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
Polycyclic aromatic hydrocarbons (PAH) are common soil pollutants, and several of the high molecular weight (HMW) PAH are classified as human carcinogens. Most of the HMW PAH are known to be degraded by microorganisms, but they generally degrade much more slowly and/or less extensively than the low molecular weight (LMW) compounds. This project is studying the factors that likely control the rate and extent of HMW PAH degradation in soils, by focusing on the basic mechanisms governing both the bioavailability and degradation kinetics for these compounds.
Project investigators have learned, for example, that the carcinogen benzo[a]pyrene is mineralized (completely oxidized to carbon dioxide and water) by a diverse group of bacteria isolated from various contaminated soils. However, scientists have found that the metabolism of benzo[a]pyrene and other HMW compounds that do not serve as growth substrates for bacteria must be induced by either low molecular weight PAH or known inducers of LMW PAH metabolism. Recent studies showed that, even under induced conditions, the rate of degradation of benzo[a]pyrene by one bacterial isolate is at least two orders of magnitude slower than the rate of degradation of the LMW compound phenanthrene at equivalent concentrations.
Scientists are also studying the formation of metabolites that accumulate when HMW PAH are degraded but not mineralized by bacteria; this was found to be the common fate of pyrene among bacterial isolates. Results to date suggest that both mineralization and incomplete metabolism of PAH will occur in complex systems containing a heterogeneous mixture of PAH, such as contaminated soil. With a better understanding of the mechanisms that control the degradation of these compounds, scientists will be better able to assess and, if desired, influence the extent to which biodegradation affects the long-term risk of PAH contamination in the subsurface.