Syntrophy, Synthrophy, 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". This term for nutritional interdependence is often used in microbiology to describe this symbiotic relationship between bacterial species. Morris et al. have described the process as "obligately mutualistic metabolism".
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. These microbes are vital, for instance for the digestive and the immune system to function.
An example of syntrophy is that of the house dust mite that lives off human skin flakes, of which a healthy human being produces about 1 gram per day. These mites can also produce chemicals that stimulate the production of skin flakes, and people can become allergic to these compounds.
Another example is the many organisms that feast on faeces or dung. A cow eats a lot 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, feast on it.
The defining feature of ruminants, such as cows and goats, is a stomach called a rumen which 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. 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)
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 interactions are very important in all living communities, and are important to the Dynamic Energy Budget theory.
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