FUNGI

 Yeasts, molds, mildews, rusts, mushrooms, and toadstools are among the many different organisms in kingdom Fungi. A Fungus is an organism made of eukaryotic cells with cell walls and that gets its nourishment by absorbing organic substances. Fungi range in size from microscopic, one-celled yeasts to large mushrooms that may be 25cm or more in diameter.

Have you ever seen a mushroom growing in a shady area after a period of warm, damp weather? If so, you will have noticed that mushrooms are like plants in some ways. Mushrooms, like plants, grow upward from the soil. The cells of a mushroom have cell walls, as do the cells of plants. For these and other reasons, biologists once classified mushrooms and other fungi as plants.

Many fungi grow as microscopic tubes, or filaments. These filaments are like those of some kinds of algae and are surrounded by cell walls. However, the cell walls of fungi are different from those of algae and plants. The cell walls of most fungi contain chitin, a substance found in the shells of crabs and the outer coverings of insects.

Unlike plants, fungi have no chlorophyll or chloroplasts. Fungi cannot make their own food. Like animals, fungi use food substances made by other organisms. Fungi also must digest their food. However, fungi do not digest food within their bodies as animals do. Fungi digest food outside their bodies and then absorb the products of digestion. Fungi secrete digestive enzymes into their surroundings. The enzymes break down the organic matter near the fungus into small molecules, which are then absorbed.

You can see that the fungi are organisms with a special mixture of traits. Most biologists now think of fungi as making up a separate kingdom.

The Nature of Fungi

Fungi use organic matter as a source of nourishment. For this reason, fungi generally live close to other life forms or near dead organic material. Fungi need moisture, and many cannot live in strong sunlight. The damp, shady floor of a forest with its covering of dead leaves gives a suitable habitat for fungi. Fungi are often found in forests, but they may be found in many other places as well. Fungi may live inside other organisms. The organisms provide sources of organic matter and the dark, moist conditions in which fungi thrive.

Most fungi are saprobes. Saprobes consume the dead remains and waste products of other organisms. Saprobic fungi break down dead organic matter. They help to put back into the soil minerals needed for the growth of plants.

Some fungi are parasites. Remember that parasites are organisms that get organic nutrients from a living host. A parasite usually harms or injures its host. Fungi may be parasitic on plants, animals, or even other fungi. For example, fungi of the genus Trichophyton are parasitic on humans and cause athlete’s foot. Wheat rust and corn smut are fungi that are plant parasites that harm important food crops.

A third group of fungi are neither saprobes nor parasites. These are fungi that get organic matter from living tissues but that do no harm. Such fungi live closely with other life forms.

General Structure of a Fungus

Most fungi are made up of branching filaments, or tubes, called hyphae. Nuclei are found throughout the cytoplasm. In some fungi the hyphae are divided into cells by cross walls called septa. In other fungi, however, no septa appear. Even in fungi with septa, the cytoplasm is continuous. Each septum has a pore in it.

The hyphae of a fungus grow and branch into a complex, tangled network. Such a network of fungal hyphae is called a mycelium. The mycelium spreads over the surface or into the food source on which the fungus is growing. Often, the mycelium is hidden from view. It may be within the soil or beneath the bark of a decaying tree.

A fungal mycelium spreads in all directions through its food source. The mycelium forms the body of the fungus. When a fungus is mature, part of the mycelium may develop into a structure specialized for reproduction. Look at the bracket fungus growing on the tree trunk in Figure. The mycelium of this fungus has spread throughout the bark of the tree. The part of the fungus that you can see in the photograph is the reproductive structure, or fruiting body. The parts of a fungus other than the fruiting body often called vegetative hyphae or vegetative mycelium.

Most fungi can reproduce both sexually and asexually. Asexual reproduction is often by fragmentation. Fragments of broken hyphae may be carried to new places by wind or water. If conditions are suitable, these fragments will grow into new fungi.

Fungi most often reproduce by producing spores. The air may be so full of fungal spores in late summer that the spores may cause problems for people with allergies. Spores are formed in the fruiting bodies of fungi by either a sexual or an asexual process. In some fungi the fruiting bodies are microscopic. In others, such as mushrooms and bracket fungi, the fruiting bodies can easily be seen with the unaided eye. The method of producing spores and the kind of fruiting body formed are features used to classify fungi.

CLASSIFICATION OF FUNGI

1. Phylum Oomycota

Phylum Oomycota is a large and varied group that includes the water molds, white rusts, and downy mildews. Organisms in phylum Oomycota are called oomycotes. The water molds are water saprobes. The white rusts and downy mildews are parasitic on plants. Other organisms in this phylum live as parasitesin, or on, algal protists, plants, small water organisms, and other fungi.

Most fungi have cell walls made of chitin. Most oomycotes have cell walls containing cellulose, a substance that is found in the cell walls of plants.

Few traits are shared by all oomycotes. One shared trait, however, is the formation of asexual spores called zoospores, each of which has two flagella. The flagella allow the zoospores to swim. Water is needed for asexual reproduction in most oomycotes. Rainwater, the water in soil, or even dew is enough water for the zoospores to swim in.

Sexual reproduction also takes place among the oomycotes. The fertilized egg of an oomycote develops into an oospore. Saprolegnia, a common freshwater mold, can often be seen as a fuzzy, whitish growth on organisms that have died. This water mold usually lives as a saprobe, but it may be a parasite on injured fish and on fish eggs.

The life cycle of Saprolegnia  - asexual reproduction is by means of diploid zoospores that have flagella. These zoospores are produced by mitosis within a spore case called a sporangium. When the sporangium is ripe, it opens and lets out the zoospores into the water. After a few hours, the zoospores become enclosed within walls to become resting spores, or cysts. The cysts later change into zoospores again.

2. Phylum Zygomycota

Phylum Zygomaycota is made up of land-dwelling fungi often called molds and blights. Members of phylum Zygomycota are called zygomycotes. Most zygomaycotes are saprobes. However, some are parasites on plants, animals, or other fungi. All zygomycotes reproduce sexually by means of thick-walled diploid spores called zygospores.

Rhizopus stolonifera is a zygomycote mold that often grows on bread and starchy fruits. It is whitish when young; it is black when it reaches maturity and forms spores. The hyphae that are parallel to the fungu’s growth medium are called stolons. The many smaller, branching hyphae growing downward from the stolons are called rhizoids. The rhizoids go into the food source the fungus is growing on ang give out enzymes that digest the food.

Rhizopus reproduces asexually by forming stalklike hyphae called sporangiophores. These structures stick up into the air from the stolons. At the top of each sporangiophore is a sporangium that has a very large number of asexual, monoploid spores. Note that phore means “carrier” and that the sporangiophore carries the spore case, or sporangium. The spores of zygomycotes, unlike those of oomycotes, have no glagella. When the spores are ripe, the sporangia burst and release the spores. These spores can then germinate into new vegetative hyphae.

Rhizopus also can reproduce sexually through the mating of hyphae of genetically different strains. Stolons from the two strains of Rhizopus grow next to each other. A swelling from each stolon grows toward the other. The two swellings meet and nuclei from each side come together and join so that a diploid cell is formed. This diploid cell forms a thick wall and becomes a zygospore.

Meiosis takes place during the germination of Rhizopus zygospore. The germinating zygospore froms a single sporangium having monoploid spores. These spores appear to be the same as those formed in asexual reproduction. When these spores are released, they grow into vegetative hyphae. They may later reproduce either sexually or asexually.

3. Phylum Ascomycota

Phylum Ascomycota is the largest group of fungi, with about 2000 genera. Members of phylum Ascomycota are called ascomycetes. The yeasts, the powdery mildews, the cup fungi, truffles, and morels are part of this phylum. Also included are a number of molds that cause food to spoil. Most ascomycetes are saprobes, living on dead plant material. However, other fungi are very destructive plant parasites. Some cause diseases such as Dutch elm disease, chestnut blight, and peach leaf curl. Most ascomycetes are adapted to land habitats.

Ascomycotes are sometimes called sac fungi because spores develop in a sac called an ascus. These monoploid, non-moving spores are called ascospores. The ascospores are formed by sexual reproduction followed by meiosis.

The pink bread mold Neurospora crassa is an ascomycote that has been used in ecperiments in genetics and biochemistry

Neurospora and many other ascomycetes reproduce asexually by forming spores called conidia. The conidia are formed on specialized hyphae called conidiophores. After the spores fall from the conidiophores, they germinate to form new vegetative hyphae.

Sexual reproduction in Neurospora begins with the mating of hyphae of different types. The ascogonia are female reproductive structures. They form tubes that fuse with antheridia on the male hyphae. Male nuclei from the antheridia cross through the tubes into the ascogonia. One male nucleus and one female nucleus become isolated in an extension of each ascogonium. The male and female nuclei join to form a zygote. The zygote goes first through meiosis and then mitosis to produce eight monoploid cells inside an ascus. The eight cells form thick walls as they ripen into ascospores. Hyphae grow around the developing asci to form a fruiting body called an ascocarp. Each ascospore is able to develop into fungal hyphae.

Yeasts, one-celled ascomycetes of the genus Saccharomyces, are used in baking and brewing. Saccharomyces means”sugar-fungus”. These fungi live on sugary food sources, such as grape juice.

Saccharomyces yeasts reproduce asexually by budding. Budding is a method of reproduction that involves an outgrowth, called a bud, from the cell. The nucleus finally divides and one of the new nuclei moves into the bud. The bud later breaks off, becoming a separate yeast cell.

Saccharomyces yeasts can also reproduce sexually by forming ascospores. In such cases the yeast cell becomes an ascus. Such an ascus may contain four or eight ascospores.

A number of common molds of the genera Penicillium and Aspergillus have no known sexual reproduction. These molds have been classified in phylum Ascomycota because they form conidiophores and reproduce asexually by means of conidia. Penicillium molds often are seen as bluish-green growths on oranges and other citrus fruits and sometimes on bread.

4. Phylum Basidiomycota

Phylum Basidiomycota includes some of the largest fungi, such a mushrooms, puffballs, and bracket fungi. Puffballs are similar to mushrooms, but the cap is roundish. Bracket fungi look like shelves growing on trees. Members of phylum Basidiomycota are called basidiomycetes.

Basidiomycetes produce spores in club-shaped structures called basidia. The monoploid spores, called basidiospores, are formed by meiosis of a zygote formed by sexual reproduction. In most basidiomycetes the structure that bears the basidia is called a basidiocarp. Because of the shape of the basidia, members of this phylum are called club fungi.

The object often called a mushroom is only a small part of the complete fungus. Most of a mushroom fungus is made of an underground mycelium. The mycelium absorbs nutrients from organic matter in the soil. The part of the mushroom seen above the ground is the basidiocarp, or fruiting body.

Mushroom do not form asexual spores. The basidiospores, or sexual spores, of mushrooms change into hyphae that develop into underground mycelia.

The young basidiocarps of some mushrooms look like little rounded balls. A sheetlike veil holds the developing mushroom together. Inside the veil may be another shetlike tissue, the annulus, which draws the edge of the mushroom cap toward the stem, or stipe. When the basidiocarp grows a bit larger, the veil and annulus tear. This tearing allows the mushroom to open like an umbrella. The cap of the mature mushroom has gills on its undersurface. The gills of a mushroom are where the basidiospores are formed.

Club-shaped basidia develop from the surface of each gill of the mushroom cap. Within each basidium, two nuclei join to form a zygote nucleus. The zygote nucleus then undergoes meiosis, forming four monoploid nuclei within the basidium. These four nuclei then travel into projections at the outer part of the basidium. The nuclei develop into four monoploid basidiospores. The basidiospores are released from the basidiocarp when they are mature.

Rusts and smuts are basidiomycetes that do not form basidiocarps. They are small and nearly always parasitic on plants. Many rusts have complex life cycles, infecting different hosts at different stages of the life cycle. Unlike mushrooms, many of these basidiomycetes produce asexual spores as well as basidiospores.

Wheat rust, Puccinia graminis, infects wheat and barberry plants. Basiodiospores develop on the infected wheat plant and are carried by wind to barberry plants. Asexual spores develop from hyphae on barberry plants. These spores are carried by wind to wheat plants.

NUTRITION OF FUNGI

A fungus is in direct contact with its food. The fungus secretes enzymes into the food. These enzymes break large food molecules into molecules that are small enough to be absorbed by the hyphae. Different enzymes act on different kinds of nutrients.

After food has been absorbed, it travels to all parts of the fungus. Think about a mushroom, a saprobic fungus. The underground mycelium of the mushroom is in contact with organic matter in the soil. Above the ground the fruiting body, or basidiocarp has no direct source of food. Food travels from the underground mycelium to the mushroom through the hyphae. Remember that some fungi lack septa in their hyphae. Thus cytoplasm is free to move back and forth through the hyphae, carrying food with it. Even in those fungi with hyphae that have septa, pores in the septa allow the cytoplasm to flow.

Although most saprobic fungi live in the soil, some live in other environments. Bracket fungi are found on both living and dead trees. Those bracket fungi that grow on living trees are believed to be saprobes rather than parasites. They usually consume only dead cells in the bark or in the dead heartwood of the tree.

Parasitic fungi obtain nutrients directly from the living cells and tissues of their hosts. Parasitic fungi often form special hyphae called haustoria. These hyphae are highly branched and specialized for absorbing nutrients. Haustoria pass into the tissues and often even enter the cells of the host.

A few filamentous fungi are predators on such small organisms as protozoans, rotifers, and roundworms. These fungi have special sticky pads or loops that trap the prey. Some types of soil-dwelling fungi form rings along the hyphae. After trapping a worm, the fungus forms haustoria, which pass into the prey. Digestion of the host’s tissues takes place, followed by absorption of the nutrients.

FUNGI AS SYMBIONTS

A close relationship between two species in which at least one species benefits is called symbiosis. A symbiotic relationship in which one organism lives on or in another and harms it is called parasitism. If both species are helped by symbiosis, the relationship is called mutualism. Some fungi are mutualistic.

A lichen is a symbiotic association between a fungus and a blue-green bacterium, an algal protist, or a green alga. The fungus may give structural support and supply certain nutrients for the partner. The bacterium, protist, or plant – unlike the fungus – is able to make its own food. It provides food for the fungus. Lichens live in many habitats, from tropical heat to arctic cold, from deserts to rain forests.

The photosynthetic species found in lichens include about 8 species of blue-green bacteria, 20 species of green algae, and 1 algal protist. The blue-green bacterium Nostoc and the green alga Trebouxia are the most common. All of the photosynthetic species can probably live without the fungi. But the fungi usually need a partner for survival. Lichens that contain Nostoc can use nitrogen from the air as a nutrient.

A mycorrhiza is a structure formed by the mutualistic association of a fungus and the roots of a plant. The fungus grows on the roots, with some hyphae reaching into the tissues of the host plant. Some plants, including elm, maple, and willow, can grow well with or without fungi. Other plants, such as some species of pine, spruce, and oak, need a growth of fungi on their roots. The fungus receives sugars from the roots of the host plant and gives growth-stimulating chemicals to the plant. The fungus also aids the plant in taking in water and minerals.

Orchids have a very close relationship with their mycorrhizal fungi. The fungus invades the cells of the orchid roots and provides organic nutrients to the orchid. Some orchids are too dependent on the fungus that they can live without chlorophyll. Most of the organic matter flows from fungus to orchid. But the fungus may be helped by receiving some amino acids and vitamins from the orchid.

Symbiotic relationships between insects and fungi are fairly common. Many insects have yeasts, along with bacteria, in their digestive tracts. The fungi aid the insect in digesting its food and are helped by receiving a share of that food.

Some kinds of burrowing beetles, termites, and ants keep fungal gardens. The insects bring food to the growing fungi. The fungi serve as food for the insects. Several species of ants of the southern and southwestern United States show such relationships with fungi.

Fungi are common parasites of insects. Sometimes the fungus helps to keep the insect population under control. The citrus mealybug in Florida is controlled by the fungus Entomophtora fumosa, which grows well in California, which has a cool, rainy season.

IMPORTANCE OF FUNGI

A great amount of dead organic material is deposited on the earth’s surface each year. If it were not for saprobes, including many fungi, this waste would build up to massive proportions. The buildup of such wastes would exclude most plants and animals from their natural habitats. Saprobes remove this material by breaking it down. They also return inorganic nutrients and minerals to the soil, where these materials can be used for more plant growth.

Saprobic fungi are able to break down organic material. In some cases this ability causes problems. Mildews and molds form quickly in moist environments. Clothing and other cloth products, such as tents, are often attacked by molds and mildew. Cotton is very often attacked because it is made of plant cellulose. Many molds can break down cellulose. These fungi often cause musty odors, stains, and weakened fabric. Wool, which is a protein fiber, is less likely to be attacked by fungi. There are fewer fungi that can digest protein.

Many foods are attacked by molds. You have probably seen moldy fruits, vegetables, and bread. The downy or cottony molds Penicillium and Rhizopus grow quickly on foods that are in even moderately humid environments. Many of these fungi affect the smell, taste, and appearance of the products they grow on.

Some fungi that attack foods produce poisons that may cause illness in people and livestock. Corn that becomes infected with certain species of Penicillium and Aspergillus may cause poisoning of pigs and cattle. Horses and other livestock may be poisoned by moldy hay. Death may take place because of the effects of the poison on the nervous and circulatory systems.

Ergotism is a disease caused by an ascomycote that is a parasite on rye. The developing ascopores invade the seeds and replace the seed material with hardened, purple-black masses of fungal hyphae. The fungus forms several poisons. People who eat the contaminated grain may have convulsions and other mental disorders.

Although some fungi destroy useful products, many valuable products are made as a result of fungal activity. These include bread, alcohol, cheese, drugs, and enzymes. Fermentation of sugar by yeast (usually Saccharomyces cerevisiae) forms carbon dioxide and ethyl alcohol. The carbon dioxide causes bread dough to rise. Brewers and winemakers use the ethyl alcohol to make beer and wine.

Cheese is formed from milk that has been coagulated to form semisolid curds and a watery liquid called whey. The curds are the newly-formed cheese, sometimes sold as cottage cheese. The curds are ripened, either by bacteria or fungi, to make other kinds of cheese. Most soft cheeses are ripened by fungi. Camembert cheese is soft but has a stiff rind containing the white mold Penicillium camemberti. This fungus gives off an enzyme that makes the cheese soft and buttery and mild in flavor. Roquefort cheese is made from sheep’s milk. The curds are broken up, drained until fairly dry, and inoculated with Penicillium roqueforti, which grows over the curds.

Other important substances are produced by fungi. Antibiotics, including penicillin, are formed by Penicillium. The drug ephedrine used in treating hay fever and asthma, and the B-vitamins are commercially made by yeasts. Some enzymes and organic acids, including citric acid, are also made using fungi.

Some fungi, such as mushrooms, have been thought of as delicacies since the time of the ancient Greeks and Romans. In North America, mushrooms are grown commercially in large mushroom farms. The commercial mushroom is an excellent source of vitamins and the minerals phosphorus and iron. These mushrooms also contain some proteins. Mushrooms are grown in trays of a mixture of straw and fertilizer. The trays are kept in a moist, dark environment which prevents the growth of soil algae and weeds. The fungal mycelium grows throughout the mixture, and the edible basidiocarps rise up above the surface.

There are many species of wild mushrooms that are edible, but some are deadly poisonous. It is difficult to tell nonpoisonous and poisonous varieties apart. Usually the killer mushroom is a species of Amanita, the “death angel.” Even one cubic centimeter of this mushroom can cause death. Since only an expert can identify wild mushrooms, you should never collect and eat wild mushrooms.