What is Symbiosis?

A symbiosis is a lasting, close association of two species from which both benefit. A lichen is a fungus housing algal cells that feed it by photosynthesis while it shelters and waters them; a coral is an animal housing algae in its tissues; the roots of most plants are wrapped in fungi that supply minerals in exchange for sugar; the guts of animals hold bacteria that digest what the animal cannot. The association produces organisms, structures and ways of life that neither partner could achieve alone.

Symbiosis is the sustained, intimate association of organisms of different species; it is a mutualism if both gain, a commensalism if one gains and the other is unaffected, and a parasitism if one gains at the other’s expense — and the same pair may move along that line with circumstances. An endosymbiont lives inside its host’s cells; an obligate partner cannot live alone. Symbionts are transmitted vertically, from parent to offspring (the aphid’s Buchnera pass through the egg), or horizontally, acquired anew each generation from the environment or from other hosts (the squid’s Vibrio, the human gut’s bacteria). The microbiome is the whole community of microbes of a habitat — a gut, a leaf surface, a gram of soil — and the holobiont is a host together with its microbiome, considered as the unit that natural selection sees.