Question

Lichens indicate pollution by
(a)${ O }_{ 3 }$
(b)${ SO }_{ 2 }$
(c)${ NO }_{ 2 }$
(d)CO

Answer 1

The major gases that are responsible for air pollution are the oxides of nitrogen and sulfur. These oxides are also responsible for acid rain. That is because they react with the water present in the atmosphere and result in the release of acidic hydrogen.

Complete answer:
Lichen is formed by the symbiotic association of fungi with algae or cyanobacteria. The algal part is responsible for making food through photosynthesis and the fungal part is responsible for absorbing nutrients. But lichens absorb their nutrients from the air instead of soil. Thus polluted air will result in deterioration of the health of lichens. Lichens are sensitive to high levels of ${ SO }_{ 2 }$ as they absorb more ${ SO }_{ 2 }$ as compared to other plants. Thus a place with high ${ SO }_{ 2 }$ concentration in air will not have lichens living in it.
Lichens are bioindicators for the pollutant ${ SO }_{ 2 }$.

Additional Information: Absorbing excessive ${ SO }_{ 2 }$ can cause the death of the lichen algae, destruction of chlorophyll thus reduction in photosynthesis, stunted growth of lichen fungus, or kill a lichen completely.
So, the correct option is ${ SO }_{ 2 }$.

Note: Lichens and mosses are also called pioneer organisms. It means they are the first organisms to start a community in an uninhabited place such as bare rock. This bare rock is then converted into a lush green forest after many years by a gradual succession known as primary succession.
Sources for the release of ${ SO }_{ 2 }$ and other sulfur oxides are the burning of coal, smelters, oil refineries, etc.
The discoloration of the Taj mahal is also because of ${ SO }_{ 2 }$.