Information courtesy of wikipedia.

Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. Lichens take the external shape of the fungal partner and hence are named based on the fungus. The fungus most commonly forms the majority of the lichen's bulk, though in filamentous and gelatinous lichens this may not always be the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota. Some lichen taxonomists place lichens in their own division, the Mycophycophyta, but this practice ignores the fact that the components belong to separate lineages.

The algal cells contain chlorophyll, permitting them to live in a purely mineral environment by producing their own organic compounds (see photosynthesis). The fungus protects the alga against drying out and, in some cases, provides it with minerals obtained from the substratum. If a cyanobacterium, such as in Terricolous Lichens, is present this can fix atmospheric nitrogen, complementing the activities of the green alga.

Morphology and Structure

Lichens live on various surfaces: soil, trees, rocks, and walls. They are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as found at high mountain elevations and at high latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the arctic regions. Recent NASA research shows that lichen can even endure extended exposure to space. Some lichens have the aspect of leaves (foliose lichens); others cover the substratum like a crust (crustose lichens); others adopt shrubby forms (fruticose lichens); and there are gelatinous lichens (see lichen forms below).

Although the form of a lichen is determined by the genetic material of the fungal partner, association with a photobiont is required for the development of that form. When grown in the laboratory in the absence of its photobiont a lichen fungus develops as an undifferentiated mass of hyphae. If combined with its photobiont under appropriate conditions the morphogenesis of the lichen occurs and its characteristic form emerges. (Brodo, Sharnoff & Sharnoff, 2001)

There is evidence to suggest that the lichen symbiosis is parasitic rather than mutualistic. The photosynthetic partner can exist in nature independently of the fungal partner but not vice versa. Furthermore, photobiont cells are routinely destroyed in the course of nutrient exchange. The association is able to continue because photobiont cells reproduce faster than they are destroyed. (Ibid.)

When seen under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced filaments (fungus). The upper layer is formed by densely agglutinated fungal hyphae building a protective outer layer called the cortex. Cyanobacteria may be held in small eruptions of or under the surface called cephalopodia. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. Each cell or group of cells of the photobiont is usually individually wrapped by hyphae and in some cases penetrated by an haustorium. Beneath the algal layer is a layer of loosely interwoven fungal hyphae without algal cells. This layer is called the medulla. Beneath the medulla the bottom surface resembles the upper surface and is called the lower cortex, consisting of densely packed fungal hyphae. The lower cortex often bears structures, such as rhizines or a tomentum, serving to attach the thallus to the substratum on which it grows. Lichens also sometimes contain structures made from fungal metabolites, for example crustose lichens sometimes have a polysaccharide layer in the cortex. Although each lichen thallus generally appears homogenous, it may consist of several different species of fungus and photobiont living together.

Reproduction

Lichens most frequently reproduce asexually, either by vegetative reproduction or through the dispersal of diaspores containing algal and fungal cells. Soredia (sing. soredium) are small groups of algal cells surrounded by fungal filaments that form in cavities called soralia, which open when the lichen dries or surrounding tissues die and release the soredia to be dispersed by wind. Another form of diaspore are isidia, elongated outgrowths from the thallus that break off for dispersal. Fruticose lichens in particular can easily fragment. Due to the relative lack of differentiation in the thallus, the line between diaspore formation and vegetative reproduction is often blurred. Many lichens break up into fragments when they dry, dispersing themselves to resume growth when moisture returns.

Lichens also reproduce sexually in a manner typical of fungi, forming fungal and algal "propagules" that following germination must meet with a compatible partner before a functional lichen can form. This is generally not a common means of reproduction for most lichens, though it is more common in basidiomycetous lichens since they appear to lack structures specifically designed for asexual reproduction. Spores are produced in spore producing bodies, the three most common spore body types are the apothecia, perithecia and the pycnidia

Medicinal Uses

Lichens produce protective secondary metabolites that serve to deter herbivory and colonization by pathogens. Usnic acid, stictic acid, and vulpinic acid are a few of the 700 plus secondary compounds that are produced by lichens. Researchers found that pure extracts of usnic acid, evernic acid, and vulpinic acid inhibited the growth of gram positive bacteria Staphylococcus aureus, Bacillus subtilis, and Bacillus megaterium, but the acids had no affect on the gram negative bacteria Escherichia coli or Pseudomonas aeruginosa. Interest in the antibiotic potential of lichen compounds was extremely high during the post-World War II era through the end of the 1950's. A secondary compound that generated a high amount of interest and considerable research was usnic acid. In fact, sixty-four papers are known to have been published on usnic acid between 1950-1959. In the 1970's, usnic acid was reported to have potential as an anti tumor drug. Once again there is an interest in the potential uses of antibiotics derived from lichens as lichens may be a valuable source of antibiotics for the pharmaceutical industry in the future. The goal of our study was to determine the potential antibiotic properties of four lichen species from the Pacific Northwest: Hypogymnia apinnata, Letharia columbiana, Lobaria pulmonaria, and Usnea filipendula, and to detect what secondary compounds may be present in the four lichen species by using Thin Layer Chromatography. Along with determining what secondary compounds are present in the four lichen species, a tincture of Usnea barbata purchased at a health food store was also analyzed for compounds present.