Chapter 29

1. Unicellular fungi absorb nutrients directly from their environment, but multicellular fungi use structures known as hyphae for absorption. In both cases, fungi secrete digestive enzymes into the environment and then absorb the breakdown products through their cell membranes. The hyphae of multicellular fungi are long, thin networks that can penetrate soil and decaying or living organic material.

2. The large surface area-to-volume ratio allows rapid absorption over a wide area, as the hyphae can be immediately adjacent to the food source. However, this can also lead to rapid water loss and drying, which is why fungi tend to be restricted to (or at least thrive best in) moist environments.

1. Fungi break down organic matter, especially the cellulose and lignin of plant walls, thus returning the carbon in dead plants back into a form that can be used again by other organisms. Fungi also break down the keratin of animals, which is resistant to bacterial decay. Without fungi, these materials would build up in the environment and be a global sink for carbon.

2. Lichens represent a mutualistic association between a fungus and a photosynthetic partner (a unicellular alga and/or a cyanobacterium). The fungus obtains fixed carbon (organic compounds) from its photosynthetic partner and provides it with minerals and water. Both the fungus and the photosynthetic partner benefit and can grow in harsh environments (such as on rocks or the bark of trees) where neither organism would survive well, if at all, on its own.

3. As with lichen associations, mycorrhizal fungi form a mutualistic partnership with a photosynthetic species, in this case a multicellular plant. Many plants depend on these mycorrhizal associations for survival. The plant provides organic compounds (the products of photosynthesis) to the fungus, and the fungus provides minerals and water from the soil to the plant. The high surface area-to-volume ratio of the fungal hyphae greatly increases the plant’s ability to absorb water and minerals. Also, the hyphae act like a sponge, holding water around the roots.

1. In mating types, there is no phenotypic distinction as exists between male and female sexes, and there are often more than two mating types. Mating types are morphologically indistinct, but within each species, each mating type is genetically different. Individuals within a species may not mate with another of the same mating type. In contrast, male and female sexes are defined by the relative size of their gametes. (Females have large gametes, and males have small gametes; reproduction requires the combination of a male and a female gamete).

2. Microsporidia are among the smallest eukaryotes known. They are obligate parasites on animals. The host cell is penetrated by a polar tube through which the contents of the spore are injected into the host. This sporoplasm then replicates within the host cell and produces new infective spores. In some insects, once an individual is infected, the microsporidian parasite may be transmitted from parent to offspring.

3. Most chytrids possess flagellated gametes and reproduce in water (or at least in wet environments, such as moist soil).

4. The zygospore is the diploid phase of the zygospore fungi life cycle. It has a thick, multilayer cell wall and can serve as a resting stage that may remain dormant for months until environmental conditions are favorable for growth. At that time, its diploid nuclei undergo meiosis, and the zygospore produces a stalked sporangiophore, which bears from one to many sporangia. Each sporangium contains haploid spores, which are the products of meiosis.

5. The haploid ascospores of sac fungi are contained in sacs known as asci. Many species of sac fungi bear these asci on cup-shaped fruiting bodies known as ascoma. In contrast to these cup-shaped fruiting bodies, the fruiting bodies of club fungi include the more familiar mushrooms, puffballs, and bracket fungi, which are called basidiomata. These basidiomata bear the basidia, in which fertilization and meiosis take place to produce the haploid basidiospores.

1. Some fungi are eaten directly (including mushrooms and a few lichens), but fungi play a bigger role in the production of bread (from baker’s yeast), cheese (fungi give many cheeses their distinctive flavors), and drinks (through fermentation of yeasts). In addition, fungi enhance and facilitate the growth of many crop plants.

2. The museum lichen collections represent an indirect record of past air quality that can be used in areas where no direct measurements of air quality were originally taken. In addition, lichen diversity surveys represent a fast and inexpensive way to monitor air quality across an environment.

3. Site 5 shows the highest diversity and density of lichens and so is probably farthest from the city center. Site 4 is next, followed by Site 1, then Site 3, and finally Site 2. In addition to distance from the city center and prevailing wind direction, other predictive factors could include distance to point-pollution sources (such as factories or power plants) and distance to major highways (a source of pollution from automobile exhaust). Other answers are also possible; it is important for such studies to control for factors such as the species of tree examined and the exposure of the branches to similar light and humidity conditions.

4. By using selective tree harvest, rather than clear-cutting, forests may be managed to retain largely intact communities of many organisms, including mycorrhizal fungi. If some of the original forest trees are retained, replacement of harvested trees is greatly facilitated by the continued presence of viable mycorrhizal fungal communities.

1. 

2. A-32

   Yes, the levels of environmental lead contamination and the ratios of lead isotopes are consistent with the identified sources. The closure of the lead smelter was offset by the use of leaded gasoline, but there was a marked drop-off of environmental lead contamination after the elimination of both of these sources. The trend lines show a peak in environmental lead contamination around the 1960s to 1980s, and then a rapid drop after the end of leaded gasoline use in the early 1980s. The measured ratios of ²⁰⁶Pb/²⁰⁷Pb are consistent with the suspected sources of lead contamination through time. One way to extend the analysis would be to analyze additional lichen samples collected during the relatively poorly sampled period between 1907 and 1978 (from which period only two samples are included). Note, however, that such an analysis would depend on the existence of appropriate samples, such as museum collections made during this time.

1. There is a strong linear relationship between the negative effects of exposure to garlic mustard on a species and its dependence on its fungal mutualists, as the plants that are most dependent on the mutualism are most negatively affected by exposure to garlic mustard. This is consistent with garlic mustard reducing growth by interfering with the mycorrhizal mutualists.

2. Trees from mature forests are most dependent on arbuscular mycorrhizal fungi. A reasonable explanation for this finding is that trees in mature forests rely heavily on drawing nutrients and water from their roots, the site of action for arbuscular mycorrhizal fungi. Plants that live on forest edges and particularly those that live in disturbed areas are less reliant on the mutualists than are those that live in mature forests. This may be due to edge species being less reliant on gathering water and nutrients via their roots.

3. Plants with active arbuscular mycorrhizal fungal mutualists are more resilient to drought and temperature extremes than those without. Thus the loss of the fungal mutualists by exposure to garlic mustard may lead to the plants being less resilient to drought and temperature extremes.

   The plants with arbuscular mycorrhizal fungi should be able to withstand extremes in climate better than those without. Loss of fungal mutualists via the action of garlic mustard should lead to greater negative consequences for the plants as a result of climate change.