Symbiont

A symbiont is an organism that lives in close and long-term association with another organism, the host. Symbiosis covers interactions where the partners live in direct contact or within each other. Relationships are commonly described by their effect on the host: mutualism (both partners benefit), commensalism (the host is unaffected), and parasitism (the symbiont benefits at the host’s expense). Symbiotic partnerships can be obligatory or facultative for one or both parties, and symbionts may reside inside host cells (endosymbionts) or on external surfaces or within body compartments (ectosymbionts).

Mutualistic associations include Buchnera bacteria providing essential amino acids to aphids in exchange for plant sugars, rhizobia forming nitrogen-fixing nodules on legume roots, and mycorrhizal fungi aiding plant nutrient uptake in return for carbon. Photosynthetic symbionts such as zooxanthellae in marine corals and certain lichen partnerships contribute energy to their hosts. Commensal symbionts benefit from habitat or nutrients with little or no effect on the host, while parasitic symbionts exploit host resources and can cause harm.

Some symbionts are intracellular, others are extracellular. Endosymbiosis has played a major role in evolution; the mitochondria and, in plants and algae, chloroplasts, originated from ancient bacterial endosymbionts and are now essential organelles in eukaryotic cells. In contemporary biology, the term symbiont often refers to members of microbiomes—gut, skin, or environmental communities—that influence host development, metabolism, and health. The designation of a partner as symbiont is contextual, reflecting the specific host–symbiont interaction.