The fungus concentrates its energy and resources on adding hyphal length rather than girth. The fungus usually gives the lichen its overall shape and structure, and tissues formed by hyphae account for most of the lichen's mass The alga always provides the fungus with food. Cyanobacteria in lichens fix nitrogen and provide organic nitrogen. The fungus provides the alga with a suitable physical environment for growth. Lichens absorb most of the minerals they need either from air or in the form of dust Fungal pigment shades the algae from intense sunlight.
Some fungal compounds are toxic and prevent lichens from being eaten by consumers .
Lichens are important pioneers on newly cleared rock and soil surfaces, such as burned forests and volcanic flows. Physical penetration of the outer crystals of rocks and chemical attack of rock by lichen acids help break down the rock and establish soil-trapping lichens. This process makes it possible for a succession of plants to grow. Nitrogen-fixing lichens also add organic nitrogen to some ecosystems.