When a fungus and algae (or autotrophic bacteria) work together,
they form a lichen. Lichens are known for their ability to survive
harsh environments. In a lichen, strands of fungal mycelium
surround, and support the host algae (or bacterium), and secrete a
substance that forces the flow of nutrients into the fungus from
the autotrophe. This results in a mutual symbiotic relationship,
because the fungus doesn't have to get its own food, and the algae
(or bacterium) is protected from predators, the elements, and it is
provided a place where it normally wouldn't be able to grow (trees,
rocks, roofs, hydrothermal springs, and siding).