Economic Importance of Lichen

 

 Economic Importance of Lichens

Lichens have little direct economic value. Even in a few cases where they are used commercially, as in perfumery and medicine, their importance is not widely recognized. However lichens do have considerable indirect importance in the primary food chain for grazing animals and invertebrates including insects, and their usefulness to nature is quite considerable,

Lichens as food

Lichens have been used as food in times of scarcity. They contain lichenin, a carbohydrate allied to starch, Cetraria islandica Cladonia spp, Evernia prunastri and Lecanorae sculenta are used as food. In India, the genus Parmelia and members of the family Usneaceae are used as food by the natives. Lichens growing on soil may contain bitter irritating acids, especially fumarprotocetraric acid, which while not poisonous, must be removed by boiling in soda. 

Lichens will, of course, never be utilized as food source by man on a large scale. Although their nutritive value compares favourably with that of cereal crops, their slow growth would prevent large-scale harvesting for human consumption. They are however undeniably important as a food source for reindeer and caribou in the tundra and subarctic forests. Lichens account for 30-60% of the total winter intake of these animals. The most cominonly grazed lichens are reindeer mosses (Cladinas, Cladonias and Cetrarias). Thamnolia vermicularis and Peltigera. If the snow cover is deep, caribou will also browse on corticolous lichens such as Bryoria and Usnea. Sheep in the Libyan deserts graze extensively on the subfoliose lichen Rhizoplaca (Lecanora) esculenta. Alectoria jubata, stereocaulon paschale and Cetraria islandica serve as fodder for horses, oxen, cows and swine.

Lichens are a significant food source for land snails and slugs and insects. Slugs and insects feed on Parmelia baltimorensis, Aspicilia calcarea and Lecanora saxicola are included in the diets of mites caterpillars, termites and snails.

Medical Uses

During the Middle Ages lichens figured prominently in the herbals used by medical practitioners. According to the Doctrine of Signatures Lobaria pulmonaria could be used to treat pulmonary diseases because of its superficial resemblance to lung tissue. Similarly Parmelia sulcata a foliose lichen with sulcate markings on the surface, was sought as a remedy for cranial maladies, and Peltigera canina was used to treat rabies. Several species of Pertusarin and Cladonia as well as Cetraria islandica were recommended in cases of intermittent fever.

Lichens have persisted in folk medicine into recent times, various lichens are used even today as expectorants. Usnea species are most commonly utilized. Cetraria islandica is claimed to be effective in treating diabetes, lung diseases and catarrh: Peltigera canina eaten in India as a remedy for liver ailments and its high content of the amino acid methionine may be the basis for its alleged curative power. Several well known lichen substances have now been identified as antibiotic agents. Gram- positive bacteria are significantly inhibited by usnic acid, protolichesterinic acid, and a few orcinol derivatives. Usnic acid also has low-level activity against lung carcinoma. Lichen substances are also being investigated experimentally as antibiotics in plant pathology, Tobacco mosaic virus is inhibited by lecanoric, psoromic and usnic acids. Sodium usnate is effective against tomato canker.

Antifungal properties of lichen substances are also being explored. Extracts of Nephroma arcticum, Hypogymnia physodes and Platismatia glauca have fungicidal and fungistatio effects on seven wood-rotting fungi.

Lichens in dyeing and perfumery:

Lichens had considerable economic importance as dyestuffs before the discovery of synthetic coal-tar dyes. Rocella and Lecanora yield purple dyes, known as orchileudbear nnd litmus Ochrolechia, Parmelia, Ramalina, Rocella, Usnea, etc., yield several dyes. Lichen extracts are commercially important in the perfume industry. Evernia prunastri and Pseudovernia furfuracea are collected in large quantities in Yugoslavia, Southern France, and Morocco. Lobaria pulmonaria is yet another lichen which yields a fine perfume Different species of lichens are ground to powders to be used for cosmetic purposes. The thalli of Usnea possess the power of retaining scent and the powdered thallus of Ramalina calicaris is used is a substitute for starch and used in perfumery. Some extracts are added to soap for scenting.

In tanning, brewing and distilling

The astringent substances extracted from the thalli of Cetraria islandica and Lobaria pulmonaria have been made use of an tanning leather. Lobaria pulmonaria has also been used in the brewing of beer. Cladonia rangiferina, Usnea florida and Ramalina fraxinea have been used in the preparation of alcohol.

Miscellaneous uses

Parmelia abessinica a crustose lichen found in rocky areas of A.P. is used as a spice, it contains atranorin, lecanoric acid, salazinic acid and isolichenin. Parnelia, Evernia and Ramalina yield gums which are applied in calicoprinting production of parchment and cardboard. Many crustaceous lichens contain oxalic acid Pertusaria spp. is used in the manufacture of oxalic acid in France.

In geological explorations, the occurrence of cetrarias is highly correlated with marble and limestone deposits

Lichens are indicators of pollution as they cannot survive environmental stress imposed on them by domestic and industrial atmospheric pollution. Areas within and around industrial cities show a central zone, the 'lichen desert from which lichens are almost entirely absent, surrounded by a belt, the 'struggle zone', within which lichens occur but in a stunted or poorly developed condition. By noting the size of thalli the number of species present and the kind of substrate on which they are growing at any particular site, it is possible to estimate very accurately the annual average levels of sulphur dioxide at the site. There are, however, toxitolerant or toxiphilous lichens which thrive in areas of high pollution. Lecanora conizaeoides is a species known to be remarkably resistant to sulphur dioxide.

Lichens useful to nature

Lichens are of considerable ecological importance as pioneers in colonization of rocky habitat. They excrete organic acids which disintegrate rocks, thus forming soil and preparing substrata in which other kinds of plants can be subsequently established. The first lichen to appear on rocks is Lichen candelarius. Other crustaceous lichens follow. When they die, their decaving romains, together with rock particles, form a soil.

Harmful effects

(1) A skin rash known as 'cedar poisoning occurs as an allergic contact dermatitis induced by usnic acid, a common lichen substance in the corticolous species of Alectoria, Evernia and Usnea which are widespread in the forests. The role of usnic acid in respiratory allergies is also being studied.

(2) As epiphytic autotrophs, lichens can have harmful effects on trees. There may be extensive penetration of the rinizines through the cork, cortex, bast and cambium, as far as the living wood.

(3) By their mechanical and chemical action, lichens have seriously corroded windows and archaeological ruins.