Certain species of lichen are found to possess greenish-yellow algae or in some other cases a brown alga. When some lichens combine both green algae and cyanobacteria as photobionts, the cianobacteria specializes in trapping the atmospheric nitrogen for metabolism. When a fungus (the mycobiont) is associated by symbiosis with a partner of photosynthesis, which is the provider for food from sunlight, lichens are formed.

Quite interestingly, lichens look like any other plant and also grow in the same way. Lichens are essentially shaped like its fungal partner on its exterior which constitutes the maximum part of the lichens. Thus lichens are named in accordance to the fungus. The association of the lichens is still not definitely categorized as mutualism or parasitism, so lichens continue to be associated and classified according to the host plant on which it thrives.

It has been proved that lichen controls the growth of the cyanobacteria. The alga and the fungus of the lichen are mutually dependent on one another. The fungus provides the alga with water and minerals which it uses to prepare nutrients for both of them.