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Ecologyecological relationship

Symbiosis

Symbiosis never meant "both benefit". A tapeworm is a symbiont.

Different species living closely together — and that is all the word means. It does not say both benefit. Parasitism is a symbiosis; so is a lichen; so is a tapeworm. What varies is the outcome, and the outcome often depends on conditions.

The word has been narrowed in popular use to mean a partnership in which both sides gain, and the narrowing costs more than it looks. When symbiosis means mutual benefit, "parasitic symbiosis" reads as a contradiction rather than as a standard category, and every close association acquires an unearned warmth. The original sense, from the lecture that introduced the term in 1879, is simply the living together of differently named organisms — with parasitism named as one of the cases it covers. What symbiosis actually describes is intimacy, not outcome: two organisms bound closely enough and long enough that each is part of the other’s circumstances. Sort by outcome and you get mutualism, commensalism and parasitism. Sort by location and you get endosymbiosis, where one partner lives inside the other’s cells or body, and ectosymbiosis, where it lives on the surface. Sort by dependence and you get obligate relationships, where neither partner can complete its life alone, and facultative ones, where they can. Those three sorts cut across each other, which is why "symbiosis" on its own tells a reader remarkably little. The most interesting recent finding about symbiosis concerns how they start. The assumption has always been that a relationship this close must be built over a long shared history. Then somebody put a yeast and an alga together in conditions where each lacked what the other released — the alga short of carbon, the yeast short of nitrogen — and the two organisms, which had no relationship of any kind, became mutually dependent inside the experiment. It happened readily, and with several other pairings. That does not explain how any real lichen arose. It does suggest the starting price is a great deal lower than anyone assumed: not a long courtship, but an environment in which one organism’s waste is another’s requirement.

Early coverage · 75% complete · reviewed 2026-09-04

What this page covers

Symbioses occur across every kingdom and the most consequential ones cross kingdoms: fungi with algae, fungi with plant roots, animals with photosynthetic cells, almost everything with bacteria.

Often confused with: Mutualism, which is one outcome of symbiosis rather than the whole of it; Cooperation, which implies shared interests the participants do not have; A fixed relationship, when the outcome often shifts with conditions

Quick facts

The word means living together
Outcome unspecified — parasitism included
Three ways to sort them
By outcome, by location, by dependence — and they cut across each other
They can start fast
A yeast and an alga became dependent inside one experiment
And they can shift
The same partnership can be beneficial or costly depending on conditions

A word that does not mean what it is used to mean

Living together. That is the whole definition.

Symbiosis means organisms of different species living closely together. It does not mean both benefit — parasitism is a kind of symbiosis, and always has been in the technical sense.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Symbiosis, as introduced by de Bary in 1879, denotes the persistent close association of differently named organisms irrespective of outcome, encompassing mutualism, commensalism and parasitism. The restricted popular usage equating symbiosis with mutual benefit is a later narrowing.

Who this applies to
The term as used across biology.
Studied in
Animalia, Plantae, Fungi, Bacteria
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

The original definition is explicit and the broad sense remains standard in the technical literature.

How far it can be extended

A definitional matter applying wherever the term is used.

Caveats

  • Some biologists do use symbiosis in the narrow mutualistic sense, and the disagreement is genuine rather than a simple popular error.
  • The broad sense still requires the association to be close and persistent; two species merely occurring together are not symbiotic.

Still unanswered

  • Where the boundary lies between a close association and mere co-occurrence, which is a matter of judgement rather than a defined threshold.

Last reviewed 2026-09-04

The evidence (1 study)

  • Supports · primary

    Die Erscheinung der Symbiose

    de Bary, 1879 · Karl J. Trübner

    The definition, which includes parasitism as one case among several.

Three ways of sorting symbioses, none of which is the others
Sorted byThe categoriesExample
OutcomeMutualism, commensalism, parasitismA lichen, a barnacle on a whale, a tapeworm
LocationEndosymbiosis (inside), ectosymbiosis (on the surface)Coral symbionts inside cells; cleaner fish outside
DependenceObligate (neither can live alone), facultative (both can)Lichen fungi mostly obligate; many pollinators facultative

Diagram

Three ways to sort a symbiosis

And a relationship has a position on all three.

Three ways to sort a symbiosis — and they cut across each otherBy outcomeMutualismCommensalismParasitismBy locationInside the cellsInside the bodyOn the surfaceBy dependenceObligate for bothObligate for oneOptional for bothA relationship has a position on all three rows, so the bare word says almost nothing.
The same explanation in words

Three rows, each a different way of sorting. By outcome: mutualism, commensalism, parasitism. By location: inside the cells, inside the body, on the surface. By dependence: obligate for both, obligate for one, optional for both. A closing note records that a relationship has a position on every row, which is why the bare word symbiosis says almost nothing on its own.

The three sorts are independent, which is why the bare word carries so little information. A relationship can be endosymbiotic and parasitic, or ectosymbiotic and mutualistic, or obligate for one partner and optional for the other. Saying two species are symbiotic tells a reader that they live closely together and nothing else at all.

Three cross-kingdom partnerships, compared honestly

Where the partner sits, what moves, and what breaks it.

Lichen, mycorrhiza and coral: the same idea, three architectures
AspectLichenMycorrhizaCoral
WhoFungus + alga and/or cyanobacteria, plus other fungiFungus + plant rootAnimal + photosynthetic dinoflagellates
Where the partner sitsAlgal cells enclosed within fungal tissueFungal hyphae around or inside root cellsInside the animal’s own cells
What movesSugars out of the alga; water, minerals and shelter from the fungusSugars from plant; phosphorus, nitrogen and water from fungusPhotosynthetic products to the animal; nitrogen and shelter to the symbiont
What breaks itProlonged drying, pollution, loss of the surface it grows onHigh soil nutrients — the plant stops needing the fungusHeat stress, which expels the symbionts
Can it be reversed?Usually not; the partners rarely live apartYes — the balance shifts with soil conditionsYes — recolonisation is possible if the animal survives

Diagram

Lichen, mycorrhiza, coral: one idea, three architectures

Compared by where the partner sits and what ends the relationship.

Lichen, mycorrhiza, coral: one idea, three architecturesLichenMycorrhizaCoralPartner sitsInside fungal tissueAround or in root cellsInside animal cellsWhat movesSugars out; shelter inSugars out; phosphorus iSugars out; nitrogen inWhat breaks itDrying, pollutionRich soil — plant stops Heat stressReversible?RarelyYes, with conditionsYes, if the animal surviThe outlined row is the one that defeats a teamwork reading: conditions set the terms.
The same explanation in words

Three partnerships compared across four rows. Where the partner sits: inside fungal tissue for a lichen, around or within root cells for a mycorrhiza, inside the animal’s own cells for a coral. What moves: sugars out and shelter in for the lichen, sugars out and phosphorus in for the mycorrhiza, sugars out and nitrogen in for the coral. What breaks it — the row marked out as the important one: drying and pollution for the lichen, rich soil for the mycorrhiza because the plant stops needing the fungus, and heat stress for the coral. Whether it is reversible: rarely for a lichen, yes with changed conditions for a mycorrhiza, yes for a coral if the animal survives. A note records that the third row is what defeats a teamwork reading, because conditions set the terms.

The bottom two rows are where the "teamwork" framing falls apart. In two of these three, the relationship’s value to each partner depends on the surrounding conditions, and in one of them abundant soil nutrients turn a helpful fungus into a cost the plant would rather not carry. These are not friendships that occasionally fail. They are exchanges whose terms are set by the environment.

Each in full

  • Lichen

    The one where the partners built a body together

  • Mycorrhizal networks

    Where the exchange is a market with sanctions

  • Mutualism

    The outcome category, and why it is not cooperation

How does something like this even begin?

Cheaper than anyone thought.

A yeast and an alga with no shared history were grown where each lacked what the other released. They became mutually dependent inside the experiment — no long coevolution required to start.

Well supported

Good evidence backs this, though some details remain open.

Co-culture of Saccharomyces cerevisiae and Chlamydomonas reinhardtii under conditions of reciprocal nutrient limitation produced mutualistic growth via exchange of carbon dioxide and nitrogenous compounds, where neither grew alone. The interaction arose readily and recurred across other fungal and algal pairings.

Who this applies to
Laboratory co-culture under engineered nutrient conditions; not a claim about how any natural association arose.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Fungi, Chlorophyta
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

A clean and replicated experimental result. Its bearing on how real lichens originated is an inference, and a loose one.

How far it can be extended

The conditions were designed to make the exchange necessary, and nothing shows such conditions are common in nature.

Caveats

  • A facultative laboratory association is not a lichen; lichens involve structural integration this does not produce.
  • It shows the starting conditions are reachable, not that this is how any particular symbiosis began.

Still unanswered

  • Whether environments imposing this kind of reciprocal limitation are common enough to seed such associations in nature.

Last reviewed 2026-09-04

The evidence (1 study)

How we know

Building a partnership that never existed

Close mutualisms are assumed to need a long shared history. How much of one does it actually take to get started?

A yeast and a green alga with no natural association were placed together in conditions of reciprocal shortage: the alga without a usable carbon source, the yeast without usable nitrogen, and gas exchange between them possible. Growth of each was compared against growing alone and across a range of nutrient conditions, and the design was then repeated with other fungal and algal species.

What happened

The two organisms grew together where neither grew alone, exchanging carbon dioxide and nitrogenous compounds, and the same relationship formed readily across several other fungal and algal pairings.

What it shows

That the entry price for this kind of mutualism is lower than anyone assumed. No shared evolutionary history was required — only an environment in which each organism’s waste was the other’s requirement. It reframes the origin question: instead of asking how partners evolved to help each other, one can ask what conditions make helping unavoidable.

What it does not show

The conditions were engineered to force the exchange, and nothing here shows such conditions are common in nature. A facultative laboratory association is also not a lichen: it lacks the structural integration, the shared body and the joint dispersal that make a lichen what it is. This is about how such relationships can start, not about how any real one did.

The controls — what makes this evidence rather than a story
  • Each organism grown alone in the same conditions, so joint growth is measured against the alternative of no growth at all.
  • Nutrient conditions varied, establishing that the interaction depends on the shortage rather than on the pairing.
  • Additional species pairings, distinguishing a general capacity from a quirk of two laboratory strains.

From Niche engineering demonstrates a latent capacity for fungal-algal mutualism

It is worth being careful about what this licenses. The experiment does not show that lichens began this way, and a facultative exchange between two laboratory organisms is not a lichen — there is no shared body, no joint dispersal, no structural integration. What it does is remove an assumption: that the first step towards a close partnership is difficult. Given an environment in which each organism needs what the other discards, the first step appears to be nearly free.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

12 studies, newest first. Each one has a page explaining what it found and what it could not show.

2017Current Biology

The origin and diversification of mitochondria

Mitochondria descend from a bacterial lineage, retain only a small remnant genome, and have transferred most ancestral genes to the host nucleus.

2017Nature

Global warming and recurrent mass bleaching of corals

Bleaching severity tracked accumulated heat exposure.

2016Science

Basidiomycete yeasts in the cortex of ascomycete macrolichens

A basidiomycete yeast was present in the outer layer of both lichens, in abundances tracking their chemical difference, and turned out to be widespread across macrolichens on six continents.

2014Science

Niche engineering demonstrates a latent capacity for fungal-algal mutualism

Under those conditions the two organisms grew together where neither grew alone, exchanging carbon dioxide and nitrogenous compounds.

2013Nature Reviews Microbiology

Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants

Nodulation proceeds through reciprocal chemical signalling and a controlled infection process, using a signalling pathway shared in part with the much older mycorrhizal symbiosis.

2008Science

Breakdown of an ant-plant mutualism follows the loss of large herbivores

With large herbivores excluded, trees reduced their production of swollen thorns and nectar.

2006Proceedings of the Royal Society B: Biological Sciences

The role of zooxanthellae in the thermal tolerance of corals: a nugget of hope for coral reefs in an era of climate change

Colonies hosting a more heat-tolerant symbiont type withstood higher temperatures than colonies of the same coral species hosting a less tolerant type, and symbiont composition shifted following transplantation.

2001Ecology Letters

The exploitation of mutualisms

Exploitation is present in most well-studied mutualisms, by partners and by outsiders, and mutualisms nevertheless persist.

1991Biogeochemistry

Nitrogen limitation on land and in the sea: how can it occur?

Nitrogen limitation persists because fixation is energetically expensive, requires other resources including phosphorus, iron and molybdenum that are themselves often scarce, and because nitrogen is readily lost from ecosystems in gaseous and dissolved forms.

1967Journal of Theoretical Biology

On the origin of mitosing cells

The features of mitochondria and chloroplasts are consistent with descent from engulfed bacteria rather than with origin from within the host cell.

1879Karl J. Trübner

Die Erscheinung der Symbiose

Associations between unlike organisms form a continuum of outcomes, and the fact of living together does not determine which outcome applies.

1869Schultze

Die Algentypen der Flechtengonidien

The green cells inside a lichen correspond to free-living algal genera, and the lichen body consists of a fungus enclosing algal cells rather than a single organism producing its own green tissue.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 75% completeness against what we would call a finished subject, and was last reviewed on 2026-09-04. It carries 9 claims and answers 1 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • Gut and skin microbiomes, the most consequential animal symbioses, are not covered here.
  • Insect endosymbionts such as Buchnera and Wolbachia are not treated.
  • Commensalism is named in the sorting table and covered on its own page rather than here.