Basically Lichen are a mutualistic combination (called Symbiosis) of 4 Organisms, Fungus (Caseomycetes), Algae (Trebouxia/Trentepohulia), Bacteria (Nostoc) and Yeast. There are 3 types, Crustose (flat), Foliose (leafy) and Fruticose (long tubular). The fungus provides the structure, but can't produce food, the Algae, through photosynthesis supplies the food and the yeast provides the CO2 for the photosynthesis. One big happy family, until it comes to reproduction. That's very complicated. Lichen are found on every part of the planet, hot or cold. They are very slow growing, but can survive anything. Here are images of some of the different types. Because of their toxicity to each other, they sometimes form black toxic barrier lines around each specie.