Lichen

A lichen looks like a single plant-like organism but is really a partnership. Most of its body, the thallus, is made of fungal hyphae, usually from an ascomycete. Held among these threads is a layer of photosynthetic partners: green algae, cyanobacteria, or both. Lichens are named and classified by their fungus, and roughly 20,000 kinds are known.

Both partners gain. The alga or cyanobacterium makes sugars by photosynthesis and shares them with the fungus, and nitrogen-fixing cyanobacteria also supply usable nitrogen. The fungus shields its partner from drying out and from strong light, gathers water and minerals, and anchors the lichen to its surface. This mutualism lets lichens live where few other organisms can, on bare rock, bark, roofs and gravestones, in deserts and in polar tundra. They survive drying out completely and revive soon after rain. Most grow only a few millimeters a year, as flat crusts, leafy sheets or branching, shrub-like tufts.

Lichens help turn bare rock into soil, an early step of ecological succession, and reindeer and caribou graze on them in winter. Many species are very sensitive to air pollution, above all to sulfur and nitrogen compounds, so lichen surveys are used to monitor air quality. The pH indicator litmus is extracted from lichens.