Symbionts

Symbiosis (from Greek συμβίωσις "living together", from σύν "together" and βίωσις "living") is any type of a close and long-term biological interaction between two different species, be it mutualistic, commensalistic, or parasitic. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms".

Symbiosis can be obligatory, which means that one or both of the symbionts entirely depend on each other for survival, or facultative (optional) when they can generally live independently.

Symbiosis is also classified by physical attachment; symbiosis in which the organisms have bodily union is called conjunctive symbiosis, and symbiosis in which they are not in union is called disjunctive symbiosis. When one organism lives on another such as mistletoe, it is called ectosymbiosis, or endosymbiosis when one partner lives inside the tissues of another, as in Symbiodinium in corals.

In 1877, Albert Bernhard Frank used the term symbiosis which previously had been used to depict people living together in community to describe the mutualistic relationship in lichens. In 1879, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms." The definition has varied among scientists with some advocating that it should only refer to persistent mutualisms, while others thought it should apply to any type of persistent biological interaction in other words mutualistic, commensalistic, or parasitic.

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