Effects of enrichment with salicylate on bacterial selection and PAH mineralization in a microbial community from a bioreactor treating contaminated soil

Abstract

We investigated enrichment with salicylate as a method to stimulate the degradation of polycyclic aromatic hydrocarbons (PAHs) by a microbial communityfrom a bioreactortreating PAH-contaminated soil. DNA-based stable isotope probing (SIP) was used to compare the effect of alternate methods of salicylate addition (spike vs slow, continuous addition) on the diversity of the enriched microbial community. After identification of salicylate degraders by SIP, real-time quantitative PCR (qPCR) primers were developed to quantify the abundances of three groups containing salicylate-utilizing organisms in the bioreactor community before and after enrichment. The different methods of salicylate addition were found to select for different microbial communities. Two groups containing salicylate-degrading bacteria increased in abundance substantially after enrichment by continuous addition of salicylate but did not increase in abundance in response to the spike addition, whereas a third group increased in abundance in response to both methods of salicylate addition. The initial rate of naphthalene mineralization increased significantly after enrichment by spike addition of salicylate, but neither phenanthrene nor benzo[a] pyrene mineralization rates were enhanced. Continuous addition of salicylate did not enhance the mineralization rate for any of the PAHs. These results suggest that enrichment with salicylate can select for naphthalene-degrading bacteria, but does not select for organisms responsible for degrading PAHs of higher molecular weight. Differences in microbial selection observed in this study that resulted from different rates of carbon source addition also have implications for the design of SIP experiments with water-soluble carbon sources.