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
Final Progress Reports
Dr. Aitken and his research team have extended their work involving the use of stable-isotope probing (SIP) to characterize the PAH-degrading microbial communities in contaminated soils. Most of their work has focused on using SIP to identify uncultivated bacteria responsible for degrading naphthalene, phenanthrene, and pyrene in a bioreactor treating field-contaminated soil from a former manufactured-gas plant site. The synthesis of phenanthrene and pyrene in uniformly 13C-labeled form by the Chemistry and Analytical Core provides project investigators a unique capability to apply SIP to the study of PAH biodegradation.
The bacteria responsible for degrading the three PAHs the research team has studied are unique to the PAH. This finding was somewhat surprising, as most isolated PAH-degrading bacteria are able to metabolize a wide range of PAHs. The two most dominant organisms whose DNA was associated with pyrene degradation are not closely related to any previously cultivated or characterized bacteria, illustrating the power of SIP for discovery of novel organisms in complex environmental systems. The investigators used a quantitative polymerase chain reaction (qPCR) method to quantify the relative abundances of the PAH degraders in the bioreactor. Most of the PAH degraders were less than 0.1% of the total bacterial community. In contrast, one of the pyrene-degrading bacteria was over 10% of the total bacterial community, suggesting that it probably is able to grow on a wide range of the contaminants in the soil.
Project personnel have also used SIP to evaluate the potential for the phenanthrene- and pyrene-degrading bacteria to simultaneously utilize these two PAHs in a mixture. Differences in the preference of one substrate over another were observed among the organisms. Understanding such bacterial responses to mixtures of carbon sources is important for elucidating biodegradation behavior in complex systems. In another application of SIP, the researchers extended their work on the potential use of salicylate to enhance the populations of PAH-degrading bacteria in contaminated systems. They evaluated the effect of the method by which salicylate was added to the bioreactor community on the selective enrichment of salicylate-degrading organisms. Different organisms were identified depending on whether salicylate was added as a single pulse (spike) or whether it was added slowly and continuously. The researchers will soon evaluate whether this difference translates into differences in PAH degradation by the two different methods of salicylate addition. Finally, in collaboration with Project 5, project investigators used SIP to identify the pyrene-degrading bacteria in a creosote-contaminated soil from a Superfund site. Nearly all of the DNA recovered in that experiment was from organisms closely related to the highly abundant pyrene degrader found in the project’s bioreactor. It is remarkable that organisms associated with pyrene degradation were very similar in two soils with different contaminant matrixes and from geographically distant areas, and that these organisms have never been isolated on pyrene as a sole carbon source. Project investigators are in the process of obtaining soil samples from other PAH-contaminated sites to evaluate the presence and abundance of these bacteria at other sites.