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Fungi are heterot rophic , unicellular or multicellular microbes that colonize both living and nonliving habitats. Fungi belong to the Eukarya domain of life, and the estimated 1.5 million species of fungi may be classified into one of five major groups: Ascomycota, Basidiomycota, Zygomycota, Oomycota, and Deuteromycota. These groups are classified on the basis of cellularity (multicellular, unicellular, or clonal), type of hyphae (septate or coenocytic), reproductive strategy, habitat (aquatic versus terrestrial), and life form. All fungi contain chitin in their cell walls, and other chemical constituents of fungal cell walls are used to classify fungi for ecological, industrial, and biotechnological purposes. All fungi are chemoorganotrophs, meaning that they lack the chlorophyll necessary to produce their own food; instead, fungi excretes extracellular enzymes that break organic material into new, simpler compounds that can then be absorbed by the fungus and used as carbon and energy sources. Many fungal species have a filamentous life form embodied as hyphae; hyphae that branch, intertwine, and grow together as tufts in soil and under decaying mats of organic matter collectively constitute fungal mycelia. Some mycelial mats are easily observable with the naked eye, but other soil fungi require chemical stains and microscopes to observe. Fungi have various reproductive strategies; all groups except the Deuteromycetes have some sexual reproduction through spores, while fungal groups that reproduce asexually produce asexual spores called conidia. Fungi colonize new areas both by the extension of hyphae-which may extend several meters from the original mycelial mat-and by spore dispersal, often over great distances.
Fungi grow in very diverse habitats, including fresh and salt water; however, most fungi are terrestrial and are commonly found in soils and on dead organic matter. Soil fungi, especially the Basidiomycetes, are instrumental in the decomposition of organic matter into simpler carbon compounds. The decay fungi are especially effective at decomposing recalcitrant (i.e., difficult to decompose) plant compounds such as cellulose and lignin. Certain fungal species are also important in the mineralization of organic compounds into inorganic nutrients and minerals that are in turn used by plants and other soil organisms. Pathogenic fungi are responsible for the majority of agricultural diseases, and can reduce crop yields and kill plants in natural systems. Many fungal pathogens of crop and noncrop plant species such as the powdery mildews (Ascomycetes), rusts (Basidiomycetes), smuts (Basidiomycetes), and blights (Ascomycetes, Oomycetes) are often transmitted by windblown spores. The fungal species Ophiostoma ulmi (syn. Ceratocystis ulmi) and Cryphonectria parasitica, which cause the tree diseases Dutch elm disease and Chestnut blight, respectively, have seriously changed the species composition of deciduous forests in the United States.
Most people associate fungi with the button mushrooms commonly used in cooking, or can envision fungal brackets that colonize dead trees or logs. Many fungi also have broad, lesser-known commercial and biotechnological uses. Secondary metabolites of the Deuteromycete fungus Penicillium chrysogenum are used industrially to produce penicillin antibiotics, and fungal metabolites have many other important industrial and commercial uses. Microscopic fungi also colonize the surfaces of rock either alone or in symbiotic association with algae or cyanobacteria, and in this way play a central role in the chemical weathering of rock into soil particles. Many people are not aware that the greatest fraction of biomass of any terrestrial fungus exists belowground as mycelia. A remarkable example of the below-ground expanse of soil fungi is a giant mycelial mat of the Basidiomycete Armillaria ostoyae, believed to be the single largest organism on earth. This single organism colonizes over 10 square kilometers of forest floor in northeast Oregon, and is believed to be between 2,000 and 8,500 years old!
- C. Brady and R.R. Weil, The Nature and Property of Soils (Prentice Hall, 2002);
- T.D. Brock, T. Madigan, J.M. Martinko and J. Parker, Biology of Microorganisms (Prentice Hall Publishers, 1994);
- Orson K. Miller and Hope Miller, North American Mushrooms: A Field Guide to Edible and Indedible Fungi (Globe Pequot Press, 2006).