Lichen 4. The Algal Layer

A lichen is a slow-growing niche-constructing organism that forms a thallus via scripted symbiotic/mutualist relationships between fungi, algae, and bacteria. Here we use quick-freeze deep-etch electron microscopy (QFDEEM) and light microscopy to probe a hallmark lichen construction wherein clusters of algae and hyphae, inter-connected by wall-to-wall junctions, form stable consortia that we call green modules. These assemble in the pseudo-meristem and then localize to the algal layer of the thallus. In the foliose lobes of Candelaria concolor, the cells in each module are enveloped in a continuous 10-nm-thick film of hydrophobin proteins, which binds to wall and matrix surfaces on its hydrophilic side and faces air or water on its hydrophobic side. We document patterned relationships between modules and associated cords of hyphae destined for the outer layers, and propose ways that these relationships could form the structural foundation for water and air regulation within foliose lobes.