Symbiosis

Symbiosis (from ancient Greek σύν sýn, German 'together', and ancient Greek βίος bíos, German 'life') refers to the association of individuals of two different species that is beneficial to both partners.

Based on his work on lichens, Anton de Bary proposed in 1878 at the 51st Assembly of German Naturalists and Physicians in Kassel to introduce the term symbiosis into biology for any coexistence of organisms of different species, i.e. also for parasitism. In this broad sense, the term symbiosis is still used in US literature for all forms of co-evolutionary coexistence, from mutualism to commensalism, neutralism and parasitism. In Europe, on the other hand, the term symbiosis is used in the narrower sense defined at the beginning.

In symbioses between living organisms that differ considerably in size, the larger partner is often referred to as the host, and the smaller partner as the symbiont.

Frequency of symbioses

Most of the biomass on earth consists of symbiotic systems, as a large proportion of trees and shrubs depend on pollination by other species. In addition, there are lichens, a symbiotic community between a fungus and green algae or cyanobacteria. Many sessile marine invertebrates living in shallow water, such as fire corals, most floral animals, and giant clams, cohabit with photosynthesizing zooxanthellae. Another example is the gastric and intestinal bacteria of animals, which enable the digestion of cellulose-rich plant food in ruminants, for example.

Distinction according to the degree of interdependence

One way of distinguishing between different forms of symbiosis is by the degree of interdependence of the species involved:

• Protocooperation (alliance): Loosest form of symbiosis: Both species derive an advantage from living together, but are equally viable without each other.
• Mutualism: regular but not vital relationship of symbionts.
• Eusymbiosis, also obligate symbiosis (ancient Greek εὖ eu "good, genuine"): In eusymbiosis, the partners are no longer viable on their own. Leafcutter ants, for example, cultivate fungi in their burrow on which they feed; the fungi, in turn, cannot reproduce without the ants.