CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
21. The Nature of Microorganisms
21.5. The Kingdom Fungi
The members of the kingdom Fungi are nonphotosynthetic, eukaryotic organisms with cell walls. The structure of the cell wall differs from that of other organisms because fungal cell walls contain chitin along with other compounds. Some are single-celled, but most are multicellular organisms composed of filaments of cells joined end-to-end. Each filament is known as a hypha. The hyphae form a network known as a mycelium (figure 21.18).
FIGURE 21.18. Mycelium
The basic structure of a fungus is a multicellular filament known as a hypha. A mass of hyphae is collectively known as a mycelium.
Even though fungi are nonmotile, they are easily dispersed, because they form huge numbers of spores. A spore is a cell with a tough, protective cell wall that can resist extreme conditions. Fungi have a variety of kinds of spores. Some spores are produced by sexual reproduction, others by asexual reproduction. An average-sized mushroom can produce over 20 billion spores; a good-sized puffball can produce as many as 8 trillion spores. When released, the spores are transported by wind or water. Because of their small size, spores can remain in the atmosphere a long time and travel thousands of kilometers. Fungal spores have been collected as high as 50 kilometers above Earth.
All fungi are heterotrophs that must obtain nutrients from organic sources. Most secrete enzymes that digest large molecules into smaller units, which the fungi absorb. Fungi grow by division of the cells at the end of a hypha. In this way they expand into new areas where there are new sources of organic matter. Fungi are free-living, parasitic, or mutualistic. Free-living fungi, such as mushrooms, decompose dead organisms as they absorb nutrients. Parasitic fungi are responsible for many plant diseases and a few human infections, such as athlete’s foot, vaginal yeast infections, and ringworm. Mutualistic fungi are commonly associated with the roots of plants.
The Taxonomy of Fungi
Fungi are divided into subgroups, based on their methods of reproduction (figure 21.19). It is assumed that fungi originated from ancestors that had flagellated swimming reproductive spores and that the most primitive groups of fungi would share this characteristic.
FIGURE 21.19. Fungus Taxonomy
There are several subgroups within the kingdom Fungi. They are distinguished from one another primarily by their methods of reproduction.
The Chytridiomycota are fungi that are aquatic or live in moist soil where they digest organic matter. They are considered one of the most primitive groups of fungi because they have a flagellated spore stage. Infections caused by one member of this group, Batrachochytrium dendrobatidis, are thought to be one of the reasons for the decline in frogs and other amphibians throughout the world.
The Neocallimastigomycota are fungi that form flagellated spores and live as mutualistic, anaerobic fungi in the rumen of ruminants. They break down cell walls of plants to simpler compounds and contribute to the nutrition of ruminants (see Outlooks 21.2).
The Zygomycota are distinguished from other fungi because they form sexual spores when the hyphae of two different strains of the species join. The spores do not have flagella. One common example is bread mold.
The Glomeromycota are fungi that form arbuscular mycorrhizae with plants. (Mycorrhizae are discussed later in this chapter.) It appears that most must obtain nutrients from the roots of plants to live. It appears that they only reproduce asexually.
The Ascomycota are distinguished from other fungi because their cells contain two distinct nuclei and they produce spores by sexual reproduction in a saclike structure. They also produce spores asexually. About 75% of all fungi are in this group. Molds, mildews, yeasts, morel mushrooms, and truffles are examples. In recent years a fungus in this group, Geomyces destructans, has been identified as the cause of white-nose syndrome in bats. Millions of bats have died because of infections with this fungus.
The Basidiomycota are unique in having cells that contain two distinct nuclei and sexual spores produced in sets of four on a club-shaped structure. Bracket fungi, most mushrooms, and puffballs are common examples. Many are parasites on plants; rusts and smuts are examples.
The term yeast is not a taxonomic term. A yeast is any fungus that is single-celled and generally reproduces by asexual reproduction. One common method of asexual reproduction is budding in which one cell grows a lobe that eventually pinches off to become a separate organism. Yeasts are found in the Basidiomycota and Ascomycota. The yeasts involved in brewing and baking are in the Ascomycota.
Mold and mildew are other terms associated with fungi. They are not taxonomic terms but simply describe growths of a fungus that forms a fuzzy growth or discolors a surface. They can be from many different taxonomic categories, not just fungi. They are common everywhere there is a source of organic molecules and moisture.
The Significance of Fungi
Fungi play many significant roles in ecosystems. They are also economically important to humans in many ways.
Because fungi produce so many spores, any dead organism is likely be colonized by a fungus. Because all fungi are capable of breaking down organic matter, fungi along with bacteria are the major decomposers of organic matter in ecosystems. This decomposer function recycles elements such as carbon, nitrogen, and phosphorus. However, fungi also colonize and break down organic matter that we do not want to have recycled and cause billions of dollars of damage each year. Clothing, wood, leather, and all types of food are susceptible to damage by fungi. One of the best ways to protect against such damage is to keep the material dry, because fungi grow best in a moist environment.
Fungi as Food
Fungi and their by-products have been used as sources of food for centuries. When we think of fungi and food, mushrooms usually come to mind. The common mushroom found in the grocer’s vegetable section is grown in many countries and has an annual market value in the billions of dollars. But there are other uses for fungi as food. Shoyu (soy sauce) was originally made by fermenting a mixture of wheat, soybeans, and an ascomycote fungus for a year. Most of the soy sauce used today is made by a cheaper method of processing soybeans with hydrochloric acid. True connoisseurs still prefer soy sauce made the original way. Another mold is important to the soft-drink industry. The citric acid that gives a soft drink its sharp taste was originally produced by squeezing juice from lemons and purifying the acid. Today, however, a mold is grown on a nutrient medium with table sugar (sucrose) to produce great quantities of citric acid at a low cost.
The primary flavors of blue cheeses, such as Danish, American, and the original Roquefort, are produced because the cheeses have been aged with the mold Penicillium roquefortii. Another species of Penicillium produces the antibiotic penicillin (How Science Works 21.3).
Yeasts are important organisms in the production of alcoholic beverages and in the making of bread. It is difficult to imagine a world in which these two food materials are not important.
HOW SCIENCE WORKS 21.3
In 1928, Dr. Alexander Fleming was working at St. Mary's Hospital in London. As he sorted through some old petri dishes on his bench, he noticed something unusual. The mold Penicillium notatum was growing on some of the petri dishes. Apparently, the mold had found its way through an open window and onto a bacterial culture of Staphylococcus aureus. The bacterial colonies that were growing at a distance from the fungus were typical, but there was no growth close to the mold.
Fleming isolated the agent responsible for this destruction of the bacteria and named it penicillin.
Through Fleming's research efforts and those of several colleagues, the chemical was identified and used for about 10 years in microbiological work in the laboratory. Many suspected that penicillin could be used as a drug, but the fungus could not produce enough of the chemical to make it worthwhile. When World War II began, and England was being firebombed, there was an urgent need for a drug that would control bacterial infections in burn wounds.
Two scientists from England were sent to the United States to begin research into the mass production of penicillin.
Their research in isolating new forms of Penicillium and purifying the drug were so successful that cultures of the mold now produce over 100 times more of the drug than the original mold discovered by Fleming. In addition, the price of the drug dropped considerably—from a 1944 price of $20,000 per kilogram to a current price of less than $250.00. The species of Penicillium used to produce penicillin today is P chrysogenum, which was first isolated in Peoria, Illinois, from a mixture of molds found growing on a cantaloupe. The species name, chrysogenum, means golden and refers to the golden-yellow droplets of antibiotic that the mold produces on the surface of its hyphae. The spores of this mold were isolated and irradiated with high dosages of ultraviolet light, which caused mutations to occur in the genes. When some of these mutant spores were germinated, the new hyphae were found to produce much greater amounts of the antibiotic.
Mycorrhizae are associations between certain fungi and the roots of plants. There are two kinds of mycorrhizae. In one kind, arbuscular mycorrhiza, the cells of fungi actually penetrate the cells of the plant roots. Members of the Glomeromycota are always found as arbuscular mycorrhizae. In the other kind, ectomycorrhiza, the fungal cells surround the root cells but do not invade them. Ectomycorrhizae are usually formed by members of the Ascomycota and Basidiomycota (figure 21.20). Mycorrhizal fungi are found in 80-90% of all plants, and many plants cannot live without their mycorrhizal fungi. The addition of these fungi to the roots of plants increases the roots’ surface area for absorption. Plants with mycorrhizal fungi can absorb as much as 10 times more minerals than those without the fungi. Some types of fungi also supply plants with growth hormones, whereas the plants supply carbohydrates and other organic compounds to the fungi.
FIGURE 21.20. Mycorrhizae
Mycorrhizae are associations between fungi and the roots of plants. Both organisms benefit. The plants have increased absorption of water and minerals, and the fungi receive organic molecules as food from the plants.
Lichens are organisms that consist of a symbiotic relationship between a fungus and either an alga or a cyanobacterium. The alga or cyanobacterium does photosynthesis and provides the fungus with organic molecules for food, while the fungus provides the moist environment required by the alga or cyanobacterium.
Because the fungi provide a damp environment and the algae produce the food, lichens require no soil for growth. For this reason, many kinds of lichens are commonly found growing on bare rock and are the pioneer organisms in the process of succession. Some lichens grow as fluffy growths on trees or as fleshy structures on the soil (figure 21.21). Lichens are important in the process of soil formation. They secrete an acid, which weathers the rock and makes minerals available for use by plants. When lichens die and decompose, their organic matter mixes with rock particles to form soil.
FIGURE 21.21. Lichens
Lichens grow in a variety of habitats. (a) There are several kinds of shrubby lichens growing on this stump in Alaska. (b) There are several kinds of lichen growing on the bark of this tree. The different colors are due to the different species of algae or cyanobacteria in the lichens.
Lichens are found in a wide variety of environments, ranging from the frigid Arctic to the scorching desert. Reindeer moss of cold northern regions is actually a lichen. One reason for this success is their ability to withstand drought conditions. Some lichens can survive with only 2% water by weight. In this condition, they stop photosynthesis and go into a dormant stage until water becomes available and photosynthesis begins again.
Another factor in the success of lichens is their ability to absorb minerals. However, because air pollution increases the amounts of minerals in the air, many lichens are damaged. Some forms of lichen absorb concentrations of sulfur 1,000 times greater than those found in the atmosphere. This increases the amount of sulfuric acid in the lichen, resulting in damage or death. For this reason, areas with heavy air pollution are “lichen deserts.” Because they can absorb minerals, certain forms of lichen have been used to monitor the amount of various pollutants in the atmosphere.
Many kinds of fungi are important pathogens of plants, and a few are human pathogens. Chestnut blight and Dutch elm disease almost caused these two species of trees to become extinct. Wheat rust gets its common name because infected wheat plants look as if they are covered with rust. Corn smut is also due to a fungal pathogen (figure 21.22).
FIGURE 21.22. Corn Smut
Most people who raise corn have seen corn smut. Besides being unsightly, it decreases the corn yield.
A fungal disease known as sudden oak death was identified in Germany, the Netherlands, California, and Oregon. It is caused by the fungus Phytophthora ramorum, which also infects rhododendrons, redwoods, and fir trees. Once a tree is infected, an oozing canker develops on the lower trunk; a few weeks later, its leaves turn yellow to brown. Control of the disease will require spending millions of dollars for fungicides. It will also be expensive and difficult to dispose of the millions of infected plants to prevent the spread of the fungus.
Pathogenic fungi that affect domestic crops cost billions of dollars yearly. Farmers and fruit growers must use large amounts of fungicides to control the spread of fungal disease in their fields and orchards.
The most common sites of human fungal infections are the skin, mouth, and lungs. For example, the organism Pneumocystis is present in the lungs of most people. However, in people with impaired immune systems, such as AIDS patients, the populations of this organism can increase and cause a form of pneumonia that is often fatal.
The growth of many kinds of molds releases large numbers of spores that are easily inhaled. These can lead to allergic reactions that cause serious illness. The flooding of homes on the Gulf Coast of the United States as a result of hurricanes resulted in many homes developing mold problems. Rehabilitation of these homes requires removal of plaster board to allow the underlying wood structures to dry out and to permit spraying with disinfectants. This costs several thousand dollars.
A number of fungi produce deadly poisons called mycotoxins. The most deadly of these fungi is Amanita verna, known as “the destroying angel”; it can be found in woodlands during the summer (figure 21.23). Mushroom hunters must learn to recognize this deadly species. It is believed to be so dangerous that food accidentally contaminated by its spores can cause illness and possibly death.
FIGURE 21.23. Poisonous Amanita Mushroom
The Amanita mushroom is deadly poisonous. It destroys the liver cells of those who eat it, causing death.
The mushroom Psilocybe mexicana has been used for centuries in religious ceremonies by certain Mexican tribes because of the hallucinogenic chemical it produces. These mushrooms have been grown in culture, and the drug psilocybin has been isolated. In the past, it was used experimentally to study schizophrenia.
Claviceps purpurea is a parasite on rye and other grains that affects the flowers of the grasses. It is commonly called ergot. The infection produces a small, hard structure similar in appearance to a seed. The metabolic activity of C. purpurea produces a toxin that can cause hallucinations, muscle spasms, insanity, and even death. However, it is also used in controlled doses to treat high blood pressure, to stop bleeding after childbirth, and to treat migraine headaches.
21.5. CONCEPT REVIEW
22. Name two beneficial results of fungal growth and activity.
23. Why are spores important in the life cycle of fungi?
24. Describe the role of fungi in lichens and mycorrhizae.
25. How do fungi obtain food?
26. What kinds of organisms are most affected by parasitic fungi?
Organisms in the domains Archaea and Bacteria and in the kingdoms Protista and Fungi rely mainly on asexual reproduction, and each cell usually satisfies its own nutritional needs. The Bacteria are prokaryotic and perform a wide variety of functions in ecosystems. Cyanobacteria are major photosynthesizers. Other functions include decomposer, mineral cycling, parasitism, and mutualism. Only a few species are pathogenic.
The Archaea are prokaryotic organisms that differ from Bacteria in the structure of cells and DNA. They are extremely common in aquatic habitats and the soil. Many live in extreme environments. Some live at high temperatures, some in high-salt environments. Many produce methane. Some are chemoautotrophs while others are decomposers. They are involved in the nitrogen cycle. None are pathogenic.
The members of the kingdom Protista are one-celled eukaryotic organisms. In some species, there is minimal cooperation between cells. The protists include: (1) algae with cells that have a cell wall and carry on photosynthesis; (2) protozoa, which lack cell walls and cannot carry on photosynthesis; and (3) funguslike protists that lack chitin and are thus distinguished from true fungi. Some species of Protista are multicellular and have cells specialized for particular functions. It is thought that the multicellular fungi, plants, and animals had Protistalike ancestors.
The kingdom Fungi consists of nonphotosynthetic, eukaryotic organisms with cell walls that contain chitin. Most species are multicellular. Fungi are nonmotile organisms that disperse by producing spores. Lichens are organisms that consist of a combination of organisms involving a mutualistic relationship between a fungus and an algal protist or a cyanobacterium. Mycorrhizae are associations between fungi and plant roots that are beneficial to both organisms.
1. The domains Bacteria and Archaea differ from the domain Eucarya in that the Eucarya
a. are eukaryotic and the Bacteria and Archaea are prokaryotic.
b. are single-celled but the others are not.
c. carry on photosynthesis but the others do not.
d. are simpler organisms than the others.
2. Some Archaea are found living in extremely salty or hot environments. (T/F)
3. The Apicomplexa are parasitic species of protozoa. (T/F)
4. Which one of the following groups does not have members that carry on photosynthesis?
a. the kingdom Protista
b. the domain Bacteria
c. the kingdom Fungi
5. The primary organisms that make up marine phytoplankton are cyanobacteria, dinoflagellates and _____.
6. Fungi are primarily dispersed by tiny structures called _____.
7. Methanogens are members of the domain Bacteria. (T/F)
8. The Chytridiomycota differ from Zygomycota in that Chytridiomycota
a. do not have chitin in their cell walls.
b. have spores with flagella.
c. do not cause disease.
d. form mushrooms.
9. Benthic algae live attached to objects. (T/F)
10. Lichens and mycorrhizae are similar in that they both involve symbiotic relationships with fungi. (T/F)
11. Both Bacteria and Archaea are involved in the nitrogen cycle. (T/F)
12. The major kind of Bacteria that carries on photosynthesis is _____.
13. Methanogens are in the domain _____.
14. Which of the following does not have pseudopods?
15. A mushroom is composed of hyphae. (T/F)
1. a 2. T 3. T 4. c 5. diatoms 6. spores 7. F 8. b 9. T 10. T 11. T 12. cyanobacteria 13. Archaea 14. d 15. T
A Mushroom Decline
Throughout much of Europe, there has been a severe decline in the mushroom population. On study plots in Holland, data collected since 1912 indicate that the number of mushroom species has dropped from 37 to 12 per plot in recent years. Along with the reduction in the number of species, there has been a parallel decline in the number of individual plants; moreover, the surviving plants are smaller. The phenomenon of the disappearing mushrooms is also evident in England. One study noted that, in 60 fungus species, 20 exhibited declining populations. Mycologists are also concerned about a decline in the United States; however, there are no long-term studies, such as those in Europe, to provide evidence for such a decline. Consider the niche of fungi in the ecosystem. How would an ecosystem be affected by a decline in fungi numbers?