Lichen-forming fungi (also termed lichen mycobionts) are, like plant pathogens or mycorrhizal fungi, a polyphyletic, taxonomically heterogeneous group of nutritional specialists (Tables 3.1 and 3.2) but otherwise normal representatives of their fungal classes. Long after the discovery of the dual nature of lichens by Schwendener (1867; Honegger 2000) and his proposal to include lichens in fungi, most biologists and even the majority of lichenologists considered lichens as a group of organisms that differ so fundamentally from all others that they had to be treated as a separate group, e.g. as a phylum “Lichenes”; this term is nowadays obsolete. Even in the early twenty-first century, many scientists consider lichens as plants, thus ignoring the fact that species names of lichens refer to the fungal partner, fungi forming a separate kingdom. It is the heterotrophic mycobiont of morphologically advanced lichens that mimics plant-like structures. In this chapter the similarities and differences between lichen-forming and nonlichenized fungi are discussed at the phylogenetic, morphological and cytological levels and also with regard to different nutritional strategies.

Lichenized versus nonlichenized fungi

Lichenization: a successful nutritional strategy

Fungi, as heterotrophic organisms, have developed various nutritional strategies for acquiring fixed carbon (Table 3.1). Lichenization, i.e. the acquisition of fixed carbon from a population of minute, living algal and/or cyanobacterial cells, is a common and widespread mode of nutrition. One out of five fungal species is lichenized (Table 3.1). Some lichen-forming fungi belong to orders with uniform nutritional strategies; others belong to orders with diverse strategies (Table 3.1).