Fungus

V

Classification of Fungi

Scientists have long disagreed about how to classify fungi, and the classification systems are still developing. The first description of fungi was published in 1729 by Italian botanist Pier Antonio Micheli. Fungi were initially classified in the Plant Kingdom, and the field of fungus study, or mycology, developed as a branch of botany. Recognition of the unique characteristics of fungi led mycologists to establish a separate kingdom, Kingdom Fungi, in the late 1960s. More recently, some mycologists have noted that some organisms, such as slime molds, downy mildews, and water molds, have characteristics that place them in the Kingdom Protista rather than the fungi. Unlike true fungi, some slime molds have a mobile, multinucleate feeding stage similar to amoebas. Downy mildews and water molds produce motile cells for part of their life cycle, have hyphal walls that lack chitin, and make an egg cell and sperm nuclei. Some scientists have proposed that downy mildews and water molds deserve to be classified in a separate kingdom, called Kingdom Stramenopila.

Fungi are classified primarily by the type of spores and fruiting bodies produced. Many mycologists divide the Kingdom Fungi into four main phyla: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. A fifth phylum, Deuteromycota, is used by some taxonomists for fungi that typically produce only asexual spores.

The phylum Chytridiomycota, commonly called Chytrids, includes approximately 800 species that are found in aquatic (freshwater and marine) or moist habitats. Chytrids are among the smallest and simplest fungi. Most have a central body with small tubelike extensions, while others produce a small network of hyphae. Chytrids develop a structure called a sporangium that has motile spores equipped with a posterior flagellum, a long, whiplike tail that aids in locomotion. Chytrids grow as saprobes in damp soils and water, or as parasites of plants, animals, algae, protists, and other fungi. Some do not require oxygen and live only in the guts of plant-eating animals, where they break down material containing cellulose and other compounds. Because chytrid spores are motile, some mycologists have classified them in the Kingdom Protista.

The Zygomycota include approximately 900 terrestrial species, including many important decomposers, mycorrhizal fungi, and parasites of spiders and insects. One of the most common zygomycetes is black bread mold, often found on bread, fruit, and other foods. The fungus looks like a fuzzy growth with tiny black dots at the tips of the fuzz. The black dots are sporangia growing at the ends of special hyphae. The sporangia produce asexual, nonswimming spores called sporangiospores. Zygomycetes reproduce sexually by forming thick-walled zygospores.

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The largest group of fungi, with around 50,000 known species, is the Ascomycota, or sac fungi. This group includes yeasts, lichens, morels, cup fungi, truffles, and a number of plant parasites such as powdery mildews. Named for the sexual spores produced inside saclike cells called asci (singular, ascus), Ascomycota also may produce very fine, almost powdery asexual spores called conidia. Certain Ascomycota such as cup fungi produce fruiting bodies with sexual spores on their upper surface, while others, including the truffles, produce sexual spores inside tuber-like fruiting bodies that develop underground.

Ascomycetes are used to produce Camembert and Roquefort cheeses. The slight grittiness in these cheeses is due to conidia being crushed between the teeth. The mold ergot, which infects the flowers of rye and other grains, produces toxins that can poison humans and other animals that eat the infected grain. The yeast Candida albicans is a common pathogen of humans, causing such ailments as oral thrush and vaginal yeast infections. In people with weakened immune systems, this yeast may spread widely throughout the body and become life threatening.

The Basidiomycota, also known as club fungi, include around 25,000 species of mushrooms, puffballs, bird’s nest fungi, jelly fungi, rusts, smuts, and shelf and bracket fungi. This division contains important plant parasites, mutualists, and saprobes, including decay fungi that cause brown rot and white rot of wood. These fungi are named for their specialized, club-shaped reproductive cells, called basidia, which form spores called basidiospores. Basidia may line gills or tubes on the underside of fleshy fruiting bodies, which consists of a stalk and cap—the familiar components of most mushrooms. Certain Basidiomycota produce spores inside tuber-like underground fruiting bodies, called “false truffles.”

Many basidiomycetes are saprobes, which play a vital role in the decomposition of litter, wood, and dung. A number of mushrooms are good to eat, such as boletes and chantarelles, both of which are highly prized for their distinct flavor. Other mushrooms are well known for their poisonous qualities, including the death cap (Amanita phalloides). Some, such as the liberty cap (Psilocybe semilanceata) and the fly agaric (Amanita muscaria), are well known for their hallucinogenic properties. Smuts—such as Ustilago, which attacks corn, and stinking smut (Tilletia), which attacks wheat—are common basidiomycetes that invade flowering plants, especially cereal grasses, and cause serious economic loss. Rusts, such as Puccinia, which attacks wheat, invade plant cells of agricultural crops and forest trees, causing millions of dollars in losses each year.

The Deuteromycota, or imperfect fungi, comprise about 25,000 species, many of which do not have a defined sexual cycle. They typically reproduce asexually by spores called conidia on specialized hyphae called conidiophores. The deuteromycetes include many molds, some of which are important to humans. Penicillium, the mold used to develop the first antibiotic, is sometimes classified in the Deuteromycota. On the other side of the ledger, the deuteromycetes also include organisms such as ringworm that are serious animal and plant pathogens.

VI

Uses of Fungi

Fungi have been used as a food source since the beginning of recorded history. Mushrooms add flavor, texture, and nutritional value to many dishes. In North America in recent years, a variety of mushrooms have gained popularity, including portabella, cremini, oyster, morel, chantarelle, wood or tree ear, truffle, matsutake, and shiitake.

Truffles—tuber-like, fleshy fungi with a characteristic taste and aroma—are highly prized by gourmet chefs. Harvested most commonly in France and northern Italy, truffles are collected with the aid of trained dogs or pigs that use scent to hunt these fungi hidden beneath the soil. The price for truffles in Europe may reach as high as $500 (U.S.) per pound in some years.

Other fungi are used in the manufacture of foods. Yeast, for example, is added to fruit juice, which it ferments to produce wine. Yeasts also are used in the manufacturing of beer, and they are added to dough to make bread rise, producing more volume and a lighter texture in the final baked product. Certain molds are used to ripen cheeses, such as Brie, Camembert, and the characteristic blue-veined Roquefort. In Asia, fungi are added to soybeans and allowed to ferment to make several food products—soy sauce is made with the mold Aspergillus, and tempeh is made with the black bread mold Rhizopus.

Many fungi also produce biologically active compounds that are useful in manufacturing. These compounds include alcohols—such as ethanol and glycerol produced during fermentation—and plant growth regulators—such as giberellic acid, which is used in the promotion of plant and fruit development. Fungi are extremely important in the production of antibiotics; for example, penicillin, griseofulvin, cyclosporine, and cephalosporin are used to fight bacterial and fungal diseases worldwide.

Fungi are becoming an increasingly important tool in cleaning the environment. The accumulation of pesticides and other chemicals in the environment is destroying many ecosystems, and placing many animal and plant species at risk. A number of fungi are used in bioremediation, in which the fungi are mixed with polluted water or soil, where they decompose the organic material in pollutants and, in the process, detoxify them. Fungi employed in this effort include many that are commonly found in soils, such as Aspergillus, Fusarium, Rhizopus, Mucor, Penicillium, and Trichoderma. Fungi also have been used successfully to control insects, fungus pathogens, roundworms, and other organisms that cause damage and disease to agricultural crops.

VII

Harmful Fungi

Fungi cause about 100,000 diseases of plants, including about 70 percent of the major crop diseases, resulting in an economic loss of billions of dollars each year. These plant pathogens cause extensive disease to seeds, seedlings, mature plants, and aging plants, resulting in decreased growth and reproduction of crop plants. Fungi also attack forest trees and wooden structures.

A number of fungi cause diseases in humans and other vertebrates. In general, these fungus infections, or mycoses, develop slowly, recur more frequently than bacterial infections, and do not produce a lasting immunity in the body. A mycosis is classified in one of two groups, depending on the part of the body that is infected. A dermatomycosis is an infection of the skin, hair, or nails, such as ringworm or athlete’s foot. These infections rarely progress to the internal organs. Most respond well to medication, although treatment may take several weeks.

A systemic mycosis, which is an infection of the entire body, is typically more serious and can be fatal for individuals whose immune system has been weakened by diseases such as acquired immunodeficiency syndrome (AIDS) or cancer. Fungal infections are typically spread by spores that enter the body through inhalation or through an opening in the skin. Some infections are passed from animals to humans or between humans. A few drugs are effective at treating systemic infections, but because treatment may last for several months to years to prevent relapse of the infection, these drugs often cause toxic side effects.

Fungi cause a number of human respiratory diseases. Coccidioidomycosis is caused by the yeast Coccidioides immitis. Although typically contracted by the inhalation of dust containing yeast spores, the fungus may also be introduced through the skin from infected soil. Initial symptoms may resemble the flu, with fever, cough, and possibly a skin rash, and the infection usually runs its course without specific treatment. In rare cases, the fungus penetrates internal tissues, such as the bones, joints, and brain, producing tumors that later form abscesses or ulcers. No treatment is available that can halt the course of this form of the disease.

Histoplasmosis is caused by the yeastlike fungus Histoplasma capsulatum, which grows in pigeon, bat, and chicken droppings. Contracted by the inhalation of dust from animal droppings, by ingestion, or through the skin. The fungus causing histoplasmosis lives as a parasite in certain tissue and blood cells of the infected person. An infection in the respiratory system is similar to tuberculosis—small spots form in the lungs—although these lesions heal on their own. A progressive form typically invades the bone marrow and is rapidly fatal.

Aspergillosis is an infection of the skin, nasal sinuses, and lungs or other internal organs caused by molds of the genus Aspergillus. The disease, contracted by the inhalation of spores, occurs most often among agricultural workers. Itching and pain are frequent symptoms, and if scratching is prolonged, the skin may thicken and become gray or black. A virulent type of pneumonia is caused by the yeastlike fungus Pneumocystis carinii, particularly prevalent in people with compromised immune systems, such as AIDS patients.

Mycotoxins are poisons produced by fungal growth in cereals, nuts, fruits, and vegetables. More than 100 species of fungi produce these toxins. The most common mycotoxin is aflatoxin, produced by Aspergillus flavus and Aspergillus parasiticus. Commonly found on corn, peanuts, and tree nuts, the toxin also can be transmitted to humans through the milk, meat, or eggs of animals fed contaminated grains. Aflatoxin is the most potent carcinogen, or potentially cancer-causing agent, yet discovered. Other mycotoxins include trichothecenes and zearalenone, compounds known to injure the intestines, bone marrow, lymph nodes, spleen, and thymus. They are produced by species of Fusarium that grow on grain, straw, or hay stored while damp. Occasionally, circumstances prevent the harvesting of grains during the autumn, and the grains lie dormant in the damp fields until they are harvested in the spring. These grains are especially vulnerable to trichothecenes and zearalenone contamination. A large outbreak of trichothecenes contamination occurred in Russia in early 1944 among hungry peasants who had been searching the winter fields for unharvested wheat and millet.

Scientific classification: Fungi are classified in the Kingdom Fungi, also known as the Kingdom Mycetae. The kingdom has five main phyla: Chytridiomycota, Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota.

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