According to published data, 147 algal species can be lichenized with asco- and basidiomycetes. Most of these algal species belonged to the Chlorophyta (108 species) and Cyanoprokaryota (36). Two different lichenization types can be distinguished (facultative and obligate). Most photobionts are facultative and can therefore also occur free-living in terrestrial habitats. In contrast, some species of the genera Trebouxia Puym., Asterochloris Tscherm.-Woess, Coccomyxa Schmidle, and Pseudococcomyxa Korschikov are only known as lichen photobionts; however, if these species are obligate photobionts, is still under controversial discussion. The morphology and life cycle of algae changed during the process of lichenization, whereas ultrastructure of algal and fungal components had not any considerable changes. The co-evolution of both partners in the lichen association led to physiological adaptations of both mycobiont and photobiont and result that both function as single organism. It is known that the photobiont is responsible for the water exchange and the transfer of carbohydrates within the lichen association whereas the mycobiont forms the typical algal-hyphae connection and regulates the synthesis of secondary metabolites.

Keywords: lichens, photobiont, mycobiont, symbiosis, co-evolution

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