Common Greenshield (Flavoparmelia caperata). Lichens are composed of two organisms; fungus and algae. The physical shape of the lichen is determined by the fungus and the algae lives within it. Algae produces its own food through photosynthesis. Fungus does not have this ability. Thus the fungus part of the lichen receives nutrients from the algae. There is a debate as to weather or not this is a mutualistic relationship or one of parasitism. This depends on weather or not the algae relies on the fungus for a place to live or if it would do just as well on its own or with another fungus. Common Greenshield is one of the more common species.