A lichen is not a single organism, but a combination of a fungus and a green alga. Lichens exist in extreme climates, from dry deserts to the icy Arctic. They grow on surfaces such as rock, bark, and soil.
The fungus forms an outer layer that protects the alga beneath from drying out and shields it from harmful amounts of light. The alga makes its own food by photosynthesis, and shares it with the fungus. In return, the fungus supplies the alga with essential minerals, such as nitrogen. This beneficial relationship between two different species is called symbiosis.