Lichen

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The lichen seen here growing on a tree is one of the fruticose lichens. It is made up of a layer of algal cells, a middle layer of fungal hyphae, and an outer layer of fungal tissue.
Lichen, living partnership of a fungus and an alga. The fungus component is called the mycobiont and is composed of intertwined, threadlike fibers called hyphae that are tightly packed into a tissuelike sheet. The fungus uses these hyphae to absorb food from its surroundings. The algal component, called the photobiont, makes its own food through photosynthesis and grows as a mass of green cells dispersed among the fungal hyphae. Lichens survive in a wide variety of environments, either forming small, circular crusts or leaflike structures attached to bark, rocks, or soil, or as hairlike structures hanging from tree branches.
A lichen is actually a combination of two separate organisms: an alga and a fungus. Most lichens are three-layered organisms, with an algal layer sandwiched between two layers of fungus. The alga produces the food for the lichen through photosynthesis, while the fungus absorbs water and other nutrients. Neither the fungus nor the alga can live independent of the other.
The relationship between the fungus and the alga in a lichen is an example of mutualism, in which both partners benefit from the partnership (see Symbiosis). This relationship enables each to tolerate harsh conditions where neither could survive alone. In this partnership, the fungus furnishes the alga with water, prevents overexposure to sunlight, and provides simple mineral nutrients, while the photosynthesizing alga supplies food to the fungus even if no other organic material is available. In dry, barren areas where plants have a hard time growing, such as polar tundra, deserts, rocky outcrops, or high mountains, lichens are the primary photosynthesizers. Some remarkable species even grow inside porous rocks, just below the surface where some light can still reach the algal cells.

Their ability to grow in severe conditions often makes lichens pioneers in plant succession, the process in which plants colonize bare rock or soil. Lichens release acids that break down inhospitable rock, permitting soil-trapping mosses and grasses to grab hold. In areas where soil gradually accumulates, such as a forest floor, the pioneering lichens are eventually replaced by plants and trees, although other lichens may grow on these plants and trees.

Although lichens have been used in folk medicine as purported cures for many ills, from headaches and toothaches to tuberculosis, diabetes, and asthma, their use in modern medicine is recent. The discovery in the 1940s that some fungi produce potent antibiotics stimulated an extensive screening of fungi and lichens. Since then, lichen extracts have found limited use in Europe, where lichen antibiotics have been used to treat tuberculosis and some skin diseases.


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