Lichens are an intimate symbiosis, in which two species live together as a type of composite organism. Lichens are an obligate mutualism between a fungus mycobiont and an alga or blue-green bacterium phycobiont.

Each lichen mutualism is highly distinctive, and can be identified on the basis of its size, shape, color, and biochemistry. Even though lichens are not true "species" in the conventional meaning of the word, lichenologists have developed systematic and taxonomic treatments of these mutualisms.

The fungal partner in the lichen mutualism gains important benefits through access to photosynthetic products of the alga or blue-green bacterium. The phycobiont profits from the availability of a relatively moist and protected habitat, and greater access to inorganic nutrients.

The most common fungi in lichens are usually species of Ascomycetes, or a few Basidiomycetes. The usual algal partners are either species of green algae Chlorophyta or blue-green bacteria of the...