Fungi

Fungi are eukaryotic organisms distinct from plants and animals and members of several other smaller kingdoms. Common fungi include mushrooms, conks, corals, jellies, puffballs, stinkhorns, morels, cups, truffles, lichens, yeasts, rusts, smuts, bread molds, mildews, and molds on bathroom tiles.

In 1959, R. H. Whittaker introduced a five-kingdom taxonomy that granted fungi equal status with plants and animals. The five-kingdom system has been supplanted by a multiple-kingdom classification, and species traditionally treated as fungi are now distributed across several kingdoms. Those believed to form a monophyletic lineage are assigned to kingdom Eumycota (often called kingdom Fungi). Mycology, the science devoted to fungi, still covers all traditional fungi.

Characteristics of Fungi

The Eumycota consist of eukaryotic, nonchlorophyllous heterotrophs that absorb nutrients from dead or living organic matter, have cell walls composed of chitin , and store excess energy as glycogen . The kingdom contains four phyla: Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. All true fungi have a definite cell wall throughout all developmental stages. Fungal cell walls are composed of chitin, the compound also found in arthropod exoskeletons (for example, lobster shells). Most fungi produce a vegetative mycelium (filamentous thallus) composed of hyphae that branch and extend via tip elongation, although some groups (like yeasts) consist only of individual cells. Hyphae (singular, hypha) are tube-like filaments with either single multinucleate cells (coenocytes) that lack septa (cross-walls) separating nuclei, or many septate cells containing one, two, or more nuclei.

Fungal Nutrition: Saprobes, Parasites, and Mutualists

A fungal thallus may be as small as a single microscopic cell (baker's yeast, Saccharomyces cerevesiae ) or exceedingly large (Armillaria gallica, the several-acre-sized "humungous fungus" reported in 1992 as the world's largest organism). Sporocarp (fruit body, or "mushroom" size) also ranges from microscopic to meters in diameter (Bridgeoporus nobilissimus, an endangered bracket fungus found on noble fir trees).

Not unexpectedly, such a diverse kingdom manifests several different life cycles. Virtually all fungi produce spores. Both asexual and sexual spores may germinate to form vegetative thalli from primary and secondary mycelia. Thalli may be haploid dominant, diploid dominant, or exhibit haplo-diploid alternation of generations. Here it is important not to confuse the chromosomal state of individual nuclei (haploid versus diploid) with the number of nuclei per cell (monokartyotic versus dikaryotic). Fungi are unusual in that they often exhibit dikaryotamy, wherein hyphal cells contain two (usually haploid) nuclei that migrate, multiply, and divide together.

Although superficially similar to plants, fungi are probably more closely related to animals. Like animals, fungi lack chlorophyll and do not photo-synthesize, must obtain nutrients from organic sources, and store energy as glycogen instead of starch. Unlike animals, however, fungi do not engulf, but rather absorb, their nutrients after breaking them down via enzymatic action, earning them the nickname "absorbotrophs."

Fungi absorb their nutrients in three different ways: (1) saprobes decompose dead organic matter; (2) parasites feed on living hosts; and (3) mutualists live in symbiotic unions with other living organisms. Saprophytic fungi, such as edible meadow mushrooms (Agaricus campestris ), shiitake (Lentinula edodes ), and oyster mushrooms (Pleurotus ostreatus ), decompose dead plant and animal tissue by releasing enzymes from hyphal tips, thereby recycling organic materials back into the surrounding environment. Parasitic fungi also use enzymes to break down living tissue, usually sapping the energy of the host and frequently causing its demise.

Lichens and mycorrhizae are two important mutualistic associations. Lichens represent partnerships between a fungus (mycobiont) and one or more algae (phycobiont). Although there are a few basidiolichens, almost all lichen mycobionts are Ascomycota (approximately 20,000 ascolichens described thus far). Mycorrhizae are symbiotic or non to slightly pathogenic fungus-plant unions formed with approximately 85 percent of the vascular plants. Mycorrhizae are identified as ectomycorrhizal, arbuscular mycorrhizal (AM), ericoid, orchid, arbutoid, and monotropoid based on anatomical form and association. Ectomycorrhizal fungi (predominantly basidiomycetes such as boletes, amanitas, and coral fungi) form thick mycelial mantles around rootlets of many trees (oaks, firs, pines, poplars) to which they transport water and minerals from the soil, receiving sugars and other organic nutrients in return. AM fungi (in the Zygomycota order Glomales) form an endo-infection by penetrating rootlets to form coils and vesicles or finely branched arbuscules. The last four mycorrhizal types are specific to individual plant groups.

Ecological and Economic Importance

Fungi have a profound biological and economic impact. As decomposers, plant pathogens, and symbiotic partners, their ability to grow anywhere, on anything, makes them both beneficial and harmful recyclers of carbon and nitrogen. Beneficially, they are used as food (mushrooms, truffles) and in baking and brewing (yeasts). They are being developed to detoxify pollutants (soil fungi), control insects (pathogenic Zygomycota), and regulate plant growth (pathogenic Ascomycota). Detrimentally, rusts, smuts, and molds cost billions of dollars through crop disease and spoilage while forest pathogens such as the honey mushroom (Armillaria ostoyae ) and root-butt rot (Heterobasidion annosum ) similarly threaten the timber industry. Some are toxic when eaten, such as the infamous destroying angel (Amanita phalloides ). Natural LSD, a hallucinogen produced by ergot (Claviceps purpurea ), is associated with medieval hysterical frenzies produced by consumption of infected grain, and the aflatoxin produced by Aspergillus flavus in improperly stored grain is one of the most potent carcinogens yet discovered. As human and animal pathogens, fungi cause infections that range from the vexing (athlete's foot, yeast infections) to life threatening (histoplasmosis). Fortunately, other fungi (such as Penicillium ) have been used to develop modern antibiotics and beneficial immunosuppressants .

Classification

With the introduction of deoxyribonucleic acid (DNA) sequence analyses, previous fungal evolutionary theory is undergoing rapid transformation, not surprising when one considers the extremely fragmentary fungal fossil record. Mycologists generally agree that true fungi (with animals and plants) diverged from the protozoan fungi (for example, slime molds and Oomycota) before fungi and animals diverged from plants. The chytrids separated from the remaining phyla approximately 550 million years ago, followed by splintering of the Zygomycota from the other two phyla, which share septate hyphae and dikaryotic stages in their life cycles. The Ascomycota split from the Basidiomycota approximately 400 million years ago, followed by an increase in fungal diversity throughout the Paleozoic. Most yeasts and "asco-molds" are thought to have evolved in approximately the last 200 million years.

While most plants and vertebrates appear to have been described, most fungi have yet to be discovered or named. The 1995 edition of the Dictionary of Fungi counts 566,360 described species (about 100,000 in the Eumycota) but notes that there may be approximately 1.5 million species of fungi in the world. Elias Fries (1794–1878), often regarded as the "father of mycology," classified fungi based primarily on spore print color and sporocarp appearance. Friesian-based nomenclature, reflecting similarity of form (phenetic) rather than genetic relationships, is still used in many field guides. Now, however, taxonomists are integrating molecular and morphological characters to develop natural classifications that more adequately reflect evolutionary relationships. Realizing that fungal taxonomy and nomenclature will remain somewhat fluid until new species and data are analyzed and integrated, most mycologists generally accept the classification below.

Chytridiomycota. The fact that chytrids alone among the Eumycota produce motile zoospores explains why their phylum is sometimes assigned with the flagellate oomycetes to kingdom Chromista. Chytrids possess posteriorly uniflagellate spores, mitochondria with flattened cristae, and cell walls composed of glucan and chitin. Among the simplest and smallest fungi, they live as saprobes in water and damp organic-rich habitats, or as parasites on invertebrates, plants, and other fungi. The so-called "frog chytrids" (such as Batrachochytrium dendrobatidis ) are implicated in the current worldwide decline of amphibian species. Other chytrids are host-specific "rumen fungi" (such as Piromyces communis ) that thrive in anaerobic conditions in the guts of herbivores, such as cattle and sheep. There are five orders and about 800 species of chytrids recognized thus far.

Unicellular members of the order Chytridiales lack a mycelial stage and consist of a central body with a few rudimentary appendages (haustoria) that attach to and invade the host tissue. Other chytrids, such as the Blastocladiales, develop true mycelia with sporangia and male/female gametangia that produce uniflagellate zoospores. Chytrid thalli may be either haploid or diploid, and some, like the aquatic chytrid Allomyces, exhibit an isomorphic alternation of generations with similar appearing sexual and asexual zoospores.

Zygomycota. The chytrids and this phylum are assigned the two "bottom" branches of the fungal evolutionary tree. There are more than 1,000 species in two classes (Trichomycetes, Zygomycetes) and ten orders, representing a diverse assemblage of saprobes, soil fungi, obligate insect and fungal parasites, and mycorrhizal formers. Common representatives of the saprobic order Mucorales include the dung-inhabiting "cap-thrower" (Pilobolus ), the black bread mold (Rhizopus stolonifera ), and Phycomyces blakesleeanus, sometimes referred to as the "body in the basement" because of the rapid growth of long, hairlike sporangiophores over a substrate under the right conditions.

Zygomycota are characterized by large, thick-walled, coenocytic zygospores and hyphae with relatively thin walls composed of chitin and chitosan. Both asexual sporangiospores and sexual zygospores germinate into haploid mycelia, with the hyphae functioning as gametangia during the sexual stage. In Rhizopus, for instance, close proximity of two hyphal strands of different mating types chemically triggers each to grow branches toward the other to form septate suspensor cells and gametangia. Eventual fusion produces a diploid zygosporangium that undergoes meiosis to become a thick-walled zygospore with large numbers of haploid nuclei.

Ascomycota. In addition to most lichens and so-called "imperfect fungi," about 33,000 species of unicellular yeasts, green and black molds, powdery mildews, morels, cup fungi, and ascotruffles ("true" truffles) belong to this phylum. The phylum is characterized by ascospores produced within a saclike sporangium called an ascus. Mycelia (more complex than Zygomycota mycelia) are composed of septate hyphae with chitin-glucan hyphal walls. Most species produce specialized fruiting bodies called ascocarps whose details of structure help define different species, classes, or orders. Nonascocarpic representatives (such as unicellular yeasts and mildews that reproduce primarily by budding) do not form mycelia. Both sexual and asexual reproduction are found within this phylum.

Basidiomycota. This phylum, which also features septate hyphae and chitin-glucan cell walls, is characterized by basidiospores borne upon a club-like structure called a basidium. Approximately 22,500 species are assigned to three classes: Basidiomycetes, Teliomycetes, and Ustomycetes. Basidiomycetes include mushrooms, polypores, crusts, corals, clubs, basidiolichens, and jellies, which propel their spores, and "gastromycetes" (or "stomach fungi") that passively release their spores (puffballs, basidiotruffles ["false truffles"], stinkhorns, and birds' nests). Teliomycetes (rusts) and Ustomycetes (smuts) are obligate parasites of insects or plants. Rusts and smuts have exceedingly complex cycles involving up to five separate spore stages and multiple hosts. This ability to produce spores on different hosts in multiple ways presents a significant economic challenge to agriculture.

Oomycota. Oomycetes (kingdom Chromista) are distinguished from true fungi by having glucan-cellulose cell walls that only occasionally incorporate small amounts of chitin. These algae-like fungi occur in aquatic or moist terrestrial habitats as single cells or mycelial mats composed of multinucleate, nonseptate hyphae. Their life cycle generally mirrors that of plants, with a transitory haploid stage. Both resting oospores and motile zoospores are diploid, the latter propelled by two unequal flagella (tinsel type plus whiplash). This phylum contains about 700 species in nine orders, including the generally saprophytic water molds (Saprolegniales) and the pathogenic Peronosporales and Pythiales. Most water molds are saprophytic, but there are a number of parasites that invade plants (white rusts, downy mildews, tobacco blue mold) or fish. Among the economically significant wilts, blights, and pathogens are Phytophthora infestans (responsible for the Irish potato famine in the 1850s), Plasmopara viticola (the causative agent of downy mildew of grapes), and the fish parasite Saprolegnia parasitica, a twenty-first-century threat to salmon migrating through dams in western North America.

see also Alternation of Generations; Kingdom; Mycorrhizae; Plant Pathogens and Pests; Symbiosis

Lorelei L. Norvell

Bibliography

Alexopoulos, C. J., C. W. Mims, and M. Blackwell. Introductory Mycology, 4th ed. New York: John Wiley & Sons, 1996.

Arora, D. Mushrooms Demystified: A Comprehensive Guide to the Fleshy Fungi. Berkeley, CA: Ten Speed Press, 1986.

Hanlin, R.T., and M. Ulloa. Atlas of Introductory Mycology, 2nd ed. Winston-Salem, MA: Hunter Textbooks Inc., 1978.

Fungi

Fungi (plural of fungus) are one of the five kingdoms of organisms. Kingdoms are the main divisions into which scientists classify all living things on Earth. The other kingdoms are: Monera (single-celled organisms without nuclei), Protista (single-celled organisms with a nucleus), Plantae (plants), and Animalia (animals).

Fungi constitute a large and diverse group of organisms. The kingdom of fungi is divided into four major groups: conjugating fungi, sac fungi, club fungi, and imperfect fungi. Mushrooms, molds, yeasts, and mildew are all fungi. Biologists have estimated that there are more than 200,000 species of fungi in nature, although only about 100,000 have been identified so far. The scientific study of fungi is called mycology.

General characteristics

The different groups of fungi have different levels of cellular organization. Some groups consist of single-celled organisms that have a single nucleus per cell. (A nucleus is a membrane-enclosed structure within a cell that contains the cell's genetic material and controls its growth and reproduction.) Other groups consist of single-celled organisms in which each cell has hundreds or thousands of nuclei. Still others consist of multicellular organisms that have one or two nuclei per cell. The bodies of multicellular fungi usually consist of slender, cottony filaments called hyphae. A mass of hyphae is called a mycelium. The mycelium carries on all the life-maintaining processes of the organism, including sexual reproduction (in most species).

Unlike plants, fungi do not contain chlorophyll (green pigment) and thus cannot create their own food through photosynthesis (the chemical process by which plants containing chlorophyll use sunlight to convert carbon dioxide and water to carbohydrates, releasing oxygen as a byproduct).

Most species of fungi grow on land and obtain their nutrients from dead organic matter. Most species feed by secreting enzymes, which partially break down the food. The fungi then absorb the partially digested food to complete digestion internally. Because fungi (along with bacteria) help decompose dead plants, animals, and other organic matter, they serve an important ecological role. They release large amounts of carbon dioxide into the atmosphere and recycle nitrogen and other important nutrients for use by plants and other organisms.

Some fungi are parasites, living in or on another organism (called a host) from which they obtain their nutrients. This relationship usually harms the host. Such parasitic fungi usually have specialized tissues called haustoria that penetrate the host's body. Most of the diseases that afflict agricultural plants are caused by parasitic fungi. Some examples are corn smut, black stem rust of wheat and barley, and cotton root rot. Some species of fungi also can parasitize animals. Fungi that parasitize humans cause diseases such as athlete's foot, ringworm, and yeast infections.

Words to Know

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Hyphae: Slender, cottony filaments making up the body of multicellular fungi.

Nucleus: Membrane-enclosed structure within a cell that contains the cell's genetic material and controls its growth and reproduction.

Parasite: Organism living in or on another organism (called a host) from which it obtains nutrients.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to convert carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

Symbiosis: Close relationship between two organisms of different species, which often benefits each member.

Conjugating fungi

There are about 600 species of conjugating fungi. Most species are land-based and feed on organic matter, although there are a few parasitic species. The algaelike conjugating fungi have a continuous mycelium containing hundreds or thousands of nuclei, with no divisions between them. Species of conjugating fungi cause potato blight, downy mildew, black bread mold, and water mold (which affects dead leaves and sticks in water).

Sac fungi

Sac fungi are so-named because many species in this group reproduce sexually by forming a spore-filled structure called an ascus, which means literally "a sac." This large group of fungi includes many species

that are beneficial to humans. For example, yeasts are a major group of sac fungi. Different yeasts are used by bakers, brewers, and vintners to make their bread, beer, or wine. Truffles, regarded as a food delicacy, are underground sac fungi that grow in association with tree roots.

Some species of sac fungi appear as blue-green molds on fruits, vegetables, and cheeses. Several other species are important for the making of cheeses, such as blue cheese.

Some other sac fungi cause plant diseases. These include chestnut blight (a disease that virtually wiped out the American chestnut as a mature forest tree) and Dutch elm disease.

Lichens

A lichen is the product of a symbiotic (mutually beneficial) relationship between fungi and blue-green or green algae. The resulting structure resembles neither species. Typically, the algae supply carbohydrates to the fungi and the fungi supply nitrogen and other nutrients to the algae. Lichens can be very colorful, ranging from bright reds and oranges to yellows and greens, with white, gray, and black hues.

Many lichens can inhabit harsh environments and withstand prolonged periods of drought. In the temperate region of North America, lichens often grow on tree trunks and bare rocks and soil. In Antarctica, they have been found growing upon and within rocks. In the Arctic, the lichen species commonly known as reindeer mosses are important in the diets of caribou and reindeer.

Club fungi

Club fungi species reproduce sexually by forming spores on top of club-shaped structures called basidia. The club fungi are believed to be closely related to the sac fungi. This large group includes species that are known as mushrooms, toadstools, earthstars, stinkhorns, puffballs, jelly fungi, coral fungi, and many other interesting names. Some species, such as the rusts and smuts, cause disease in agricultural grains. Other species, such as the fly agaric, produce chemical hallucinogens (chemicals that induce visions) and have been used by numerous cultures in their religious ceremonies.

A significant species of club fungi is called mycorrhizae, which means "fungus root." Mycorrhizal fungus form a symbiotic relationship with many types of plant roots. (Symbiosis is the close association between two organisms of different species, which often benefits each member.) The fungus typically supplies nitrogen-containing compounds to the plant, and the plant supplies carbohydrates and other organic compounds to the fungus. Mycorrhizal fungus are very important for the growth of orchids and many trees, including pines and beeches.

Imperfect fungi

Mycologists have never observed the sexual reproduction of fungi in the imperfect fungi group. Since this part of their life cycle is missing, they are referred to as imperfect fungi. These fungi may have lost their sexual phase through the course of evolution. Species in this group produce plant and animal diseases. Athlete's foot and ringworm in humans are caused by imperfect fungi.

Some species in this group appear as blue-green molds on fruits, vegetables, and cheeses. Several other species are important for the making of cheeses such as blue cheese, Roquefort, and Camembert. Certainly the best known product obtained from this group of fungi is penicillin, the first widely used antibiotic. Penicillin was first discovered in the mold Penicillium notatum by Scottish bacteriologist Alexander Fleming (1881–1955) in 1928. Scientists now know it is produced by other species in this group, as well.

[See also Fermentation; Hallucinogens; Parasite; Yeast ]

Fungi , kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites , symbionts, or saprobes (see saprophyte ). Previously classified in the plant kingdom, fungi are nonmotile, like plants, but lack the vascular tissues (phloem and xylem) that form the true roots, stems, and leaves of plants. Most coenocytic (multinucleated) or multicelluar fungi are composed of multiple filaments, called hyphae, grouped together into a discrete organism called a mycelium. The cell walls of most fungi are of chitin compounds instead of cellulose; a group fungi known as cryptomycota lack chitinous cell walls. In many ways fungi are more closely related to animals than to plants, and they have been thought to share a common protist ancestor with animals. A recent classification system suggested by nucleic acid (genetic material) comparisons places the fungi with the animals and the plants in an overarching taxonomic group called the eukarya.

Most fungi are capable of asexual and sexual reproduction . Asexual reproduction is by fragmentation or spore formation. Those that reproduce sexually produce gametes in specialized areas of the hyphae called gametangia. The gametes may be released to fuse into spores elsewhere, or the gametangia themselves may fuse. In some cases dikaryons [ di  = two, karyo  = nucleus], which are found only among fungi, result when unspecialized hyphae fuse but their nuclei remain distinct for part of the life cycle.

Unlike algae or plants, fungi lack the chlorophyll necessary for photosynthesis and must therefore live as parasites or saprobes (see parasite ). Typically they release digestive enzymes onto a food source, partially dissolving it to make the necessary organic or inorganic nutrients available. Some parasitic types obtain their food directly from the cells of a living food source. Some types of fungi are involved in symbiotic relationships, for example, lichens (a combination of a fungus and a green alga or a cyanobacterium ) and the mycorrhizae (symbiosis between a fungus and the roots of a vascular plant).

Some fungi are pathogenic to humans and other animals. Such diseases are called mycoses or fungal infections . Some molds, in particular, release toxic chemicals called mycotoxins that can result in poisoning or death. Various fungi can also cause serious damage to fruit harvests and other crops (see diseases of plants ).

Types of Fungi

The 100,000 identified species of organisms commonly classed together as fungi are customarily divided into four phyla, or divisions: Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota.

Zygomycota includes black bread mold and molds, such as those of the genus Glomus, that form important symbiotic relationships with plants. Most are soil-living saprobes that feed on dead animal or plant remains. Some are parasitic of plants or insects. They reproduce sexually and form tough zygospores from the fusion of neighboring gametangia. There is no distinguishable male or female.

Ascomycota includes yeasts , the powdery mildews, the black and blue-green molds, edible types such as the morel and the truffle , and species that cause such diseases of plants as Dutch elm disease, chestnut blight, apple scab, and ergot . There are over 50,000 species, about 25,000 of which occur only in lichens. In ascomycetes, the hyphae are subdivided by porous walls through which the cytoplasm and the nuclei can pass. Their life cycle is a complex combination of sexual and asexual reproduction.

Basidiomycota includes the gill fungi (most mushrooms ), the pore fungi (e.g., the bracket fungi, which grow shelflike on trees, and an edible type called tuckahoe), and the puffballs . It also includes the fungi that cause smut and rust in plants. Like ascomycetes, the hyphae are subdivided by porous walls. In basidiomycetes, two hyphae fuse to form a dikaryotic mycelium (a mycelium in which both nuclei remain distinct). These mycelia differentiate into reproductive structures called basidia that make up the basidiocarp (the body popularly known as the mushroom cap). The nuclei then fuse and undergo meiosis, creating spores with one nucleus each. When these spores germinate, they produce hyphae, and the process begins again.

Deuteromycota comprises a miscellaneous assortment of fungi that do not not fit neatly in other divisions; they have in common an apparent lack of sexual reproductive features. Also called Fungi Imperfecti, the group includes species that help create Roquefort and Camembert cheeses, that cause diseases of plants and of animals (e.g., athlete's foot and ringworm ), and that produce penicillin. A number of the fungi classified as deuteromycetes have been found to be asexual stages of species in other groups, and some classification schemes consider the deuteromycetes a class under Ascomycota.

Usefulness of Fungi

Fungi are valuable economically as a source of antibiotics, of vitamins, and of various industrially important chemicals, such as alcohols, acetone, and enzymes, as well as for their role in fermentation processes, as in the production of alcoholic beverages, vinegar, cheese, and bread dough. They are extremely important in soil renewal, through the decomposition of organic matter (see humus )—a function unwelcome when it results in the rotting of clothing and other goods and the spoilage of foods.

Bibliography

See C. M. Christensen, The Molds and Man (3d. rev. ed. 1965); J. Webster, Introduction to Fungi (1980); B. Kendrick, The Fifth Kingdom (1985); A. Chandra, Elsevier's Dictionary of Edible Mushrooms (1989); C. T. Ingold and H. J. Hudson, The Biology of Fungi (6th ed. 1993); G. W. Hudler, Magical Mushrooms, Mischievous Molds (1998); P. Roberts and S. Evans, The Book of Fungi (2011).

Fungi

Fungi play an essential role in breaking down organic matter and thereby allowing nutrients to be recycled in nature. As such, they are important decomposers and without them living communities would become buried in their own waste. Some fungi, the saprobes, get their nutrients from nonliving organic matter, such as dead plants and animal wastes, clothing, paper, leather, and other materials. Others, the parasites , get nutrients from the tissues of living organisms. Both types of fungi obtain nutrients by secreting enzymes from their cells that break down large organic molecules into smaller components. The fungi cells can then absorb the nutrients.

Although the term fungus invokes unpleasant images for some people, fungi are a source of antibiotics , vitamins, and industrial chemicals. Yeast , a kind of fungi, is used to ferment bread and alcoholic beverages. Nevertheless, fungi also cause athlete's foot, yeast infections, food spoilage, wheat and corn diseases, and, perhaps most well known, the Irish potato famine of 1843–1847 (caused by the fungus Phytophthora infestans ), which contributed to the deaths of 250,000 people in Ireland.

Fungi are not plants, and are unique and separate forms of life that are classified in their own kingdom. Approximately 75,000 species of fungi have been described, and scientists estimate that more than 90% of all fungi species on the planet have yet to be discovered. The fungi body, called mycelium , is composed of threadlike filaments called hyphae . All fungi can reproduce asexually by cell division, budding, fragmentation, or spores, although some reproduce sexually.

The main groups of fungi are chytrids, water molds, zygosporangium-forming fungi, sac fungi, and club fungi. Chyrids live in muddy or aquatic habitats and feed on decaying plants, though some live as parasites on living plants, animals, and other fungi. Water molds, distantly related to other fungi, play an important role as decomposers in aquatic habitats. Some, however, live as parasites on aquatic animals and terrestrial plants, including potato plants that can be destroyed by certain types of water molds. Zygosporangium-forming fungi also can be either saprobes, such as the well-known black bread mold , or parasites on insects, such as houseflies. Sac fungi, of which more than 30,000 species are known, include the yeast used to leaven bread and alcoholic beverages. However, many of these fungi also cause diseases in plants. Club fungi, numbering more than 25,000 species, include mushrooms, stinkhorns, and puffballs. While some fingi are edible, others produce deadly poisons.

See also Candidiasis; Chitin; Fermentation; Fungal genetics; History of the development of antibiotics; Lichens; Winemaking

fungi Subdivision of Thallophyta, plants without differentiation into root, stem, and leaf; they cannot photosynthesize, and all are parasites or saprophytes. Microfungi are moulds, as opposed to larger fungi, which are mushrooms and toadstools. Yeasts are sometimes classed with fungi.

Species of moulds such as Penicillium, Aspergillus, etc., are important causes of food spoilage in the presence of oxygen and relatively high humidity. Those that produce toxins (mycotoxins) are especially problematical. On the other hand species of Penicillium such as P. cambertii and P. roquefortii are desirable and essential in the ripening of certain cheeses.

A number of larger fungi (mushrooms) are cultivated, and other wild species are harvested for their delicate flavour. The mycelium of smaller fungi (including Graphium, Fusarium, and Rhizopus species) are grown commercially on waste carbohydrate as a rich source of protein for food manufacture. See mycoprotein.

Fungi

Mycology is the study of fungi (mykes, Greek for "fungi," and ology, meaning "study of"). Most contemporary mycologists consider the fungi to be in two kingdoms: kingdom Fungi with five phyla and kingdom Stramenopila with three phyla. The total number of fungi in the world is estimated to be over 1.5 million with less than 5 percent of the species described. Some mycologists believe that the total number of fungi may be more than 2 million. Two other kingdoms are sometimes mistaken for fungi: the slime molds (kingdom Myxomycota), which have a creeping plasmodium, and the bacteria and actinomycetes (kingdom Monera).

Structure and Life Cycle

Fungi are nonphotosynthetic, lacking the chlorophyll of higher plants and algae, and are recognized by their fruiting bodies, which is the visible part of the fungus. Examples include mushrooms, puffballs, molds, cup fungi, and morels. The vegetative structure consists of minute filamentous cells called hyphae, which are microscopic in size, usually from 1 micron to 10 microns in diameter. An aggregate of hyphae is called a mycelium, which is the thallus or vegetative part of the fungus plant known as spawn in the mushroom industry. In the kingdom Fungi, the mycelium has one haploid nucleus per cell (only one set of chromosomes) or is dikaryotic (two haploid nuclei per cell). In contrast, in the kingdom Stramenopila, mycelium has diploid nuclei (one nucleus with chromosomes from both parents). In both kingdoms, the mycelium has rigid cell walls usually composed of chitin (a complex carbon compound ), although it is infrequently made up of cellulose in kingdom Fungi.

In both kingdoms, fungi obtain their nutrition by excreting enzymes into the host or any organic material, which is then broken down and absorbed into the hyphal cell to provide the nutrition necessary for growth. Fungi function in the ecosystem as saprophytes, or decomposers. They break down dead organic matter as parasites by attacking living hosts or host cells, and as mycorrhizae (mycor, meaning "fungi," and rhizae, meaning "root") by forming jointly beneficial unions with the roots of higher plants. Fungi and algae combine to form a plant called a lichen. Only fungi and bacteria decompose various kinds of organic matter and change complex organic structures, such as plant cell walls containing lignin or the chitinous exoskeletons of insects, into simple carbohydrates that can then be assimilated by a wide variety of organisms.

The hyphae grow until they form an extensive mycelium of fungal tissue. At this point a young fruiting body initial (or button) begins to form and develops into a mature fruiting body. In some phyla fruiting bodies are large and variously recognized as mushrooms, boletes, puffballs, conks, cup fungi, morels, false morels, truffles, and witches' butter, to mention only a few. However, many of the aquatic fungi, molds, and other fungi (such as the yeasts) form minute fruiting structures that can only be seen with the aid of a magnifying glass or a microscope.

The function of the fruiting body is to form a tissue in or on which the spore-bearing surface is formed. The spore-bearing surface covers the gills of a mushroom, is inside the tubes of the bolete, or forms a spore mass inside the puffball and truffle. The spore of the fungus serves the same purpose as the seed of the green plants, but the spore is composed of only one or several simple cells. The spore forms following meiosis in sexual cells located in the spore-bearing surface. In the mushrooms, boletes, cup fungi, and morels, for example, the nearly mature spores are forcibly discharged at maturity from the spore-bearing surface. If one blows over the surface of a cup fungus at maturity, a small cloud (the puffing or a discharge of the spores) can be seen. However, in other fungi such as the puffballs, stinkhorns, and truffles, no forcible discharge occurs. The powdery spore mass of the puffball is often discharged through a pore in the top that forms at maturity. The greenish-gray spore mass of the stinkhorn emits a strong odor, which attracts insects that eat, contact, and spread the spores. The truffle, which is found at the surface of or beneath the soil, gradually matures and produces strong smells that attract small rodents that dig up and eat the fruiting bodies and distribute the spores.

Molds, such as Penicillium, produce microscopic asexual fruiting bodies that in turn produce asexual spores called conidia on structures known as conidiophores. Some yeast cells bud and reproduce asexually. Other fungi, such as the bread mold Rhizopus, produce asexual fruiting structures known as sporangiophores that support sacs called sporangia in which asexual spores are produced. Aquatic fungi also produce a variety of asexual spores, some of which are motile (called zoospores ). These spores swim to a potential host, retract their flagella , and enter the host producing an oval fruiting body with a feeding tube or minute root-like rhizoids. The zoospores of the kingdom Fungi have one whiplash flagellum, while in the kingdom Stramenopila the zoospores have two flagella, one whiplash and one tinsel type, that move rapidly to propel the zoospore. Spores, either sexual or asexual, motile or nonmotile, usually germinate to form thin cylindric hyphal cells that rapidly elongate and branch to form the mycelium of the new fungus plant.

Nutrition

The fungus cell must grow into the host plant or a bit of organic material in order to gain nutrition from it. This is achieved by discharging enzymes (called exoenzymes) from the cells. Complex carbohydrates and proteins are broken down by this process and then are absorbed by the hyphae. The nutrients can then be translocated from one cell to another. The growth of most fungi is indeterminate (that is, it never stops) because the fungus must continue to grow into new areas to seek new sources of food. The typical fairy ring represents a visible bright green grass ring where the active mycelium is, and it is along this ring that the mushrooms will fruit. Each year the diameter of the ring will increase while the mycelium dies out in the middle because the food base is exhausted.

Mycorrhizae

Mycorrhizal fungi invade the healthy outer cells of the tiny rootlets of higher plants. Ectomycorrhizae surround the rootlet with a sheath of fungal cells, and special hyphae penetrate between the cortical cells of the root-let and exchange nutrients with the higher plant, usually a tree or a shrub. Endomycorrhizae called VA (vesicular arbuscular) mycorrhizae form oval storage cells (vesicles) and minute branchlike processes (arbuscules) in the root cells of the host where nutrients are exchanged. Because fungi do not carry out photosynthesis and cannot make their own sugar, the mycorrhizal fungus obtains moisture and carbohydrates from its green plant host and, in return, provides the host with nitrogen, phosphorus, zinc, and other essential compounds. It does this using the miles of tiny mycelium to successfully compete for phosphorus and nitrogen, which extends the root system of the green plant. Most of the woody plants such as the pine, oak, birch, and beech have ectomycorrhizae, and most herbaceous plants such as grass, corn, wheat, and rye have VA endomycorrhizae.

Food, Drugs, and Poisons

Fungi play a major role in the diet of humans. Yeasts (Saccaromyces cerevisiae ) are used in the process of fermentation, in which they break down carbohydrates to liberate carbon dioxide and to produce alcohol. Gin is made when juniper berries are fermented, wine from grapes, beer from grains, bourbon from corn, and scotch from barley. Yeasts are also used in making Limburger cheese, yogurt, and Kombucha tea. Baker's yeast produces a high proportion of CO₂, which causes the dough to rise. Molds, generally species of Penicillium, are used to produce cheese such as blue, Roquefort, and Camembert.

The new age of antibiotics was ushered in with Sir Alexander Fleming's discovery of penicillin in 1929. It was first produced by the blue-green mold Penicillium notatum. Many other antibiotics are produced from Actinomycetes. On the other hand, aflatoxins produced by species of Aspergillus cause food spoilage and are carcinogenic. Mushrooms also produce toxins that only affect humans when they are eaten. Examples of these are the amatoxins and phallotoxins produced by a mushroom, Amanita virosa, that are often fatal to humans; muscarine and muscimol produced by the fly agaric, Amanita muscaria, are usually not fatal. Hallucinogens such as psilocybin and psilocin are produced by several species of mushrooms including Psilocybe cubensis and the protoplasmic poison monomethyhydrozine (MMH) by the false morel Gyromitra esculenta.

Fungal Diseases

Fungi that are parasitic on humans include the common dermatophytes on the skin, hair, and nails, causing such diseases as barber's itch and athlete's foot (Microsporium canis ). More serious diseases, such as Histoplasma capsulatum or histoplasmosis found in warm temperate climates and coccidiomycosis (Coccidioides immitis ) in arid areas, grow in bird dung and soil, producing a respiratory infection in humans that is occasionally fatal. North and South American blastomycosis, sporotrichosis, and other diseases caused by fungi attack tissues and organs within the body and are incapacitating or fatal to their victims.

Diseases that affect major economic plants have historically impacted people. The ergot (Claviceps purpurea ), which infects the grains of rye, produces deadly brown specks in bread and led to deformity and the death of thousands of people in the Middle Ages. The European grape was saved from the grips of the downy mildew (Plasmopara viticola) in the 1800s by the discovery of Bordeaux Mixture (copper sulfate and lime); the discovery gave birth to plant pathology as a science. The European potato famine, caused by the potato blight fungus (Phytophthora infestans ), in the years 1845 to 1847 forced more than a million Irish to flee from Ireland. In the United States, the chestnut blight (Cryphonectria parasitica ) has reduced the tall and highly valued American chestnut from the eastern forests to a rare shrub, while the Dutch elm disease (Ceratocystis ulmi ) threatens to eliminate the American elm. Scientists struggle continually to produce resistant strains of wheat that will not be parasitized by the wheat rust (Phytophthora infestans ) and corn that will be resistant to the corn smut (Ustilago maydis ). Mexicans and Hispanic Americans cook the infected ears in many ways and consider them to be a delicacy.

The shelves of every supermarket have the meadow mushroom (Agaricus bisporus ) and specialty mushrooms like Shiitake (Lentinus edodes ), oyster shell (Pleurotus ostreatus ), and the Portabello (Agaricus sp.) for sale. In fact, the leading agricultural crop in Pennsylvania is mushrooms.

see also Chestnut Blight; Dutch Elm Disease; Interactions, Plant-Fungal; Lichen; Mycorrhizae; Pathogens; Plant Systematics; Potato Blight; Taxonomy; Taxonomy, History of.

Orson K. Miller Jr.

Bibliography

Alexopoulos, C. J., C. W. Mims, and M. Blackwell. Introductory Mycology. New York:John Wiley & Sons, 1996.

Kavaler, L. Mushrooms Molds and Miracles. New York: The New American Library, 1965.

Large, E. C. The Advance of the Fungi. New York: Henry Holt & Co., 1940.

Miller, O. K. Mushrooms of North America. New York: E. P. Dutton Inc., 1973.

Rolfe, R. T. and F. W. Rolfe. The Romance of the Fungus World. New York: Dover Publications Inc., 1925.

Schaechter, E. In the Company of Mushrooms. Cambridge, MA: Harvard University Press, 1997.

Stamets, P. Growing Gourmet & Medicinal Mushrooms. Berkeley, CA: Ten Speed Press,1993.

fungi A group of organisms formerly regarded as simple plants lacking chlorophyll but now classified in a separate kingdom, Fungi. They can either exist as single cells or make up a multicellular body called a mycelium, which consists of filaments known as hyphae. Most fungal cells are multinucleate and have cell walls composed chiefly of chitin. Fungi exist primarily in damp situations on land and, because of the absence of chlorophyll, are either parasites or saprotrophs on other organisms. The principal criteria used in classification are the nature of the spores produced and the presence or absence of cross walls within the hyphae (see Ascomycota; Basidiomycota; Deuteromycota; Zygomycota). See also lichens.

Fungi The taxonomic kingdom that comprises eukaryotic, non-photosynthetic (see photosynthesis) organisms, which obtain nutrients by absorbing organic compounds from their surroundings. Fungi may be unicellular, filamentous (see mycelium), or plasmodial (i.e. forming an acellular, mobile feeding structure consisting of a mass of naked protoplasm with many nuclei), and most have cell walls containing chitin. Many fungi live as saprotophs and are important agents of organic decomposition. Others live as symbionts or parasites (see parasitism). Some can cause disease in plants or animals. Fossils are rare, but fungi are believed to have left the sea about 400 Ma ago, when the first plants colonized dry land.

Fungi One of the taxonomic kingdoms, comprising eukaryotic, non-photosynthetic organisms, which obtain nutrients by the absorption of organic compounds from their surroundings. Fungi usually have chitin-containing cell walls and may be unicellular, filamentous (mycelial), or plasmodial. They may live saprotrophically, symbiotically, parasitically, etc. Some can cause disease in plants or animals, including humans. Distribution is cosmopolitan. As fungi generally lack hard parts, they are rarely found as fossils, but possible thread-like representatives have been found in Precambrian rocks. They probably left the sea about 400 million years ago, when the first plants colonized the land.

Fungi One of the three multicellular kingdoms, along with the Plantae (plants) and Animalia (animals). Although resembling plants, Fungi feed by ingesting organic matter, whereas plants are autotrophic and require only inorganic substances as nutrients. As fungi generally lack hard parts they are rarely found as fossils, but thread-like representatives have been found in Precambrian rocks. They probably left the sea about 400 million years ago, when the first plants colonized the land.

fun·gi / ˈfənˌjī; -ˌgī/ • plural form of fungus.

fungi •Haggai • Belgae • gilgai • fungi •sarcophagi • mamaguy • assegai