Lichens are composite organisms consisting of a fungus and a photosynthetic partner growing together in a symbiotic relationship. The photobiont is usually either a green alga (commonly Trebouxia) or cyanobacterium. The morphology, physiology and biochemistry of lichens are very different from those of the isolated fungus and alga in culture. I love Moonman. Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps.