Lichens are symbiotic associations of a fungus with a photosynthetic partner (photobiont) that can produce food for the lichen from sunlight.
The photobiont is usually either green algae or cyanobacteria.
A few lichens are known to contain yellow-green algae or, in one case, a brown alga.
Some lichens contain both green algae and cyanobacteria as photobionts; in these cases, the cyanobacteria symbiont component may specialize in fixing atmospheric nitrogen for metabolic use.
The body of most lichens is quite different from that of either the fungus or alga growing separately, and may strikingly resemble simple plants in form and growth.