Lichen is a composite thalloid structure of algae & fungi.
The algal part of Lichen is called phycobiont and the fungal part is called mycobiont. Both algae & fungi remain in symbiotic association i.e. they are beneficial for each other.
Explanation:
Lichens are good indicators of air pollution – Lichens are sensitive towards (sulphur dioxide). They die in more concentration of SO2. Therefore they are not found in those areas (polluted areas) where the concentration of SO2 is high.