Lichens

Lichens

Lichens are the "dynamic duo" of the plant world. They consist of a fungus and a photosynthetic partner (green algae or cyanobacteria , or sometimes both) that live and grow so intimately interconnected that they appear to be a single organism. The fungus surrounds its green partner and shares in the sugars and other carbohydrates that the alga or cyanobacterium produces by photosynthesis. At the same time the fungus provides a protected environment for its food-producing partner and expands its potential habitats. Lichen fungi have a range of nutritional relationships with their associated algae or cyanobacteria from almost pure parasitism to a very benign association called symbiosis, or, more specifically, mutualistic symbiosis, wherein both partners benefit equally from the partnership. Lichens are an extremely successful life form, with thousands of species throughout the world. Some are extremely tiny and inconspicuous, little more than a black or gray smudge, but others can form broad, brightly colored patches or grow to be up to 3 meters long.

Fungal and Algal Components of Lichens

The fungi that form lichens mainly belong to the sac fungi or Ascomycetes, although a few are mushroom-forming fungi, the Basidiomycetes. Each recognizable lichen (with a few interesting exceptions) represents a separate species of fungus; about fourteen thousand are known. The name we give to each lichen is actually the name of its fungal component. There are, however, only a few hundred species of photosynthetic symbionts (photobionts for short) that are involved in lichen partnerships. Lichen fungi are very choosy about their photobionts, and so each recognizable lichen generally contains a specific photobiont. Any given photobiont may, however, be found in many different lichens. A number of lichens associate with a green alga as their main photosynthetic partner but also produce small warts or gall-like bumps containing cyanobacteria, which contribute to the lichen's nutrition and survival.

Lichen Types and Reproduction

Lichens come in many shapes and sizes. They can be roughly grouped into four growth types: crustose, foliose, squamulose, and fruticose. Crustose lichens form a thin or thick crust so tightly attached to the material on which it grows (the substrate) that one has to remove the substrate together with the lichen to make a collection. A foliose lichen is leaflike; it is flat and has a clearly distinguishable upper and lower surface. Foliose lichens are attached to the substrate directly by the lower surface or by means of tiny hairlike structures called rhizines. Squamulose lichens are scalelike with flat lobes as in foliose lichens but more like crustose lichens in size and stature. Fruticose lichens are clearly three dimensional, growing vertically as stalks or shrubby cushions or hanging down from branches or rock faces with hair- or strap-shaped branches.

The arrangement of tissues within most lichens follows the same basic plan. In a typical foliose lichen, a relatively tough upper cortex functions as a protective layer. Below the cortex is a green layer formed by the photo-biont, then comes a cottony medulla , and, finally, on the lower surface, there is usually a protective lower cortex. The rhizines develop from the lower cortex.

Lichen reproduction is rather complex because at least two organisms are involved. The lichen fungus can produce sexual fruiting bodies and spores, but the photobionts reproduce only by cell division within the lichen. When a fungal spore is dispersed by wind or water, it can germinate almost anywhere, but it will form a new lichen only if it encounters the right kind of photobiont. This is a chancy business, and the vast majority of spores perish without forming new lichens.

There is, however, a less perilous way for lichens to reproduce. Any fragment of a lichen containing both the fungus and photobiont has the potential of developing into a new lichen. Many lichens have, in fact, evolved special, easily dispersed fragments in the form of powdery particles (soredia) or spherical to elongated granules or outgrowths (isidia).

Ecology of Lichens

Although lichens as a whole can be found growing on a wide variety of surfaces including rock, bark, wood, leaves, peat, and soil, individual species are more or less confined to specific substrates. Lichens are most conspicuous where other forms of vegetation are sparse, such as the bark of roadside trees or the surface of granitic boulders. They are usually the first organisms to invade entirely bare rock, contributing to the first particles of soil on the rock surface. Lichens carpet the ground in the vast boreal forests of the north, drape the trees and shrubs of foggy coastal regions and tropical cloud forests, and cover the exposed rocks on mountaintops and in the Arctic. They occur from the tropics to the polar regions and from lake edges and seashores to the desert. In general, however, lichens do best where there is much light, moist air, and cool temperatures. Lichens are notoriously sensitive to even small amounts of air pollution, especially the sulfur dioxide so common in cities and near factories, and large cities often have no lichens at all. Their disappearance from an area is an early sign of deteriorating air quality.

Importance and Economic Uses of Lichens

The importance of lichens to the natural world and to humans is not well appreciated except, perhaps, for their role in soil formation. Lichens containing cyanobacteria are important sources of nitrogen in certain forest and desert ecosystems . The ground-dwelling boreal lichens preserve the ground's moisture. Lichens growing in the dry soils of the interior prairies and foothills prevent erosion.

Although lichens have usually been used as human food only in times of emergency (they are unpalatable and have very little nutritional value), a few lichen delicacies are enjoyed by native people of western North America and by the Japanese. Reindeer lichens in the boreal forest, however, are essential as winter forage for caribou herds, which are, in turn, basic to the survival and culture of northern native people. Some lichens yield a chemical called usnic acid, which is an effective antibiotic against certain types of bacteria. Other chemicals produced only by lichens have been used as a source of rusty red, yellow, and purple dyes for coloring wool and silk. Extracts of oakmoss lichens have been used for generations in the perfume industry. The litmus used to determine the acidity of solutions comes from a lichen. The most important use of lichens today, however, is for detecting and monitoring air pollution.

see also Algae; Boreal Forest; Fungi; Plant Community Processes.

Irwin M. Brodo

Bibliography

Casselman, Karen L. Craft of the Dyer: Colour from Plants and Lichens, 2nd ed. New York: Dover Publications Inc., 1993.

Hale, Mason E. How to Know the Lichens, 2nd ed. Dubuque, IA: W. C. Brown Co., 1979.

McCune, Bruce, Linda Geiser, Alexander Mikulin, and Sylvia D. Sharnoff. Macrolichens of the Pacific Northwest. Corvallis, OR: Oregon State University Press, 1997.

Nash, Thomas H., III, ed. Lichen Biology. Cambridge: Cambridge University Press, 1996.

Richardson, D. H. S. "Pollution Monitoring with Lichens." Naturalists' Handbook 19. Slough, England: Richmond Publishing Co., 1992.

Sharnoff, Sylvia D., and Stephen Sharnoff. "Lichens of North America Project" [On-line] 1997. Available at http://www.lichen.com.