Chapter 16: Lichenized fungi (chiefly Hymenoascomycetes: Lecanorales)

Lichens represent a sophisticated symbiotic partnership between a fungal component (mycobiont) and a photosynthetic partner, either a green alga or cyanobacterium (photobiont), producing a composite organism capable of thriving in extraordinarily harsh environments ranging from polar regions to arid deserts. The estimated thirteen thousand five hundred described lichen species demonstrate remarkable physiological adaptations enabling survival under desiccation, intense ultraviolet radiation, and nutrient-limited conditions. Common photobiont partners include Trebouxia and Trentepohlia algae, while Nostoc represents the predominant cyanobacterial associate. Lichens function as pioneer colonizers on barren rock and soil substrates, actively contributing to pedogenesis through the secretion of organic acids like oxalic acid. The three primary thallus morphologies, crustose, foliose, and fruticose, reflect adaptations to different microhabitats and environmental pressures. Vegetative propagation occurs through soredia and isidia structures, dispersed by wind or invertebrate vectors, while sexual reproduction follows ascomycete patterns with ascus production in various fruit body types. The symbiotic relationship depends on precise biochemical signaling and carbohydrate allocation, wherein algal photobionts export polyols while cyanobacteria provide fixed nitrogen compounds. Fungal pigments including usnic acid and vulpinic acid, along with distinctive lichen acids derived from polyketide biosynthesis, offer photoprotection and antimicrobial properties. Lichens serve dual roles as sensitive indicators of atmospheric pollution, particularly sulfur dioxide concentrations, and as tools for lichenometry dating geological surfaces. Representative genera including Lecanora, Xanthoria, Peltigera, and Cladonia exemplify the ecological and cultural significance of lichens, from food sources to environmental monitoring organisms.