Superfund Research Program
Beyond Parent Compound Disappearance in the Bioremediation of PAH-Contaminated Soil
Project Leader: Michael D. Aitken
Grant Number: P42ES005948
Funding Period: 1995-2018
Project Summary (2011-2018)
PAHs are among the top 10 contaminants of concern at Superfund sites. Although bioremediation has been used at PAH-contaminated sites, there is an inherent assumption that the disappearance of the PAHs corresponds to a reduction in risk. In Dr. Aitken’s work he and his team of researchers have observed that soil from a former manufactured-gas plant (MGP) site subjected to simulated in situ bioremediation was more genotoxic than the untreated soil. Such a finding is consistent with the known propensity of some microorganisms to transform PAHs and other contaminants to products that can be as toxic as or more toxic than the parent compounds. The researchers' underlying hypothesis is that there are metabolic bottlenecks in the aerobic bacterial metabolism of PAHs and other contaminants that lead to the accumulation of these products in contaminated soil. In this project they are exploring the genetic basis for incomplete bacterial metabolism of PAHs and the conditions that can lead to increases in the toxicity of contaminated soil treated by bioremediation.
The current state of knowledge of PAH metabolism by bacteria is based on a relatively limited range of microbial genera that are amenable to cultivation in the laboratory. The researchers used a molecular method called stable-isotope probing (SIP) to identify major PAH-degrading bacteria in field-contaminated soils without having to isolate them first on artificial media. Their SIP results have revealed a number of organisms that have not been associated with PAH degradation in the past, suggesting the possibility that genetic determinants of PAH metabolism remain to be discovered as well. In this project they are using whole genome sequencing as a basis for identifying genes associated with PAH metabolism in selected PAH-degrading cultures identified by SIP. Differences in the ability to metabolize various PAHs are being correlated to differences in key genes. The effects of bioremediation conditions on genotoxicity, selection of PAH-degrading organisms, and expression of key genes associated with PAH biodegradation are being evaluated.
The researchers are also evaluating the effects of the same conditions on the expression of genes associated with xenobiotic metabolism in skin as potential biomarkers of dermal exposure to contaminants in the soil. Their ultimate goal is to understand the relationship between the potential for incomplete PAH metabolism by bacteria based on genetic determinants and observed effects of bioremediation on the toxicity of the soil.