A lichen is an association between one or two fungus species and an alga or cyanobacterium (blue-green alga) that results in a form distinct from the symbionts. Although lichens appear to be single plantlike organisms, under a microscope the associations are seen to consist of millions of cells of algae (called the phycobiont) woven into a matrix formed of the filaments of the fungi (called the mycobiont). Many mycobionts are placed in a single group of Ascomycota called the Lecanoromycetes, which are characterized by an open, often button-shaped fruit called an apothecium. 

Although lichens had long been assumed to consist of a single fungus species and a single phycobiont, research suggests that many macrolichens also feature specific basidiomycete yeasts in the cortex of the organism. There are various types of phycobionts, though half the lichen associations contain species of Trebouxia, a single-celled green alga. There are about 15 species of cyanobacteria that act as the photobiont in lichen associations, including some members of the genera Calothrix, Gloeocapsa, and Nostoc.

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