View source for Anaerobic respiration

{{Short description|Respiration using electron acceptors other than oxygen}}
{{Distinguish|Fermentation}}

'''Anaerobic respiration''' is [[Cellular respiration|respiration]] using [[Oxidizing agent|electron acceptors]] other than [[Allotropes of oxygen|molecular oxygen]] (O<sub>2</sub>) in its [[electron transport chain]].<ref>{{cite book|last1=Slonczewski|first1=Joan L.|last2=Foster|first2=John W.|title=Microbiology: An Evolving Science|date=2011|publisher=W.W. Norton|location=New York|isbn=9780393934472|page=166|edition=2nd}}</ref>

In [[aerobic organisms]], electrons are shuttled to an electron transport chain, and the final electron acceptor is [[oxygen]]. Molecular oxygen is an excellent electron acceptor. [[Anaerobes]] instead use less-oxidizing substances such as [[nitrate]] ({{chem|NO|3|-}}), [[fumarate]] ({{chem|C|4|H|2|O|4|2-}}), [[sulfate]] ({{chem|SO|4|2-}}), or elemental [[sulfur]] (S). These terminal electron acceptors have smaller [[reduction potential]]s than O<sub>2</sub>. Less energy per oxidized molecule is released. Therefore, anaerobic respiration is less efficient than aerobic.{{Citation needed|date=July 2025}}

== As compared with fermentation ==
{{Further|Fermentation}}
Anaerobic cellular respiration and [[fermentation]] generate [[adenosine triphosphate|ATP]] in very different ways, and the terms should not be treated as synonyms. Cellular respiration (both [[aerobic respiration|aerobic]] and anaerobic) uses highly reduced chemical compounds such as [[NADH]] and [[FADH2|FADH<sub>2</sub>]] (for example produced during [[glycolysis]] and the [[citric acid cycle]]) to establish an [[electrochemical gradient]] (often a proton gradient) across a membrane. This results in an [[electrical potential]] or ion [[concentration]] difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratory [[integral membrane proteins]] with sequentially increasing [[reduction potential|reduction potentials]], with the final electron acceptor being oxygen (in [[aerobic respiration]]) or another chemical substance (in anaerobic respiration). A [[proton motive force]] drives [[protons]] down the gradient (across the membrane) through the proton channel of [[ATP synthase]]. The resulting current drives ATP synthesis from [[Adenosine diphosphate|ADP]] and inorganic phosphate.{{Citation needed|date=July 2025}} 

[[Fermentation (biochemistry)|Fermentation]], in contrast, does not use an electrochemical gradient but instead uses only [[substrate-level phosphorylation]] to produce ATP. The electron acceptor [[NAD+|NAD<sup>+</sup>]] is regenerated from [[NADH]] formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of [[glyceraldehyde-3-phosphate]] is oxidized back to NAD<sup>+</sup> by the reduction of [[pyruvate]] to [[lactic acid]] at a later stage in the pathway. In [[yeast]], [[acetaldehyde]] is reduced to [[ethanol]] to regenerate NAD<sup>+</sup>.{{Citation needed|date=December 2023}}

There are two important anaerobic microbial methane formation pathways, through [[carbon dioxide]] / [[bicarbonate]] ({{chem|HCO|3|-}}) reduction (respiration) or acetate fermentation.<ref>{{cite journal|author=Sapart|display-authors=et al|year=2017|title=The origin of methane in the East Siberian Arctic Shelf unraveled with triple isotope analysis|journal=Biogeosciences|volume=14|issue=9|pages=2283–2292|bibcode=2017BGeo...14.2283S|doi=10.5194/bg-14-2283-2017|doi-access=free|hdl=2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/253190|hdl-access=free}}</ref>

==Ecological importance==

Anaerobic respiration is a critical component of the global [[nitrogen cycle|nitrogen]], [[iron]], [[sulfur cycle|sulfur]], and [[carbon cycle|carbon]] cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds.  The [[Biogeochemical cycle|biogeochemical cycling]] of these compounds, which depends upon anaerobic respiration, significantly impacts the [[carbon cycle]] and [[global warming]]. Anaerobic respiration occurs in many environments, including freshwater and marine sediments, soil, subsurface aquifers, deep subsurface environments, and biofilms. Even environments that contain oxygen, such as soil, have micro-environments that lack oxygen due to the slow diffusion characteristics of [[oxygen]] gas.{{Citation needed|date=December 2023}}

An example of the ecological importance of anaerobic respiration is the use of nitrate as a [[Electron acceptor|terminal electron acceptor]], or dissimilatory [[denitrification]], which is the main route by which fixed [[nitrogen fixation|nitrogen]] is returned to the atmosphere as molecular nitrogen gas.<ref>{{Cite journal|last1=Simon|first1=Jörg|last2=Klotz|first2=Martin G.|date=2013-02-01|title=Diversity and evolution of bioenergetic systems involved in microbial nitrogen compound transformations|journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics|volume=1827|issue=2|pages=114–135|doi=10.1016/j.bbabio.2012.07.005|pmid=22842521|doi-access=free}}</ref> The denitrification process is also very important in host-microbe interactions. Like mitochondria in oxygen-respiring microorganisms, some single-cellular anaerobic ciliates use denitrifying endosymbionts to gain energy.<ref>{{cite journal |last1=Graf |first1=Jon S. |last2=Schorn |first2=Sina |last3=Kitzinger |first3=Katharina |last4=Ahmerkamp |first4=Soeren |last5=Woehle |first5=Christian |last6=Huettel |first6=Bruno |last7=Schubert |first7=Carsten J. |last8=Kuypers |first8=Marcel M. M. |last9=Milucka |first9=Jana |title=Anaerobic endosymbiont generates energy for ciliate host by denitrification |journal=Nature |date=3 March 2021 |volume=591 |issue=7850 |pages=445–450 |doi=10.1038/s41586-021-03297-6|pmid=33658719 |pmc=7969357 |bibcode=2021Natur.591..445G |doi-access=free }}</ref> Another example is [[methanogenesis]], a form of carbon-dioxide respiration, that is used to produce [[methane]] gas by [[anaerobic digestion]]. Biogenic methane can be a sustainable alternative to fossil fuels. However, uncontrolled methanogenesis in landfill sites releases large amounts of methane into the atmosphere, acting as a potent [[greenhouse gas]].<ref>{{Cite journal|last1=Bogner|first1=Jean|last2=Pipatti|first2=Riitta|last3=Hashimoto|first3=Seiji|last4=Diaz|first4=Cristobal|last5=Mareckova|first5=Katarina|last6=Diaz|first6=Luis|last7=Kjeldsen|first7=Peter|last8=Monni|first8=Suvi|last9=Faaij|first9=Andre|date=2008-02-01|title=Mitigation of global greenhouse gas emissions from waste: conclusions and strategies from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. Working Group III (Mitigation)|journal=Waste Management & Research|volume=26|issue=1|pages=11–32|issn=0734-242X|pmid=18338699|doi=10.1177/0734242x07088433|bibcode=2008WMR....26...11B |s2cid=29740189}}</ref>  [[Sulfate-reducing bacteria|Sulfate respiration]] produces [[hydrogen sulfide]], which is responsible for the characteristic 'rotten egg' smell of coastal wetlands and has the capacity to precipitate heavy metal ions from solution, leading to the deposition of [[sulfide minerals|sulfidic metal ores]].<ref>{{Cite journal|last1=Pester|first1=Michael|last2=Knorr|first2=Klaus-Holger|last3=Friedrich|first3=Michael W.|last4=Wagner|first4=Michael|last5=Loy|first5=Alexander|date=2012-01-01|title=Sulfate-reducing microorganisms in wetlands – fameless actors in carbon cycling and climate change|journal=Frontiers in Microbiology|volume=3|pages=72|doi=10.3389/fmicb.2012.00072|issn=1664-302X|pmc=3289269|pmid=22403575|doi-access=free}}</ref>

==Economic relevance==
[[File:Anaerobic Denitrification (ETC System).svg|thumb|Anaerobic Denitrification (ETC System)
<br>
<br>The model above shows the process of anaerobic respiration through ''[[denitrification]]'', which uses nitrogen (in the form of nitrate, {{chem|NO|3|-}}) as the electron acceptor. {{chem|NO|3|-}} goes through respiratory dehydrogenase and reduces through each step from the ubiquinose through the bc1 complex through the ATP synthase protein as well. Each reductase removes oxygen step by step so that the final product of anaerobic respiration is N<sub>2</sub>.<br><br>1. Cytoplasm<br>2. Periplasm

Compare to the [[Electron transport chain|aerobic electron transport chain]].]]
Dissimilatory [[denitrification]] is widely used in the removal of [[nitrate]] and [[nitrite]] from municipal wastewater. An excess of nitrate can lead to [[eutrophication]] of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas.<ref>{{Cite journal|last1=Nancharaiah|first1=Y. V.|last2=Venkata Mohan|first2=S.|last3=Lens|first3=P. N. L.|date=2016-09-01|title=Recent advances in nutrient removal and recovery in biological and bioelectrochemical systems|journal=Bioresource Technology|volume=215|pages=173–185|doi=10.1016/j.biortech.2016.03.129|issn=1873-2976|pmid=27053446|bibcode=2016BiTec.215..173N }}</ref>

Specific types of anaerobic respiration are also critical in [[bioremediation]], which uses microorganisms to convert toxic chemicals into less-harmful molecules to clean up contaminated beaches, aquifers, lakes, and oceans. For example, toxic [[arsenate]] or [[selenate]] can be reduced to less toxic compounds by various anaerobic bacteria via anaerobic respiration. The reduction of [[Organochloride|chlorinated chemical pollutants]], such as [[vinyl chloride]] and [[carbon tetrachloride]], also occurs through anaerobic respiration.{{Citation needed|date=December 2023}}<ref>{{Cite journal|doi=10.1016/j.hazl.2021.100039 |title=Vinyl chloride and 1,4-dioxane metabolism by Pseudonocardia dioxanivorans CB1190 |date=2021 |last1=Polasko |first1=Alexandra Lapat |last2=Miao |first2=Yu |last3=Kwok |first3=Ivy |last4=Park |first4=Keunseok |last5=Park |first5=Junyoung O. |last6=Mahendra |first6=Shaily |journal=Journal of Hazardous Materials Letters |volume=2 |bibcode=2021JHzML...200039P |s2cid=239140980 |doi-access=free }}</ref>

Anaerobic respiration is useful in generating electricity in [[microbial fuel cell]]s, which employ bacteria that respire solid electron acceptors (such as oxidized iron) to transfer electrons from reduced compounds to an electrode. This process can simultaneously degrade organic carbon waste and generate electricity.<ref>{{Cite journal|last1=Xu|first1=Bojun|last2=Ge|first2=Zheng|last3=He|first3=Zhen|date=2015-05-15|title=Sediment microbial fuel cells for wastewater treatment: challenges and opportunities|journal= Environmental Science: Water Research & Technology|language=en|volume=1|issue=3|pages=279–284|doi=10.1039/c5ew00020c|issn=2053-1419|doi-access=free|hdl=10919/64969|hdl-access=free}}</ref>

==Examples of electron acceptors in respiration==

{| class="wikitable sortable"
|-
! Type
! Lifestyle
! Electron acceptor
! Products
! [[Reduction potential|E<sup>o′</sup>]] (V)
! Example organisms
|-
| [[Aerobic respiration]]
| [[Obligate aerobe]]s and [[facultative anaerobe]]s
| {{O2}}
| {{H2O}}
| +0.82
| [[Aerobic organism]]s such as [[Escherichia coli]]
|-
| (Per)chlorate respiration
| [[Facultative anaerobe]]s 
| {{chem2|[[Perchlorate|ClO4-]], [[Chlorate|ClO3-]]}}
| {{H2O}}, {{O2}}, {{chem2|Cl−|link=Chloride}}
| +0.797
| ''[[Azospira suillum]]'', ''[[Sedimenticola selenatireducens]]'', ''[[Sedimenticola thiotaurini]]'', and other [[gram negative]] [[prokaryotes]]<ref name=":1">{{Cite journal|last1=Melnyk|first1=Ryan A.|last2=Engelbrektson|first2=Anna|last3=Clark|first3=Iain C.|last4=Carlson|first4=Hans K.|last5=Byrne-Bailey|first5=Kathy|last6=Coates|first6=John D.|title= Identification of a Perchlorate Reduction Genomic Island with Novel Regulatory and Metabolic Genes|journal=Applied and Environmental Microbiology|year=2011|volume=77|issue=20|pages=7401–7404|doi =10.1128/AEM.05758-11|pmid=21856823|pmc=3194888|bibcode=2011ApEnM..77.7401M|url=}}</ref> 
|-
| Iodate respiration
| [[Facultative anaerobe]]s 
| {{chem2|IO3-|link=Iodate}}
| {{H2O}}, {{chem2|[[Hydrogen peroxide|H2O2]], [[Iodide|I−]]}}
| +0.72
| ''[[Denitromonas]]'',<ref>{{Cite journal|last1=Reyes-Umana|first1=Victor|last2=Henning|first2=Zachary|last3=Lee|first3=Kristina|last4=Barnum|first4=Tyler P.|last5=Coates|first5=John D.|date=2021-07-02|title=Genetic and phylogenetic analysis of dissimilatory iodate-reducing bacteria identifies potential niches across the world's oceans|journal=The ISME Journal|volume=16|issue=1 |language=en|pages=38–49|doi=10.1038/s41396-021-01034-5|pmid=34215855|pmc=8692401 |s2cid=235722250|issn=1751-7370}}</ref> ''[[Azoarcus]]'', ''[[Pseudomonas]]'', and other [[gram negative|prokaryotes]]<ref name=":2">{{Cite bioRxiv|last1=Reyes-Umana|first1=Victor|last2=Henning|first2=Zachary|last3=Lee|first3=Kristina|last4=Barnum|first4=Tyler|last5=Coates|first5=John|title = Genetic and phylogenetic analysis of dissimilatory iodate-reducing bacteria identifies potential niches across the world's oceans|year = 2020|biorxiv= 10.1101/2020.12.28.424624}}</ref>
|-
| Iron reduction{{br}}([[Dissimilatory iron reducing bacteria]])
| [[Facultative anaerobe]]s and [[obligate anaerobe]]s 
| {{chem2|Fe(3+)|link=Ferric}}
| {{chem2|Fe(2+)|link=Ferrous}}
| +0.75
| Organisms within the order ''[[Desulfuromonadales]]'' (such as  ''[[Geobacter]]'', ''[[Geothermobacter]]'', ''[[Geopsychrobacter electrodiphilus|Geopsychrobacter]]'', ''[[Pelobacter carbinolicus|Pelobacter]]'') and ''[[Shewanella]]'' species <ref name=":0">{{Cite journal|last1=Richter|first1=Katrin|last2=Schicklberger|first2=Marcus|last3=Gescher|first3=Johannes|date=2012-02-01|title=Dissimilatory reduction of extracellular electron acceptors in anaerobic respiration|journal=Applied and Environmental Microbiology|volume=78|issue=4|pages=913–921|doi=10.1128/AEM.06803-11|issn=1098-5336|pmc=3273014|pmid=22179232|bibcode=2012ApEnM..78..913R}}</ref>
|-
| Manganese reduction{{br}}([[dissimilatory metal-reducing microorganisms]])
| [[Facultative anaerobe]]s and [[obligate anaerobe]]s
| {{chem2|Mn(4+)|auto=y}}
| {{chem2|Mn(2+)|auto=y}}
| 
| ''[[Desulfuromonadales]]'' and ''[[Shewanella]]'' species <ref name=":0" />
|-
| Cobalt reduction{{br}}([[dissimilatory metal-reducing microorganisms]])
| [[Facultative anaerobe]]s and [[obligate anaerobe]]s
| {{chem2|Co(3+)|auto=y}}
| {{chem2|Co(2+)|auto=y}}
|
| ''[[Geobacter sulfurreducens]]''
|-
| Uranium reduction{{br}}([[dissimilatory metal-reducing microorganisms]])
| [[Facultative anaerobe]]s and [[obligate anaerobe]]s
| {{chem2|U(6+)|auto=y}}
| {{chem2|U(4+)|auto=y}}
|
| ''[[Geobacter metallireducens]]'', ''[[Shewanella oneidensis]]''<ref>{{Cite journal|last1=Wall|first1=Judy D.|last2=Krumholz|first2=Lee R.|date=13 October 2006|title=Uranium Reduction|journal=Annual Review of Microbiology|volume=60|pages=149–166|pmid=16704344|doi=10.1146/annurev.micro.59.030804.121357}}</ref>
|-
| [[Denitrification]]{{br}}(nitrate reduction)
| [[Facultative anaerobe]]s
| {{chem2|NO3-|link=Nitrate}}
| (Ultimately) N<sub>2</sub>
| +0.40
| ''[[Paracoccus denitrificans]]'', ''[[Escherichia coli]]''
|-
| [[Fumarate reductase|Fumarate respiration]]
| [[Facultative anaerobe]]s
| [[Fumarate]]
| [[Succinate]]
| +0.03
| ''[[Escherichia coli]]''
|-
| [[Sulfate-reducing microorganisms|Sulfate respiration]]
| [[Obligate anaerobe]]s 
| {{chem2|SO4(2-)|link=Sulfate}}
| {{H2O}}, {{chem2|HS-|link=Sulfide}}
| −0.22
| Many [[Deltaproteobacteria]] species in the orders [[Desulfobacterales]], [[Desulfovibrionales]], and [[Syntrophobacterales]]
|-
| [[Methanogenesis]] (carbon dioxide reduction)
| [[Methanogens]] 
| {{chem2|CO2|link=Carbon dioxide}}
| {{chem2|CH4|link=Methane}}
| −0.25
| ''[[Methanosarcina barkeri]]''
|-
| [[Sulfur-reducing bacteria|Sulfur respiration]] (sulfur reduction)
| [[Facultative anaerobe]]s and [[obligate anaerobe]]s
| [[Sulfur|S]]<sup>0</sup>
| {{chem2|HS-|link=Sulfide}}
| −0.27
| ''[[Desulfuromonadales]]''
|-
| [[Acetogenesis]] (carbon dioxide reduction)
| [[Obligate anaerobe]]s 
| {{chem2|CO2|link=Carbon dioxide}}
| [[Acetate]]
| −0.30
| ''[[Acetobacterium woodii]]''
|-
| [[Halorespiration]]
| [[Facultative anaerobe]]s and [[obligate anaerobe]]s
| [[Halocarbon|Halogenated organic compound]]s{{br}}({{chem2|R\sX|auto=y}})
| [[Halide]] ions, dehalogenated compounds{{br}}({{chem2|X− + R\sH|auto=y}})
| +0.25 – +0.60<ref name=Hollinger1998>{{Cite journal | last1 = Holliger | first1 = C. | last2 = Wohlfarth | first2 = G. | last3 = Diekert | first3 = G. | doi = 10.1111/j.1574-6976.1998.tb00377.x | title = Reductive dechlorination in the energy metabolism of anaerobic bacteria | journal = FEMS Microbiology Reviews | volume = 22 | issue = 5 | pages = 383 | year = 1998 | s2cid = 85965965 | url = https://doc.rero.ch/record/295119/files/22-5-383.pdf }}</ref>
| ''[[Dehalococcoides]]'' and ''[[Dehalobacter]]'' species 
|}

== See also ==
*[[Hydrogenosome]]s and [[mitosome]]s – mitochondrion-related organelles (MROs)
*[[Spinoloricus cinziae]] – an anaerobic microbial animal with hydrogenosome-like organelles
*[[Monocercomonoides]] – genus of eukaryotes that lack mitochondria and MROs entirely
*[[Anaerobic digestion]]
*[[Microbial fuel cell]]
*[[Standard electrode potential (data page)]]
*[[Table of standard reduction potentials for half-reactions important in biochemistry]]
*[[Lithotroph]]s

== Further reading ==
*{{Cite journal|last1=Gregory|first1=Kelvin B.|last2=Bond|first2=Daniel R.|last3=Lovley|first3=Derek R.|date=June 2004|title=Graphite electrodes as electron donors for anaerobic respiration|journal=Environmental Microbiology|volume=6|issue=6|pages=596–604|doi=10.1111/j.1462-2920.2004.00593.x|pmid=15142248|bibcode=2004EnvMi...6..596G |issn=1462-2912}}

==References==
{{Reflist|25em}}

{{metabolism}}
{{MetabolismMap}}

{{DEFAULTSORT:Anaerobic Respiration}}
[[Category:Anaerobic digestion]]
[[Category:Biodegradation]]
[[Category:Cellular respiration]]
[[Category:Anaerobic respiration]]