A lichen (pronounced “LIE-ken”) is a remarkable living partnership between a fungus and an alga (or sometimes a bacterium called a cyanobacterium) that work together as one organism. The fungus provides structure and protection, like a house, while the alga makes food through photosynthesis, like a cook. Neither partner could survive alone in the harsh places where lichens thrive.

Lichens grow on rocks, tree bark, and even bare ground in some of Earth's toughest environments: Arctic tundra, scorching deserts, and high mountain peaks. They can survive being frozen solid, then spring back to life when conditions improve. Some lichens are thousands of years old, growing just a millimeter or two each year.

You've probably seen lichens without realizing it. Those crusty gray-green patches on rocks or the frilly orange-yellow growths on tree branches are lichens. They come in many colors and forms: some look like tiny leaves, others like crusty paint splattered on stones, and still others like delicate miniature forests.

Lichens are extremely sensitive to air pollution, so scientists use them as natural pollution detectors. Where air is clean, lichens flourish. Where pollution is heavy, they disappear. This makes them valuable indicators of environmental health, silently reporting on the quality of the air we all breathe.