Option 1: Lichen. Lichen is a symbiotic organism composed of a fungus and a photosynthetic partner (usually an alga or a cyanobacterium). Lichen has been widely used as a bioindicator to monitor air pollution. It is sensitive to air pollutants such as sulfur dioxide, nitrogen oxides, and heavy metals. When exposed to high levels of air pollution, lichen can show changes in color, size, and growth, or it may even die off. By studying the health and diversity of lichen populations in a particular area, scientists can assess the levels of air pollution and its impact on the environment. Lichen monitoring helps to identify areas with high pollution levels and can guide efforts to minimize air pollution and protect human health.