The Hidden Lives of Lichens: Two Kingdoms, One Body

The lichen is the original challenge to the idea that an organism is a single species. Walk into a forest, look at a fallen log, and the gray-green crust on the bark is a lichen. The crust is two organisms living so tightly entwined that for centuries even biologists thought it was one. The fungus provides the body and the protection from desiccation. The alga (or in some cases a cyanobacterium) does photosynthesis and provides the sugars. Neither can live as a lichen alone. Together they form a stable composite that has been evolving as a unit for over 400 million years.

This is a useful object lesson in how the species concept fails to handle the world's actual diversity. The lichen is not a metaphor. It is something the biology textbooks of 1860 had no category for, and even now, with the category invented, the boundaries keep moving.

Schwendener's heresy

The dual nature of lichens was first proposed in 1867 by Simon Schwendener, a Swiss botanist who suggested the green cells inside lichen tissue were a separately-living alga, captive within a fungus. The lichen was, in Schwendener's framing, not an organism at all but a relationship.

The proposal was treated as a moral failing. Schwendener was accused of denying the unity of life. The leading lichenologist of the era, James Crombie, called the dual hypothesis "a marriage between a captive Algerian damsel and a tyrant fungal master." The metaphor was unkind to both partners and revealing about the era's discomfort: composite organisms violated a tacit assumption that species were neat units evolution worked on, not committees.

By 1900 the dual hypothesis was confirmed beyond doubt. Each lichen species could be traced to a specific fungus and a specific photosynthetic partner; the two could be separated in the laboratory and grown as independent cultures (with difficulty); resyntheses had been performed. The lichen was a relationship.

What the relationship looks like

The fungus, called the mycobiont, makes up roughly 90% of the lichen body. It forms the visible structure: the leaflike thallus, the crustose patches, the upright fruticose forms. The fungal hyphae weave a structural matrix and form a protective outer layer (cortex) that controls water loss and UV exposure.

The photobiont, embedded in a layer just below the cortex, photosynthesizes and donates a fraction of its sugars to the fungus. The donation is not voluntary in any meaningful sense; the fungus secretes enzymes and chemicals that increase the photobiont's permeability and channel sugars across the membrane. Whether this is mutualism, controlled parasitism, or something in between is genuinely debated. Different lichens behave differently.

The arrangement allows lichens to live in places neither partner could survive alone. They colonize bare rock, the underside of glaciers, the fringe of deserts, the canopy of cloud forests. The fungus alone needs sugars (and would die without an external source); the alga alone needs constant moisture and is intolerant of UV (and would die in most lichen habitats). The composite endures freezing, drying, and direct sun for years and resumes metabolism within minutes of rehydration.

The third partner

The dual hypothesis was clean: one fungus, one photobiont, end of story. In 2016, a team led by Toby Spribille at the University of Montana looked at why two seemingly identical lichen species (one toxic, one not) had different chemistry despite the same fungal and algal partners. They found a third organism. Each lichen contained a previously-unnoticed yeast embedded in the cortex. The toxic species had one yeast; the non-toxic species had another. The yeast was contributing significantly to the chemistry that distinguished them.

The Spribille discovery has since been replicated across many lichen lineages. Lichens are routinely tripartite: one ascomycete fungus (the dominant partner), one photosynthetic partner, and at least one basidiomycete yeast playing a role that ranges from clear (chemical defense) to mysterious (presence with no known function).

Some lichens are quartite or quintic. Cyanobacteria show up alongside algae, contributing nitrogen fixation. The microbiome of a lichen, when sequenced, reveals dozens of bacterial species in stable association. The "lichen organism" is genuinely a community, and the discovery of new partners has been a steady drumbeat for the last decade.

What this does to the species concept

Lichens are named, in formal taxonomy, after their dominant fungus. Cladonia rangiferina, the reindeer lichen, is technically the name of the fungus. The alga gets a different name (often Trebouxia) and is not part of the lichen species name despite being half the organism.

This convention papers over a genuine problem. If a lichen species is its fungus, then a fungus that pairs with two different algae would be one species and a fungus that pairs with the same alga as another, distinct fungus would be two species. Both situations are real. Some "species" are actually fungi capable of pairing with several photobionts; some "species" share a photobiont with relatives but are differentiated by other partners or by their fungal genome.

The species concept assumes you can ask "is this organism the same kind as that one?" and get a yes-or-no answer. For most familiar life, this works approximately. For lichens, it requires a choice: which of the partners do you mean? The answer changes which "species" you find.

Old and slow

Lichens are among the slowest-growing organisms on Earth. Crustose lichens on rock advance their margins at a rate measured in tenths of a millimeter per year. The patches become datable: a lichen of a known species, ten centimeters across, has been there for a century or more. Lichenometry is a real dating technique used for glacial moraines, archaeological sites, and earthquake-deformed surfaces.

The slow growth is paired with extreme longevity. Individual lichens have been carbon-dated to over 5000 years old, comparable to the oldest bristlecone pines. They survive what would kill the rapidly-growing partners separately because the relationship is so finely tuned to local stress. Energetic minimalism is a strategy.

What is lost

Lichens are sensitive to air pollution in a way that has made them indicator species. Sulfur dioxide poisons the photobiont; nitrogen oxides shift the competitive balance among lichen species; heavy metals accumulate. The lichen flora of a city in 1920 is now found only in the surrounding countryside. The lichen flora of a forest near a coal plant in 1970 is in some cases not found anywhere within fifty kilometers today.

The British Lichen Society maintains a record of distribution that doubles as a long-term air quality monitor. Comparing maps from 1900, 1970, and 2020 shows the rise of acid rain, the partial recovery after sulfur emissions were reduced, and the new pressure from nitrogen pollution. The lichens are recording, the way snowflakes record meteorology, what the air has been doing.

The lesson runs through everything written about composite organisms: the unit of selection is not always one genome. The lichen, the human-microbiome composite, the eukaryotic cell with its mitochondria, the social insect colony, are all cases where evolution has produced something that behaves like an organism but contains many. The species concept is a useful approximation that is wrong at every interesting boundary, and lichens have been making this point quietly for half a billion years.