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Claim check

Is a lichen two organisms — a fungus farming algae?

ExaggeratedSomething true sits underneath, stretched well past what was shown.

The composite idea is right and the count is not. Many lichens carry a second fungus in their outer layer as well as the algae, plus bacterial communities — and "farming" is a Victorian polemic against the discovery, not a description of it.

The claim as it circulates

“A lichen is a fungus and an alga living together, with the fungus providing shelter and effectively farming the algae for the sugars they make.”

Where you may have met it: Field guides and school biology; Popular natural-history writing; Museum and nature-reserve interpretation panels

Taking it apart

What was claimed
That a lichen consists of exactly two organisms, a fungus and an alga, with the fungus cultivating the alga for the products of its photosynthesis.
What was actually observed
Microscopy in 1869 established that the green cells within lichen tissue correspond to free-living algae enclosed by fungal filaments — a composite, not a single organism. Sequencing much later found basidiomycete yeasts embedded in the cortex of many macrolichens across lineages and continents, in species whose composition had been considered settled. Variable bacterial communities are also present, and some lichens carry cyanobacteria in addition to or instead of green algae.
What the evidence supports
That a lichen is a fungus that has built a body around photosynthetic partners, with further organisms living in that body. The number varies between species and within them, which is why counting is not the useful question.
What it does not support
It does not make the two-partner account an error. It was a genuine and hard-won discovery, rejected for years by specialists, and it is incomplete rather than wrong. Nor is the role of the additional fungal partner established: its presence is far better documented than its function, and nothing here says what it does.

The farming metaphor has a specific history worth knowing. When the composite nature of lichens was proposed, it was resisted hard — a described species was supposed to be one organism — and one of the phrases deployed against the idea was that it turned the fungus into a farmer keeping algae as slaves. The framing was coined to make the hypothesis sound ridiculous. It lost, and then survived as the standard way of explaining the thing it was invented to attack.

The rest of the answer

  • Lichen
  • Symbiosis

    Three cross-kingdom partnerships compared

  • Fungi

    What the partner building the body actually is

The claims underneath

Each one carries its own evidence, scope and caveats. Expand any of them to reach the studies.

A lichen is a fungus with photosynthetic partners inside it — and usually more than two organisms, since many carry a second fungus in the outer layer as well. "One organism or two" was the wrong question.

Well supported

Good evidence backs this, though some details remain open.

Macrolichens comprise an ascomycete host, one or more photosynthetic partners (green algae, cyanobacteria, or both), and in many species basidiomycete yeasts embedded in the cortex, alongside variable bacterial communities. Partner composition varies within a single lichen species.

Who this applies to
Established for many macrolichens; the number and identity of partners vary between and within species.
Studied in
Ascomycota, Basidiomycota, Chlorophyta, Cyanobacteria
Why we rate it this way, and what the caveats are
Well supportedHigh confidence

Sequencing and localisation evidence across many lichen species, replicated since.

How far it can be extended

The additional fungal partner was found across many macrolichen lineages on multiple continents.

Caveats

  • The two-partner account was a real discovery, not an error; it was incomplete rather than wrong.
  • How much the additional partners contribute varies and is not established for most species.

Still unanswered

  • What the basidiomycete partner does, which is not established — its presence is far better documented than its function.

Last reviewed 2026-09-04

The evidence (2 studies)

A lichen is not one fungus and one alga — many have a third organism too

Well supported

Good evidence backs this, though some details remain open.

Metatranscriptome analysis of macrolichens revealed basidiomycete yeasts embedded in the cortex, present across many lichen taxa on six continents, with abundance tracking chemical and morphological differences between lichens previously indistinguishable by fungal and algal markers.

Who this applies to
macrolichens, with the decisive case in two Bryoria species
Studied in
Bryoria fremontii, Bryoria tortuosa
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

Presence and distribution are firmly established by sequencing and imaging. The functional role is inferred from the correlation with chemistry rather than demonstrated, because the three-partner system is hard to culture.

How far it can be extended

The yeasts were subsequently found across a wide sample of macrolichen taxa on six continents, though not in all lichens.

Caveats

  • The yeast’s function is inferred, not demonstrated.
  • Not present in all lichens.
  • Lichens may involve bacteria and additional fungi as well, so "three" is likely also provisional.

Still unanswered

  • What does the yeast actually do?
  • How many partners does a lichen have, once bacteria are counted?

Last reviewed 2026-08-09

The evidence (1 study)

Go deeper

Lichens

No roots, no waterproofing. That is why a lichen can live on bare rock — and why it tells you what the air has been like.

Related things people ask

Last reviewed 2026-09-04