A lichen is an organism whose growth, form and reproduction are utterly dependent upon the intimate association between a fungus and a green alga or cyanobacterium. In this partnership, the fungus apparently is dependent upon its algae or cyanobacteria, for lichen fungi are not found in nature without their partners. However, the algae and cyanobacteria that occur in lichens also occur in the free-living state. About 25,000 species of lichen fungi are known, but the majority of their photosynthetic partners are members of only two genera of algae and one genus of cyanobacteria.

A lichen fungus obtains nutrients from its alga or cyanobacterium. When a cyanobacterium such as Nostoc is present, it performs nitrogen fixation and transfers its products to its fungus. If one experimentally isolates lichen algae or cyanobacteria, they grow more rapidly as isolates than they do in the lichen association, suggesting the fungus is in some sense parasitic on the photosynthetic cells. However, the fungus may protect algae from desiccation or other environmental threats. Examine the lichens in the laboratory.