Symbioses

Symbiosis is a close and long-term interaction between two different organisms. Interactions can be categorized by their effect on the participants’ fitness: mutualism, where both benefit; commensalism, where one benefits and the other is unaffected; and parasitism, where one benefits at the expense of the other. These relationships can be obligate or facultative. Obligate symbioses require the partners for survival, while facultative symbioses are optional.

Symbioses occur across all domains of life and can be endosymbiotic (one partner inside the other, as in organelles) or ectosymbiotic (on the surface). Examples: lichens are a mutualism between fungi and photosynthetic algae or cyanobacteria; mycorrhizal fungi form mutualistic associations with plant roots, aiding nutrient uptake in exchange for carbohydrates; nitrogen-fixing rhizobia live in root nodules of legumes. Coral reefs rely on endosymbiotic algae (zooxanthellae) that supply energy through photosynthesis.

Commensal associations include many gut or surface bacteria on animals where the host is unaffected. Parasitism includes a wide range of organisms, from tapeworms in intestines to parasitoid wasps that develop inside hosts; parasites may cause disease or reduce host fitness, sometimes with complex life cycles.

Symbiotic relationships have shaped evolution, exemplified by symbiogenesis, the idea that the integration of formerly independent organisms gave rise to new cell compartments, such as mitochondria and chloroplasts. In modern ecosystems, symbioses influence nutrition, protection, reproduction, and ecosystem productivity, and many relationships are context-dependent, changing with environment and life stage.