Fungi

V

Fungus Ecology

Spores and hyphal fragments of fungi are carried for long distances in the atmosphere.

Water habitats often abound with chytrids and water moulds. A number of ascomycetes and fungi lacking a sexual phase also frequent fresh or salt water. Many fungi have been discovered in polluted rivers and streams. These fungi participate in the natural decomposition of sewage. Some of these species are of special interest because they may cause disease in human beings.

Soil is a natural habitat for saprophytic fungi, which live on organic remains, as well as a reservoir for parasitic fungi, which infect living plants and animals. The water moulds and downy mildews are common soil inhabitants, as are various other ascomycetes and many fungi that do not have a sexual phase. Many such fungi decompose cellulose and proteins, and thus are important in the formation of humus.

Certain fungi live in a symbiotic association with algae (see Symbiosis), forming characteristic organisms known as lichens. Most lichen fungi are ascomycetes, but a few species are basidiomycetes. Fungi that are intimately associated with roots of higher plants form mycorrhiza, a specialized type of hyphal growth in which a portion of the mycelium either wraps itself around the tips of roots, forming a velvety white cover, or penetrates into the cortex of the root. A number of plants, including economically important crop plants, seem to be dependent on this relationship for satisfactory development. Certain species of mushrooms are prominent in forming mycorrhizae.

Some fungi, which ordinarily grow on dead organic matter, are capable of infecting live plants when given the opportunity. Others cannot exist except as parasites of living plants. Diseases caused by chytrids, oomycetes, and other simple fungi include clubroot of cabbage, powdery scab of potatoes, potato wart, white rusts, potato late blight, and downy mildews. Diseases caused by ascomycetes and their conidial stages include the spot anthracnoses, chestnut blight, Dutch elm disease, oak wilt, ergot, brown rot of stone fruits, and numerous others. The rusts and smuts are basidiomycetes. (See articles on some individual diseases of plants.)

Some soil-inhabiting fungi trap microscopic organisms such as amoebae and nematodes (such as roundworms). Most of these predacious fungi seem to be deuteromycetes or conidial stages of zygomycetes, but some appear to be conidial stages of basidiomycetes. Nematodes are trapped by networks of hyphae covered by an adhesive substance, by knob-like outgrowths that come into contact with the prey, or by hyphal rings that in some instances swell shut abruptly after the nematodes have entered. When an amoeba or nematode has been trapped, special hyphae grow into its body and deplete it of protoplasm.

Many small animals, including insects and millipedes, eat fungi and thus are instrumental in spore distribution. Some groups of insects cultivate fungi as food. Notable among these are the ambrosia beetles, tropical leaf-cutting ants, and certain groups of termites. Numerous fungi are parasites of insects.

VI

Uses of Fungi

The hydrolytic enzymes of fungi are useful for a number of industrial processes. When grown on steamed wheat bran or rice bran, one fungal species produces an amylase product useful in alcoholic fermentation. Proteases obtained from another fungus are used in the manufacture of liquid glue. Commercial production of industrial ethyl alcohol (ethanol) is accomplished by fermentation of sugar cane molasses or hydrolysed starch by means of enzymes formed by another fungus. In the process of making bread, yeast is added to dough to produce carbon dioxide.

Fungus is used for the commercial production of citric acid and in the production of gluconic acid and of gallic acid, which is still sometimes used in the manufacture of inks and dyes. Synthetic resins are manufactured from fumaric acid formed by black bread mould. Gibberellic acid, which promotes increased growth of plant cells, is formed by a fungus-causing disease in rice plants. Commercially usable oils have been obtained from species of several genera, and one species is a practical source of edible proteins. Vitamin D is prepared by irradiation of ergosterol, a substance that may be obtained from the waste brewer’s yeast. A yeast-like fungus is a source of riboflavin, and biotin accumulates during production of fumaric acid by another fungus. Fungi are also used to produce Roquefort cheese and to ripen Camembert cheese.

Fungi have been used medicinally since ancient times. The use of fungi as a purgative is no longer prevalent, but the alkaloid in the sclerotium of ergot is still used to produce uterine contractions in childbirth. Ergot alkaloids are also a source of lysergic acid diethylamide (LSD), which produces hallucinogenic effects, often of a severe nature. The use of antibiotics in medical practice dates from recognition of the antibiotic properties of penicillin. Many antibiotics today are produced by non-fungal micro-organisms. Griseofulvin, however, is an anti-fungal antibiotic formed by several species of a genus of fungi.

VII

Classification

With advances in biochemical and molecular biology, it is no longer possible to classify fungi as a separate kingdom of life. Today, these organisms are found in at least three different kingdoms: Fungi, Protozoa, and Chromista. Within the Fungi, four major phyla are recognized: Ascomycota, Basidiomycota, Chytridiomycota, and Zygomycota. Several other phyla are considered fungi in other kingdoms. The Protozoa now include the fungi Myxomycota, Acrasiomycota, and Plasmodiophoromycota; and Chromista includes the Oomycota and Labyrinthulomycota.

The Myxomycota, or true slime moulds, have a nutritional phase of an unwalled mass of amoeba-like protoplasm, called a plasmodium. The reproductive phase includes swimming cells, called swarm cells, which are propelled by two flagella of unequal length. The Plasmodiophoromycota resemble Myxomycota in having swarm cells and a plasmodial stage. The Acrasiomycota have some slime mould characteristics, but their nutritional stage (the pseudoplasmodium) is different.

The Chromista kingdom includes fungi that resemble algae that have lost their chloroplasts. The phylum of Oomycota is composed of fungi ranging from a single cell to a complex mass of hyphae that are not walled off by septa (non-septate mycelium). Besides forming oospores, the oomycetes form zoospores that move about by two flagella. Included in the phylum are water moulds, white rusts, and downy mildews. Most water moulds live on dead matter, but Saprolegnia parasitica invades living fish. The white rusts and downy mildews, belonging to the order Peronosporales, are parasitic on plants. In some downy mildews, including Phytophthora and Peronospora, the spore cases containing the zoospores may be modified to resemble and function as conidia.

A

Ascomycota

Ascomycetes, also called sac fungi, bear a definite number of ascospores inside a bladder-like sac called an ascus. Except for some yeasts and a few other types, ascomycetes have well-developed hyphae, usually with a single nucleus in each hyphal cell. Certain cells become binucleate shortly before formation of the spore sacs. Nuclear union occurs in the young ascus; division usually produces eight daughter nuclei, which become centres of ascospore formation. Some ascomycetes have only one ascospore; others may have up to several hundred. Yeasts and similar fungi do not have asci formed within or on a supporting hyphal mass. Brewer’s yeast (Saccharomyces cerevisiae), in addition to forming ascospores, reproduces by forming protuberances, or buds, that eventually pinch off from the parent cell. The yeasts of the genus Schizosaccharomyces divide by fission. The Taphrinales, such as the parasite causing peach leaf curl, are often included as an order here, but their true relationship is uncertain.

The forms of the order Eurotiales have asci scattered throughout the interior of a ball of hyphae called a cleisthothecium. Penicillium and Aspergillus are conidial stages of the Eurotiales. The Erysiphales, a group of plant parasites called the powdery mildews, have cleistothecia of specialized form. Some ascomycetes have asci formed inside flask-shaped structures called perithecia. Many perithecia may be borne on a supporting mass of hyphae called an ascoma. The morels, truffles, and cup fungi are well-known ascomata, with asci borne at the upper surface. The genus Neurospora—familiar as a red bread mould—has been used extensively in the study of heredity.

B

Basidiomycota

The phylum Basidiomycota comprises numerous and varied types of fungi, reproductive structures of which are the basidia, located at the tips of the hyphae and usually bearing four basidiospores on stalk-like protrusions. The basidia may be club-shaped, cylindrical, or oval, and either four-celled or one-celled.

Some important plant parasites are included, such as the rusts in the order Uredinales and the smuts in the order Ustilaginales. These groups have basidia that are either deeply cleft or divided into several cells, usually four, each of which produces a spore.

Many rusts, including Puccinia graminis, the black stem rust of wheat and other grains, have a complicated life cycle, requiring growth on two different hosts for production of the various spore forms. In the black stem rust, small, flask-shaped structures, known as the spermagonia, bear numerous tiny, spore-like bodies, called spermatia, on the upper surfaces of berberis leaves. On the lower surfaces develop cup-shaped structures called aecia, from the bases of which arise rows of aeciospores. The aeciospores never reinfect berberis, but attack only grain plants, producing clusters of red, spore-containing pustules called uredia, which give a rust-like appearance to the plant stems and leaves. Later in the season another type of spore, known as the teliospore, or winter spore, which is black and thick walled, is produced on the wheat stem. In the following spring the teliospores develop cylindrical projections, each of which divides into four cells bearing individual basidiospores. Rusts that alternate between two hosts are termed heteroecious; those that have all stages of development confined to one host are known as monoecious.

In the smuts the teliospores are known as chlamydospores. These spores may soon reinfect the host plant but usually germinate in the soil the next spring and produce a short filament of approximately four cells, which bear basidiospores called sporidia.

Within the Basidiomycota there are many sapropytic fungi that live on dead or decaying organic matter, including the mushrooms, the coral fungi, and the pore, or bracket, fungi, which differ in the type of fruiting body, or basidioma. In the mushrooms, which are known as gill fungi, Agaricaceae, the hymenium is formed along the sides of elongated blades, or gills. Coral fungi, Clavariaceae, have a multi-branched basidioma, with the hymenium on its smooth surface. In the bracket fungi, Polyporaceae, common on rotting logs, the hymenium lines the inside of tubes. Tooth fungi, Hydnaceae, have the hymenium on spiny outgrowths.

The Gasteromycetes include such familiar forms as the puffballs in the order Lycoperdales and stinkhorns in the order Phallales. The basidiomata of puffballs are often large, globular structures, containing enormous numbers of spores. The fruiting body of the stinkhorns is a cylindrical structure, and the spore-bearing surface at the apex of the structure gives off a foul odour that attracts carrion-feeding insects and ensures dissemination of the spores.