A lichen is not a single organism; it is a stable symbiotic association between a fungus and algae and/or cyanobacteria. Lichens can be used as to tell us if there is a high amount of air pollution in and around the area, including all the sulfur dioxide concentration in the atmosphere, as Lichen are sensitive to all the Sulfur Dioxide in the air. Lichens are plants that grow in exposed places such as rocks, tree bark and bare ground. They need to be very good at absorbing water and nutrients to grow there. Impacts of Acid Rain has meant that Lichen on tree bark have been affected and therefore destroying its chlorophyll, resulting in no more Photosynthesis.