The Lichen is two organisms functioning as a single, stable unit. Lichens comprise of a fungus living in a symbiotic relationship with an algae or cyanobacterium (or both in some instances). There are about 20,000 species of lichens worldwide.
Lichens have different properties from those of its component organisms. They come in many colors, sizes, and forms and are sometimes plant-like, but lichens are not plants. It is estimated that 6% of Earth’s land surface is covered by lichens. Lichens can be seen as being relatively self-contained miniature ecosystems.
Why form a dual organism?
Fungi are incapable of photosynthesis because they lack the green pigment chlorophyll. So, they have to seek outside sources of food. On the other hand, algae and cyanobacteria can conduct photosynthesis, similar to plants.
So when a fungus, which is the dominant partner in this relationship, associates with an alga or cyanobacterium to form lichen, it is providing itself with constant access to a source of nourishment. Cyanobacteria also provide fungi with the additional benefit of nitrogen fixation. The algae or cyanobacteria benefit by being protected from the environment, especially from damaging ultraviolet rays by the filaments of the fungi, which also gather moisture and nutrients from the environment, and provide an anchor to it. Fungi often form a protective cortex with pigments that absorb ultraviolet light.
Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface. Lichens grow abundantly on bark, leaves, and mosses. Different kinds of lichens have adapted to survive in some of the most extreme environments on Earth: arctic tundra, hot deserts, and rocky coasts. They can even live inside solid rock.
Some lichens are considered the oldest living things. They are among the first living things to grow on fresh rock exposed. The long life-span and slow and regular growth rate of some lichens can be used to date events. This is called Lichenometry.
Lichens do not grow in polluted areas as it is sensitive to atmospheric pollution including nitrogen and sulfur emissions. This sensitivity makes lichen a valuable biological indicator of air quality.