Lichen 1. Solo fungal and algal partners

Abstract Lichens are slow-growing niche-constructing organisms that form a thallus via scripted symbiotic/mutualistic relationships between fungi, algae, and bacteria, and that are distributed across nearly all terrestrial ecosystems. Here we use quick-freeze deep-etch electron microscopy (QFDEEM) to probe the ultrastructure of a lichen-forming fungus, Cladonia grayi, and its partner alga, Asterochloris glomerata, grown separately (solo) in the laboratory. The solo fungus resembles its lichenized counterpart in: a) general cellular organization; b) the capacity to form lateral branches; c) the production of extracellular materials; and d) the generation of acellular hyphae that we call struts. It differs in carrying a fibrillar coat on its wall exterior and in often adopting two novel plasma-membrane configurations called pleated and pitted. The solo alga also resembles its lichenized counterpart in general cellular organization, but its algaenan-based wall carries a fibrillar coat not evident in the lichen, and its pyrenoid and plastoglobule endowments are less well developed. These findings are followed by four reports on the QFDEEM ultrastructure of four species of lichens, where we describe the differentiations that occur when the fungi and algae are living together in community with bacteria.