Lichens

Categories: Algae; fungi; taxonomic groups

Symbiosis

Symbiosis is an extreme form of an ecological relationship known as mutualism between members of different species, in which each partner in the union derives benefits from the other. In symbiotic unions, the partners are so dependent on each other they can no longer independently survive.

In lichens, the fungal (mycobiont) symbiont provides protection, while the green-algal or bacterial (photobiont) symbiont provides sugars, created by photosynthesis. It is often suggested that the fungus in lichen species might also pass water and nutrients to the photobiont, but this function is less well documented. This special relationship allows lichens to survive in many environments, such as hot deserts and frozen Arctic tundra, that are inhospitable to most other life-forms. As a result, the lichen whole is greater than the sum of its parts. While in nature lichen partners always exist together, under laboratory conditions it is possible to take the lichen apart and grow the two partners separately.

Anatomy

Whereas in most plant species the anatomy of the organism is identified with structures associated with a single vegetative body, the “lichen body” is more aptly described as a colony of cells that share a variety of associations with one another that vary from one species of lichen to the next. In some species of lichen, fungal and algal cells merely coexist. Coenogonium leprieurii, for example, is a lichen that lives in low-light tropical and subtropical forests in which the filamentous green algal partner (Trebouxia) is dominant.

In most lichen species, however, the relationship between the symbiotic partners is more intimate, with the lichen body appearing to be a single entity. In these species the algal symbiont has no cell walls and is penetrated by filaments from the fungal symbiont called haustoria, which pass sugars from the algal cell to the fungal cell and may have a role in the transportation of water and nutrients from the fungal cell. This integration is so complete that many naturalists prior to the nineteenth century mistakenly classified lichens as mosses.

In most lichen species it is nevertheless possible, with a good magnifying device, to identify several distinct regions of the thallus or lichen body. The outermost region is the cortex, a compacted layer composed of short, thick hyphae (widely dilated filaments) of the fungal symbionts that protect the lichen from abiotic factors in its environment. These hyphae extend downward into a second region, the photobiont layer, where they surround the algal symbionts. Below this is a third region, the medulla, composed of a loosely woven network of hyphae.

Underneath this is a fourth region, the undercortex, that is similar in appearance and structure to the cortex. The bottom of the lichen body is composed of rhizines, rootlike structures composed of bundles of hyphae that attach the lichen to its substrate (the rock, bark, or other support on which it resides). This arrangement of regions into layers serves to prevent water loss. Many species can survive complete desiccation, coming back to life when water becomes available again. The cortex also contains pseudocyphellae, which are pores that allow for the exchange of gases necessary for photosynthesis.

Life Cycle

Lichens typically live for ten years or more, and in some species the lichen body can survive for more than a hundred years. Reproduction in most fungal species proceeds by the development of a cup- or saucer-shaped fruiting body called an apothecium, which releases fungal spores to its surroundings. Procreation in lichens is more problematic in that the fungal offspring must also receive the right algal symbiont if they are to survive. The most common form of dispersion in lichen is by the accidental breaking off of small pieces of the thallus called isidia, which are then spread by wind to new substrates. In some species, small outgrowths of the thallus known as soralia arise, composed of both fungi and algae and surrounded by hyphae, to form soredia, which after dispersion give rise to a new thallus.

Biological and Agricultural Importance

Lichens are excellent bioindicators of air pollution, as many species are particularly sensitive to certain contaminants in their surroundings, such as sulfur dioxide. They represent a major food source for reindeer in Lapland and are used as cattle fodder there as well. One species of lichen (Umbilicaria esculenta) is considered a delicacy in Japan. Historically, lichens have been used as pigments for the dying of wool. The medical properties of some species of lichens for the treatment of lung disease and rabies have led to a renewed interest in them.

Bibliography

Brodo, Irwin M., Sharnoff, Sylvia Duran, and Stephen Sharnoff. Lichens of North America. New Haven, Conn.: Yale University Press, 2001. A comprehensive field guide to lichens and their natural history.

Dobson, Frank. Lichens: An Illustrated Guide. Richmond, Surrey, England: Richmond, 1981. A massive (795-page) field guide to lichens in the British Isles. Includes more than nine hundred color photographs, as well as maps, bibliography, and index.

Hale, Mason E., Jr. The Biology of Lichens. 3d ed. London: Edward Arnold, 1983. A standard college-level text focused primarily on anatomy and physiology that discusses lichens as pollution monitors.

Hawksworth, D. L., and D. J. Hill. The Lichen-Forming Fungi. New York: Blackie & Son, 1984. A college-level text focused primarily on the life cycle of lichens, their ecology, and biogeography.

Purvis, William. Lichens. Washington, D.C.: Smithsonian Institution Press, 2000. Presents an introductory overview of lichen biology, ecology, and evolution, information on ecological and economic importance, how they are studied, and extensive illustrations and photographs.