Definition
A lichen is a stable mutualistic association between a filamentous fungus (the mycobiont) and one or more photosynthetic partners (the photobiont), which may be green algae or cyanobacteria. Together they form a composite organism distinct from either partner alone.
Explanation
Lichens arise when a fungal partner envelops and penetrates colonies of algae or cyanobacteria, creating a thallus with distinct morphological types: crustose forms adhere tightly to substrates, foliose forms are leaf like and lobed, and fruticose forms are branched and shrubby. The fungus provides structure, water retention and protection, while the photobiont performs photosynthesis or nitrogen fixation and supplies organic carbon. Most lichen fungi are ascomycetes, with a minority basidiomycetes; common photobionts belong to the genera Trebouxia and Nostoc. Lichens colonize harsh environments such as rocks, bark, tundra and deserts, tolerating extreme desiccation and temperature fluctuations by entering anhydrobiosis. They reproduce via fragmentation, soredia (clusters of hyphae and algal cells) or sexual spores released by the fungus that must later encounter a compatible photobiont. Lichens contribute to soil formation by secreting organic acids that weather minerals, and many fix atmospheric nitrogen. They are sensitive to air pollutants, making them bioindicators of environmental quality. Lichen substances such as usnic acid have antibiotic properties and have been used historically for dyes, perfumes and traditional medicines.
Ecological and applied aspects
• Reindeer moss (Cladonia rangiferina), a fruticose lichen, is a major winter food source for reindeer and caribou.
• Crustose lichens like Rhizocarpon geographicum colonize exposed rocks, creating concentric growth rings used in lichenometry to date geological surfaces.
• Cyanobacterial lichens such as Peltigera contribute to nitrogen input in boreal forests through symbiotic nitrogen fixation.
• Litmus, a pH indicator, is extracted from species of Roccella and Lasallia and has been used since medieval times.
• Lichens are sensitive to sulfur dioxide and heavy metals and decline in polluted urban areas, so surveys of lichen diversity are used to monitor air quality.
Lichens exemplify a successful symbiosis between kingdoms, allowing partners to inhabit ecological niches neither could occupy alone. Their diverse forms, resilience and ecological roles make them important components of many ecosystems.
Related Terms: symbiosis, mycobiont, photobiont, mutualism, thallus