A lichen is more than the classic pair

The definition of a lichen as a symbiosis between one ascomycete fungus and a photosynthesising partner had held since the 1870s. In 2016 Toby Spribille's team published a result in Science that widened it: the cortex of many common lichens also contains specific basidiomycete yeasts. Not as incidental contamination, but as a consistent, repeatable component.

The starting point was a puzzle that had been set aside. Two lichens in the genus Bryoria look different and differ chemically, although they share the same ascomycete and the same algal partner. Sequencing the whole sample showed that what separates them is the abundance of basidiomycete yeasts. Those yeast cells sit embedded in the cortical layer — precisely where the shape and surface of the thallus are formed.

The scale of the phenomenon turned out to be global. Yeast lineages maintain close associations with particular lichen species across large geographical distances and have been found on six continents. The cortex, described in textbooks as a layer of differentiated cells of a single ascomycete, consistently contains two unrelated fungi at once.

"Three" is a convenient shorthand rather than a new canonical number. A lichen also hosts bacteria, algae from other lineages and further fungi, so the right conclusion is not to replace two with three but to stop quoting a single number at all. Spribille's paper changed something other than the arithmetic: it showed that a trait treated as a property of the fungal species can come from a partner nobody had been looking for.

It is worth asking why the yeasts stayed hidden for so long. Classical mycology described a lichen by culturing the isolated ascomycete and by examining a section of the thallus under a microscope. In such a section the yeasts are single cells embedded in a gelatinous cortical layer, and they do not look like a separate organism. Only reading the whole genetic material of the thallus, without first assuming how many partners to expect, made them visible.

That pattern recurs in mycology. The method decides what can be found, and the limits of the method end up written into the textbook as properties of nature. Environmental sequencing has repeatedly revealed organisms nobody could culture — and each time the question has had to be asked whether the definition was merely describing what happened to be visible at the time.

What the evidence actually shows

The study did not show that every lichen consists of exactly three organisms. Spribille and colleagues detected basidiomycete yeasts in the class Cystobasidiomycetes within the cortex of many macrolichens, alongside the familiar ascomycete fungus and photobiont. In the pairs studied, yeast abundance correlated with phenotype and vulpinic-acid content, while in-situ hybridization located yeast cells embedded in the thallus cortex. This expanded the classic two-partner model but did not replace it with a universal “one plus one plus one” formula. A thallus may contain multiple photobionts, bacteria, and additional fungi, and the importance of each partner varies among lichen species. A lichen is best described as a stable symbiosis whose dominant mycobiont builds the structure within a broader microbiome—not as a single organism with an invariably fixed number of components.

Another partner, further questions

In 2019, Veera Tuovinen and colleagues detected Tremella fungi in 95 percent of the studied Letharia thalli. Yeast cells also occurred without visible fruiting bodies of the additional fungus. Its hyphae contacted algal cells, challenging the automatic treatment of such inhabitants solely as fungal parasites. The finding concerned the sampled Letharia. It neither established a fixed composition for every lichen nor resolved all the detected partner's functions.

For a reader, three questions remain separate: was an organism detected, where are its cells, and what do they do within the association? A headline announcing another inhabitant may answer only some of these. Look for the method and the organisms studied before turning “detected” into “essential”. This is a practical way to follow new discoveries without writing a new rigid definition after each result. It also helps explain why finding an additional organism can open a research question rather than close it.

What a walk can reveal

Without laboratory equipment, start with the thallus form. Crustose lichens adhere closely to their substrate; foliose lichens have flattened, often lobed parts; fruticose lichens form three-dimensional branches, upright or hanging. These are useful descriptions, not species names. In some lichens, visible discs are apothecia, structures where the fungal partner produces spores. Distinguish the whole thallus from the individual fruiting body of a cap-and-stem mushroom.

Try describing one thallus beside a path without removing it. Keep a photograph showing the substrate and a closer view of the surface. Record which parts could be examined and whether discs, branches or small granular structures were visible. You do not have to name every structure on the spot. A description and a photograph let you return to the question later, when you can compare your evidence with a reference.

Fictional example: “Grey, foliose thallus on bark, with some edges lifting away; underside not visible. Several brown discs on the surface. Species undetermined.” This note lets another person reconstruct the visible subject. It cannot count its microscopic partners. That boundary is precisely what connects a simple field observation with the questions investigated in the laboratory.