# Syntrophy

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Cooperation between microbial species to digest a substrate

In [biology](/source/Biology), **syntrophy**,[1][2][3][4] **syntrophism**,[1][5][6] or **cross-feeding**[1] (from [Greek](/source/Greek_language) *syn* 'together' and *trophe* 'nourishment') is the cooperative interaction between at least two [microbial](/source/Microbial) species to degrade a single [substrate](/source/Substrate_(biology)).[2][3][4][7] This type of [biological interaction](/source/Biological_interaction) typically involves the transfer of one or more [metabolic intermediates](/source/Metabolic_intermediate) between two or more metabolically diverse microbial species living in close proximity to each other.[3][5] Thus, syntrophy can be considered an obligatory interdependency and a mutualistic metabolism between different microbial species, wherein the growth of one partner depends on the [nutrients](/source/Nutrient), [growth factors](/source/Growth_factor), or substrates provided by the other(s).[8][9]

## Microbial syntrophy

Syntrophy is often used synonymously for mutualistic [symbiosis](/source/Symbiosis) especially between at least two different bacterial species. Syntrophy differs from [symbiosis](/source/Symbiosis) in a way that syntrophic relationship is primarily based on closely linked metabolic interactions to maintain thermodynamically favorable lifestyle in a given environment.[10][11][12] Syntrophy plays an important role in a large number of microbial processes especially in oxygen limited environments, methanogenic environments and anaerobic systems.[13][14] In anoxic or methanogenic environments such as wetlands, swamps, paddy fields, landfills, digestive tract of [ruminants](/source/Ruminant), and anerobic digesters syntrophy is employed to overcome the energy constraints as the reactions in these environments proceed close to [thermodynamic equilibrium](/source/Thermodynamic_equilibrium).[9][14][15]

### 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.[15] This obligate metabolic cooperation is required to facilitate the degradation of complex organic substrates under anaerobic conditions. Complex organic compounds such as ethanol, [propionate](/source/Propionate), [butyrate](/source/Butyrate), and [lactate](/source/Lactic_acid) cannot be directly used as substrates for [methanogenesis](/source/Methanogenesis) by methanogens.[9] On the other hand, [fermentation](/source/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.[16] The interspecies electron transfer can be carried out via three ways: [interspecies hydrogen transfer](/source/Interspecies_hydrogen_transfer), interspecies formate transfer and interspecies direct electron transfer.[16][17] [Reverse electron transport](/source/Reverse_electron_flow) is prominent in syntrophic metabolism.[13]

The metabolic reactions and the energy involved for syntrophic degradation with H2 consumption:[18]

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](/source/Ethanol) into acetate and [methane](/source/Methane) mediated by [interspecies hydrogen transfer](/source/Interspecies_hydrogen_transfer). Individuals of organism S are observed as obligate [anaerobic bacteria](/source/Anaerobic_bacteria) that use ethanol as an [electron donor](/source/Electron_donor), whereas M.o.H are [methanogens](/source/Methanogens) that oxidize hydrogen gas to produce methane.[18][19][9]

**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](/source/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.[18] Syntrophic degradation of substrates like butyrate and benzoate can also happen without hydrogen consumption.[15]

An example of propionate and butyrate degradation with interspecies formate transfer carried out by the mutual system of *[Syntrophomonas wolfei](/source/Syntrophomonas_wolfei)* and *Methanobacterium formicicum*:[16]

- 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*](/source/Methanosaeta) or *[Methanosarcina](/source/Methanosarcina)*[16][20]

## Examples

### In ruminants

The defining feature of [ruminants](/source/Ruminant), such as cows and goats, is a stomach called a [rumen](/source/Rumen).[21] The rumen contains billions of microbes, many of which are syntrophic.[14][22] Some anaerobic fermenting microbes in the rumen (and other gastrointestinal tracts) are capable of degrading organic matter to [short chain fatty acids](/source/Short_chain_fatty_acids), and hydrogen.[14][9] The accumulating [hydrogen](/source/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.[22] In addition, fermentative bacteria gain maximum energy yield when [protons](/source/Protons) are used as electron acceptor with concurrent [H](/source/Hydrogen)2 production. Hydrogen-consuming organisms include [methanogens](/source/Methanogens), sulfate-reducers, [acetogens](/source/Acetogens), and others.[23]

Some fermentation products, such as [fatty acids](/source/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](/source/Methanogenesis).[24] In [acetogenesis](/source/Acetogenesis) processes, these products are oxidized to [acetate](/source/Acetate) and H2 by obligated proton reducing bacteria in syntrophic relationship with methanogenic [archaea](/source/Archaea) as low H2 partial pressure is essential for acetogenic reactions to be thermodynamically favorable (ΔG < 0).[25]

### Biodegradation of pollutants

Syntrophic microbial [food webs](/source/Food_webs) play an integral role in bioremediation especially in environments contaminated with crude oil and petrol. Environmental contamination with [oil](/source/Petroleum) is of high ecological importance and can be effectively mediated through syntrophic degradation by complete mineralization of [alkane](/source/Alkane), [aliphatic](/source/Aliphatic) and [hydrocarbon](/source/Hydrocarbon) chains.[26][27] The hydrocarbons of the oil are broken down after activation by [fumarate](/source/Fumarate), a chemical compound that is regenerated by other microorganisms.[26] 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](/source/Bioremediation) and global carbon cycling.[26]

Syntrophic microbial communities are key players in the breakdown of [aromatic compounds](/source/Aromatic_compounds), which are common pollutants.[27] The degradation of aromatic [benzoate](/source/Benzoic_acid) to [methane](/source/Methane) produces intermediate compounds such as [formate](/source/Formate), [acetate](/source/Acetate), [CO2](/source/Carbon_dioxide) and H2.[27] The buildup of these products makes benzoate degradation thermodynamically unfavorable. These intermediates can be metabolized syntrophically by [methanogens](/source/Methanogens) and makes the degradation process thermodynamically favorable[27]

### Degradation of amino acids

Studies have shown that bacterial degradation of [amino acids](/source/Amino_acids) can be significantly enhanced through the process of syntrophy.[28] Microbes growing poorly on amino acid substrates [alanine](/source/Alanine), [aspartate](/source/Aspartate), [serine](/source/Serine), [leucine](/source/Leucine), [valine](/source/Valine), and [glycine](/source/Glycine) can have their rate of growth dramatically increased by syntrophic H2 scavengers. These scavengers, like *[Methanospirillum](/source/Methanospirillum)*and*[Acetobacterium](/source/Acetobacterium),* metabolize the H2 waste produced during amino acid breakdown, preventing a toxic build-up.[28] Another way to improve amino acid breakdown is through interspecies [electron transfer](/source/Electron_transfer) mediated by formate. Species like *[Desulfovibrio](/source/Desulfovibrio)* employ this method.[28] Amino acid fermenting anaerobes such as *[Clostridium](/source/Clostridium)* species, *Peptostreptococcus asacchaarolyticus*, *Acidaminococcus fermentans* were known to breakdown amino acids like [glutamate](/source/Glutamic_acid) with the help of hydrogen scavenging methanogenic partners without going through the usual [Stickland fermentation](/source/Stickland_fermentation) pathway[14][28]

### 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[4][18]

## Syntrophic theories of eukaryogenesis

Many [symbiogenetic](/source/Symbiogenesis) models of [eukaryogenesis](/source/Eukaryogenesis) propose that the first [eukaryotic](/source/Eukaryote) cells were derived from [endosymbiosis](/source/Endosymbiont) facilitated by microbial syntrophy between [prokaryotic](/source/Prokaryote) cells. Most of these models involve an [archaeon](/source/Archaea) and an [alphaproteobacterium](/source/Alphaproteobacteria), where the dependence of the [archaeon](/source/Archaea) on the [alphaproteobacterium](/source/Alphaproteobacteria) leads the former to engulf the latter, the [alphaproteobacterium](/source/Alphaproteobacteria) then eventually becoming the [mitochondria](/source/Mitochondrion). While these models share the concept of syntrophic interaction as a key driver of [endosymbiosis](/source/Endosymbiont), they often differ on the exact nature of the metabolic interactions involved and the mechanisms by which [eukaryogenesis](/source/Eukaryogenesis) occurred.

### Hydrogen hypothesis

In 1998, William F. Martin and Miklós Müller introduced the hydrogen hypothesis, proposing that [eukaryotes](/source/Eukaryote) arose from syntrophic associations based on the transfer of H2.[29] In this model, an syntrophic association arose where a anaerobic [autotrophic](/source/Autotroph) [methanogenic](/source/Methanogenesis) [archaeon](/source/Archaea) was dependent on the H2 made as a byproduct of [anaerobic respiration](/source/Anaerobic_respiration) by a facultatively anaerobic [alphaproteobacterium](/source/Alphaproteobacteria).[29] This syntrophy led the [alphaproteobacterium](/source/Alphaproteobacteria) to become an [endosymbiont](/source/Endosymbiont) of the [archaeon](/source/Archaea), serving as the precursor to the [mitochondria](/source/Mitochondrion).

### Dennis Searcy model

Dennis Searcy proposed that the precursors to [mitochondria](/source/Mitochondrion) were [parasitic](/source/Parasitism) [bacteria](/source/Bacteria) that developed a syntrophy with their hosts based upon the transfer of organic acids, H2 transfer, and the reciprocal exchange of sulfur compounds.[30]

### Reverse flow model

The reverse flow model was created based on the metabolic analysis of [Asgard archaea](/source/Asgard_(Archaea)), which is thought to be the kingdom from which [eukaryotes](/source/Eukaryote) emerged.[31][32][33] This model proposes that a syntrophic association arose where anaerobic ancestral [Asgard archaea](/source/Asgard_(Archaea)) generated and provided reducing equivalents that facultative anaerobic [alphaproteobacteria](/source/Alphaproteobacteria) used in the form of H2, small reduced compounds, or by direct [electron](/source/Electron) transfer.[31]

### Entangle-Engulf-Endogenize model

The Entangle-Engulf-Endogenize (E3) model was created in 2020 based on the isolation of syntrophic [archaea](/source/Archaea) from deep sea marine sediment.[34] Unlike most other symbiogenetic models, the E3 model involves three separate types of microbes: a [fermentative](/source/Fermentation) [archaeon](/source/Archaea), a facultatively aerobic [organotroph](/source/Organotroph) (which was acts as the precursor of the mitochondria), and [sulfur-reducing bacteria](/source/Sulfur-reducing_bacteria) (SRB).[34] This model proposes that, originally, the [fermentative](/source/Fermentation) [archaeon](/source/Archaea) may have degraded [amino acids](/source/Amino_acid) via syntrophic association with [SRB](/source/Sulfur-reducing_bacteria) and the facultatively aerobic [organotroph](/source/Organotroph).[34] As [oxygen](/source/Oxygen) levels began to rise, however, the interaction with the facultatively aerobic [organotroph](/source/Organotroph) (which is thought to have made the [archaeon](/source/Archaea) more aerotolerant) became stronger became stronger until it was engulfed (a process facilitated by syntrophic interaction with [SRB](/source/Sulfur-reducing_bacteria)).[34] Additionally, the E3 model suggests that, instead of [phagocytizing](/source/Phagocytosis) the facultatively aerobic [organotroph](/source/Organotroph), the [archaeon](/source/Archaea) used extracellular structures to enhance interactions and engulf the facultatively aerobic [organotroph](/source/Organotroph).[34]

### Syntrophy hypothesis

The syntrophy hypothesis was proposed in 2001 by researchers Purificación López-García and David Moreira before being refined in 2020 by the same researchers.[35][36] Similarly to the E3 model, the syntrophy hypothesis suggests that [eukaryogenesis](/source/Eukaryogenesis) involved three different types of microbes: a complex sulfate-reducing [deltaproteobacterium](/source/Myxococcota) (the precursor to the [cytoplasm](/source/Cytoplasm)), an H2-producing [Asgard archaeon](/source/Asgard_(Archaea)) (the precursor to the [nucleus](/source/Cell_nucleus)), and a facultatively aerobic sulfide-oxidizing [alphaproteobacterium](/source/Alphaproteobacteria) (the precursor to [mitochondria](/source/Mitochondrion)).[36] In this model, the [deltaproteobacteria](/source/Myxococcota) forms syntrophic associations with both the [Asgard archaeon](/source/Asgard_(Archaea)) (based on the transfer of H2) and the [alphaproteobacterium](/source/Alphaproteobacteria) (based on the redox of sulfur), leading both to become [endosymbionts](/source/Endosymbiont) of the [deltaproteobacteria](/source/Myxococcota).[36] In this now obligatory [symbiosis](/source/Symbiosis), organic compounds were degraded in the [periplasmic space](/source/Periplasm) of the [deltaproteobacteria](/source/Myxococcota) before being moved to the [archaeon](/source/Archaea) for further degradation.[36] This interaction drove the [periplasm](/source/Periplasm) to develop and expand in close proximity with the [archaeon](/source/Archaea) to facilitate molecular exchange, resulting in an [endomembrane system](/source/Endomembrane_system), transport channels, and the loss of the [archaeal](/source/Archaea) membrane.[36] Ultimately, the archaeon became the nucleus while the periplasmic endomembrane system became the endoplasmic reticulum.[36] Meanwhile, the consortium lost the metabolic capability for [bacterial](/source/Bacteria) sulfate reduction and [archaeal](/source/Archaea) energy [metabolism](/source/Metabolism) as it became more reliant on aerobic [respiration](/source/Cellular_respiration) performed by the [alphaproteobacterium](/source/Alphaproteobacteria) which, ultimately, became the [mitochondrion](/source/Mitochondrion).[36]

## Examples of syntrophic organisms

- *[Syntrophomonas wolfei](/source/Syntrophomonas_wolfei)* is a gram-negative, anaerobic, fatty-acid oxidizing bacterium that forms syntrophic associations with H2-using bacteria.[37]

- *[Syntrophobacter fumaroxidans](/source/Syntrophobacter_fumaroxidans)* is a gram-negative anaerobic bacterium that can oxidize propionate in pure cultures or in syntrophic association with *Methanospirillum hungateii.*[3][38]

- *[Pelotomaculum thermopropionicum](/source/Pelotomaculum_thermopropionicum)* is a thermophilic, anaerobic, syntrophic propionate-oxidizing bacterium that, in co-culture with *Methanothermobacter thermautotrophicus*, can grow on propionate, ethanol, lactate, 1-butanol, 1-pentanol, 1,3-propanediol, 1-propanol, and ethylene glycol.[3][39]

- *[Syntrophus aciditrophicus](/source/Syntrophus_aciditrophicus)* is a gram-negative, obligately anaerobic, nonmotile, rod-shaped bacterium that, in syntrophic association with hydrogen/formate-using methanogens or sulfate reducers, degrades benzoate and fatty acids.[15][40][41]

- *[Syntrophus buswellii](/source/Syntrophus_buswellii)* is a gram-negative, anaerobic, motile, rod-shaped bacterium that, in syntrophic association with H2-using bacteria, degrades benzoate.[14][42]

- *Syntrophus gentianae* is a obligately anaerobic bacterium that ferments benzoate in syntrophic association with H2-using bacteria.[43]

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1. **[^](#cite_ref-43)** Schöcke L, Schink B (September 1998). ["Membrane-bound proton-translocating pyrophosphatase of Syntrophus gentianae, a syntrophically benzoate-degrading fermenting bacterium"](http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-59985). *European Journal of Biochemistry*. **256** (3): 589–594. [doi](/source/Doi_(identifier)):[10.1046/j.1432-1327.1998.2560589.x](https://doi.org/10.1046%2Fj.1432-1327.1998.2560589.x). [PMID](/source/PMID_(identifier)) [9780235](https://pubmed.ncbi.nlm.nih.gov/9780235).

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Adapted from the Wikipedia article [Syntrophy](https://en.wikipedia.org/wiki/Syntrophy) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Syntrophy?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.