Surface interactions in lichens

Abstract

A considerable number of algal species inhabit a quite specialised ecological niche: the thallus of lichen-forming fungi. About 21% of fungi live as lichens in an ecologically obligate symbiosis with algal and/or cyanobacterial photobionts (Hawksworth 1988). More than 85% of lichen-forming fungi associate with unicellular or filamentous green algae, and about 10% with heterocystforming cyanobacteria, which provide photosynthates and, in the latter case, fixed nitrogen as well. About 3–4% of lichen mycobionts, the so-called cephalodiate species, associate with green and blue-green photobionts simultaneously (Tschermak-Woess 1988). In contrast to the genetically uniform thalli of multicellular algae, lichen thalli represent genetically heterogenous systems; the exact number of participants being unknown since several fungal and algal or cyanobacterial genotypes, originating from different sexual or asexual propagules, can and often do, participate in the formation of individual thalli (Jahns 1988). The species name given to lichens refers to the mycobiont (Art. 13.1d in Greuter 1988). Several investigators conclude from biogeographical studies that some ascomycetous lichen families might have arisen in palaeozoic times (summary: Hawksworth 1988). However, fossil records are largely missing because most lichen thalli contain no degradation-resistant biopolymers.