symbios

Symbios is the long-term interaction between different biological species living in direct contact. The term is widely used in biology and ecology; in some contexts, "symbios" is used as a shorthand for symbiosis, though the standard term is symbiosis. Relationships can be mutually beneficial, neutral for one partner, or detrimental for the other, leading to the classic categories of mutualism, commensalism, and parasitism.

Mutualism describes interactions in which both partners benefit, such as mycorrhizal fungi associated with plant roots, which aid nutrient uptake, or gut microbes that synthesize essential vitamins for their host. Commensalism involves one partner gaining benefit while the other is largely unaffected, examples including certain barnacles attaching to whales. Parasitism occurs when one organism benefits at the expense of the other, as with many intestinal worms or external parasites. Some interactions can shift along a continuum depending on environmental conditions or the partners involved.

Key examples and mechanisms include lichens, which are partnerships between a fungus and a photosynthetic partner (an alga or cyanobacterium); endosymbiosis, where bacteria live inside host cells and give rise to organelles such as mitochondria and chloroplasts; and gut symbionts in various animals that aid digestion, nutrient processing, or defense.

Symbiosis has profound implications for evolution, ecology, and health. It drives coevolution, shapes ecosystem functioning, and informs practices in agriculture, medicine, and conservation, where manipulating symbiotic associations can enhance growth, resilience, or disease resistance.