Lichen
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Introduction
Lichens have traditionally been referred to as a prime example of a symbiotic relationship. Each lichen consists of an intimate association between a fungus and a species of algae. The algae within the lichen photosynthesizes, providing food for both symbionts. The fungus protects the alga from harmful light intensities, produces a substance that accelerates photosynthesis in the algae, and absorbs and retains water and minerals for both organisms. There is physiological and ultrastructural evidence that suggests the fungus parasitizes the algae in a controlled fashion and, in some instances, actually destroys the algal cells. There are about 25,000 species of lichens known and they are capable of living in environmental conditions that kill most other forms of life. The number of aquatic lichens is limited as most live under the blazing sun often on bare rocks. Aquatic lichens typically live in the intertidal zone along sea shores or in shallow streams.
Morphology
The fungus and algal cells of lichens are associated together in a spongy thallus which can range in diameter from less than 1 millimeter (mm) to more than 2 meters (m). The thallus consists of 3 or 4 layers of cells or hyphae. The upper layer, called the upper cortex, contains gelatinous substances which perform protective functions. Below the upper cortex is the algal layer, in which algal cells are scattered among strands of hyphae. Next comes the medulla, which consists of loosely packed hyphae and within which a number of substances produced by the lichen are stored. The fourth layer, called the lower cortex, may not always be present. It resembles the upper cortex but is thinner and is often covered in anchoring strands of hyphae called rhizines.
Lichens have been categorized into three major groups depending on their type of growth. Crustose lichens are attached or embedded on the substrate and form colored, crusty patches. Foliose lichens have leaf-like thalli whose edges are crinkly or divided into lobes. Fruticose lichens may resemble tiny upright shrubs or may hang down in festoons from tree branches.
Reproduction
Although lichens can reproduce sexually, they are predominantly asexual reproducers. In the latter case, small powdery clusters of hyphae and algae, called soredia are formed and cut off from the thallus as it grows. These soredia are dispersed by wind or water and take up residence elsewhere. Sexual reproduction occurs when the fungal ascomata produce spores which germinate and parasitize independently living algae upon contact with them. Lichen algae reproduce by mitosis and simple cell division.
Importance
Lichen are good measures of air quality as they are very sensitive to pollution, in particular sulfur dioxide. Their sensitivity to this air pollutant is so great that it has been possible to calculate the amount of atmospheric sulfur dioxide by mapping the occurrence or disappearance of certain lichens in a given area. They are also very sensitive to nuclear radiation and can be used to monitor radioactive contamination.
Many lichens have antibiotic properties and are used to treat various ailments. One type in Europe is used to treat tuberculosis, while others are used to produce salves effective in treating cuts and skin diseases.
Lichens have also been used as dyes. Greeks, Romans and Native Americans all utilized lichens to make dyes of many colors.