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Syntrophy, synthrophy,^[1]cross-feeding, or cross feeding [Greek syn meaning together, trophe meaning nourishment] is the phenomenon that one species lives off the products of another species. In this association, the growth of one partner is improved, or depends on the nutrients, growth factors or substrate provided by the other partner. Jan Dolfing described syntrophy as "the critical interdependency between producer and consumer".^[2] This term for nutritional interdependence is often used in microbiology to describe this symbiotic relationship between bacterial species.^[3][4] Morris et al. have described the process as "obligately mutualistic metabolism".^[5]

Microbial syntrophy

Syntrophy plays an important role in a large number of microbial processes.

The defining feature of ruminants, such as cows and goats, is a stomach called a rumen. The rumen contains billions of microbes, many of which are syntrophic. One excellent example of this syntrophy is interspecies hydrogen transfer. Some anaerobic fermenting microbes in the rumen (and other gastrointestinal tracts) are capable of degrading organic matter to short chain fatty acids, and hydrogen. The accumulating hydrogen inhibits the microbe's ability to continue degrading organic matter, but syntrophic hydrogen-consuming microbes allow continued growth by metabolizing the waste products. 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. 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. In acetogenesis process, 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). (Stams et al., 2005)

The number of bacterial cells that live on or in the human body, for example throughout the alimentary canal and on the skin, is in the region of 10 times the total number of human cells in it.^[6] These microbes are vital, for instance for the digestive and the immune system to function.^[7]

Another example is the many organisms that feed on faeces or dung. A cow diet consists mainly of grass, the cellulose of which is transformed into lipids by micro-organisms in the cow's large intestine. These micro-organisms cannot use the lipids because of lack of oxygen in the intestine, so the cow does not take up all lipids produced. When the processed grass leaves the intestine as dung and comes into open air, many organisms, such as the dung beetle, feed on it.

Yet another example is the community of micro-organisms in soil that live off leaf litter. Leaves typically last one year and are then replaced by new ones. These micro-organisms mineralize the discarded leaves and release nutrients that are taken up by the plant. Such relationships are called reciprocal syntrophy because the plant lives off the products of micro-organisms. Many symbiotic relationships are based on syntrophy.

Biodegradation of pollutants

Syntrophic microbial food webs can play an integral role in the breakdown of organic pollutants such as oils, aromatic compounds, and amino acids.^[8][9][10]

Environmental contamination with oil is of high ecological importance, but can be mediated through syntrophic degradation.^[8] Alkanes are hydrocarbon chains that are the major chemical component of crude oils, and have been experimentally verified to be broken down by syntrophic microbial food webs.^[8] The hydrocarbons of the oil are broken down after activation by fumarate, a chemical compound that is regenerated by other microorganisms.^[8] 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.^[8]

Syntrophic microbial communities are key players in the breakdown of aromatic compounds, which are common pollutants.^[9] The degradation of aromatic benzoate to methane produces many intermediate compounds such as formate, acetate, CO2 and H2.^[9] The build up of these products makes benzoate degradation progressively less favourable. These intermediates can then be taken up and metabolized syntrophically by methanogens to make the whole process more thermodynamically favourable.^[9]

Studies have shown that bacterial degradation of amino acids can be significantly enhanced through the process of syntrophy.^[10] 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.^[10] Another way to improve amino acid breakdown is through interspecies electron transfer mediated by formate. Species like Desulfovibrio employ this method.^[10]

Metabolic mechanism

The main motive behind a syntrophic relation between two bacterial organisms is generalized as a relationship where each participant's metabolic activity cannot independently overcome the thermodynamic pressure of the reaction under standard conditions even when a cosubstrate or nutrient is added into the environment. Therefore, the cooperation of the other participant is required to reduce the intermediate pool size.^[11]
The Methanobacillus omelianskii culture is a classic example in demonstrating how two separate unfavourable reactions can be carried out by syntrophic interactions.^[12] Strain S and strain M.o.H of Methanobacillus omelianskii oxidize ethanol into acetate and methane by a process called interspecies hydrogen transfer. Individuals of strain S are observed as obligate anaerobic bacteria that use ethanol as an electron donor, whereas organisms of strain M.o.H are methanogens that oxidize hydrogen gas to produce methane.^[1][13] These two metabolic reactions can be shown as follows:

Strain S: 2 CH₃CH₂OH + 2 H₂O → 2 CH₃COO⁻ + 2 H⁺ + 4 H² (ΔG°' = +19 kJ)

Strain M.o.H.: 4 H₂ + CO₂ → CH₄ + 2 H₂O (ΔG°' = -131 kJ)^[11][14]

Complex organic compounds such as ethanol, propionate, butyrate, and lactate cannot be directly used as substrates for methanogenesis by methanogens. 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.^[15] In this case, hydrogen, an electron-carrying compound (mediator) is transported from the fermenting bacteria to the methanogen through a process called mediated interspecies electron transfer (MIET), where the mediator is carried down a concentration gradient created by a thermodynamically favourable coupled redox reaction.^[16]

References