What is lichen on a rock?

Lichen on a rock is a symbiotic relationship between a fungus and a photosynthetic partner, such as algae or cyanobacteria. The fungus provides structure and protection while the photosynthetic partner produces energy through photosynthesis. Lichen is commonly found on rocks, especially in harsh environments where other plants cannot survive. It can be used as an indicator of air quality, as it is sensitive to pollution. Lichen on a rock comes in a variety of shapes and colors and is an important component of many ecosystems.