(inadequate food or water), these individuals aggregate to form a macroscopic pseudo-plasmodium (because the cells remain individuals with separate membranes), or ''slug'' (unrelated to true slugs, which are mollusks), which crawls to an exposed location, forms a fruiting body, and releases spores. (In this way they resemble superficially the prokaryotic myxobacteria.) The spores can later germinate to produce new amoeboid cells. In addition to this asexual process, cellulose encased macrocysts may occasionally form in which two amoeba fuse.
Fungi are chemoorganotrophic heterotrophs. Most are saprobic, but some are parasites (the majority of plant pathogens are fungi, as are some animal pathogens) or symbionts. Although some are found in aquatic or marine environments, most are terrestrial. In addition to not being photosynthetic, they also differ from plants and algae in having cell walls composed primarily of chitin rather than cellulose. Fungi are nonmotile, unlike protozoans and slime molds (and some algae). Mitosis in fungi is different from that in plants and animals.
Like some bacteria, fungi excrete enzymes to digest food externally, then absorb the smaller organic molecules produced. This makes them especially suited for degradation of organic solids and for their important ecological role as decomposers of dead biomass (such as leaves and wood) as well as high-molecular-weight anthropogenic compounds. They play a major role in the global carbon cycle, especially because of the ability of many to degrade cellulose and of some to attack lignin. Fungi store energy either as gly-cogen or in lipids.
Fungi usually have one of two growth forms, although a few can take both. Both may reproduce sexually through spores. Yeasts are unicellular and reproduce asexually by budding (see below). Molds, which are the much more common form, are multicellular, filamentous organisms and usually reproduce asexually with spores. As with the bacterial actinomycetes (Section 10.5.7), an individual filament is referred to as a hypha (plural, hyphae), and the intertwined, matted growths they form are known as mycelia (singular, mycelium). However, unlike with bacterial filaments, which are essentially colonies of single-celled organisms, molds are multicellular. Typically, there is considerable interchange of materials among cells within the filament. In fact, often the cross-walls separating cells have holes in them or disappear completely. This can lead to the presence of several nuclei within a single "cell," a growth form referred to as coenocytic. Hyphae are usually hap-loid and produce microscopic spores, usually at the tips of specialized filaments called conidia (singular, conidium; Figure 10.42) or in sacs called sporangia (singular, sporangium). After dispersal, the spores may germinate and grow into new hyphae if deposited on a suitable substrate.
The fungal mycelium typically permeates the soil or substrate on which the organism is growing. Mushrooms are the familiar, large, fruiting bodies of some fungi and do not represent the major portion of the organism, which consists of the mycelia below the surface.
Both bacteria and fungi degrade organic material, but bacteria typically grow faster. However, fungi are less inhibited by dry conditions than bacteria, and so often make up a majority of the living biomass in drier soils and at the dry edges of compost piles. Fungi are also able to withstand higher concentrations of salt and sugar, allowing them to cause spoilage in foods that most bacteria cannot grow on. Some thermophilic fungi can grow at temperatures up to 62°C. As a group, fungi also can tolerate lower pH than many common organotrophic bacteria, allowing them to grow competitively, for example, on fermented fruit. In activated sludge plants for wastewater treatment, if the pH drops below 5.5 to 6, fungi may grow excessively and interfere with the settling process (fungal bulking).
The fungi are now classified into four phyla or divisions: Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes (Table 10.11). Previously, another phylum, the Oomycetes, was included, and is discussed here, but they are now considered a separate protist group.
Zygomycota The Zygomycetes, or lower fungi, include about 800 known species, including Rhizopus stolonifer, the common black bread mold. Zygomycete sporangia consist of small stalks that produce spores. In Pilobolus, the "shotgun fungus,'' which grows on horse and cow dung, spore release is dramatic—the sporangium can be shot to a height of 6 feet! Sexual reproduction occurs through contact of two hyphae (which are haploid) of opposite mating types (called + and —) and fusing of their nuclei, leading to the production of a diploid zygospore. This resistant form later undergoes meiosis, germination, mitosis, production of a sporangium, and release of haploid spores. Most zygomycetes, such as the common Mucor, are decomposers in soils and decaying plant material, with some causing food spoilage. One group forms mycorrhyzae (see below), and a few are parasitic on plants or animals. Entomophthora, for example, parasitizes houseflies, eventually killing them. (Look for a white powdery halo of spores around the next dead fly you find on a windowsill.)
Ascomycota The Ascomycetes, or sac fungi, is the largest fungal phylum, with 30,000 species known, including most yeasts, a few mushrooms (such as truffles, morels, and cup fungi), and many molds. The blue-green mold that grows on citrus fruit and the pink bread mold Neurospora are Ascomycetes. Many plant diseases are caused by ascomycetes, including powdery mildew, apple scab, and several rots. Chestnut blight, caused by Endothia parasitica, has virtually eliminated the American chestnut, once a major forest tree in the northeastern United States. Ceratocystis ulmi, the cause of Dutch elm disease, has had a similar effect on the American elm, another major forest and shade tree. Cla-viceps purpurea infects rye and other cereals, leading to production of a structure called an ergot in the seed head. Ingestion by cattle or humans can lead to severe nervous system disorders, including delusions, spasms, and often, death. [One of the chemicals present is lysurgic acid, a precursor of the hallucinogen, lysurgic acid diethylamide (LSD).] In
Europe in the Middle Ages, whole villages would sometimes appear to go insane from eating bread made with infected rye.
Ascomycete molds (yeasts are described below) reproduce asexually by pinching off conidiospores from special hyphae called conidia. Sexual reproduction occurs through fusing of two hyphae to form a sac called an ascus (plural, asci), in which (after meiosis and mitosis) ascospores develop.
Basidiomycota The Basidiomycetes, or club fungi, consist of over 16,000 species, including most of the familiar mushrooms, toadstools, stinkhorns, puffballs, and jelly and shelf fungi. In fact, Basidiomycetes are thought to form the major part of living biomass in most soil. Although some mushrooms (e.g., the commercially produced Agaricus campestris bisporus) are edible, others are poisonous (e.g., Amanita verna, the destroying angel) or hallucinogenic (e.g., Psilocybe). Mushrooms are the fruiting bodies that result from sexual reproduction, serving to release the haploid spores. Some important crop parasites, such as the rusts and smuts, also are basidiomycetes.
White rot fungi (e.g., Phanerochaete) are basidiomycetes that cause decay of wood by digesting lignin and cellulose. Because the lignin-degrading enzymes are also effective against many xenobiotic compounds, P. chrysosporium is being studied for use in engineered processes to degrade industrial pollutants. Brown rot fungi attack cellulose but not lignin.
Deuteromycota The Deuteromycetes are also referred to as fungi imperfecti, reflecting the fact that they have no known sexual reproduction phase. This is not a true phylum but rather a form-phylum, containing more than 11,000 "form-species." Once the sexual phase is discovered, species in this group are reclassified into one of the other phyla. Almost all appear to be Ascomycetes, but a few are Basidiomycetes. Included are molds of the genus Penicillium, well known as the source of the antibiotic penicillin and also the source of the flavor and color of Roquefort and other "blue" cheeses. Aspergillus (Figure 10.42) is a common soil saprobe, but during growth on peanuts or other stored foods, some make aflatoxins, among the most toxic natural compounds known. Aspergillus also can be an aeroallergen (cause allergic reactions) when the spores become airborne in agriculture and at composting sites, and occasionally can be responsible for lung disease in immunocompromised hosts. Other species of Aspergillus are used in fermentation of soy sauce and saki. Verticillium cause pink rot of apples and also a wilt that is affecting many Norway maples planted as municipal street trees in the northeastern United States. Fusarium causes wilts of several plants, and Rhizoctonia (a Basidiomycete) can cause damping off and root rot.
On the other hand, some fungi have been found to be helpful in controlling nematode damage of plants. Arthrobotrys and Dactylella, for example, trap and feed on these small animals—serving as the Venus flytraps of the microbial world! These fungi also are occasionally observed in wastewater treatment systems (Figure 10.43). Geotrichum is another fungus that is sometimes found in activated sludge plants, where its excessive growth (at low pH) leads to a problem referred to as fungal bulking.
Among the Fungi Imperfecti that are human pathogens are several species (e.g., Epidermophyton and Trichophyton) that cause the skin infections known as athlete's foot and ringworm (Section 12.7.7). Histoplasmosis, which is a potentially fatal respiratory disease (Section 12.4.7), is caused by Histoplasma capsulatum. Candida is a common source of vaginal yeast infections (Section 12.6.7). Both Histoplasma and
Candida are soil organisms and opportunistic pathogens and can invade a variety of tissues in compromised hosts. They also grow in both yeast and mold form.
Oomycota Because they are nonphotosynthetic, have a cell wall, and produce a mycelium, the Oomycetes, or water molds, were considered fungi until fairly recently. However, unlike fungi, they produce motile spores, and whereas the cell wall contains chitin in some species, it is made of cellulose in others. Analysis of their 18S rRNA has now confirmed that they are a separate group. Asexual reproduction is by release of biflagellated zoospores from a sporangium that forms at a hyphal tip. Sexual reproduction involves contact of male and female hyphae, leading to production of oospores. Most oomycetes are aquatic, with a few, such as Saprolegnia parasitica, parasitic on fish. A few are terrestrial, including some very important plant pathogens. Late potato blight, which may have led to the starvation of as many as 1 million Irish in the 1840s (and prompted emigration of millions more), is caused by Phytophthora infestans. Pythium causes damping-off disease of seedlings, and Plasmopara and other Oomycetes cause downy mildew.
Yeasts Yeasts are single-celled fungi. There are 60 genera with 500 known species. Most are ascomycetes, but some are basidiomycetes, zygomycetes, or deuteromycetes. They reproduce asexually by budding, a mitotic division producing a large and a small cell (the bud). Yeasts can respire using oxygen, but in the absence of oxygen, they are limited to glycolysis, which feeds into a fermentation pathway to produce ethanol and CO2. The ascomycete Saccharomyces cerevisiae has been "domesticated" and developed into both baker's and brewer's yeast.
Mycorrhyzae Some molds, called mycorrhyzae (''fungus roots''), grow either within or surrounding the roots of many plants as a symbiotic infection. Their mycelia extend into the soil, increasing the surface area available for absorption. This increases a plant's ability to extract nutrients. In return, the fungus shares in the primary productivity of the
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