Fungus, any member of a diverse group of organisms that—unlike plants and animals—obtain food by absorbing nutrients from an external source. The fossil record suggests that fungi were present 550 million years ago and may have evolved even earlier. Today thousands of different types of fungi grow on and absorb food from substances such as soil, wood, decaying organic matter, or living plants and other organisms. They range from tiny, single-celled organisms invisible to the naked eye to some of the largest living multicellular organisms. In Michigan for example, the underground portion of an individual Armillaria mushroom, a type of fungus, extends more than 12 hectares (30 acres). Other fungi are among the longest-lived organisms on Earth—some lichens, a living partnership of a fungus and an alga, are thought to be more than 4,500 years old. Lichens of the Splash Zone of the Intertidal Region Lichens are actually a combination of two entirely different types of living organisms, an alga and a fungus. This association is called a symbiotic relationship. In this particular type of symbiotic relationship neither partner can live independent of the other. Lichens are very slow growing and can survive very harsh environmental conditions and are ecologically significant in that they contribute to the weathering of solid rock.G.I. Bernard/Oxford Scientific Films Common fungi include mushrooms, puffballs, truffles, yeasts, and most mildews, as well as various plant and animal pathogens (disease agents), such as plant rusts and smuts. Some experts estimate that there are 1.5 million fungus species, of which approximately 100,000 have been identified. The unique characteristics of fungi led scientists to classify these important organisms into a separate kingdom, Kingdom Fungi (also known as Mycetae). Certain fungus-like organisms, such as downy mildews, water molds (also known as oomycetes), and slime molds, once classified as fungi, are now placed in the Kingdom Protista. II UNIQUE FEEDING METHOD Fungi lack chlorophyll, the green pigment that enables plants to make their own food. Consequently, fungi cannot synthesize their own food the way plants do. In order to feed, fungi release digestive enzymes that break down food outside their bodies. The fungus then absorbs the dissolved food through its cell walls. Depending as they do on outside sources for food, fungi have developed various living arrangements that enhance their opportunities for food absorption. Some fungi live as parasites, feeding on living plants, animals, and even other fungi. Certain fungus parasites injure plants and animals, causing millions of dollars of damage to farm animals, crops, and trees each year. For example, the fungus Ophiostoma ulmi, which causes Dutch elm disease, has killed more than half of the elm trees in North America since it was accidentally introduced there in the 1930s. Fungi that obtain their food by breaking down dead organisms or substances that contain organic compounds, such as starch and cellulose, are called saprobes or saprophytes. While they are invaluable decomposers of organic material, saprobes can also cause food spoilage and destroy wood products. During the American Revolution (1775-1783), more British ships were destroyed by wood-digesting saprobes than by enemy attack. Some saprobes even grow in aviation fuels, where they breakdown the fuels, destroying their usefulness. Some fungi also form highly specialized relationships with other organisms (see Symbiosis). For example, the roots of most plants develop a mutually beneficial association with fungi to form mycorrhizae. Mycorrhizae greatly increase the nutrient-absorbing capacity of the plant root—the fungus absorbs minerals from the soil and exchanges them for organic nutrients synthesized by the plant. Fungi also form mutualistic associations with various animals. For example, leaf-cutting ants cut pieces of leaves and bring them into their underground nests, where they feed them to certain fungi. These fungi primarily live in ant nests, and the ants eat nothing but the fungi. Some termites and wood-boring beetles use fungi to break down the cellulose in wood, making the wood easier for the insects to digest. Plant parasites such as rusts invade plant cells via specialized structures called haustoria that absorb nutrients from the cell.
III FUNGI STRUCTURE Structure of a Fungus Fungi are made of filamentous tubes called hyphae. In many species, perforated sidebar Hyphae grow by elongation at the tips and by branching to form an interwoven mat known as the mycelium. As the mycelium develops, it may produce large fruiting bodies or other structures that contain reproductive spores. Fruiting bodies are often the most visible structure of a fungus, usually growing above the soil or other surfaces so that the spores can be dispersed by air currents or other mechanisms. In contrast, the mycelium is usually hidden beneath the surface of the plant, animal, or other material it is decomposing. For example, a mushroom mycelium is typically buried beneath the soil surface, while its fruiting body, the familiar umbrella-shaped structure, sprouts from the ground. IV REPRODUCTION Puffball Mushroom Releasing Spores Many fungi reproduce by releasing vast numbers of spores, often numbering in the billions or trillions. The spores, which develop in the fruiting body of the mushroom, are minute spherical structures that contain a small amount of protoplasm. Mushrooms, such as the puffball, produce spores both sexually and asexually.Richard Packwood/Oxford Scientific Films Fungi typically follow a reproductive cycle that involves the production of sexual spores. These spores contain one or more nuclei and are usually haploid—that is, their nuclei contain one set of chromosomes. When environmental conditions are favorable, the spores germinate and develop into a mycelium that produces fruiting bodies with enormous numbers of sexual spores, which repeat the reproductive cycle. Some fungi produce asexual spores directly from hyphae, which then germinate to produce additional mycelium. The mycelium spreads rapidly, aiding the fungus in dispersal and colonization. Life Cycle of a Mushroom Most mushrooms undergo a complex reproductive cycle in which they produce reproductive spores that undergo sexual reproduction under proper environmental conditions. Mushrooms belong to the phylum Basidiomycetes, named for the tiny, club-shaped structures called basidia that these fungi use to produce spores. The basidia line the undersides of a mushroom cap on thin gills that radiate from the mushroom's center. Many fungi can reproduce by the fragmentation of their hyphae. Each fragment develops into a new individual. Yeast, a type of unicellular fungus, reproduce by budding, in which a bump forms on the yeast cell, eventually partitioning from the cell and growing into a new yeast cell. V CLASSIFICATION OF FUNGI Fungi Classification 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 herbivores, 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 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. Sulfur Fungus A member of the phylum Basidiomycetes, the sulfur fungus is one of the largest of the edible fungi, reaching a width of several meters and a weight of several kilograms. It typically grows on trees, sometimes at heights that make harvesting these fungi difficult.Oxford Scientific Films 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 Chanterelle Mushroom The chanterelle mushroom is one of the most popular edible mushrooms. Harvested since Roman times throughout Europe, it is prized for its nutlike flavor. Unlike some species of edible fungus, larger specimens of chanterelles provide a better texture and flavor than do smaller specimens.Oxford Scientific Films Truffle Hunting A farmer leads a pig in a search for truffles in France. Though the fungus grows underground, its scent is so strong and distinctive that dogs and pigs can be trained to detect it.Adam Woolfitt/Woodfin Camp and Associates, Inc. Bread Yeast Bread yeast, Saccharomyces cerevisiae, or baker’s yeast, a type of fungi, reproduces by a process called budding. Bread yeast causes bread to rise by releasing carbon dioxide, which gets trapped in the dough. Here, microscopic yeast (left) have a macroscopic effect (right) on bread dough.Michael A. McClure. PHD/Phototake NYC;Safra Nimrod/Phototake NYC Penicillum Mold Penicillin is an important antibiotic derived from the mold Penicillum notatum, pictured here. Penicillin is effective against a wide range of disease-causing bacteria. It acts by killing bacteria directly or by inhibiting their growth.Andrew McClenaghan/Science Source/Photo Researchers, Inc. 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 Corn Smut The corn smut is a parasitic fungus that attacks the ears, stalks, and tassels of corn. Large, unsightly mycelial, or fungal, masses develop that eventually produce large quantities of black spores. Occasionally smut galls, or swellings, are produced, which are used as food in some areas of Central and South America.Gilbert Grant/Photo Researchers, Inc. 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 number of fungi cause fungal infections Fly Agaric Mushroom The brightly colored fly agaric mushroom is a deadly poisonous mushroom. Containing ibotenic acid and a number of other organic poisons, the fly agaric can cause severe damage to the central nervous system, blood vessels, kidneys, liver, and muscles. Symptoms, which may not become apparent for 8 to 12 hours or longer, include nausea, vomiting, and severe diarrhea and can lead to coma and death.David Thompson/Oxford Scientific Films 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. Powdery Mildew on a Plant Mildew is a parasitic fungi. The powdery mildew shown here often attacks the leaves of plants, usually those in shaded areas in humid regions. The white spores of the fungi produce a powdery pattern on the leaves and cause the leaves to curl and wither.Nigel Cattlin; Holt Studios International/Photo Researchers, Inc. 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. Contributed By:
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walls, or septa, divide the hyphae into cells containing one or two nuclei. Protoplasm flows through the opening in the septa to provide the cells with nutrients, which are stored in the hyphal walls as glycogen. Hyphae elongate from the tip. The entire mass of hyphae is collectively called the mycelium.
Black Bread Mold A member of the phylum Zygomycota, this black bread mold growing on a piece of stale bread shows the mycelium, or interwoven filaments that make up the vegetative portion of the fungus. The small dark spots are the fruiting bodies, or sporangia, from which the spores are released.John Cooke/Oxford Scientific Films  |
