Biochemical Diversity and Ecology of Lichen-Forming Fungi: Lichen Substances, Chemosyndromic Variation and Origin of Polyketide-Type Metabolites (Biosynthetic Pathways)

Abstract

Many organisms containing high-value compounds are difficult to culture or are becoming endangered or even extinct by over-harvesting. Lichens, in general, are slow-growing organisms and the extraction of the naturally grown, composite thalli, in many cases is economically not feasible and profitable which may also be very limited. Mycobiont cultures are an attractive alternative to the extraction of naturally grown thalli. In Europe, the laboratory at the University of Salzburg has established a worldwide recognized unique culture collection of c. 150 different mycobionts. The modulation (“regulative manipulation”) of growth conditions of microorganism and fungi is a common strategy used in biotechnology and applied microbiology to improve yields and diversity of secondary metabolites of therapeutic value. Interest in polyketide-type metabolites is considerable, as many of these natural products are of medical, and industrial, and/or agricultural importance involve polyketide synthase (PKS) pathway. In case studies, growth and culture conditions have been modulated and optimized culture conditions have been adopted to obtain increased biomass production for several selected mycobionts (e.g., Roccella decipiens, species of the genus Xanthoparmelia) by adopting particular environmental conditions in one of the culture chambers. In recent investigations, by exploring further possibilities to optimize culture conditions and biomass production, it turned out that axenically cultured mycobionts can be triggered to produce single or a whole pattern of secondary metabolites. Polyketides, as has been demonstrated, are only biosynthesized under “permissive” ecological conditions. By using the knowledge from preliminary investigations and doing further extensive test series, it was possible to achieve the production of one particular polyketide and even the production of a predictable pattern of polyketides, depending upon the investigated lichen chemotypes. Such studies could also help to elucidate the often observed variation in secondary products (chemosyndromic variation) within a lichen population growing under heterogeneous environmental conditions. Variations in chemistry actually mirror physiological, ecological, and even evolutionary responses to change in the environment and climate. The repeatable and in vitro production of higher quantities of lichen metabolites in fungal cell cultures has already and could further become a milestone elucidating the architecture and function of PKS genes that are involved in polyketide and “still unknown” genes that control other metabolic pathways, e.g., shikimate and pulvinic acid compound production. In a novel and holistic approach, functional genomics could be used to understand the molecular mechanisms that are involved in the desiccation tolerance of some lichens. By constructing a cDNA library from selected species of Xanthoparmelia and its cultured mycobiont, it could be of high interest to perform transcriptome sampling which could be used to detect and identify further “new genes” responsible for the control of desiccation resistance in lichens.