Briefly explain Lichens organism?

Lichens are unique organisms consisting of a symbiotic relationship between a fungus and a photosynthetic organism. They represent a type of symbiotic relationship known as mutualism, whereby both partners in the relationship benefit from living together, without adversely affecting each other. The fungus is usually an ascomycete, and the photosynthetic partner is often a blue-green alga or a green alga.

Lichens are able to grow in harsh conditions, and therefore are often "pioneers," the first to inhabit newly formed substrate. They can be found on bare rocks in cold and drought conditions. The fungal partner provides the algal partner with a tough substrate that collects water and minerals from the atmosphere while the algal partners contribute their photosynthetic products to the fungus. Lichens, therefore are also very sensitive to air pollution, and can be used as convenient indicators of air quality.