Metabolic diversity of lichen-forming ascomycetous fungi: culturing, polyketide and shikimate metabolite production, and PKS genes.

Lichens are composite and symbiotic organisms. Biologically, they often have been interpreted as one organism (fungi and algae associated within a common thallus), but taxonomically as a life form of ascomycetous fungi; as the lichen-forming fungus or "mycobiont" has been, in most cases, classified as the dominant symbiotic partner. About 46% of the ascomycota are lichen-forming, however, about 2-3% of the lichen fungi are basidiomycota. Lichen-forming fungi produce a great variety of secondary metabolites, biosynthetically derived from the acetyl polymalonyl, mevalonic and shikimate pathways. Thus, secondary metabolites comprise a significant proportion of the lichen thallus dry weight (0.1-5% or even more). The majority of secondary lichen products are aromatic polyketides, and a number of them has been shown to exhibit marked biological activity.