symbiose

Symbiose, or symbiosis in English, refers to a close and long-lasting interaction between two or more different biological species. Such associations may involve exchange of nutrients, protection, or other benefits, and they can occur at the level of organisms or cells. The concept emerged in the 19th century as scientists studied lichens and other intimate relationships, and the term derives from Greek roots meaning "living together." In French, the term is symbiose; in English, symbiosis.

Symbioses are commonly categorized into mutualism, commensalism, and parasitism. In mutualism, both partners benefit; in commensalism, one benefits while the other is unaffected; in parasitism, one benefits at the expense of the other. Many interactions are context-dependent and can shift along these categories; some relationships are obligate (partners depend on each other) while others are facultative.

Prominent examples include lichens (a symbiosis between a fungus and photosynthetic algae or cyanobacteria), mycorrhizal associations between fungi and plant roots that aid nutrient uptake, nitrogen-fixing bacteria (Rhizobium) in legume nodules, and coral-algae symbioses that support reef ecosystems. In animals and humans, gut microbiota form complex mutualistic communities essential to digestion and immune function.

A key evolutionary example is endosymbiosis, in which one organism lives inside another; this process is thought to have given rise to mitochondria and chloroplasts in eukaryotic cells. The study of symbiosis informs ecology, evolution, and medicine by highlighting interdependence in life on Earth.