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Merganser (fish-eating duck)

Figure 31.10 Comparison of the Rates of Disappearance of Two Structurally Related Herbicides, 2,4-D, and 2,4,5-T The addition of a third chlorine atom in the 2,4,5-T molecule blocks the enzyme that degrades the substance, so that compound remains in the environment.

to the additional chlorine atom, which blocks the enzyme that makes the initial attack on 2,4-D.

Most herbicides and insecticides not only are toxic to their target, but also have far-ranging deleterious effects on fish, birds, and other animals. DDT is an example of such a compound. This pesticide accumulates in the fat of predatory birds through biological magnification (table 31.1). Small amounts of the pollutant that contaminate water are concentrated in minute plankton, which are eaten by minnows, accumulating even more in the fish. When large birds eat the fish, the amount of chemical in tissues is tremendously magnified. The continuing ingestion of DDT, which accumulates in fat, results in an ever-greater concentration of the DDT as it passes upward through the food chain.

DDT, as well as other chlorinated hydrocarbon insecticides, also interferes with the reproductive process of birds, leading to the production of fragile eggs, which break before the young can hatch. Although banned in the United States, DDT is still heavily used in other countries where its effects can be far-reaching. For example DDT use in South America depleted predatory birds that normally summer in Greenland. The result was an overpopulation of Greenland's rodents.

Means of Bioremediation

Many factors influence the degradation rate of pollutants. As a general rule, any practice that favors multiplication of microorganisms will increase the rate of degradation. Thus, providing adequate nutrients, maintaining the pH near neutrality, raising the temperature, and providing an optimal amount of moisture are all likely to promote pollutant degradation. ■ environmental factors that influence microbial growth, p. 86 ■ nutritional factors that influence microbial growth, p. 90

There are two general bioremediation strategies—bio-stimulation and bioaugmentation. Biostimulation enhances growth of indigenous microbes in a contaminated site by providing additional nutrients. For example, petroleum-degrading bacteria are naturally present in seawater, but they degrade oil at a very slow rate because the low levels of nitrogen and phosphorus in the environment limit their growth. To enhance bioremediation of oil spills, a fertilizer containing these nutrients, and which adheres to oil, was developed. When this fertilizer is applied to an oil spill, microbial growth is stimulated, leading to at least a threefold increase in the speed of degradation by the bacteria. (figure 31.11). Bioaugmentation exploits the

Nester-Anderson-Roberts: I V. Applied Microbiology I 31. Environmental I I © The McGraw-Hill

Microbiology, A Human Microbiology: Treatment of Companies, 2003 Perspective, Fourth Edition Water, Wastes and

Polluted Habitats

798 Chapter 31 Environmental Microbiology: Treatment of Water,Wastes, and Polluted Habitats

798 Chapter 31 Environmental Microbiology: Treatment of Water,Wastes, and Polluted Habitats

Figure 31.11 Oil Spill Bioremediation Bioremediation was used to clean the shoreline contaminated by oil spilled from the Exxon Valdez.The growth of indigenous oil-degrading bacteria was stimulated by addition of nutrients. Comparing the uncleaned rocks on the left to the rocks cleaned by bioremediation on right shows the dramatic efficiency of bioremediation.

Figure 31.11 Oil Spill Bioremediation Bioremediation was used to clean the shoreline contaminated by oil spilled from the Exxon Valdez.The growth of indigenous oil-degrading bacteria was stimulated by addition of nutrients. Comparing the uncleaned rocks on the left to the rocks cleaned by bioremediation on right shows the dramatic efficiency of bioremediation.

activities of microorganisms that are added to the contaminated material, complementing the resident population. The activated sludge process used during secondary treatment of wastewater is a form of bioaugmentation. A great deal of research is underway to develop microbial strains suited for bioaugmentation. One example, Burkholderia (Pseudomonas) cepacia, is capable of growth on 2,4,5-T and has been used successfully to remove this chemical from soil samples in the laboratory. A limitation of using specially selected strains to breakdown pollutants, however, is that microbes that thrive under laboratory conditions may not compete well in natural habitats. ■ activated sludge processes, p. 786

Successful bioremediation may also involve controlling metabolic processes by manipulating the availability of O2 and specific growth substrates. For example, anaerobic degradation of trichloroethylene (TCE), a solvent used to clean metal parts, results in the accumulation of vinyl chloride, a compound more toxic than TCE. Aerobic conditions are important to prevent this buildup. Some pollutants are degraded only when specific substrates are made available to the microbes. This phenomenon, called co-metabolism, occurs because the enzyme produced by the microbe to degrade the additional substrate fortuitously degrades the pollutant as well. From the microbe's perspective, the breakdown of the pollutant by the enzyme is an accident. For example, the enzymes produced by some microbes to degrade methane also degrade TCE. In this case, adding methane enhances the degradation of TCE.

Bioremediation may be done either in situ or off-site. In situ bioremediation generally relies on biostimulation and is less disruptive. Oxygen (O2) can be added to contaminated groundwater and soil either by injecting hydrogen peroxide, which rapidly decomposes to liberate O2 and water, or pumping air into soil, bioventing. Off-site processes may be performed using a bioreactor, a large tank designed to accelerate microbial processes. Both nutrients and oxygen may be added to facilitate microbial growth and metabolism, while the slurry is agitated to ensure that the microbes remain in contact with the contaminants. A slower process involves mounding the contaminated soil over a layer that traps seeping chemicals. To provide O2, the soil may be turned occasionally or air may be forced through.

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