Lichen (pronounced lie’ken, from the Greek Λειχήν = tree moss) is not a single entity. Lichens are fungi that live in a symbiotic association with - a green alga or a cyanobacterium or both to fulfill its nutritional requirements. In the lichen association the fungal partner is known as the mycobiont and the green alga or a cyanobacterium is known as the photosynthetic partner or the photobiont.
The mycobiont provide the necessary substratum and also helps in the assimilation of moisture, micro and macronutrients for the photosynthetic partner to grow and in turn receive carbohydrates for their metabolic activity. This constant supply of carbohydrates enables the fungal partner to continuously grow and reproduce, unlike the free living fungi that appear only upon the moisture and nutrients availability. However lichens are extremely slow growing and long lived.
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The scientific name of a lichen species is the name of the fungus (mycobiont). The estimated number of lichen species may range between 15,000 and 20,000 species world wide.
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In India, around 2000 - 2200 species are known to occur and in Tamil Nadu a total of 555 lichen species have been reported.
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The algal or cyanobacterial partner (photobiont) has a different binomial. Lichen symbiosis is one of the most successful symbioses known in nature.
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This relationship enables lichens to colonize a vast spectrum of habitats and climates all over the world including extreme environments - from the Polar regions to the Equator and inter-tidal zones to mountain peaks.
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Lichens colonize natural substrata such as tree bark, wood, rock, soil, leaf surfaces, and shells / hard surfaces of living animals like carapaces, weevils and man-made substrates like walls, mortar, asbestos, glass, Iron poles, plastics etc.
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Lichens are vital components of ecosystem functions - in food web and food chains, nutrient cycling and are used by many animal groups as brooding/nest building spaces.
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Lichens are used by man through various ways - Refer to the Economic Importance link.