The fungi are an extremely diverse division of the Eucarya that includes both unicellular species and species that grow as finely divided networks of filaments called hyphae. Unicellular species are all microscopic, but hyphal networks can support fruiting bodies that are readily visible without magnification. All are included among the microorganisms on the basis of their lack of differentiated tissues. Primary features that distinguish the fungi from other Eucarya are: (i) use of chitin as a cell wall polymer (but not in all fungi); and (ii) the absence of photopigments. Fungal nutritional requirements and core metabolic processes are neither unusual for eukaryotes nor particularly diverse. However, their morphologies and life cycles display considerable diversity, providing the basis for classical fungal taxonomy. This scheme has a serious limitation in that mature reproductive structures are required for correct identification of some organisms and these structures can be difficult to find or are produced haphazardly by some fungi. The operational solution to this problem has been inclusion of all fungi for which sexual reproductive structures have not been seen in a single polyphyletic subdivision. The seven classically defined fungal taxonomic subdivisions are given here.
The Ascomycota (Ascomycetes or sac fungi) include about 35,000 described species. Many species of the red, brown, and blue-green molds that cause food spoilage, as well as the powdery mildews that cause Dutch elm disease and chestnut blight, are Ascomycetes. The Basidiomycota (Basidiomycetes or club fungi) include about 30,000 known species. Among these are the smuts, rusts, shelf fungi, stinkhorns, puffballs, toadstools, mushrooms, and bird's nest fungi. The human pathogen Cryptococcus neoformans, which causes cryptococcosis, is also a Basidiomycete, as are the important plant pathogens, the smuts and rusts. The Zygomycota (Zygomycetes) include about 600 known species. Among these are the common bread molds and a few species parasitic on plants and animals. The Deuteromycota (Deuteromycetes or fungi imperfecti) include all species for which a sexual reproductive phase has not been observed—about 30,000 known species. Several human pathogens, including the organisms causing ringworm, athlete's foot, and histoplasmosis, are included in this group. The aflatoxin-producing species Aspergillis flavus and A. parasiticus, important organisms in fungal food poisonings, are also in this group. Other Deuteromycetes are responsible for production of antibiotics or are used in production of foods, such as cheeses and soy sauce. When the reproductive (perfect) stage of a Deuteromycete is characterized, it is transferred from the fungi imperfecti to the appropriate group. These four subdivisions of fungi form a monophyletic cluster of true fungi. Molecular phylogenetic analysis has revealed that the remaining three classically defined fungal groups are more closely related to other types of organisms than to the four true fungal subdivisions. The Oomycota (Oomycetes or water molds) resemble fungi in that they grow in a finely branched network of hyphal filaments. However, the Oomycetes have cell walls composed of cellulose, produce motile asexual zoospores, and are closely related to the algal Phaeophyta and Chrysophyta. The Oomycetes are saprotrophs and several are important plant pathogens, the most famous of which is Phytophthora infestans, the organism responsible for the Irish potato famine. The Myxomycota (plasmodial slime molds; about 700 known species) and Acrasiomycota (cellular slime molds; about 50 known species) lack cell walls of any kind during vegetative growth, display amoeboid motility, and are related to the protozoa. Both types are saprotrophic and feed by phagocytosis. When food resources become limiting, both types of slime molds will form fruiting structures. None of the slime molds are known to be important plant or animal parasites or to produce commercially valuable products. However, slime molds are significant participants in organic matter turnover in soils.
The major ecological functions of fungi are well understood. Soil fungi are ubiquitous saprotrophs and responsible for much of the decay of soil organic matter. These organisms are highly adapted to the saprotroph function, producing a variety of important extracellular enzymes for degradation of insoluble substrates and infiltrating the soil matrix and decaying organic matter through hyphal growth. Although primarily terrestrial, fungi are also found in aquatic ecosystems, including highly saline intertidal soils, and are particularly significant in the decay of plant materials in these environments. Fungi are also important members of the digestive tract flora of many animals and actively participate in the breakdown of plant materials, particularly in the ruminant animals and many insects.
Fungal growth at the expense of plant biomass is not confined to dead biomass. Some fungi are also very significant plant parasites responsible for economically important damage to crop plants. The complex relationship of the mycorrhizal fungi with their plant hosts, which can be characterized as mutualis-tic, parasitic, or fluctuating between these roles, should not be overlooked in this regard. The mycor-rhizae promote plant growth in many cases and are very important agents in the structuring of plant communities, but they can also damage their hosts under stressful environmental conditions. Fungal parasitism in some cases results in spoilage of food products, particularly the seed heads of maturing grain crops. An interesting fungus important in the spoilage of grains is the Ascomycete Claviceps purpurea, which produces the alkaloid mycotoxin ergot. Ergot is a potent hallucinogen and consumption of ergot-contaminated rye is thought to have been the cause of medieval dancing fits, in which the populations of whole villages danced wildly until exhausted. Accusations of witchcraft and the resulting Salem witch trials and executions may also have been due to ergotism.
Although they are less frequently cited as inhabitants of extreme environments than the extremist species among the Bacteria or Archaea, the fungi can grow across a broad range of environmental conditions and include species that could certainly be considered extremophiles. In general, fungi can tolerate greater extremes of salinity, pH, and desiccation than non-extremophilic species of bacteria. The diverse array of fungi participating in the lichen symbiosis can be found from temperate to very cold and dry environments worldwide. The microbiota of the rock varnishes common in hot deserts and the endolithic microflora growing within rocks in the Antarctic dry valleys both include fungi. Fungi are found in anaerobic environments including subsurface marine sediments, water-saturated decaying organic matter, the rumens of ruminant mammals, and the hindguts of numerous insect species. Fungi inhabit soils having temperatures from below freezing up to about 60°C. Any effort to catalog total fungal diversity should certainly include such extreme environments, which are clearly more common than is usually realized.
There are between 72,000 and 100,000 known species of fungi, but total species numbers have been estimated at approximately 1.5 million, approximately six times the estimated number of vascular plant species (Hawksworth and Rossman, 1997). This estimate should certainly be considered conservative since it does not include the mostly undescribed fungi growing commensalistically, mutualistically, or para-sitically in or on animals. The estimated millions of undescribed insect species in the tropics are particularly noteworthy in this regard. Since mutualistic and parasitic fungi are typically quite host specific, many of these insect species may harbor unknown fungi. Given the important ecological functions of fungi as saproptrophs in organic matter decomposition and symbionts of terrestrial plants, it is sensible to consider plants first in estimates of fungal diversity. However, the many interactions of fungi with animals are easily overlooked and a great many undescribed fungal species are likely to be involved in them. The tropics, with their abundance of plant and insect species that are found nowhere else, likely represent the largest global reservoir of undescribed fungi, and characterization of the diversity of tropical fungi has only started.
The introduction of molecular biological methods for phylogenetic characterization of fungi should greatly facilitate identification of unknown species, particularly those that do not reliably produce fruiting bodies. Classical description of new species on the basis of morphological and life cycle characteristics continues at a respectable rate, but most fungal species, including many already available in culture, herbarium, and private collections, have yet to be validly described. It is certain that discovery of new fungal species will be limited primarily by the resources available for this effort for many years to come.
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