Give scientific reasons: Lichens are good pollution indicators.

(N/A) Lichens are symbiotic associations between algae (phycobiont) and fungi (mycobiont).
They are considered excellent indicators of air pollution,particularly sulfur dioxide $(SO_2)$ pollution,for the following reasons:
$1$. Lichens do not have roots and absorb nutrients and water directly from the atmosphere through their surface.
$2$. They lack a protective cuticle or stomatal regulation,making them highly sensitive to the accumulation of toxic gases and heavy metals present in the air.
$3$. In areas with high levels of $SO_2$ pollution,lichens fail to grow or die,as $SO_2$ interferes with their chlorophyll and metabolic processes.
$4$. Their absence in a specific area serves as a biological marker for high air pollution levels.