Lichens are composite, symbiotic organisms made up from members of as many as three biological kingdoms . The lichen fungi cultivate partners that manufacture food by photosynthesis. Sometimes the partners are algae, other times cyanobacterium, formerly called blue-green algae. Some enterprising fungi exploit both at once.
The alga or cyanobacterium converts sunlight and carbon dioxide to food for the lichen fungus and in return the lichen fungus protects the alga/cyanobacterium from drying out. The fungus obtains water and minerals from the air and the material it is growing on. The alga provides carbohydrates and vitamins. Some blue-green algae fix nitrogen that is used by both the alga and the fungus. Nitrogen is also obtained from bird excrement, organic debris, or plant leachate . It has been said that lichens are fungi that have discovered agriculture.
Lichens are found on rocks, tortoise shells, window panes, and plants. As plants are stressed and begin to decline, the reduced canopy allows sunlight to enter and support photosynthesis for the lichen. The presence of lichens are often an indicator of poor plant health but it is never the cause. Lichens are harmless to plants and, if overall plant health is improved, the vibrant canopy should inhibit any sunlight available for lichen photosynthesis.