What Is a Lichen?

Lichens are not a single organism but a mutualistic symbiosis: the fungal partner (mycobiont) provides structure, mineral uptake, and protection from UV and desiccation, while the photosynthetic partner (photobiont) provides sugars through photosynthesis. Recent research has revealed that many lichens also include a third partner — a basidiomycete yeast embedded in the cortex. There are roughly 20,000 known lichen species, colonising nearly every terrestrial habitat from Antarctic rocks to tropical tree bark. Lichens are pioneer organisms that can grow on bare rock, slowly dissolving it with organic acids and beginning soil formation. They are extremely sensitive to air pollution (especially sulfur dioxide), making them valuable bioindicators of air quality. Some lichens grow less than 1 mm per year and can be thousands of years old — a specimen of Rhizocarpon geographicum in the Swedish Lapland was estimated at over 8,600 years.