Marine Animal Encyclopedia
Gray Lichen Pyrenocollema halodytes
Seen on hard, calcareous rocks, where it forms small, black-brown patches, gray lichen is unusual in being an association of three organisms—a fungus, a cyanobacteria, and an alga. The fungus anchors the gray lichen to the rock; the cyanobacteria and the alga contain chlorophyll and make food by photosynthesis. The cyanobacteria can also utilize nitrogen, a process that uses a lot of energy, and this comes from the sugar made during photosynthesis.