Bioaugmentation with metal-resistance microorganisms in the remediation of metal and organic contaminated soils

Abstract

Current thinking is that co-contaminated sites (i.e., sites with both organic and metallic pollutants) are difficult to bioremediate because the metal toxicity is such that organic degradation is inhibited. The objective of this research was to evaluate the potential of bioaugmentation with metal-detoxifying microbial populations as a viable remediative approach for such sites. Divided into three sections, this research found that metal-detoxifying microorganisms could facilitate the remediation of co-contaminated systems. The objective of the first study was to compare the microbial community response to cadmium exposure between metal-contaminated and uncontaminated soils. This study found that while cadmium adversely affected the numbers of culturable microorganisms in all soils, cadmium-resistant isolates were found in each soil, regardless of prior metal exposure. However, the metal-contaminated soil microbial communities were more resistant than the uncontaminated soil community. In one metal-stressed soil, resistance increased with increasing cadmium stress. A cadmium-resistant Pseudomonas spp. was found to increase in numbers with increasing cadmium, suggesting a different mechanism of cadmium resistance at high cadmium concentrations. The second study evaluated the diversity of cadmium-resistance/detoxification mechanisms in six cadmium-resistant isolates found in the first study. Genetic and microscopic analyses found several different approaches to cadmium resistance. Two mechanisms known to confer resistance were observed, including exopolymer and biosurfactant production. Two other isolates demonstrated intracellular cadmium accumulation via as yet unknown mechanisms. The mechanism of resistance for one isolate could not be identified. Four out of the six isolates detoxified cadmium as part of their resistance. Since metal detoxification is necessary to allow for organic degradation, these four isolates were included in 2,4-D degradation studies under co-contaminated conditions. The last study examined the use of cadmium-detoxifying microorganisms to enhance organic degradation under co-contaminated conditions. In pure culture and laboratory soil microcosms with cadmium and 2,4-dichlorophenoxyacetic acid (2,4-D) as model contaminants, four cadmium-detoxifying isolates supported the degradation of 2,4-D by the cadmium-sensitive 2,4-D degrader Alcaligenes eutrophus JMP134 in the presence of toxic levels of cadmium. Ina pilot field study, a cadmium-detoxifying Pseudomonas isolate enhanced 2,4-D degradation by A. eutrophus JMP 13 4 in the presence of cadmium.