Map lichen (Rhizocarpon geographica) growing on a granite gravestone. This type of lichen is known as saxicolous lichen as it grows on or among rocks. A lichen is a mutualistic organism comprising an alga and a fungus that live in a mutually-beneficial relationship. The alga produces carbohydrates for the fungus by photosynthesis, and the fungus protects the alga from harsh conditions. Lichens are highly sensitive to pollution: several are used as indicator species for sulphur dioxide levels in the atmosphere.