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Fungi - Mycophycophyta, Lichens

species fungus relationship fungal

A lichen is a symbiotic relationship between a fungus and an alga, or between a fungus and a photosynthetic cyanobacterium. They constitute a very diverse and polyphyletic group of organisms and are classified together simply because they all result from a fungus-alga symbiosis. In most lichens, the fungal species is in the Ascomycota phylum and the photosynthetic species is a green alga from the Chlorophyta phylum. Typically, the photosynthetic species supplies carbohydrates to the fungus and the fungus supplies nitrogen and other nutrients to the alga. The morphology of a lichen differs from its component species.

Lichens can reproduce by several methods. The fungal component of the lichen can produce spores which are dispersed, germinate, and then recombine with the algal component. Alternatively, the lichen can produce soredia, specialized reproductive and dispersal structures in which the algal component is engulfed by fungal mycelium. Typically, the soredia break off from the thallus, the main body of the lichen.

Ecologists have shown that many species of lichens are very sensitive to air pollutants, such as sulfur dioxide. Thus, they are often used as indicator species for air pollution; the presence of certain lichen species correlates with the cleanliness of the air.

Many lichens can inhabit harsh environments and withstand prolonged periods of desiccation. In the temperate region of North America, lichens often grow on tree trunks and bare rocks and soil. In the arctic and antarctic regions, lichens constitute a large proportion of the ecosystem biomass. Many lichens are even found growing upon and within rocks in Antarctica. In the arctic region, the lichen species known colloquially as reindeer mosses (Cladonia rangifera and several other species) are an important food for caribou and reindeer.

Studies of the symbiotic nature of lichens in the late 1800s laid an important foundation for development of the theory of symbiogenesis. This theory says that new life forms can evolve from the symbiotic relationship of two or more independent species. Nearly all modern biologists now agree that symbiogenesis of different bacteria led to the origin of eukaryotic cells, which contain many different organelles, intracellular" small organs" which are specialized for different functions.


Resources

Books

Griffin, D.H. Fungal Physiology. New York: Wiley-Liss, 1993.

Margulis, L., and K.V. Schwartz. Five Kingdoms. New York: W. H. Freeman and Company, 1988.

Soothill, E., and A. Fairhurst. The New Field Guide to Fungi. Transatlantic Arts, 1993.


Peter A. Ensminger

KEY TERMS

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Biomass

—Total weight, volume, or energy equivalent of all living organisms within a given area.

Clamp connection

—Loop-like hypha which connects the cytoplasm of adjacent cells. Characteristic feature of Basidiomycota.

Coenocytic

—Lacking walls for separation of the nuclei of cytoplasm.

Cyanobacteria (singular, cyanobacterium)

—Photosynthetic bacteria, commonly known as blue-green alga.

Diploid

—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.

Haploid

—Nucleus or cell containing one copy of each chromosome.

Hypha (plural, hyphae)

—Cellular unit of a fungus, typically a branched and tubular filament. Many strands (hyphae) together are called mycelium.

Mycorrhiza

—Subterranean symbiotic relationship between a fungus, typically a species of Basidiomycota, and a plant root.

Phylum

—Broadest taxonomic category within a kingdom.

Septum

—Wall that separates the cells of a fungal hypha into segments.

Symbiogenesis

—Evolutionary origin of a completely new life form from the symbiosis of two or more independent species.

Symbiosis

—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

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