Lichen 5. Medullary and bacterial biofilm layers

Abstract A lichen is a slow-growing niche-constructing organism that form a thallus via scripted symbiotic/mutualist relationships between fungi, algae, and bacteria. In preceding papers we have presented quick-freeze deep-etch EM (QFDEEM) images of the algal and outer layers of squamulose, foliose and fruticose lichens. Here we examine the remaining layers, one occupied by medullary fungal hyphae, and the others including a stunning variety of bacterial biofilms and novel extracellular matrix (ECM) materials. We document that the medullary compartment is filled with an ECM wherein fungal hyphae and secondary metabolites, many crystalline, are suspended in a ground substance that we call fog. We propose that fog is a liquid-glass-like mixture of secondary metabolites (synthesized by the fungi), polyols (synthesized by the algae), and polyol-sequestered water. Bacteria are described in several contexts. In the outer cortical layers of foliose Candelaria and Physcia lobes, they form patchy biofilm islands atop the fungal walls and the polysaccharide-based ECM. In the inner surface of Cladonia podetia they form long heterospecific biofilms at the lichen/external water boundary and at the fog boundary, reinforcing the intrinsic fog/water phase separation and preventing the fog from leaking out. In the outer layer of Usnea fibrils and the inner cortex of foliose lobes, they join fungi and extracellular materials to form surface boundaries. Hence lichenized bacteria not only participate in metabolic exchange but also serve architectural roles in lichen construction and maintenance.