Symbiosis is any close, long-term interaction between two different species. The term was coined in 1879 by the German botanist Heinrich Anton de Bary, who defined it simply as “the living together of unlike organisms.” That definition is deliberately broad: symbiosis covers relationships where both species benefit, where only one benefits, and even where one is actively harmed. What ties them together is intimacy and duration, not whether the arrangement is friendly.
The Main Types of Symbiosis
Biologists sort symbiotic relationships along a spectrum based on who benefits and who pays a cost. The three major categories are mutualism, commensalism, and parasitism, but the boundaries between them can blur depending on conditions.
Mutualism is a relationship that benefits both species. The partnership between flowering plants and their pollinators is a classic case: the plant gets its pollen carried to another flower, and the pollinator gets food. Mutualism can be obligate, meaning neither species can survive without the other, or facultative, meaning both benefit but could get by on their own.
Commensalism benefits one species while leaving the other unaffected. Barnacles hitching a ride on a whale gain access to nutrient-rich water currents as the whale swims. The whale, meanwhile, is neither helped nor harmed in any measurable way.
Parasitism benefits one species at the expense of the other. Ticks feeding on a deer, tapeworms living in a mammal’s gut, and mistletoe drawing water from a host tree are all parasitic. The parasite gains resources; the host loses them.
A less commonly discussed category is amensalism, where one species is harmed while the other is completely unaffected. Amensalism works through two basic modes. The first is competition, where a larger or stronger organism simply crowds out a smaller one. The second is antibiosis, where one organism produces a chemical that damages or kills another without gaining anything from the interaction. The bread mold Penicillium, for example, secretes a substance (penicillin) that kills nearby bacteria. Black walnut trees release a compound called juglone into the soil that destroys many smaller plants growing within their root zone. In both cases, the organism doing the damage isn’t responding to the victim; it’s simply going about its life.
Symbiosis That Shaped All Complex Life
Perhaps the most profound example of symbiosis in the history of life is the origin of the energy-producing structures inside your own cells. Endosymbiotic theory, which goes back over a century, proposes that mitochondria and chloroplasts were once free-living bacteria that took up residence inside larger cells. Over billions of years, these guests became permanent residents, losing the ability to live independently while providing their hosts with energy. Mitochondria power virtually all animal and plant cells through oxygen-based metabolism. Chloroplasts do the same for plants and algae, capturing sunlight and converting it to chemical energy.
The strongest evidence for this theory comes from the protein import machinery these organelles use. Mitochondria and chloroplasts still carry their own small genomes, separate from the DNA in the cell’s nucleus, and their gene sequences closely resemble those of free-living bacteria. This single evolutionary event, a bacterium living inside another cell and never leaving, gave rise to all complex life on Earth.
Mycorrhizal Fungi and Plant Roots
More than 80% of land plants form partnerships with a type of soil fungus called arbuscular mycorrhizal fungi. The fungus threads through the soil and extends far beyond the reach of the plant’s own roots, absorbing phosphate and nitrogen and delivering them to the plant. In return, the plant feeds the fungus organic carbon in the form of carbohydrates and fatty acids. Neither partner could thrive as well alone. Plants in phosphorus-poor soils are especially dependent on this exchange, and many ecosystems would collapse without it.
Lichens: Two Organisms That Look Like One
Lichens are what you get when a fungus and a photosynthetic organism, either an alga or a cyanobacterium, form such a tight partnership that they look and function like a single organism. The fungal partner provides physical structure, shelter, and minerals. The photosynthetic partner captures carbon dioxide from the air and converts it into organic carbon through photosynthesis, feeding both itself and the fungus. If the photosynthetic partner is a cyanobacterium, it also fixes atmospheric nitrogen, adding another nutrient to the partnership. Lichens colonize bare rock, tree bark, and arctic tundra, surviving in places neither partner could manage alone.
Coral Reefs and Their Algae
Coral reefs exist because of a mutualism between coral animals and tiny photosynthetic algae called Symbiodinium that live inside coral cells. The algae convert sunlight and carbon dioxide into organic carbon and oxygen, which fuel the coral’s growth and its ability to build a calcium carbonate skeleton. The oxygen produced as a byproduct may also help corals calcify at their maximum rate. In return, corals give the algae a safe, sunlit home and supply essential nutrients in the otherwise nutrient-poor tropical ocean.
This partnership is also a vivid example of how fragile symbiosis can be. When water temperatures rise even slightly over a prolonged period, especially during calm, clear weather, the algae produce harmful reactive oxygen molecules that damage proteins, fats, and DNA inside coral cells. The coral responds by expelling its algae, turning white in a process called coral bleaching. Without the algae, the coral loses its primary energy source and can die if conditions don’t return to normal quickly.
Your Gut as a Symbiotic Ecosystem
The trillions of microorganisms in your digestive tract form one of the most important symbiotic systems in human biology. These microbes sit at the surface of your intestinal lining and perform functions your body cannot handle on its own. They break down certain dietary fibers, produce vitamins, and help regulate your immune system. The relationship spans the full spectrum from mutualism, where both you and the microbes benefit, to commensalism, where certain microbes gain a stable habitat without noticeably affecting you.
Your gut microbes also protect you through a process called colonization resistance. They prevent harmful bacteria from gaining a foothold in three ways: by producing substances that directly inhibit pathogen growth, by consuming nutrients before invaders can use them, and by stimulating both your innate and adaptive immune responses. Early-life disruptions to this microbial community may alter immune development. One hypothesis, sometimes called the microflora hypothesis, suggests that reduced microbial exposure in childhood can shift the immune system toward allergic hypersensitivity.
How Symbiosis Drives Co-evolution
When two species depend on each other over long periods, they often shape each other’s evolution. This process, called co-evolution, creates reciprocal adaptations: changes in one species that are mirrored by complementary changes in the other. Over time, this can produce remarkable specificity, where a host and its symbiont are so finely tuned to each other that neither works as well with a different partner. The resulting divergence across populations means that the same host species in different regions may rely on genetically distinct symbionts.
Co-evolution also leaves molecular fingerprints. Partner species develop complementary genomes and patterns of gene expression that only activate during the symbiotic interaction. This reciprocal selection has allowed hosts across a wide range of symbioses to adapt to diverse conditions by maximizing the benefits they obtain from their partners. The process can move in the other direction too: what starts as a loose, facultative interaction can evolve into an obligate interdependency where neither species can survive alone.
Ecto- and Endosymbiosis
Symbiotic relationships also vary by where the partners physically sit relative to each other. In ectosymbiosis, one organism lives on the surface of another. Cleaner fish that pick parasites off larger fish are ectosymbionts. In endosymbiosis, one organism lives entirely inside the cells or body of another. The algae inside coral tissue and the mitochondria inside your cells are both endosymbionts. Endosymbiotic relationships tend to be more tightly integrated and more likely to become obligate over evolutionary time, because the internal partner loses genes it no longer needs when the host provides those functions.