Since the nutritional requirements and metabolic activities of organisms differ, it is clear that two or more different organisms growing relatedly may produce different overall changes in the environment. A rough example is provided by a balanced aquarium, in which aquatic plants utilize light and the waste products of animals—e.g., carbon dioxide, water, ammonia—to synthesize cellmaterials and generate oxygen, which in turn provide the materials necessary for animal growth. Such relationships are common among microorganisms; i.e., intermediate or end products of metabolism of one organism may provide essential nutrients for another. The mixed populations that result in nature provide examples of this phenomenon, which is called syntrophism; in some instances, the relationship may be so close as to constitute nutritional symbiosis, or mutualism. Several examples of this phenomenon have been found among thiamin-requiring yeasts and fungi, certain of which (group A) synthesized the thiazole component of thiamin molecule but require the pyrimidine portion preformed; for a second group (group B), the relationship is reversed. When group A and group B are grown together in a thiamin-free medium, both types of organisms survive, since each organism synthesizes the growth factor required by its partner; neither organism grows alone under these same conditions. Thus, two or more types of microorganisms frequently grow in situations in which only one species would not.
Such nutritional interrelationships may explain the fact that the nutritionally demanding lactic-acid bacteria are able to coexist with the nutritionally nondemanding coliform bacteria in the intestinal tracts of animals. It is known that the bacterial flora of the intestinal tract synthesize sufficient amounts of certain vitamins (e.g., vitamin K, folic acid) so that detection of deficiency symptoms in rats requires special measures, and the role of rumen bacteria in ruminant animals (e.g., cows, sheep) in rendering otherwise indigestible cellulose and other materials available to the host animal is well-known. These few examples indicate that syntrophic interrelationships are widespread in nature and may contribute substantially to the nutrition of a wide variety of species.
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]
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.
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 methaneproduces many intermediate compounds such as formate, acetate, CO 2 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]
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:
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]
Syntrophism, mutual dependence of different types of organisms for the satisfaction of their respective nutritional needs. The intermediate or end products of metabolism of one organism are essential materials for another. Syntrophism is exemplified in the mixed population of an ecosystem (q.v.).
Key Points
Methanogenic bacteria are only found in the domain Archea, which are bacteria with no nucleus or other organelles.
Methanogenesis is a form of respiration in which carbon rather than oxygen is used as an electron acceptor.
Bacteria that perform anaerobic fermentation often partner with methanogenic bacteria. During anaerobic fermentation, large organic molecules are broken down into hydrogen and acetic acid, which can be used in methanogenic respiration.
There are other examples of syntrophic relationships between methanogenic bacteria and mircoorganisms: protozoans in the guts of termites break down cellulose and produce hydrogen which can be used in methanogenesis.
Key Terms
Archea: A domain of single-celled microorganisms. These microbes have no cell nucleus or any other membrane-bound organelles within their cells.
syntrophy: A phenomenon where one species lives off the products of another species.
methanogenesis: The generation of methane by anaerobic bacteria.
Syntrophy or cross feeding is when one species lives off the products of another species. A frequently cited example of syntrophy are methanogenic archaea bacteria and their partner bacteria that perform anaerobic fermentation.
Figure: Methanogenic Bacteria in Termites: Methanogenic bacteria have a syntrophic relationship with protozoans living in the guts of termites. The protozoans break down cellulose, releasing H2 which is then used in methanogenesis.
Methanogenesis in microbes is a form of anaerobic respiration, performed by bacteria in the domain Archaea. Unlike other microorganisms, methanogens do not use oxygen to respire; but rather oxygen inhibits the growth of methanogens. In methanogenesis, carbon is used as the terminal electron receptor instead of oxygen. Although there are a variety of potential carbon based compounds that are used as electron receptors, the two best described pathways involve the use of carbon dioxide and acetic acid as terminal electron acceptors.
Acetic Acid: CO2+4H2→CH4+2H2OCO2+4H2→CH4+2H2O
Carbon Dioxide: CH3COOH→CH4+CO2CH3COOH→CH4+CO2
Many methanogenic bacteria that live in close association with bacteria produce fermentation products such as fatty acids longer than two carbon atoms, alcohols longer than one carbon atom, and branched chain and aromatic fatty acids. These products cannot be used in methanogenesis. Partner bacteria of the methanogenic archea therefore process these products. By oxydizing them to acetate, they allow them to be used in methanogenesis.
Methanogenic bacteria are important in the decomposition of biomass in most ecosystems. Only methanogenesis and fermentation can occur in the absence of electron acceptors other than carbon. Fermentation only allows the breakdown of larger organic compounds, and produces small organic compounds that can be used in methanogenesis. The semi-final products of decay (hydrogen, small organics, and carbon dioxide) are then removed by methanogenesis. Without methanogenesis, a great deal of carbon (in the form of fermentation products) would accumulate in anaerobic environments.
Methanogenic archea bacteria can also form associations with other organisms. For example, they may also associate with protozoans living in the guts of termites. The protozoans break down the cellulose consumed by termites, and release hydrogen, which is then used in methanogenesis.
