TABLE 112 continued

Process

Biodegradation (naphthalene)

Biodegradation (various polychlorinated biphenyl isomers) Biodegradation (various polychlorinated biphenyl isomers) Biodegradation (2- and

3-chlorophenol) Biodegradation (2,4-DME)

Biodegradation (2- and

3-chlorophenol) Biodegradation (various polychlorinated biphenyl isomers) Biodegradation (toluene)

Biodegradation (various polychlorinated biphenyl isomers) Biodegradation (toluene)

Biodegradation (2,4-DME)

Biodegradation (various polychlorinated biphenyl isomers)

Inhibitory Metal Concentration

Microbe Studied

Burkholderia sp.

Cd2+ 100-1000 ^M Alcaligenes spp. and Pseudomonas spp.

Co2+ 100-10,000 ^M Alcaligenes spp. and Pseudomonas spp.

Cr6+ 385 |M Indigenous community Cr3+ 25.0 |M Indigenous community Cu2+ 315 |M Indigenous community

Cu2+ 100-1000 |M Alcaligenes spp. and Pseudomonas spp.

Ni2+ 606 |M Bacillus sp.

Ni2+ 100-2000 ^M Alcaligenes spp. and Pseudomonas spp.

Zn2+ 403 ^M Bacillus sp.

Zn2+ 7.0 ^M Indigenous community

Zn2+ 200-5000 ^M Alcaligenes spp. and Pseudomonas spp.

Environment Reference

Mineral salts medium 85

containing 1.05 mM phosphate

Tris-buffered minimal 84

medium

Tris-buffered minimal 84

medium

Sediment slurry 88

Sediment microcosm 75

Sediment slurry 88

Tris-buffered minimal 84

medium

Mineral salts medium 89

containing 36 mM phosphate

Tris-buffered minimal 84

medium

Mineral salts medium 89

containing 36 mM phosphate

Sediment microcosm 75

Tris-buffered minimal 84

medium

1 mg Ni g-1 but decreased over time in uncontaminated soil samples. Nitrate concentrations increased over time regardless of the nickel concentration. Decreases in cell biomass and urea hydrolysis were also observed in response to lead toxicity.71

11.3.2 Metals and Bioremediation

Many different heavy metal and organic pollutants, located at hazardous waste sites throughout the world, pose threats to wildlife and human health.79,80 Excavation and incineration are two methods for eliminating organic wastes from these sites, but these methods can be inefficient and costly.81 Bioremediation, which involves using microorganisms to break down organic pollutants, has been considered an attractive alternative.82 One potential drawback to using bioremediation is that 40% of the sites on the U.S. Environmental Protection Agency's (EPA) National Priorities List (NPL) are cocontaminated with both organics and heavy metals.83 Metals have the potential to affect organic pollutant degradation at these sites; however, the effects of metals on biodegradation are not completely understood.76

Numerous studies have investigated the effects of heavy metals on biodegradation. Studies examining biodegradation84,85 often utilize bacteria that can use organic contaminants as sole carbon sources. Growth (an indicator of biodegradation in these studies) is often measured by optical density, protein content, oxygen consumption, or the amount of target organic compound that is removed over time.76,86 Most studies have shown that metals inhibit biodegradation of various aromatic and aliphatic hydrocarbons.87-90 One issue with these studies is that the metal may bind to the organic compound, preventing entry of the carbon source into the cell.91 It is not clear whether reductions in biodegradation in these studies were due to direct toxicity or an inhibition of uptake of the target organic pollutant.

Not all studies have shown a connection between metal toxicity and inhibition of biodegradation. Some studies have shown that heavy metals do not affect the degradation of organic compounds.92,93 For example, Baldrian et al.94 showed that cadmium and mercury did not inhibit the ability of the fungus Pleurotus ostreatus to degrade polycyclic aromatic hydrocarbons. It is possible that the metals were being sequestered by a component of the soil medium used in these studies. In addition, both physiological (e.g., the use of a different enzyme system by the fungus in comparison with bacteria) and anatomical (e.g., the compartmentalized nature of eukaryotic cells of the fungus) features unique to fungi may account for the lack of an effect.

Interestingly, some studies75,95 have shown increased biodegradation by bacteria in response to metals. In most cases, there is a dose-dependent relationship between metal concentration and biodegradation. Said and Lewis,75 however, observed that an aerobic consortium of microbes degraded 2,4-dichlorophenoxyacetic acid methyl ester (2,4-DME) at a faster rate when exposed to 100 pM cadmium per liter than when exposed to 10 pM cadmium. These results could be due to selective pressures faced by the microbial consortium at higher metal concentrations. Perhaps, only the most resistant microbes survived when exposed to higher metal concentrations. These microbes no longer had to compete with other microbes for the carbon source. Another possible explanation is that metal resistance mechanisms may be more rapidly induced in response to higher metal concentrations.

A wide range of inhibitory metal concentrations has been reported in the literature. For example, reported concentrations of zinc that reduce biodegradation range from 9.18 x 10-2 75 to 1.13 x 104 pM.84 This is likely due to a lack of standardized experimental conditions. Varying carbon sources, environmental conditions (e.g., medium composition and pH), and organisms of interest among studies likely account for the wide range of inhibitory concentrations reported in the literature. Some studies were performed using a single strain of bacteria, while others used an indigenous community of microbes. Some organisms were grown in soil or sediment microcosms, whereas others were grown in minimal salts media. Environmental conditions can affect metal toxicity by altering metal speciation and bioavailability.

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