Lichen: a composite organism formed by a stable symbiotic association between a fungus (the mycobiont) and one or more photosynthetic partners (the photobiont — usually a green alga or cyanobacterium). The fungus provides the structural matrix of the lichen body (the thallus), protecting the photobiont from desiccation and UV radiation. The photobiont provides photosynthetically fixed carbon to the fungus. When the photobiont is a cyanobacterium, it may also fix atmospheric nitrogen. The thallus is a structure that neither partner produces alone — it is an emergent form arising from the association itself.
Lichens colonize bare rock, tree bark, soil, and man-made surfaces. They are among the first colonizers of new substrates — appearing on fresh lava flows, newly exposed rock faces, and cleared ground — and play important roles in soil formation by slowly breaking down rock through chemical and mechanical weathering. There are roughly 20,000 known lichen species. Most lichen-forming fungi belong to the Ascomycota, though a smaller number are Basidiomycota. Lichens grow slowly, sometimes less than a millimeter per year, but can persist for centuries. They are extremely sensitive to air pollution, particularly sulfur dioxide, making them useful bioindicators of air quality.
The lichen challenges basic categories of biological individuality. It is not one organism, not two, but a relational entity constituted by the association itself. If autopoiesis defines a living system as one that continuously produces its own components and thereby maintains its organization, the lichen poses the question sharply: whose autopoiesis is this? The fungus alone cannot produce the thallus. The alga alone cannot produce the thallus. The thallus is produced by the relation — and the relation produces the conditions for its own continuation. The autopoietic closure belongs to neither partner individually but to the composite.
Recent work has complicated the picture further: many lichens harbor additional fungal partners (basidiomycete yeasts in the cortex), bacterial communities, and other microorganisms, making the lichen less a dual symbiosis than a small ecosystem — a holobiont in miniature. Toby Spribille and colleagues’ 2016 discovery of basidiomycete yeasts embedded in the cortex of macrolichens overturned over a century of lichenology that had described lichens as two-partner systems. This connects to broader questions about what counts as an individual in biology — questions explored in fungal symbiosis and relevant to philosophical discussions of mereology and part-whole relations.
Related terms
- Mycorrhiza — another major fungal symbiosis, with plant roots rather than algae
- Chitin — the structural polymer of the fungal partner’s cell walls
- Fungal Symbiosis — the broader concept of symbiotic relations in fungi
- Holobiont — multi-species assemblages, of which lichens are a vivid case
- Autopoiesis — self-production, whose locus in lichens belongs to the relation rather than either partner
- Fungal Taxonomy — most lichen-forming fungi belong to the Ascomycota
- Saprotroph — a contrasting nutritional strategy (decomposition rather than symbiotic photosynthesis)