The algal component of a lichen is called Phycobiont.
It may be free-living.
They mostly belong to Chlorophyceae (green algae) or cyanobacteria (blue-green algae).
Functions of Phycobiont:
Phycobiont is responsible for providing food for the fungus by undergoing photosynthesis.
Algae contain chlorophyll to prepare food for fungi.
Phycobiont forms simple carbohydrates by photosynthesis.
Phycobiont help in nitrogen fixation i.e. the conversion of inert atmospheric nitrogen or dinitrogen into utilizable compounds of nitrogen. It is performed by the class of phycobionts called cyanobacteria.