Lichens are mutualistic associations of a fungus and an algae or cyanobacterium and occur as crusty patches or bushy growths on trees, rocks and bare ground. Lichens are very sensitive to sulphur dioxide pollution in the air. Since industrialization, many lichen species have become extinct in large areas of lowland Britain, one example being the beard moss Usnea articulata. Lichens are sensitive to sulphur dioxide because their efficient absorption systems result in rapid accumulation of sulphur when exposed to high levels of sulphur dioxide pollution. The algal partner seems to be most affected by the sulphur dioxide chlorophyll is destroyed and photosynthesis is inhibited. Lichen also absorbs sulphur dioxide dissolved in water.

HOW DOES LICHEN HELPS AS A POLLUTION INDICATOR:

Lichen are used as a pollution indicator.In highly polluted region there is absence or presence of algal part only.  If the air is clean, shrubby, hairy and leafy lichens become abundant.Lichen are sensitive to environment.

   Clean air: Bushy lichens 

   small amount of pollution : leafy lichens