Concept 22.4: Fungi Can Be Sensitive Indicators of Environmental Change

We’ve already noted the important roles that fungi play in ecosystems, from decomposers to pathogens to plant mutualists. These diverse ecological roles have led to the use of fungi in studies of environmental change and in environmental remediation.

Lichen diversity and abundance indicate air quality

Lichens can live in many harsh environments where few other species can survive, as we saw in Concept 22.2. In spite of their hardiness, however, lichens are highly sensitive to air pollution because they are unable to excrete any toxic substances they absorb. This sensitivity means that lichens are good biological indicators of air pollution levels. It also explains why they are not commonly found in heavily industrialized regions or in large cities.

Monitoring the diversity and abundance of lichens growing on trees is a practical and inexpensive system for gauging air quality around cities (FIGURE 22.20). Maps of lichen diversity provide environmental biologists with a tool for tracking the distribution of air pollutants and their effects. Sensitive biological indicators of pollution, such as lichen growth, allow biologists to monitor air quality without the use of specialized equipment. Lichens are naturally distributed across the environment, and they can also provide a long-term measure of the effects of air pollution across many seasons and years.

Figure 22.20: More Lichens, Better Air Lichen abundance and diversity are excellent indicators of air quality. (A) Many lichen species show luxuriant growth on trees in suitable environments with few pollutants in the air. (B) As air quality declines, so do the number and diversity of lichens.

Fungi record and help remediate environmental pollution

Each year, biologists deposit samples of many groups of organisms in the collections of natural history museums. These museum collections serve many purposes. Biologists borrow specimens from these museums to study many aspects of evolution and ecology, and the collections document changes in the biota of our planet over time.

Collections of fungi made over many decades or centuries provide a record of the environmental pollutants that were present when the fungi were growing. Biologists can analyze these historical samples to see how different sources of pollutants were affecting our environment before anyone thought to take direct measurements. These long-term records are also useful for analyzing the effectiveness of cleanup efforts and regulatory programs for controlling environmental pollutants.

We have already seen that fungi are critical to the planetary carbon cycle because of their role in breaking down dead organic matter. Fungi are also used in remediation efforts to help clean up sites that have been polluted by oil spills or contaminated with toxic petroleum-derived hydrocarbons. Many herbicides, pesticides, and other synthetic hydrocarbons are broken down primarily through the action of fungi.

Reforestation may depend on mycorrhizal fungi

When a forest is cut down, it is not just the trees that are lost. A forest is an ecosystem that depends on the interaction of many species. As we have discussed, many plants depend on close relationships with mycorrhizal fungal partners. When trees are removed from a site, the populations of mycorrhizal fungi there decline rapidly. If we wish to restore the forest on the site, we cannot simply replant it with trees and other plants and expect them to survive. The mycorrhizal fungal community must be reestablished as well. For large forest restoration projects, a planned succession of plant growth and soil improvement is often necessary before forest trees can be replanted. As the community of soil fungi gradually recovers, trees that have been inoculated with appropriate mycorrhizal fungi in tree nurseries can be planted to reintroduce greater diversity to the soil fungal community.

APPLY THE CONCEPT: Fungi can be sensitive indicators of environmental change

Biologists analyzed museum samples of lace lichens (Ramalina menziesii) collected near San Francisco, California, from 1892 to 2006 for evidence of lead contamination. They measured concentrations of lead (Pb) as well as the ratios of its isotopes ²⁰⁶Pb and ²⁰⁷Pb. The latter measurement was used to determine the source of lead contamination. Possible sources included a lead smelter that operated in the area from 1885 to 1971 (which produced emissions with a ²⁰⁶Pb/²⁰⁷Pb ratio of about 1.15–1.17); leaded gasoline in use from the 1930s to the early 1980s, peaking in 1970 (with a ²⁰⁶Pb/²⁰⁷Pb ratio of 1.18–1.23); and resuspension of historic lead contamination as atmospheric aerosols in recent decades (with an intermediate ²⁰⁶Pb/²⁰⁷Pb ratio of about 1.16–1.19).^a

Before analyzing the data, use the information provided above to formulate hypotheses about these questions: What trends in atmospheric lead concentrations would you expect to see? What ²⁰⁶Pb/²⁰⁷Pb ratios would you expect to find at different times from the late 1800s to the early 2000s?

1. Plot lead concentration in the lichen samples against year of sample collection. Make a second plot of ²⁰⁶Pb/²⁰⁷Pb ratio against year of sample collection.
2. Do your analyses support the hypotheses you formulated? Are your hypotheses consistent with your analyses of both lead concentrations and ²⁰⁶Pb/²⁰⁷Pb ratios through time? If not, how would you modify your hypotheses, and what additional tests can you design to test your ideas?

Whether living on their own or in symbiotic associations, fungi have spread successfully over much of Earth since their origin from a protist ancestor. An earlier ancestor of fungi also gave rise to the choanoflagellates and the animals, as we will describe in Chapter 23.

ANSWER Beginning in the 1940s, antibiotics derived from fungi ushered in a “golden age” of freedom from bacterial infections. Today, however, that golden age may be coming to an end. Many antibiotics are losing their effectiveness as pathogenic bacteria evolve resistance to these drugs (FIGURE 22.21). Some bacterial diseases, such as tuberculosis, are increasingly serious health problems because of the evolution of new strains that are resistant to most classes of antibiotics.

Figure 22.21: Penicillin Resistance In a petri dish similar to those in Alexander Fleming’s lab, four strains of a pathogenic bacterium have been cultured along with Penicillium mold. One strain is resistant to the mold’s antibiotic substance, as is evidenced by its growth up to the mold.

Why do bacteria evolve resistance to antibiotics? Mutations that allow bacteria to survive in the presence of an antibiotic are favored by selection in a bacterial population whenever an antibiotic is used. Such mutations often carry a cost to the bacteria, so they may be selected against in the absence of regular antibiotic use. To reduce the rate of evolution of antibiotic resistance, antibiotics should be used only for the treatment of appropriate bacterial diseases, and then used to completely clear the bacterial infection.

Most medical antibiotics are chemically modified forms of the substances that are found naturally in fungi and other organisms. Fungi naturally produce antibiotic compounds to defend themselves against bacterial growth and to reduce competition from bacteria for nutritional resources. These naturally occurring compounds are usually chemically modified to increase their stability, improve their effectiveness, and facilitate synthetic production.

From the late 1950s to the late 1990s, no new major classes of antibiotics were discovered. In recent years, however, three new classes of antibiotics have been synthesized based on the information learned from naturally occurring antibiotics, leading to improved treatment of some formerly resistant strains of bacteria.

Fungi have also been used to combat non-bacterial diseases. One of the more unusual applications of fungi is in the war against malaria, one of the biggest killers of humans in sub-Saharan Africa. Biologists have discovered that two species of fungi, Beauveria bassiana and Metarhizium anisopliae, can kill malaria-causing mosquitoes when applied to mosquito netting. Mosquitoes have not yet shown evidence of developing resistance to these biological pathogens, as they have to most chemical pesticides.