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In [[biology]], '''syntrophy''', '''synthrophy''', or '''cross-feeding''' (from [[List of Greek and Latin roots in English|Greek]] ''syn'' meaning together, ''trophe'' meaning nourishment) is the phenomenon of one [[species]] feeding on the metabolic products of another species to cope up with the energy limitations by [[electron transfer]].<ref name=":72">{{ |
In [[biology]], '''syntrophy''', '''synthrophy''', or '''cross-feeding''' (from [[List of Greek and Latin roots in English|Greek]] ''syn'' meaning together, ''trophe'' meaning nourishment) is the phenomenon of one [[species]] feeding on the metabolic products of another species to cope up with the energy limitations by [[electron transfer]].<ref name=":72">{{cite book | vauthors = Kamagata Y | chapter = Syntrophy in Anaerobic Digestion |date=2015-03-15 |chapter-url=https://www.worldscientific.com/doi/abs/10.1142/9781783267910_0002 | title = Anaerobic Biotechnology |pages=13–30 |publisher=Imperial College Press |doi=10.1142/9781783267910_0002 |isbn=978-1-78326-790-3 |access-date=2022-11-11}}</ref><ref>{{cite journal | vauthors = Hao L, Michaelsen TY, Singleton CM, Dottorini G, Kirkegaard RH, Albertsen M, Nielsen PH, Dueholm MS | display-authors = 6 | title = Novel syntrophic bacteria in full-scale anaerobic digesters revealed by genome-centric metatranscriptomics | journal = The ISME Journal | volume = 14 | issue = 4 | pages = 906–918 | date = April 2020 | pmid = 31896784 | pmc = 7082340 | doi = 10.1038/s41396-019-0571-0 }}</ref> In this type of [[biological interaction]], metabolite transfer happens between two or more metabolically diverse [[microbial]] species that lives in close proximity to each other.<ref name=":02">{{cite book | vauthors = Schink B, Stams AJ | chapter = Syntrophism Among Prokaryotes |date=2013 | title = The Prokaryotes: Prokaryotic Communities and Ecophysiology |pages=471–493 | veditors = Rosenberg E, DeLong EF, Lory S, Stackebrandt E |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-642-30123-0_59 |isbn=978-3-642-30123-0 }}</ref> The growth of one partner depends on the [[nutrient]]s, [[growth factor]]s, or [[Substrate (biology)|substrates]] provided by the other partner. Thus, syntrophism<ref name=":02" /> can be considered as an obligatory interdependency and a mutualistic metabolism between two different bacterial species.<ref>{{cite journal | vauthors = Dolfing J | title = Syntrophy in microbial fuel cells | journal = The ISME Journal | volume = 8 | issue = 1 | pages = 4–5 | date = January 2014 | pmid = 24173460 | pmc = 3869025 | doi = 10.1038/ismej.2013.198 }}</ref><ref name=":12">{{cite journal | vauthors = Morris BE, Henneberger R, Huber H, Moissl-Eichinger C | title = Microbial syntrophy: interaction for the common good | journal = FEMS Microbiology Reviews | volume = 37 | issue = 3 | pages = 384–406 | date = May 2013 | pmid = 23480449 | doi = 10.1111/1574-6976.12019 }}</ref> |
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== Microbial syntrophy == |
== Microbial syntrophy == |
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Syntrophy is often used synonymously for mutualistic [[symbiosis]] especially between at least two different bacterial species. Syntrophy differs from [[symbiosis]] in a way that syntrophic relationship is primarily based on closely linked metabolic interactions to maintain thermodynamically favorable lifestyle in a given environment.<ref>{{ |
Syntrophy is often used synonymously for mutualistic [[symbiosis]] especially between at least two different bacterial species. Syntrophy differs from [[symbiosis]] in a way that syntrophic relationship is primarily based on closely linked metabolic interactions to maintain thermodynamically favorable lifestyle in a given environment.<ref>{{cite journal | vauthors = Sieber JR, McInerney MJ, Gunsalus RP | title = Genomic insights into syntrophy: the paradigm for anaerobic metabolic cooperation | journal = Annual Review of Microbiology | volume = 66 | pages = 429–452 | date = 2012 | pmid = 22803797 | doi = 10.1146/annurev-micro-090110-102844 }}</ref><ref name="McInerney_2009">{{cite journal | vauthors = McInerney MJ, Sieber JR, Gunsalus RP | title = Syntrophy in anaerobic global carbon cycles | journal = Current Opinion in Biotechnology | volume = 20 | issue = 6 | pages = 623–632 | date = December 2009 | pmid = 19897353 | pmc = 2790021 | doi = 10.1016/j.copbio.2009.10.001 }}</ref><ref>{{cite journal | vauthors = McInerney MJ, Rohlin L, Mouttaki H, Kim U, Krupp RS, Rios-Hernandez L, Sieber J, Struchtemeyer CG, Bhattacharyya A, Campbell JW, Gunsalus RP | display-authors = 6 | title = The genome of Syntrophus aciditrophicus: life at the thermodynamic limit of microbial growth | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 18 | pages = 7600–7605 | date = May 2007 | pmid = 17442750 | pmc = 1863511 | doi = 10.1073/pnas.0610456104 | doi-access = free | bibcode = 2007PNAS..104.7600M }}</ref> Syntrophy plays an important role in a large number of microbial processes especially in oxygen limited environments, methanogenic environments and anaerobic systems.<ref name=":8">{{cite journal | vauthors = McInerney MJ, Sieber JR, Gunsalus RP | title = Syntrophy in anaerobic global carbon cycles | journal = Current Opinion in Biotechnology | volume = 20 | issue = 6 | pages = 623–632 | date = December 2009 | pmid = 19897353 | pmc = 2790021 | doi = 10.1016/j.copbio.2009.10.001 | series = Chemical biotechnology ● Pharmaceutical biotechnology }}</ref><ref name=":3">{{Cite book | vauthors = Worm P, Müller N, Plugge CM, Stams AJ, Schink B | chapter = Syntrophy in methanogenic degradation. | title = (Endo)symbiotic Methanogenic Archaea | date = 2010 | pages = 143-173 | publisher = Springer | location = Berlin, Heidelberg |series=Microbiology Monographs |volume=19 |language=en |doi=10.1007/978-3-642-13615-3_9 |isbn=978-3-642-13614-6 }}</ref> In anoxic or methanogenic environments such as wetlands, swamps, paddy fields, landfills, digestive tract of [[ruminant]]s, and anerobic digesters syntrophy is employed to overcome the energy constraints as the reactions in these environments proceed close to [[thermodynamic equilibrium]].<ref name=":12" /><ref name=":3" /><ref name=":42">{{cite journal | vauthors = Jackson BE, McInerney MJ | title = Anaerobic microbial metabolism can proceed close to thermodynamic limits | journal = Nature | volume = 415 | issue = 6870 | pages = 454–456 | date = January 2002 | pmid = 11807560 | doi = 10.1038/415454a | s2cid = 9126984 | bibcode = 2002Natur.415..454J }}</ref> |
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=== Mechanism of microbial syntrophy === |
=== Mechanism of microbial syntrophy === |
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The main mechanism of syntrophy is removing the metabolic end products of one species so as to create an energetically favorable environment for another species.<ref name=":42" /> This obligate metabolic cooperation is required to facilitate the degradation of complex organic substrates under anaerobic conditions. Complex organic compounds such as ethanol, [[propionate]], [[butyrate]], and [[Lactic acid|lactate]] cannot be directly used as substrates for [[methanogenesis]] by methanogens.<ref name=":12" /> On the other hand, [[fermentation]] of these organic compounds cannot occur in fermenting microorganisms unless the hydrogen concentration is reduced to a low level by the methanogens. The key mechanism that ensures the success of syntrophy is interspecies electron transfer.<ref name=":5">{{Cite journal | |
The main mechanism of syntrophy is removing the metabolic end products of one species so as to create an energetically favorable environment for another species.<ref name=":42" /> This obligate metabolic cooperation is required to facilitate the degradation of complex organic substrates under anaerobic conditions. Complex organic compounds such as ethanol, [[propionate]], [[butyrate]], and [[Lactic acid|lactate]] cannot be directly used as substrates for [[methanogenesis]] by methanogens.<ref name=":12" /> On the other hand, [[fermentation]] of these organic compounds cannot occur in fermenting microorganisms unless the hydrogen concentration is reduced to a low level by the methanogens. The key mechanism that ensures the success of syntrophy is interspecies electron transfer.<ref name=":5">{{Cite journal |vauthors = Zhang M, Zang L |date=2019 |title=A review if interspecies electron transfer in anaerobic digestion |url=https://iopscience.iop.org/article/10.1088/1755-1315/310/4/042026/pdf |journal=IOP Conf. Ser: Earth Environ|volume=310 |issue=4 |page=042026 |doi=10.1088/1755-1315/310/4/042026 |bibcode=2019E&ES..310d2026Z |s2cid=202886264 }}</ref> The interspecies electron transfer can be carried out via three ways: [[interspecies hydrogen transfer]], interspecies formate transfer and interspecies direct electron transfer.<ref name=":5" /><ref>{{cite journal | vauthors = Rotaru AE, Shrestha PM, Liu F, Ueki T, Nevin K, Summers ZM, Lovley DR | title = Interspecies electron transfer via hydrogen and formate rather than direct electrical connections in cocultures of Pelobacter carbinolicus and Geobacter sulfurreducens | journal = Applied and Environmental Microbiology | volume = 78 | issue = 21 | pages = 7645–7651 | date = November 2012 | pmid = 22923399 | pmc = 3485699 | doi = 10.1128/AEM.01946-12 | bibcode = 2012ApEnM..78.7645R }}</ref> [[Reverse electron flow|Reverse electron transport]] is prominent in syntrophic metabolism.<ref name=":8" /> |
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The metabolic reactions and the energy involved for syntrophic degradation with H<sub>2</sub> consumption:<ref name=":6">{{Cite journal | |
The metabolic reactions and the energy involved for syntrophic degradation with H<sub>2</sub> consumption:<ref name=":6">{{Cite journal | vauthors = Zhang Y, Li C, Yuan Z, Wang R, Angelidaki I, Zhu G |date=2023-01-15 |title=Syntrophy mechanism, microbial population, and process optimization for volatile fatty acids metabolism in anaerobic digestion |journal=Chemical Engineering Journal |language=en |volume=452 |pages=139137 |doi=10.1016/j.cej.2022.139137 |s2cid=252205776 |issn=1385-8947}}</ref> |
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A classical syntrophic relationship can be illustrated by the activity of ‘''Methanobacillus omelianskii''’. It was isolated several times from anaerobic sediments and sewage sludge and was regarded as a pure culture of an anaerobe converting ethanol to acetate and methane. In fact, however, the culture turned out to consist of a methanogenic archaeon "organism M.o.H" and a Gram-negative Bacterium "Organism S" which involves the oxidization of [[ethanol]] into acetate and [[methane]] mediated by [[interspecies hydrogen transfer]]. Individuals of organism S are observed as obligate [[anaerobic bacteria]] that use ethanol as an [[electron donor]], whereas M.o.H are [[methanogens]] that oxidize hydrogen gas to produce methane.<ref name=":6" /><ref>{{ |
A classical syntrophic relationship can be illustrated by the activity of ‘''Methanobacillus omelianskii''’. It was isolated several times from anaerobic sediments and sewage sludge and was regarded as a pure culture of an anaerobe converting ethanol to acetate and methane. In fact, however, the culture turned out to consist of a methanogenic archaeon "organism M.o.H" and a Gram-negative Bacterium "Organism S" which involves the oxidization of [[ethanol]] into acetate and [[methane]] mediated by [[interspecies hydrogen transfer]]. Individuals of organism S are observed as obligate [[anaerobic bacteria]] that use ethanol as an [[electron donor]], whereas M.o.H are [[methanogens]] that oxidize hydrogen gas to produce methane.<ref name=":6" /><ref>{{cite journal | vauthors = Wrede C, Dreier A, Kokoschka S, Hoppert M | title = Archaea in symbioses | journal = Archaea | volume = 2012 | pages = 596846 | date = 2012 | pmid = 23326206 | pmc = 3544247 | doi = 10.1155/2012/596846 }}</ref><ref>{{cite journal | vauthors = Morris BE, Henneberger R, Huber H, Moissl-Eichinger C | title = Microbial syntrophy: interaction for the common good | journal = FEMS Microbiology Reviews | volume = 37 | issue = 3 | pages = 384–406 | date = May 2013 | pmid = 23480449 | doi = 10.1111/1574-6976.12019 }}</ref> |
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'''Organism S:''' 2 Ethanol + 2 H<sub>2</sub>O → 2 Acetate<sup>−</sup> + 2 H<sup>+</sup> + 4 H<sub>2</sub> (ΔG°' = +9.6 kJ per reaction) |
'''Organism S:''' 2 Ethanol + 2 H<sub>2</sub>O → 2 Acetate<sup>−</sup> + 2 H<sup>+</sup> + 4 H<sub>2</sub> (ΔG°' = +9.6 kJ per reaction) |
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Butyrate＋2H2O＋2CO<sub>2</sub> → 2Acetate- ＋3Formate- ＋3H<sup>+</sup> ΔG°'=＋38.5 kJ/mol) |
Butyrate＋2H2O＋2CO<sub>2</sub> → 2Acetate- ＋3Formate- ＋3H<sup>+</sup> ΔG°'=＋38.5 kJ/mol) |
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Direct interspecies electron transfer (DIET) which involves electron transfer without any electron carrier such as H<sub>2</sub> or formate was reported in the co-culture system of ''Geobacter mettalireducens'' and [[Methanosaeta|''Methanosaeto'']] or ''[[Methanosarcina]]''<ref name=":5" /><ref>{{ |
Direct interspecies electron transfer (DIET) which involves electron transfer without any electron carrier such as H<sub>2</sub> or formate was reported in the co-culture system of ''Geobacter mettalireducens'' and [[Methanosaeta|''Methanosaeto'']] or ''[[Methanosarcina]]''<ref name=":5" /><ref name="pmid26337845">{{cite journal | vauthors = Dubé CD, Guiot SR | title = Direct Interspecies Electron Transfer in Anaerobic Digestion: A Review | journal = Advances in Biochemical Engineering/biotechnology | volume = 151 | issue = | pages = 101–15 | date = 2015 | pmid = 26337845 | doi = 10.1007/978-3-319-21993-6_4 }}</ref> |
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== Examples |
== Examples == |
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=== |
=== In ruminants === |
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The defining feature of [[ruminant]]s, such as cows and goats, is a stomach called a [[rumen]].<ref>{{Cite web | |
The defining feature of [[ruminant]]s, such as cows and goats, is a stomach called a [[rumen]].<ref>{{Cite web |work = AnimalSmart.org |title=What's a Rumen |url=https://animalsmart.org/species/what%27s-a-rumen- |access-date=2022-11-21 |language=en}}</ref> The rumen contains billions of microbes, many of which are syntrophic.<ref name=":3" /><ref name=":7">{{cite journal | vauthors = Ng F, Kittelmann S, Patchett ML, Attwood GT, Janssen PH, Rakonjac J, Gagic D | title = An adhesin from hydrogen-utilizing rumen methanogen Methanobrevibacter ruminantium M1 binds a broad range of hydrogen-producing microorganisms | journal = Environmental Microbiology | volume = 18 | issue = 9 | pages = 3010–3021 | date = September 2016 | pmid = 26643468 | doi = 10.1111/1462-2920.13155 }}</ref> Some anaerobic fermenting microbes in the rumen (and other gastrointestinal tracts) are capable of degrading organic matter to [[short chain fatty acids]], and hydrogen.<ref name=":3" /><ref name=":12" /> The accumulating [[hydrogen]] inhibits the microbe's ability to continue degrading organic matter, but the presence of syntrophic hydrogen-consuming microbes allows continued growth by metabolizing the waste products.<ref name=":7" /> In addition, fermentative bacteria gain maximum energy yield when [[protons]] are used as electron acceptor with concurrent [[Hydrogen|H]]<sub>2</sub> production. Hydrogen-consuming organisms include [[methanogens]], sulfate-reducers, [[acetogens]], and others.<ref>{{Cite web | vauthors = Sapkota A |date=2022-07-12 |title=Syntrophism or Syntrophy Interaction- Definition, Examples |url=https://thebiologynotes.com/syntrophism-or-syntrophy/ |access-date=2022-11-21 |website=The Biology Notes |language=en-US}}</ref> |
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Some fermentation products, such as [[fatty acids]] longer than two carbon atoms, alcohols longer than one carbon atom, and branched chain and aromatic fatty acids, cannot directly be used in [[methanogenesis]].<ref>{{Cite journal | |
Some fermentation products, such as [[fatty acids]] longer than two carbon atoms, alcohols longer than one carbon atom, and branched chain and aromatic fatty acids, cannot directly be used in [[methanogenesis]].<ref>{{Cite journal | vauthors = Kang D, Saha S, Kurade MB, Basak B, Ha G, Jeon B, Lee SS, Kim JR | display-authors = 6 |date= July 2021 |title=Dual-stage pulse-feed operation enhanced methanation of lipidic waste during co-digestion using acclimatized consortia |journal=Renewable and Sustainable Energy Reviews |language=en |volume=145 |pages=111096 |doi=10.1016/j.rser.2021.111096 |s2cid=234830362 |issn=1364-0321}}</ref> In [[acetogenesis]] processes, these products are oxidized to [[acetate]] and H<sub>2</sub> by obligated proton reducing bacteria in syntrophic relationship with methanogenic [[archaea]] as low H<sub>2</sub> partial pressure is essential for acetogenic reactions to be thermodynamically favorable (ΔG < 0).<ref name=":11">{{cite journal | vauthors = Stams AJ, de Bok FA, Plugge CM, van Eekert MH, Dolfing J, Schraa G | title = Exocellular electron transfer in anaerobic microbial communities | journal = Environmental Microbiology | volume = 8 | issue = 3 | pages = 371–382 | date = March 2006 | pmid = 16478444 | doi = 10.1111/j.1462-2920.2006.00989.x }}</ref> |
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=== Biodegradation of pollutants === |
=== Biodegradation of pollutants === |
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Syntrophic microbial [[food webs]] play an integral role in bioremediation especially in environments contaminated with crude oil and petrol. Environmental contamination with [[Petroleum|oil]] is of high ecological importance and can be effectively mediated through syntrophic degradation by complete mineralization of [[alkane]], [[aliphatic]] and [[hydrocarbon]] chains.<ref>{{ |
Syntrophic microbial [[food webs]] play an integral role in bioremediation especially in environments contaminated with crude oil and petrol. Environmental contamination with [[Petroleum|oil]] is of high ecological importance and can be effectively mediated through syntrophic degradation by complete mineralization of [[alkane]], [[aliphatic]] and [[hydrocarbon]] chains.<ref>{{cite journal | vauthors = Callaghan AV, Morris BE, Pereira IA, McInerney MJ, Austin RN, Groves JT, Kukor JJ, Suflita JM, Young LY, Zylstra GJ, Wawrik B | display-authors = 6 | title = The genome sequence of Desulfatibacillum alkenivorans AK-01: a blueprint for anaerobic alkane oxidation | journal = Environmental Microbiology | volume = 14 | issue = 1 | pages = 101–113 | date = January 2012 | pmid = 21651686 | doi = 10.1111/j.1462-2920.2011.02516.x }}</ref><ref name="Ferry_1976">{{cite journal | vauthors = Ferry JG, Wolfe RS | title = Anaerobic degradation of benzoate to methane by a microbial consortium | journal = Archives of Microbiology | volume = 107 | issue = 1 | pages = 33–40 | date = February 1976 | pmid = 1252087 | doi = 10.1007/BF00427864 | s2cid = 31426072 }}</ref> The hydrocarbons of the oil are broken down after activation by [[fumarate]], a chemical compound that is regenerated by other microorganisms.<ref name="Callaghan_2012">{{cite journal | vauthors = Callaghan AV, Morris BE, Pereira IA, McInerney MJ, Austin RN, Groves JT, Kukor JJ, Suflita JM, Young LY, Zylstra GJ, Wawrik B | display-authors = 6 | title = The genome sequence of Desulfatibacillum alkenivorans AK-01: a blueprint for anaerobic alkane oxidation | journal = Environmental Microbiology | volume = 14 | issue = 1 | pages = 101–113 | date = January 2012 | pmid = 21651686 | doi = 10.1111/j.1462-2920.2011.02516.x }}</ref> Without regeneration, the microbes degrading the oil would eventually run out of fumarate and the process would cease. This breakdown is crucial in the processes of [[bioremediation]] and global carbon cycling.<ref name="Callaghan_2012" /> |
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Syntrophic microbial communities are key players in the breakdown of [[aromatic compounds]], which are common pollutants.<ref name=" |
Syntrophic microbial communities are key players in the breakdown of [[aromatic compounds]], which are common pollutants.<ref name="Ferry_1976" /> The degradation of aromatic [[Benzoic acid|benzoate]] to [[methane]] produces intermediate compounds such as [[formate]], [[acetate]], {{CO2|link=yes}} and H<sub>2</sub>.<ref name="Ferry_1976" /> The buildup of these products makes benzoate degradation thermodynamically unfavorable. These intermediates can be metabolized syntrophically by [[methanogens]] and makes the degradation process thermodynamically favorable<ref name="Ferry_1976" /> |
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=== Degradation of amino acids === |
=== Degradation of amino acids === |
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Studies have shown that bacterial degradation of [[amino acids]] can be significantly enhanced through the process of syntrophy.<ref name=":10">{{Cite journal | |
Studies have shown that bacterial degradation of [[amino acids]] can be significantly enhanced through the process of syntrophy.<ref name=":10">{{Cite journal | vauthors = Zindel U, Freudenberg W, Rieth M, Andreesen JR, Schnell J, Widdel F |date= July 1988 |title=Eubacterium acidaminophilum sp. nov., a versatile amino acid-degrading anaerobe producing or utilizing H2 or formate |journal=Archives of Microbiology |language=en |volume=150 |issue=3 |pages=254–266 |doi=10.1007/BF00407789 |issn=0302-8933 |s2cid=34824309}}</ref> Microbes growing poorly on amino acid substrates [[alanine]], [[aspartate]], [[serine]], [[leucine]], [[valine]], and [[glycine]] can have their rate of growth dramatically increased by syntrophic H<sub>2</sub> scavengers. These scavengers, like ''[[Methanospirillum]] ''and'' [[Acetobacterium]],'' metabolize the H<sub>2</sub> waste produced during amino acid breakdown, preventing a toxic build-up.<ref name=":10" /> Another way to improve amino acid breakdown is through interspecies [[electron transfer]] mediated by formate. Species like ''[[Desulfovibrio]]'' employ this method.<ref name=":10" /> Amino acid fermenting anaerobes such as ''[[Clostridium]]'' species, ''Peptostreptococcus asacchaarolyticus'', ''Acidaminococcus fermentans'' were known to breakdown amino acids like [[Glutamic acid|glutamate]] with the help of hydrogen scavenging methanogenic partners without going through the usual [[Stickland fermentation]] pathway<ref name=":3" /><ref name=":10" /> |
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=== Anaerobic digestion === |
=== Anaerobic digestion === |
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== Examples of syntrophic organisms == |
== Examples of syntrophic organisms == |
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* ''[[Syntrophomonas wolfei]]''<ref>{{ |
* ''[[Syntrophomonas wolfei]]''<ref>{{cite journal | vauthors = McInerney MJ, Bryant MP, Hespell RB, Costerton JW | title = Syntrophomonas wolfei gen. nov. sp. nov., an Anaerobic, Syntrophic, Fatty Acid-Oxidizing Bacterium | journal = Applied and Environmental Microbiology | volume = 41 | issue = 4 | pages = 1029–1039 | date = April 1981 | pmid = 16345745 | pmc = 243852 | doi = 10.1128/aem.41.4.1029-1039.1981 | bibcode = 1981ApEnM..41.1029M }}</ref> |
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* ''[[Syntrophobacter fumaroxidans|Syntrophobacter funaroxidans]]''<ref name=":02" /> |
* ''[[Syntrophobacter fumaroxidans|Syntrophobacter funaroxidans]]''<ref name=":02" /> |
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* ''Pelotomaculum thermopropinicium''<ref name=":02" /> |
* ''Pelotomaculum thermopropinicium''<ref name=":02" /> |
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* ''[[Syntrophus aciditrophicus]]''<ref name=":42" /> |
* ''[[Syntrophus aciditrophicus]]''<ref name=":42" /> |
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* ''[[Syntrophus buswellii]]''<ref name=":3" /> |
* ''[[Syntrophus buswellii]]''<ref name=":3" /> |
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* ''Syntrophus gentianae''<ref>{{ |
* ''Syntrophus gentianae''<ref>{{cite journal | vauthors = Schöcke L, Schink B | title = Membrane-bound proton-translocating pyrophosphatase of Syntrophus gentianae, a syntrophically benzoate-degrading fermenting bacterium | journal = European Journal of Biochemistry | volume = 256 | issue = 3 | pages = 589–594 | date = September 1998 | pmid = 9780235 | doi = 10.1046/j.1432-1327.1998.2560589.x }}</ref> |
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== References == |
== References == |
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Revision as of 08:08, 23 December 2022
In biology, syntrophy, synthrophy, or cross-feeding (from Greek syn meaning together, trophe meaning nourishment) is the phenomenon of one species feeding on the metabolic products of another species to cope up with the energy limitations by electron transfer.[1][2] In this type of biological interaction, metabolite transfer happens between two or more metabolically diverse microbial species that lives in close proximity to each other.[3] The growth of one partner depends on the nutrients, growth factors, or substrates provided by the other partner. Thus, syntrophism[3] can be considered as an obligatory interdependency and a mutualistic metabolism between two different bacterial species.[4][5]
Microbial syntrophy
Syntrophy is often used synonymously for mutualistic symbiosis especially between at least two different bacterial species. Syntrophy differs from symbiosis in a way that syntrophic relationship is primarily based on closely linked metabolic interactions to maintain thermodynamically favorable lifestyle in a given environment.[6][7][8] Syntrophy plays an important role in a large number of microbial processes especially in oxygen limited environments, methanogenic environments and anaerobic systems.[9][10] In anoxic or methanogenic environments such as wetlands, swamps, paddy fields, landfills, digestive tract of ruminants, and anerobic digesters syntrophy is employed to overcome the energy constraints as the reactions in these environments proceed close to thermodynamic equilibrium.[5][10][11]
Mechanism of microbial syntrophy
The main mechanism of syntrophy is removing the metabolic end products of one species so as to create an energetically favorable environment for another species.[11] This obligate metabolic cooperation is required to facilitate the degradation of complex organic substrates under anaerobic conditions. Complex organic compounds such as ethanol, propionate, butyrate, and lactate cannot be directly used as substrates for methanogenesis by methanogens.[5] On the other hand, fermentation of these organic compounds cannot occur in fermenting microorganisms unless the hydrogen concentration is reduced to a low level by the methanogens. The key mechanism that ensures the success of syntrophy is interspecies electron transfer.[12] The interspecies electron transfer can be carried out via three ways: interspecies hydrogen transfer, interspecies formate transfer and interspecies direct electron transfer.[12][13] Reverse electron transport is prominent in syntrophic metabolism.[9]
The metabolic reactions and the energy involved for syntrophic degradation with H2 consumption:[14]
A classical syntrophic relationship can be illustrated by the activity of ‘Methanobacillus omelianskii’. It was isolated several times from anaerobic sediments and sewage sludge and was regarded as a pure culture of an anaerobe converting ethanol to acetate and methane. In fact, however, the culture turned out to consist of a methanogenic archaeon "organism M.o.H" and a Gram-negative Bacterium "Organism S" which involves the oxidization of ethanol into acetate and methane mediated by interspecies hydrogen transfer. Individuals of organism S are observed as obligate anaerobic bacteria that use ethanol as an electron donor, whereas M.o.H are methanogens that oxidize hydrogen gas to produce methane.[14][15][16]
Organism S: 2 Ethanol + 2 H2O → 2 Acetate− + 2 H+ + 4 H2 (ΔG°' = +9.6 kJ per reaction)
Strain M.o.H.: 4 H2 + CO2 → Methane + 2 H2O (ΔG°' = -131 kJ per reaction)
Co-culture:2 Ethanol + CO2 → 2 Acetate− + 2 H+ + Methane (ΔG°' = -113 kJ per reaction)
The oxidization of ethanol by organism S is made possible thanks to the methanogen M.o.H, which consumes the hydrogen produced by organism S, by turning the positive Gibbs free energy into negative Gibbs free energy. This situation favors growth of organism S and also provides energy for methanogens by consuming hydrogen. Down the line, acetate accumulation is also prevented by similar syntrophic relationship.[14] Syntrophic degradation of substrates like butyrate and benzoate can also happen without hydrogen consumption.[11]
An example of propionate and butyrate degradation with interspecies formate transfer carried out by the mutual system of Syntrophomonas wolfei and Methanobacterium formicicum:[12]
Propionate＋2H2O＋2CO2 → Acetate- ＋3Formate- ＋3H+ (ΔG°'=＋65.3 kJ/mol)
Butyrate＋2H2O＋2CO2 → 2Acetate- ＋3Formate- ＋3H+ ΔG°'=＋38.5 kJ/mol)
Direct interspecies electron transfer (DIET) which involves electron transfer without any electron carrier such as H2 or formate was reported in the co-culture system of Geobacter mettalireducens and Methanosaeto or Methanosarcina[12][17]
Examples
In ruminants
The defining feature of ruminants, such as cows and goats, is a stomach called a rumen.[18] The rumen contains billions of microbes, many of which are syntrophic.[10][19] Some anaerobic fermenting microbes in the rumen (and other gastrointestinal tracts) are capable of degrading organic matter to short chain fatty acids, and hydrogen.[10][5] The accumulating hydrogen inhibits the microbe's ability to continue degrading organic matter, but the presence of syntrophic hydrogen-consuming microbes allows continued growth by metabolizing the waste products.[19] In addition, fermentative bacteria gain maximum energy yield when protons are used as electron acceptor with concurrent H2 production. Hydrogen-consuming organisms include methanogens, sulfate-reducers, acetogens, and others.[20]
Some fermentation products, such as fatty acids longer than two carbon atoms, alcohols longer than one carbon atom, and branched chain and aromatic fatty acids, cannot directly be used in methanogenesis.[21] In acetogenesis processes, these products are oxidized to acetate and H2 by obligated proton reducing bacteria in syntrophic relationship with methanogenic archaea as low H2 partial pressure is essential for acetogenic reactions to be thermodynamically favorable (ΔG < 0).[22]
Biodegradation of pollutants
Syntrophic microbial food webs play an integral role in bioremediation especially in environments contaminated with crude oil and petrol. Environmental contamination with oil is of high ecological importance and can be effectively mediated through syntrophic degradation by complete mineralization of alkane, aliphatic and hydrocarbon chains.[23][24] The hydrocarbons of the oil are broken down after activation by fumarate, a chemical compound that is regenerated by other microorganisms.[25] Without regeneration, the microbes degrading the oil would eventually run out of fumarate and the process would cease. This breakdown is crucial in the processes of bioremediation and global carbon cycling.[25]
Syntrophic microbial communities are key players in the breakdown of aromatic compounds, which are common pollutants.[24] The degradation of aromatic benzoate to methane produces intermediate compounds such as formate, acetate, CO2 and H2.[24] The buildup of these products makes benzoate degradation thermodynamically unfavorable. These intermediates can be metabolized syntrophically by methanogens and makes the degradation process thermodynamically favorable[24]
Degradation of amino acids
Studies have shown that bacterial degradation of amino acids can be significantly enhanced through the process of syntrophy.[26] Microbes growing poorly on amino acid substrates alanine, aspartate, serine, leucine, valine, and glycine can have their rate of growth dramatically increased by syntrophic H2 scavengers. These scavengers, like Methanospirillum and Acetobacterium, metabolize the H2 waste produced during amino acid breakdown, preventing a toxic build-up.[26] Another way to improve amino acid breakdown is through interspecies electron transfer mediated by formate. Species like Desulfovibrio employ this method.[26] Amino acid fermenting anaerobes such as Clostridium species, Peptostreptococcus asacchaarolyticus, Acidaminococcus fermentans were known to breakdown amino acids like glutamate with the help of hydrogen scavenging methanogenic partners without going through the usual Stickland fermentation pathway[10][26]
Anaerobic digestion
Effective syntrophic cooperation between propionate oxidizing bacteria, acetate oxidizing bacteria and H2/acetate consuming methanogens is necessary to successfully carryout anaerobic digestion to produce biomethane[1][14]
Examples of syntrophic organisms
- Syntrophomonas wolfei[27]
- Syntrophobacter funaroxidans[3]
- Pelotomaculum thermopropinicium[3]
- Syntrophus aciditrophicus[11]
- Syntrophus buswellii[10]
- Syntrophus gentianae[28]
References
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- ^ a b c d Zhang Y, Li C, Yuan Z, Wang R, Angelidaki I, Zhu G (2023-01-15). "Syntrophy mechanism, microbial population, and process optimization for volatile fatty acids metabolism in anaerobic digestion". Chemical Engineering Journal. 452: 139137. doi:10.1016/j.cej.2022.139137. ISSN 1385-8947. S2CID 252205776.
- ^ Wrede C, Dreier A, Kokoschka S, Hoppert M (2012). "Archaea in symbioses". Archaea. 2012: 596846. doi:10.1155/2012/596846. PMC 3544247. PMID 23326206.
{{cite journal}}: CS1 maint: unflagged free DOI (link) - ^ Morris BE, Henneberger R, Huber H, Moissl-Eichinger C (May 2013). "Microbial syntrophy: interaction for the common good". FEMS Microbiology Reviews. 37 (3): 384–406. doi:10.1111/1574-6976.12019. PMID 23480449.
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