Unit Four. The Evolution and Diversity of Life
Fungi, together with bacteria, are the principal decomposers in the biosphere. They break down organic materials and return the substances that had been locked in those molecules to circulation in the ecosystem. Fungi are virtually the only organisms capable of breaking down lignin, one of the major constituents of wood. By breaking down such substances, fungi make carbon, nitrogen, and phosphorus from the bodies of dead organisms available to other organisms.
In breaking down organic matter, some fungi attack living plants and animals as a source of organic molecules, whereas others attack dead ones. Fungi often act as disease-causing organisms for both animals and plants. The fungus Armillaria, shown in figure 18.12, is infecting a stand of conifers. The fungus originates in the center of an area indicated by the circles, and grows outward. Fungi are responsible for billions of dollars in agricultural losses every year.
Figure 18.12. World's largest organism?
Armillaria, a pathogenic fungus shown here afflicting three discrete regions of coniferous forest in Montana, grows from a central focus as a single clone. The large patch at the bottom of the picture is almost 8 hectares.
The same aggressive metabolism that makes fungi ecologically important has been put to commercial use in many ways. The manufacture of both bread and beer depends on the biochemical activities of yeasts, single-celled fungi that produce abundant quantities of ethanol and carbon dioxide. Cheese and wine achieve their delicate flavors because of the metabolic processes of certain fungi. Vast industries depend on the biochemical manufacture of organic substances such as citric acid by fungi in culture. Many antibiotics, including penicillin, are derived from fungi.
Edible and Poisonous Fungi
Many types of ascomycete and basidiomycete fungi are edible (figure 18.13a, b). They are commercially grown and can also be picked from the wild. The basidiomycete Agaricus bisporus grows in the wild but is also one of the most widely cultivated mushrooms in the world. Known as the “button mushroom” when it is small, it is also sold as the portobello mushroom when larger. Other examples of edible fungi include the yellow chanterelle (Cantharellus cibarius), morels (see figure 18.7b), and shiitake (Lentinula edodes). A great deal of care must be taken when selecting mushrooms for consumption, as many species are poisonous due to toxins they contain. Poisonous mushrooms (figure 18.13c) cause a range of symptoms, from slight allergic and digestive reactions, to hallucination, organ failure, and death.
Figure 18.13. Edible and poisonous mushrooms.
Edible mushrooms include (a) button mushrooms (Agaricus bisporus) and (b) yellow chanterelles (Cantharellus cibarius). Poisonous mushrooms include (c) Amanita muscaria.
Fungi are involved in a variety of intimate associations with algae and plants that play very important roles in the biological world. These associations typically involve a sharing of abilities between a heterotroph (the fungus) and a photosynthesizer (the algae or plant). The fungus contributes the ability to absorb minerals and other nutrients very efficiently from the environment; the photosynthesizer contributes the ability to use sunlight to power the building of organic molecules. Alone, the fungus has no source of food, the photosynthesizer no source of nutrients. Together, each has access to both food and nutrients, a partnership in which both participants benefit.
Mycorrhizae. Associations between fungi and plant roots are called mycorrhizae (Greek myco, fungus, and rhizos, roots). The roots of about 80% of all kinds of plants are involved in such associations. In fact, it has been estimated that fungi account for as much as 15% of the total weight of the world’s plant roots! Figure 18.14 shows how extensive this relationship can be. The roots on the left are roots from pine trees not associated with fungi. The roots in the middle and on the right exhibit mycorrhizae. You can see how the mycorrhizae greatly increase the surface area of the root. In a mycorrhiza, filaments of the fungus act as superefficient root hairs, projecting out from the epidermis, or outermost cell layer, of the terminal portions of the root. The fungal filaments aid in the direct transfer of phosphorus and other minerals from the soil into the roots of the plant, while the plant supplies organic carbon to the symbiotic fungus.
Figure 18.14. Mycorrhizae on the roots of pines.
From left to right are pine roots not associated with a fungus, white mycorrhizae formed by Rhizopogon, and yellow-brown mycorrhizae formed by Pisolithus.
The earliest fossil plants often have mycorrhizal roots, which are thought to have played an important role in the invasion of land by plants. The soils of that time would have completely lacked organic matter, and mycorrhizal plants are particularly successful in such infertile soils. The most primitive vascular plants surviving today continue to depend strongly on mycorrhizae.
Lichens A lichen is an association between a fungus and a photosynthetic partner. Ascomycetes are the fungal partners in all but 20 of the 15,000 different species of lichens that have been characterized. Most of the visible body of a lichen consists of its fungus, but interwoven between layers of hyphae within the fungus are cyanobacteria, green algae, or sometimes both. Enough light penetrates the translucent layers of hyphae to make photosynthesis possible. Specialized fungal hyphae envelop and sometimes penetrate the photosynthetic cells, serving as highways to collect and transfer to the fungal body the sugars and other organic molecules manufactured by the photosynthetic cells. The fungus transmits special biochemical signals that direct the cyanobacteria or green algae to produce metabolic substances that they would not if growing independently of the fungus. Indeed, the fungus is not able to grow or survive without its photosynthetic partner. Many biologists characterize this particular symbiotic relationship as one of slavery rather than cooperation, a controlled parasitism of the photosynthetic organism by the fungal host.
The durable construction of the fungus, combined with the photosynthetic abilities of its partner, has enabled lichens to invade the harshest of habitats, from the tops of mountains to dry, bare rock faces in the desert. The orange substance growing on the rocks in figure 18.15 is a lichen. In such harsh, exposed areas, lichens are often the first colonists, breaking down the rocks and setting the stage for the invasion of other organisms.
Figure 18.15. Lichens growing on a rock.
Lichens are extremely sensitive to pollutants in the atmosphere because they readily absorb substances dissolved in rain and dew. This is why lichens are generally absent in and around cities—they are acutely sensitive to sulfur dioxide produced by automobile traffic and industrial activity. Such pollutants destroy their chlorophyll molecules and thus decrease photosynthesis and upset the physiological balance between the fungus and the algae or cyanobacteria.
Key Learning Outcome 18.9. Fungi are key decomposers and play many other important ecological and commercial roles. Mycorrhizae are symbiotic associations between fungi and plant roots. Lichens are symbiotic associations between a fungus and a photosynthetic partner (a cyanobacterium or an alga).
Inquiry & Analysis
As you learned earlier in this chapter, chytrid fungi are thought to be playing a major role in a worldwide wave of amphibian extinctions, discussed in much more detail in chapter 38 (page 799). Our awareness of the possible role of chytrids began in Queensland (the northeastern portion of Australia) in 1993, when a mass die-off of frogs was reported. All different kinds of frogs seemed to be affected, and entire populations were wiped out. In the rainforests of northern Queensland, populations of the sharp-nosed torrent frog (Taudactylus acutirostris) were found to be so seriously affected as to be in danger of extinction. Captive colonies were set up at James Cook University and at the Melbourne and Taronga zoos in an attempt to preserve the species. Unfortunately, the preservation of the species failed. Every frog in the colonies died.
What was killing the frogs? The answer to this question came in 1998, when researchers examined the epithelium (skin) of sick frogs under the scanning electron microscope and saw what you can see in the photomicrographs to the right. Normally a relatively smooth surface, the epithelium of the dying frogs was roughened, with spherical bodies protruding from the surface.
These protrusions are zoosporangia, asexual reproductive structures of a chytrid fungus. One is shown up-close (inset). Each zoosporangium is roughly spherical, with one or more small projecting tubes. Millions of tiny zoospores develop in each zoosporangium. When the plug blocking the tip of a tube disappears, the spores are discharged onto the surface of adjacent skin cells, or into the water, where their flagella allow them to swim until they encounter another host. When one of the zoospores contacts the skin of another frog, it attaches and forms a new zoosporangium in the subsurface layer of the skin, renewing the infection cycle.
Study of the infecting chytrids revealed them to be members of the species Batrachochytrium dendrobatidis. This was unexpected. Chytrids are typically found in water and soil, and although there are several types known to infect plants and insects, no chytrid had ever been known to infect a vertebrate.
These initial scanning electron micrograph results seemed to make a pretty convincing case that chytrids had caused the mass die-off of frogs in Queensland. However, in order to provide more direct evidence, a series of experiments were carried out in which the ability of the chytrid fungus to kill frogs was directly assessed.
In one such experiment, typical of many, some frogs of the genus Dendrobates were exposed to chytrids and others were not. After three weeks, all frogs were examined for shed skin, a clinical sign of the frog-killing disease. The results are seen in the pie charts above.
1. Applying Concepts. In this study, is there a dependent variable? If so, what is it?
2. Interpreting Data. What is the incidence of disease in nonexposed frogs? In exposed frogs?
3. Making Inferences. Is there any association between exposure to the chytrid B. dendrobatidis and development of the skin infection that is a clinical sign of life-threatening illness in frogs?
4. Drawing Conclusions. What is the impact of exposure to chytrids upon the likelihood of developing the frog-killing disease?
5. Further Analysis
a. Many kinds of frogs and salamanders are dying all over the world. Does this experiment suggest a way to determine how general is the susceptibility of amphibians to chytrid infection?
b. While a few frog die-offs have occurred in the past, none have been nearly this serious. Do you think B. dendrobatidis is a new species, or do you think environmental changes like global warming or increased UV radiation resulting from ozone depletion might be the cause? Discuss.
1. The most important characteristic of complex multicellular organisms is
a. intercellular communication.
b. cell development.
c. cell specialization.
d. cell reproduction.
2. Which of the following is not a characteristic of the fungi kingdom?
b. cellulose cell walls
c. nuclear mitosis
d. nonmotile sperm
3. The main body of a fungus is the
4. Fungi reproduce
a. both sexually and asexually.
b. sexually only.
c. asexually only.
d. by fragmentation.
5. Morels and truffles belong to the fungus phylum
6. Zygomycetes are different from other fungi because they do not produce
b. fruiting bodies.
c. a heterokaryon.
d. a sporangium.
7. Ascomycetes form reproductive spores in
a. a special sac called the ascus.
b. gills on the basidiocarp.
d. the mycelium.
8. Meiosis in basidiomycetes occurs in the
9. Lichens are mutualistic associations between
a. plants and fungi.
b. algae and fungi.
c. termites and fungi.
d. coral and fungi.
10. Mycorrhizae help plants obtain
c. carbon dioxide.