Lichen 2. Constituents

Lichens are slow-growing niche-constructing organisms that form a thallus via scripted symbiotic/mutualist relationships between fungi, algae, and bacteria. Here we use quick-freeze deep-etch electron microscopy (QFDEEM) to document the ultrastructure of the eukaryotic organisms and the extracellular matrix materials that are found in four lichens. Previous studies have shown that each thallus in a complex lichen consists of a central medullary layer containing aligned fungal hyphae. These medullary hyphae send lateral branches to an upper layer, some making contact with resident algae, and usually to a lower layer. As the thallus ages, such "regular" branches transform into acellular versions called "struts" and "honeycombs." We conclude with a consideration of two materials that are synthesized and secreted by lichenized fungi: abundant secondary products/metabolites that often crystallize, and hydrophobin proteins that self-assemble into films.