Lichen

Lichen, any of a group of composite organisms made up of a fungus and one or more algae or cyanobacteria, forming a symbiotic association. The fungus provides a structure that may protect the alga from drying and from harsh conditions; the alga synthesizes and secretes carbohydrate that is taken up and utilized as food by the fungus.

About 15,000 kinds of lichens have been described. They are found throughout the world, especially in harsh environments, usually on rock, bark, or nutrient-poor soil. The body, or thallus, has a characteristic growth form: crust-like (crustose), leaf-like (foliose), or stalked, bush- or beard-like (fruticose). A crust-like lichen is usually composed of three layers: an upper layer containing fungal hyphae, a middle layer of algal cells intermingled with fungal hyphae, and a lower layer of fungal hyphae that penetrates the surface on which the lichen grows. Leaf-like lichens, such as the dog lichen, are also typically composed of three layers. Stalked lichens—for example, the beard lichen—consist of a central zone of loose fungal hyphae, an intermediate zone of algal cells, and an outer zone of compressed fungal hyphae. Unique chemical substances synthesized by lichens are responsible for their colour. The lichens range in size from less than 1 mm to more than 3 m (10 ft) across.

The most common method of reproduction involves the formation of a small powdery granule, or soredium, composed of a tuft of fungal hyphae surrounding one or more algal cells. The soredium detaches, disperses, and (under favourable conditions) develops into a lichen. The algal component of a lichen is capable of reproducing independently, but lichen fungi cannot live independently. Some lichens have been resynthesized by growing cultures of their algal and fungal components and then uniting the two cultures.

In Arctic and alpine regions such lichens as reindeer lichen serve as food for caribou and reindeer. Iceland moss, a bush-like lichen native to alpine and Arctic regions, has been used as food by human beings. A desert species, which can be blown away by the wind because it is only loosely attached, may be the manna described in the Bible. In the past, lichens have been used in dyeing and as a source of litmus, the acid-alkaline indicator. Lichens are sensitive to air pollution, especially sulphur dioxide produced by the burning of fossil fuels. The types of lichens present in any environment can indicate the purity of the air. Through their absorption of heavy metals and radioactive compounds, lichens may indicate environmental contamination.

Scientific classification: Lichens are classified according to the type of fungal component (called the mycobiont). The mycobiont of most lichens is an ascomycete; that of a few, mostly tropical lichens is a basidiomycete. The algal component (called the photobiont) is usually a single-celled green alga such as Trebouxia or Coccomyxa, or a cyanobacterium such as Nostoc or Scytonema. The dog lichen is classified in the genus Peltigera and the beard lichen in Usnea. Iceland moss is classified as Cetraria islandica, and the desert species that may be the manna of the Bible as Lecanora esculenta.