Syntrophy
Encyclopedia
Syntrophy, Cross-feeding, or Cross feeding is the phenomenon that one species lives off the products of another species.

For example house dust mite
House dust mite
The house dust mite is a cosmopolitan guest in human habitation. Dust mites feed on organic detritus such as flakes of shed human skin and flourish in the stable environment of dwellings. House dust mites are a common cause of asthma and allergic symptoms worldwide...

s live off human skin flakes, of which a healthy human being produces about 1 gram
Gram
The gram is a metric system unit of mass....

 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 are the many organisms that feast on faeces or dung. A cow eats a lot of grass, the cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 of which is transformed into lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

s by micro-organisms in the cow's large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...

. 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
Dung beetle
Dung beetles are beetles that feed partly or exclusively on feces. All of these species belong to the superfamily Scarabaeoidea; most of them to the subfamilies Scarabaeinae and Aphodiinae of the family Scarabaeidae. This beetle can also be referred to as the scarab beetle. As most species of...

, 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
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 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
Dynamic energy budget
The Dynamic Energy Budget theory aims to identify simple quantitative rules for the organization of metabolism of individual organisms that can be understood from basic first principles...

theory.
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