=Y. |title=Hypoxia {{!}} Anaerobic Metabolism in Fish |date=2011-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780123745538001544 |encyclopedia=Encyclopedia of Fish Physiology |pages=1757–1763 |editor-last=Farrell |editor-first=Anthony P. |access-date=2023-04-18 |place=San Diego |publisher=Academic Press |language=en |doi=10.1016/b978-0-12-374553-8.00154-4 |isbn=978-0-08-092323-9 |last2=Richards |first2=J. G.|url-access=subscription }}</ref> This process in animals is seen to be coupled with metabolic suppression to allow certain fish, such as [[goldfish]], to survive environmental anoxic conditions for a short period.<ref>{{Cite journal |last1=van den Thillart |first1=G. |last2=van Waarde |first2=A. |last3=Muller |first3=H. J. |last4=Erkelens |first4=C. |last5=Addink |first5=A. |last6=Lugtenburg |first6=J. |date=1989-04-01 |title=Fish muscle energy metabolism measured by in vivo 31P-NMR during anoxia and recovery |url=https://www.physiology.org/doi/10.1152/ajpregu.1989.256.4.R922 |journal=American Journal of Physiology. Regulatory, Integrative and Comparative Physiology |language=en |volume=256 |issue=4 |pages=R922–R929 |doi=10.1152/ajpregu.1989.256.4.R922 |pmid=2705580 |issn=0363-6119|url-access=subscription }}</ref>

== Culturing anaerobes ==
[[File:Diagnostic algorithm of possible bacterial infection.png|thumb|Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria, etc.), with most common situations and agents seen in a New England community hospital setting. Multiple anaerobic growth media are mentioned among agar plate cultures. Anaerobes may also be identified by [[MALDI-TOF]] as shown at bottom right.]]
Since normal microbial culturing is done in atmospheric air, which contains molecular oxygen, culturing anaerobes requires special techniques. Several techniques are employed by microbiologists when culturing anaerobic organisms, for example, handling the bacteria in a [[glovebox]] filled with nitrogen or the use of other specially sealed containers, or techniques such as injection of the bacteria into a [[Dicotyledon|dicot]] plant, which is an environment with limited oxygen. The [[Gas-pak]] System is an isolated container that achieves an anaerobic environment by the reaction of water with [[sodium borohydride]] and [[sodium bicarbonate]] tablets, which produce hydrogen gas and carbon dioxide. [[Hydrogen]] then reacts with oxygen gas on a palladium catalyst to produce more water, thereby removing oxygen gas. The issue with the Gas-Pak method is that an adverse reaction can occur, leading to bacterial death; therefore, a [[thioglycolate broth|thioglycolate medium]] should be used. The thioglycolate supplies a medium mimicking that of a [[Dicotyledon|dicot]] plant, thus providing not only an anaerobic environment but all the nutrients needed for the bacteria to multiply.<ref>[http://ftp.ccccd.edu/dcain/CCCCD%20Micro/gaspakjar.htm "GasPak System"] {{webarchive|url=https://web.archive.org/web/20090928043031/http://ftp.ccccd.edu/dcain/CCCCD%20Micro/gaspakjar.htm |date=2009-09-28 }}. Accessed May 3, 2008.</ref>

On May the 6 2018, a French team evidenced a link between [[redox]] and [[Gastrointestinal tract|gut]] anaerobes<ref>{{Cite journal|last1=Million|first1=Matthieu|last2=Raoult|first2=Didier|date=December 2018|title=Linking gut redox to human microbiome|journal=Human Microbiome Journal|volume=10|pages=27–32|doi=10.1016/j.humic.2018.07.002|doi-access=free}}</ref> based on clinical studies of severe acute malnutrition.<ref>{{Cite journal|last1=Million|first1=Matthieu|last2=Tidjani Alou|first2=Maryam|last3=Khelaifia|first3=Saber|last4=Bachar|first4=Dipankar|last5=Lagier|first5=Jean-Christophe|last6=Dione|first6=Niokhor|last7=Brah|first7=Souleymane|last8=Hugon|first8=Perrine|last9=Lombard|first9=Vincent|last10=Armougom|first10=Fabrice|last11=Fromonot|first11=Julien|date=May 2016|title=Increased Gut Redox and Depletion of Anaerobic and Methanogenic Prokaryotes in Severe Acute Malnutrition|journal=Scientific Reports|volume=6|issue=1|article-number=26051|doi=10.1038/srep26051|issn=2045-2322|pmc=4869025|pmid=27183876|bibcode=2016NatSR...626051M}}{{Retracted|doi=10.1038/s41598-023-44597-3|pmid=37903811|https://retractionwatch.com/?s=Didier+Raoult ''Retraction Watch''|intentional=yes}}</ref><ref group=note>This study was later retracted over ethical oversight concerns.</ref> These findings led to the development of an aerobic culture of "anaerobes" by the addition of [[antioxidant]]s in the culture medium.<ref>{{Cite journal|last1=Guilhot|first1=Elodie|last2=Khelaifia|first2=Saber|last3=La Scola|first3=Bernard|last4=Raoult|first4=Didier|last5=Dubourg|first5=Grégory|date=March 2018|title=Methods for culturing anaerobes from human specimen|journal=Future Microbiology|volume=13|issue=3|pages=369–381|doi=10.2217/fmb-2017-0170|pmid=29446650|issn=1746-0913}}</ref>

== Multicellularity ==
Very few multicellular life forms are anaerobic, since only aerobic respiration can provide enough energy for a complex metabolism. Exceptions include three species of ''[[Loricifera]]'' (< 1&nbsp;mm in size) and the 10-cell ''[[Henneguya zschokkei]]''.<ref>[https://edition.cnn.com/2020/02/26/world/first-animal-doesnt-breathe-oxygen-scn-trnd/index.html Scientists discovered the first animal that doesn't need oxygen to live ]</ref>

In 2010, three species of anaerobic ''Loricifera'' were discovered in the [[hypersaline]] [[Anoxic waters|anoxic]] [[L'Atalante basin]] at the bottom of the [[Mediterranean Sea]]. They lack [[mitochondria]], which contain the [[oxidative phosphorylation]] pathway, which in all other animals combines [[oxygen]] with [[glucose]] to produce metabolic energy; thus, they consume no oxygen. Instead, these ''loricifera'' derive their energy from [[hydrogen]], using [[hydrogenosomes]].<ref>[https://web.archive.org/web/20100418230614/http://news.nationalgeographic.com/news/2010/04/100416-oxygen-free-complex-animals-mediterranean/ Oxygen-Free Animals Discovered-A First, National Geographic news]</ref><ref name="Kristensen-2010"/>

''Henneguya zschokkei'' also lack mitochondria, [[mitochondrial DNA]], and oxidative pathways. The microscopic, parasitic [[cnidarian]] is observed to contain mitochondria-related [[organelle]]s. These organelles harbour genes encoding metabolic functions, such as those involved in the [[amino acid]] metabolism. However, these specialized organelles lack the key features of typical mitochondria found in the closely related aerobic ''[[Myxobolus squamalus]]''. Due to the difficulty of culturing ''H. zschokkei'', there is little understanding of its anaerobic pathway.<ref>{{Cite journal |last1=Yahalomi |first1=Dayana |last2=Atkinson |first2=Stephen D. |last3=Neuhof |first3=Moran |last4=Chang |first4=E. Sally |last5=Philippe |first5=Hervé |last6=Cartwright |first6=Paulyn |last7=Bartholomew |first7=Jerri L. |last8=Huchon |first8=Dorothée |date=2020-03-10 |title=A cnidarian parasite of salmon (Myxozoa: Henneguya ) lacks a mitochondrial genome |journal=Proceedings of the National Academy of Sciences |language=en |volume=117 |issue=10 |pages=5358–5363 |doi=10.1073/pnas.1909907117 |pmid=32094163 |pmc=7071853 |bibcode=2020PNAS..117.5358Y |issn=0027-8424 |doi-access=free }}</ref>

== Symbiosis ==
Anaerobic respiration and its end products can facilitate [[symbiosis]] between anaerobes and aerobes. This occurs across [[Taxon|taxa]], often in compensation for nutritional needs.<ref name="Moran-2006">{{Cite journal |last=Moran |first=Nancy A. |date=2006-10-24 |title=Symbiosis |journal=Current Biology |language=English |volume=16 |issue=20 |pages=R866–R871 |doi=10.1016/j.cub.2006.09.019 |issn=0960-9822 |pmid=17055966|s2cid=235311996 |doi-access=free |bibcode=2006CBio...16.R866M }}</ref>

Anaerobiosis and symbiosis are found in interactions between [[ciliate]]s and [[prokaryote]]s. Anaerobic ciliates interact with prokaryotes in an [[Endosymbiont|endosymbiotic]] relationship. These relationships are mediated in which the ciliate leaves end products that its prokaryotic symbiont utilizes. The ciliate achieves this through fermentative metabolism. The [[rumen]] of various animals houses this ciliate alongside many other anaerobic bacteria, protozoans, and fungi.<ref>{{Cite journal |last=Flint |first=Harry J. |date=September 1994 |title=Molecular genetics of obligate anaerobes from the rumen |journal=FEMS Microbiology Letters |volume=121 |issue=3 |pages=259–267 |doi=10.1111/j.1574-6968.1994.tb07110.x |pmid=7926679 |s2cid=24273083 |issn=0378-1097|doi-access=free }}</ref> In specific, methanogenic [[archaea]] found in the rumen acts as a symbiont to anaerobic ciliates.<ref>{{Cite journal |last1=Rotterová |first1=Johana |last2=Edgcomb |first2=Virginia P. |last3=Čepička |first3=Ivan |last4=Beinart |first4=Roxanne |date=September 2022 |title=Anaerobic ciliates as a model group for studying symbioses in oxygen-depleted environments |url=https://onlinelibrary.wiley.com/doi/10.1111/jeu.12912 |journal=Journal of Eukaryotic Microbiology |language=en |volume=69 |issue=5 |article-number=e12912 |doi=10.1111/jeu.12912 |pmid=35325496 |s2cid=247677842 |issn=1066-5234|url-access=subscription }}</ref> These anaerobes are useful to those with a rumen due to their ability to break down cellulose, making it bioavailable when otherwise indigestible by animals.<ref name="Moran-2006" />

[[Termite]]s utilize anaerobic bacteria to fix and recapture nitrogen. Specifically, the termite's hindgut is full of nitrogen-fixing bacteria, whose functions depend on the nitrogen concentration of the diet. [[Acetylene]] reduction in termites was observed to upregulate in termites with nitrogen-poor diets, meaning that nitrogenase activity rose as the nitrogen content of the termite was reduced.<ref name="Breznak-1973">{{Cite journal |last1=Breznak |first1=John A. |last2=Brill |first2=Winston J. |last3=Mertins |first3=James W. |last4=Coppel |first4=Harry C. |date=August 1973 |title=Nitrogen Fixation in Termites |url=https://www.nature.com/articles/244577a0 |journal=Nature |language=en |volume=244 |issue=5418 |pages=577–580 |doi=10.1038/244577a0 |pmid=4582514 |bibcode=1973Natur.244..577B |s2cid=4223979 |issn=1476-4687|url-access=subscription }}</ref> One of the functions of termite microbiota is to recapture nitrogen from the termite's uric acid. This allows nitrogen conservation from a diet otherwise low in nitrogen.<ref name="Breznak-1973" /><ref name="Thong-On-2012">{{Cite journal |last1=Thong-On |first1=Arunee |last2=Suzuki |first2=Katsuyuki |last3=Noda |first3=Satoko |last4=Inoue |first4=Jun-ichi |last5=Kajiwara |first5=Susumu |last6=Ohkuma |first6=Moriya |date=2012 |title=Isolation and Characterization of Anaerobic Bacteria for Symbiotic Recycling of Uric Acid Nitrogen in the Gut of Various Termites |url=https://www.jstage.jst.go.jp/article/jsme2/27/2/27_ME11325/_article |journal=Microbes and Environments |volume=27 |issue=2 |pages=186–192 |doi=10.1264/jsme2.ME11325 |pmc=4036019 |pmid=22791052}}</ref> The hindgut microbiome of different termites has been analyzed, showing 16 different anaerobic species of bacteria, including [[Clostridia]], [[Enterobacteriaceae]], and [[Coccus|Gram-positive cocci]].<ref name="Thong-On-2012" />

== See also ==
* [[Aerobic organism]]
* [[Redox gradient]]

== Notes ==
{{Reflist|group=note}}

== References ==
{{Reflist}}

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[[Category:Microbiology]]
[[Category:Anaerobes| ]]