Fungi From: Encyclopedia of Life Science. Members of the eukaryotic kingdom Fungi are unique and diverse. Fungi include molds, mushrooms, yeasts, rusts, smuts, blights, morels, and truffles. Biologists once considered fungi as plants because they were immobile, appeared to have roots, and had cell walls. Plants, however, are photosynthetic, meaning they can harvest light as an energy source and use it to synthesize organic molecules. Fungi are chemoheterotrophs, meaning they cannot synthesize their own organic molecules from inorganic substances and therefore must obtain energy and nourishment from organic substances present in the environment. The bodies of fungi consist of long, slender filaments and are mostly multicellular, though one group, the yeasts, are unicellular. The polysaccharide chitin, the same material found in arthropod exoskeletons, composes fungal cell walls, compared to cellulose in plants. Unlike other eukaryotic organisms, fungi are haploid. They only exist as diploids during a brief phase of sexual reproduction. Fungi cannot undergo photosynthesis; nor can they engulf food. Instead, they obtain their nutrition by secreting, into the environment, enzymes that digest organic matter. After the enzymes break down the organic matter into smaller components, the fungi absorb the organic molecules into their cells. Many fungi are saprophytic, meaning they obtain nourishment from decaying organic matter, dead organisms, or organic material from other organisms such as animal carcasses, leaf litter, and eliminated waste. Parasitic fungi absorb nutrients from living hosts, including both plants and animals, and can cause infectious diseases. Fungi can also grow on foods such as bread or fruit and even in substances such as house paint. Slightly acidic conditions (pH of about 5) that are unfavorable for most bacteria favor fungal growth. Fungi are also more resistant to low-moisture and high-salt environments. Molds and fleshy fungi consist of filaments called hyphae, which range in size from microscopic to covering acres of land. Septate hyphae are composed of uninucleate (containing one nucleus) cells separated by septa, or walls, though openings between cells join the cytoplasms of
adjacent cells. Coenocytic hyphae do not have septa; they consist of extended cells containing many nuclei. Hyphae grow by elongation at their tips, but if a piece of hyphae breaks, the new fragment can grow into another organism. Specialized types of hyphae carry out the functions of obtaining nutrition and reproduction. Vegetative hyphae must contact the surface on which they grow, so nutrients can be absorbed. When environmental conditions favor growth, the fungus spreads over the surface by branching out to form an intertwining network, or colony, called a mycelium. Reproductive hyphae produce reproductive spores and project outward into the air, so when the spores are released, the air can carry them away. Yeasts are nonfilamentous, unicellular spherical or oval-shaped fungi. Budding yeasts, such as Saccharomyces cerevisiae (baker's yeast), can reproduce asexually by forming a small protrusion from the parent cell. After the parent nucleus divides, the bud receives one-half, continues growing, and eventually pinches off into a smaller but fully functional yeast cell. Structures called pseudohyphae form when the buds fail to separate completely and allow pathogenic yeasts such as Candida albicans to penetrate into tissues. Fission yeasts divide into two equally sized daughter cells. Yeasts are facultative anaerobes, meaning they can grow in the presence or absence of oxygen. When grown anaerobically (in the absence of oxygen), they undergo fermentation, a catabolic process that results in the production of ethanol and carbon dioxide. Yeasts are also easy to grow in the lab and are widely used in cell, molecular, and genetic research. Some fungi are dimorphic—they can grow like molds with hyphae or like yeasts that bud. Temperature serves as the deciding factor. At 77°F (37°C) they resemble yeasts, but at 98.6°F (25°C) they resemble molds. Fungal Reproduction and Classification
Fungi reproduce by forming and releasing spores from hyphal tips. Because fungal spores are very small, air currents easily transport them over long distances. Fungal spores differ from the more resistant bacterial endospores in that fungal spores are true reproductive structures. Each spore gives rise to a new organism; thus the production of fungal spores increases the number of organisms. Bacterial
endospores allow cells to survive harsh or unfavorable conditions but do not increase the number of bacterial cells. In asexual reproduction, the offspring are genetically identical to the parent. Filamentous fungi can reproduce asexually by fragmentation, when a separated piece of mycelium develops into an entire new colony. Asexual fungal spores form by mitosis and cell division. The two main subtypes of asexual fungal spores are conidiospores (also called conidia) and the less common sporangiospores. Conidiospores occur in chains at the end of aerial hyphae called conidiophores and are not enclosed by sacs. They develop by pinching off of the tip or by segmentation of hyphae. The several forms of conidiospores include arthrospores, chlamydospores, blastospores, phialospores, microconidia, macroconidia, and porospores. Sporangiospores develop by successive cleavages inside a sac called a sporangium at the end of aerial hyphae called sporangiophores. When the sporangium breaks open, the spores are released. In sexual reproduction, two different mating types (a donor cell "+" and a recipient cell "-") of hyphae join together, fuse, and develop into a new organism that forms spores containing genetic information from both parents. The formation of sexual spores occurs in three main stages: plasmogamy, karyogamy, and meiosis. During plasmogamy the haploid nucleus from a "+" mating type invades the cytoplasm of a "-" mating type. The two haploid nuclei fuse in a process called karyogamy, resulting in a diploid nucleus. Meiosis occurs, forming nuclei of sexual spores. Because sexual spores develop in a variety of diverse ways, one means of classifying fungi is by their mode of sexual reproduction. Three of the most common divisions are Zygomycota, Ascomycota, and Basidiomycota. Members of the phylum Zygomycota usually reproduce asexually, but when opposite mating types fuse, sexual spores called zygospores form. The zygospores are large, thick-walled spores that undergo meiosis and germination. The resulting mycelium gives rise to haploid sporangia that resemble the asexual sporangium, except that it contains nuclei with genetic information from both parents. Hyphae of zygomycetes usually have no cell walls. Black bread molds such as Rhizopus stolonifer belong to the phylum Zygomycota. Members of the phylum Ascomycota generally produce
ascospores inside a tubelike sac called an ascus that forms when two mating types join to form a diploid nucleus. Subsequent meiosis and germination result in the formation of haploid ascospores that disperse when the ascus breaks open. Examples of ascomycetes include Histoplasma (the causative organism of Ohio River Valley fever, or histoplasmosis), truffles, and the yeasts. The phylum Basidiomycota includes the mushrooms, rusts, puffballs, and smuts. The fusion of hyphae of different mating types forms a fruiting body (such as a mushroom) that is common to members of this phylum. Club-shaped structures called basidia line the gills underneath the cap of the fruiting body. Fusion of haploid nuclei occurs within a basidium. Meiotic division produces haploid spores that germinate to form haploid hyphae. Generalized Life Cycle of a Zygomycete
Enlarge Image Two less well-known phylum are Deuteromycota and Chytridiomycota. Deuteromycota, commonly called the imperfect fungi, includes various fungi that are incapable of sexual reproduction or organisms for which the means of sexual reproduction is not yet known. The organisms that cause athlete's foot and that give Camembert and Roquefort cheeses their unique flavors belong to this phylum. Members of the phylum Chytridiomycota are usually unicellular and aquatic and produce flagellated, motile gametes and spores. Slime molds and water molds were once classified as fungi because they appeared to share similar life
cycles and under certain conditions form structures resembling sporangia. Biologists now believe they are unrelated to fungi. Ecological Importance
Fungi closely interact with other organisms in many different ways. A mycorrhiza is a mutualistic symbiotic association formed between a fungus and a plant. The fungus grows inside or wraps around the roots of the plant, and the mycelia branch out to increase the surface area through which absorption of water and nutrients such as phosphorus and minerals occurs. The fungus benefits from carbohydrates synthesized by the plant by photosynthesis. Plants with mycorrhizae can inhabit soil that is less fertile or live in environments that are drier than plants without them. Lichens are symbiotic associations formed by a fungus with a photosynthetic organism such as a photosynthetic bacterium or an alga. The photosynthetic organism provides a food source for the fungus, while in return the fungus protects its partner from the environment, allowing it to live in habitats that would normally be too harsh, such as on the surface of a rock, in an arid desert, or on a tree trunk. The mycelia absorb moisture and carbon dioxide from the atmosphere for use in photosynthesis. Lichens play an important role in ecological succession, the replacement of one type of community by another at a single location over time, such as after a forest burns down. Lichens help prepare the previously barren terrain for other species by secreting an acid that breaks down rocks into soil and liberates nutrients. Over time, the organic material from dead lichens nourishes future inhabitants. Lichens also serve an important role monitoring the air for pollutants such as sulfur dioxide. Because they are sensitive to such manufactured pollutants, their growth is a good indicator of air quality. Because most fungi are saprophytes and obtain their nutrition from decomposing organic material of other species, they play a crucial role in the food chain. Plants contain cell walls made of cellulose, a carbohydrate that animals are incapable of digesting. Fungi break down the decaying leaf litter on the forest floors, recycling the nutrients back into the environment in a form that other organisms can utilize. Without fungi, many nutrients would become depleted as they became
incorporated into forms unusable to other organisms. Fungi are essential to maintaining conditions that support other life-forms. Impact of Fungi
The ecological role played by fungi translates into important economic consequences. The maintenance of soil conditions that support many crops depends on relationships that fungi engage in with plants and other microorganisms. Some fungi inflict devastating damage to crops by causing diseases such as potato blight, black stem rust of wheat, covered smut of barley, powdery mildew, fruit rots, and Dutch elm disease. Other fungi are used in the biological control of costly plant diseases. For example, the yeast Candia oleophila protects fruits from harmful molds. The fungus Trichoderma harzianum not only protects fruits and vegetables from botrytis, a fungus that causes gray mold, but is also believed to enhance plant growth, degrade pesticides, and prevent the synthesis of toxins produced by other fungi. A mycorrhiza that grows on the roots of trees helps extract calcium from the mineral apatite in forest soils that are calcium poor because of the leaching effects of acid rain. The fungus Entomophaga maimaiga attacks the caterpillars of gypsy moths, insects that destroy entire forests by defoliation. The production of many types of foods depends on fungi. Food and beverage manufacturers exploit the ability of yeasts to produce ethanol and carbon dioxide, two end products of fermentation. The formation of carbon dioxide gas causes bread dough to rise, and ethanol is the form of alcohol found in beer, wine, and liquor. Different fungi impart unique flavors and textures to cheeses, and edible fungi such as mushrooms are a food source themselves. Other fungi cause fruit to rot or bread and cheese to mold. Acidic conditions, as in jams and jellies and inside fruit, inhibit bacterial growth but encourage fungal growth. Fungi can also withstand high solute concentrations used to preserve many foods. Antibiotics are chemicals produced by one organism that kill or inhibit the growth of another. Many antibiotics are produced by fungi or are derivatives of fungal products. The mold Penicillium produces penicillin, the first antibiotic discovered. Other fungi synthesize other commercially available chemical compounds. Taxomyces andreanne, a
fungus that grows on the bark of Pacific yew trees, secretes taxol, an important anticancer drug. The immunosuppressant drug cyclosporine is derived from the fungusTolypocladium inflatum. Aspergillus niger is the source for the flavoring ingredient citric acid and proteases, enzymes that digests proteins. Less than 1 percent of fungal species are pathogenic to humans and animals. Some species of fungi that are pathogenic to humans cause conditions such as athlete's foot, ringworm, yeast infections, Pneumocystis pneumonia (commonly found in acquired immunodeficiency syndrome [AIDS] patients), and histoplasmosis. Diseases caused by fungi are called mycoses and are usually chronic as a result of the slow-growing nature of fungi. Many mycoses are superficial or penetrate to just below the surface of the skin, but some, including histoplasmosis and coccidioidomycosis, are systemic and affect a number of organs.