Symbiosis refers to a close and sustained biological interaction between organisms of different species. Depending on the nature of the relationship, it can be mutually beneficial, neutral for one partner, or detrimental to one partner.
Explanation
German botanist Anton de Bary popularised the term symbiosis in the nineteenth century to describe long‑term associations between unlike organisms. Symbiotic relationships encompass mutualism, in which both partners derive a benefit; commensalism, in which one organism benefits while the other is neither helped nor harmed; and parasitism, in which one organism benefits at the expense of the other. Symbioses can be obligate, meaning the partners cannot survive without each other, or facultative, in which the association is beneficial but not essential. Many microbes form intimate symbioses with plants, animals and other microbes, exchanging nutrients, metabolites or protection. These interactions drive evolutionary change, influence ecosystem dynamics and, in some cases, gave rise to new cellular structures through endosymbiosis.
Illustrative examples
Leguminous plants form nodules on their roots that house nitrogen‑fixing Rhizobium bacteria; the plant supplies carbon compounds while the bacteria convert atmospheric nitrogen into forms the plant can use. Lichens are mutualistic partnerships between a fungus and a photosynthetic alga or cyanobacterium that together colonise harsh environments. Reef‑building corals depend on intracellular dinoflagellate symbionts (Symbiodinium) that provide photosynthetically derived nutrients, while the coral provides a protected habitat. Ruminant animals, such as cows and deer, rely on gut microbes to ferment cellulose from plant material. In human intestines, a diverse microbiota engages in mostly commensal and mutualistic interactions that influence digestion and immunity. Endosymbiotic events in the deep past led to the evolution of mitochondria and chloroplasts from bacteria. Conversely, parasitic associations such as tapeworms in vertebrate intestines or rust fungi on cereal crops represent symbioses that harm the host.
Symbiosis is a broad concept encompassing the many ways that organisms live together, from mutually beneficial partnerships to exploitative associations, and it underpins the ecology and evolution of life on Earth.
Related Terms: Mutualism, Commensalism, Parasitism, Microbiome, Endosymbiosis.