Lichens are composed of fungus and one species of cyanobacterium (blue-green algae) (canalising, for example, Peltigera horizontalis), or algae (finalising, for example, Cetraria islandica) of one kind, called two-component; a lichen consisting of a fungus of the same species, and two types of photobionts (one cyanobacteria and one Alga, but never two of two algae or cyanobacteria) are called ternary (e.g., Stereocaulon alpinum). Algae or cyanobacteria, two-component lichens feed on autotrophic. In ternary lichen feeds on autotrophic algae and cyanobacteria, apparently, feeds on heterotrophic by nitrogen fixation. The fungus feeds on heterotrophic assimilatory partner(s) on symbiosis. Consensus about the possibility of the existence of free-living forms of symbionts is not currently achieved. Had experience of selecting all the components of lichens in culture and the subsequent reconstruction of the original symbiosis.
Of the known species of fungi in the formation of lichens is involved about 20 %, mostly Ascomycetes (~98 %), the rest of the basidiomycetes (~0,4 %), some of them, not having sexual reproduction, which formally belong to deuteromycetes. There are also actinolite in which the place of the fungus is mealanie prokaryotes actinomycetes. Photobiont 85 % is represented by a green Alga, meet 80 species of 30 genera, of which the most important is Trebouxia (included in more than 70% of lichen species). Of the cyanobacteria (10-15 % of lichens) involves representatives from all major groups, in addition to the Oscillatoriales, is the most common Nostoc. Frequent heterocyst forms Nostoc, Scytonema, Calothrix, and Fischerella. In the thalli of lichen cells cyanobiont can structurally and functionally modified to: increase their size, change the shape, reduced the number of carboxysome and the number of cladding materials, slows the growth and division of cells.
Relationship photobiont and mushroom can be described as controlled parasitism by the latter. The contact between the components of a lichen might be different: 1) no direct contact, 2) through the surface, 3) the fungus through haustoria enters the body of the algae. In the relationship of the components is observed a delicate balance, so the cell division of photobiont consistent with the growth of the fungus. Mycobiont receives from photobiont nutrients produced in the photosynthesis. Fungus algae also creates a more optimal microclimate, protects it from drying out, screen from UV radiation, ensure the life on acid substrates (supplying phosphates) mitigates the influence of other adverse factors. Green algae come polyhydric alcohols, such as ribitol, erythritol or sorbitol, which are easily digested by the fungus. Cyanobacteria supply the fungus mainly glucose and nitrogen-containing substances formed due to their ongoing nitrogen fixation. The flow of substances from the fungus in photobiont not detected.