Lichen is an organism that is composed of fungus and photosynthetic components. These components interact in a symbiotic relationship. Lichens are often found in the most difficult environments for life on earth. They are also found in many less extreme environments. Most lichens are composed of fungus surrounding algal cells. The fungus benefits the photosynthetic component with its water retention properties. On the other hand, the photosynthetic component produces carbon sugars. These symbiotic properties allow for lichen to survive in many difficult situations. Lichen can be crusty or squishy. The lichen pictured above was found on a rock.