Soils co-contaminated with metals and organics present special problems for remediation. Metal contamination can delay or inhibit microbial degradation of organic pollutants such that for effective in situ biodegradation, bioaugmentation is necessary. We monitored the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) or 3-chlorobenzoate (3-CB) in two different soils with and without cadmium (Cd) contamination. Additionally, we evaluated the ability of bioaugmentation to enhance organic degradation in these co-contaminated soils. Finally, we determined whether enhanced degradation was due to survival of the introduced organism (cell bioaugmentation) or plasmid transfer to indigenous microbial populations (gene bioaugmentation). In Brazito soil, dual inoculation with a Cd-resistant bacterium plus a known 2,4-D-degrading bacterium, Ralstonia eutropha JMP134, enhanced 2,4-D degradation. Escherichia coli D11, which lacks chromosomal genes necessary for complete 2,4-D mineralization, was used for gene bioaugmentation in Madera soil. Significant gene transfer of the plasmid to the indigenous populations was observed, and the rate of 2,4-D degradation was enhanced relative to that of controls. Cell bioaugmentation was further demonstrated when (Comamonas testosteroni was used to enhance biodegradation of 3-CB in Madera soil. In this case no transfer of plasmid pBRC60 to indigenous soil recipients was observed. For the Madera soil, nonbioaugmented samples ultimately showed complete 2,4-D degradation. In contrast, nonbioaugmented Brazito soils showed incomplete 2,4-D degradation. These studies are unique in showing that both cell bioaugmentation and gene bioaugmentation can be effective in enhancing organic degradation in co-contaminated soils. Ultimately, the bioaugmentation strategy may depend on the degree of contamination and the time frame available for remediation.
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