Lichens sometimes are composite organisms consisting of a fungus (the mycobiont) and a photosynthetic partner
(the photobiont or phycobiont) growing together in a symbiotic relationship.
The photobiont is usually either a green alga (commonly Trebouxia) or
cyanobacterium (commonly Nostoc). The morphology, physiology and
biochemistry of lichens are very different from those of the isolated fungus and alga in culture.