A lichen is a complex symbiotic combination of a fungus and one or more photosynthetic organisms, typically a green algae and/or cyanobacteria but also sometimes a yeast. The fungus forms the structural body and provides protection and moisture retention for its partner, which performs photosynthesis to produce sugars that serve as food for the fungus.
This arrangement enables lichens to absorb moisture and nutrients from the atmosphere and to thrive in a wide range of environments, including extremely harsh regions where few if any other organisms can survive and where neither of the two symbiotic partners could survive by themselves. Examples of the diverse environments include coastal areas, arid deserts, rainforests, Arctic and Antarctic tundra, mangroves and mountain cloud forests.
Another major feature of lichens is their production of a variety of organic acids. Some of these acids play an essential role in the weathering of rocks and minerals and thus in the absorption of nutrients. Some serve to protect the lichens from herbivores and microbes and still others provide protection from ultraviolet radiation.
Further contributing to the adaptability of lichens is the fact that they exhibit various growth forms, including crust-like and tightly adhering to surfaces, leaf-like that can be easily peeled off, and shrub-like branches that extend outward. Despite their appearance, lichens are not plants, this because they lack roots, stems, and leaves and because their chloroplasts are contained only in the algae or other partner cells.
Lichens have several critical roles in ecosystems. One is habitat formation, as they can be among the first organisms to colonize bare rock surfaces, paving the way for other plant life. They perform nutrient recycling through helping to weather rocks and then contributing minerals that they have absorbed to soil formation when they decompose.
Lichens are also an important food source for a variety of animals, depending on the particular ecosystem and season. They can include mammals such as reindeer, musk oxen, llamas, squirrels and some monkeys. A variety of invertebrates also consume lichens, including snails, slugs and various insects. Numerous bird species utilize lichens as a nesting material and some may also consume them.
Lichens are also useful for environmental monitoring because they are sensitive to air quality and thus can indicate levels of pollution. Moreover, they could offer a rich source of bioactive compounds with significant potential for pharmaceutical applications, especially for cancer and antimicrobial therapies. While their practical use still faces major challenges, such as low growth rates, advances in biotechnology and genetic engineering could eventually allow their mass production and widespread use.
Current estimates of the number of known species of lichens worldwide range from about 15,000 to 20,000 and new species continue to be discovered. About twenty percent of these are considered endangered, particularly as a result of habitat destruction, climate change and acid_rain. An example is the unique boreal felt lichen (Erioderma pedicellatum) found in the cool Atlantic coastal areas of Eastern Canada, for which only about three hundred individuals are known to remain.