“Lichens are made up of two or more different organisms living together, a fungus and an alga. The fungus provides the body (thallus) in which the algal partner can live, protected from damaging conditions such as high levels of light (ultraviolet radiation) and lack of water (drought). The algal partner provides the essential carbohydrates (food for the fungus) from carbon dioxide and water, with the aid of sunlight. This close, interdependent relationship is referred to as a symbiosis.” https://www.opalexplorenature.org/sites/default/files/7/image/AIR%204pp%20chart.pdf
I also learned that some lichens are highly sensitive to air pollution and can be used to detect sources of it!