Syntrophy, Cross-feeding, or Cross feeding 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. This term for nutritional interdependence is often used in microbiology to describe this symbiotic relationship between bacterial species.

For example house dust mites live 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 dioxygen 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.

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. Finally, anaerobic fermentation/methanogenesis is an example of a syntrophic relationship between different groups of microorganism. Although fermentative bacteria are not strictly dependent on syntrophyic relationships, they still gain profit from the activities of the hydrogen-scavenging organisms, as the fermentative bacteria gain maximum energy yield when protons are used as electron acceptor with concurrent H2 production (Dolfing, 1988; Schink, 1997). Also, acetogenic bacteria and methanogenic archea are the two groups of microorganisms living in syntrophy during the methanogenesis. 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) (Schink, 1997; Stams et al., 2005).

Syntrophic interactions are very important in all living communities, and are important to the Dynamic Energy Budget theory.

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... transfer takes place among different species), and the syntrophy hypothesis ... via an ectosymbiotic process based on a similar syntrophy of sulfur existing between the two types of bacteria – Desulphobacter and Spirochaeta ... The syntrophy hypothesis was developed in contrast to the hydrogen hypothesis and proposes the existence of two symbiotic events ...
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