Bioindicators are living organisms that respond in a specific way to variations in the environment. The hardy lichens are beneficial bioindicators of air pollution. Lichens are a successful association between a fungus and an alga, each undertaking what it does best, and thriving as an outcome of natural teamwork. They live as one organism, both occupying the same body! Fungal mycelium contributes in absorbing atmospheric moisture for algal photosynthesis and delivers protection to the alga from intense light and UV. Algae can photosynthetically produce organic matter. They live on surfaces of bark, rock, soil and several substrates. Lichens depend on atmospheric moisture, rain, fog and dew for growth and development. Lichens do not harm the substrates they are attached to. They are also sensitive to air pollution. Pollutants in the air or dissolved in atmospheric water are harmful to lichens, and they are differentially sensitive to air pollution. For example, Fruticose: The most sensitive, Foliose: The second and Crustose: The most resistant.
There are two different approaches to use lichens as bioindicators for air pollution: a. Community changes (slow response): changes in species composition at different levels of pollution. b. Physiological changes (fast response): changes in chlorophyll content and photosynthesis at various pollution levels. So, compared with most physical/chemical monitors, lichens are economical to use in measuring air pollution.