Lichens are associations between fungi and cyanobacteria (formerly called blue‐green algae). The cyanobacteria are the photosynthetic elements in the association, which is an example of mutualism (a living arrangement where both partners benefit). The photosynthetic organisms provide nutrients for themselves and the fungus, while the fungus provides water and minerals for the photosynthetic metabolism.

Lichens exist in harsh habitats with extreme climatic conditions. They are found on mountaintops, on rock faces in the desert, and on tree bark. Commonly, lichens set the stage for the growth of more complex plants. Lichens can recover from freezing and drying to resume normal metabolic activities. Their growth is so slow that some patches of lichens may be thousands of years old.