Lichen
A lichen is an organism typically found growing on tree bark or standing stones. Lichens result from a symbiotic relationship between a fungus and a green algae or cyanobacterium. The fungi collect water and minerals from the air and the colonized surface, while the algae/cyanobacterium contribute photosynthates and energy. Lichens are not parasites; they grow on the surface of a tree, stone or other host without causing it any harm. They are found in wide variety of environmental conditions attached to almost any kind of surface. Lichens tend to grow very slowly but are highly adaptable to environmental stresses and can be very long-lived, with some considered to be among the oldest living organisms.
In environmental science, lichens are well known as an indicator of certain types of air pollution. Lichens exhibit the ability to concentrate nutrients from very dilute sources and indiscriminately absorb many toxic substances from the atmosphere. Many lichen species are highly susceptible to air pollution, especially to pollution by sulfur dioxide.
The sulfur in sulfur dioxide damages the chlorophyll in lichen, and with a sufficiently high sulfur level the photobiont (the photosynthesis-capable partner of the organism) will die, thereby bringing about the death of the fungal component as it is incapable of surviving alone. Different species show varying levels of sensitivity to pollutants, and by noting the species occurring in an area and their state of health it is possible to monitor pollutant levels. The idea of using lichens as pollution monitors was systematically developed in 1866 by the Finnish lichenologist W. Nylander, as a result of his studies of the lichens near Paris, France.