Symbiosis describes close interactions between two or more different species. It is different from regular interactions between species, because in a symbiotic relationship, the two species in the relationship live together. Many organisms are involved in symbiotic relationships because this interaction provides benefits to both species. However, there are types of symbiosis that are not beneficial and may in fact harm one or both of the species.
lichens is one of the symbiotic life form which grow as the slow growing  cloured patches on rocks,bark of tree trunks,and even on the ground. in lichens,blue green algae or cynobacteria and fungi live in symboisis.The algal component of the lichen is known as phycobiont and the fungal component is known as mycobiont.
they occurs in hostile habitats such as barren rocks,walls,tree trunkand icy regions.they are sensitive to air pollution.