What can lichens tell us about atmospheric pollution?

Subject: Environment

Section: Ecosystem

Context:

During 2000s, in Kodaikanal, the protests were triggered by a thermometer factory near Kodaikanal Lake, owned by Hindustan Unilever, which was found to be polluting the environment with mercury through improper waste disposal.
This pollution affected the soil, water, and air in the area. The factory’s illegal activities were exposed, leading to penalties.
A key evidence in the mercury poisoning case was the discovery of mercury in lichens, a plant-like substance, around the factory.

What are lichens?

Lichens are unique organisms often seen on tree barks and rocks in moist environments, contributing to the beauty of landscapes like the Himalayas with their varied colours.
They are formed from a mutualistic relationship between a fungus and a photosynthetic partner (algae or cyanobacteria), as described by the British Lichen Society.
The fungus provides a protective cover, while the photosynthetic partner contributes organic compounds through photosynthesis.
Lichens thrive in various environments, from mountains to Arctic tundras, and absorb water and nutrients directly from the atmosphere.
This direct absorption makes them sensitive to air pollution, leading to damage and bleaching.
India is home to about 3,000 lichen species, some with medicinal properties such as antibacterial, anti-inflammatory, and anti-cancer effects.
A notable example is Parmotrema perlatum (black stone flower), used by tribals for medicinal purposes and as a flavour enhancer in Indian cuisine, including biryani.

Why are lichens considered good biomonitors?

Lichens are resilient to pollution and are used as biomonitors and bioindicators due to their sensitivity to environmental changes.
They are essential for monitoring air quality and ecological health. Scientists use both living and non-living entities to assess environmental quality and pollution levels.
Bioindicators are organisms that provide information about environmental quality, while biomonitors also give quantitative data about environmental conditions. For example, fish are excellent bioindicators for aquatic ecosystems, revealing the health of these systems through their tissues, blood, and structural anomalies.
However, not all bioindicators are biomonitors. While fish can indicate the health of an environment, they don’t provide quantitative data on pollutant levels. In contrast, lichens and mosses are effective biomonitors because they can both indicate environmental quality and be analyzed to measure pollution levels.

How are lichens studied in the scientific world?

Scientists assess environmental damage to lichens using parameters like photosynthesis, chlorophyll content and degradation, ATP decrease, ethylene production, and membrane damage.
Lichens have shown a positive correlation between their sulfur content and atmospheric sulfur dioxide levels. Exposure to vehicular and urban emissions also alters their chlorophyll concentration.
Lichens are used to analyze atmospheric conditions and can estimate the age of the surfaces they colonize.
Studies on lichens are also crucial for understanding their response to global warming. A 2022 study suggests that lichens might struggle to adapt to a one-degree Celsius temperature increase within a million years.
While lichens are diverse in India, they are primarily studied for academic purposes. In contrast, European countries integrate lichen studies into air pollution policy documents.