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.

Identification of Benzo[ a ]pyrene-Metabolizing Bacteria in Forest Soils by Using DNA-Based Stable-Isotope Probing

“…The bacteria capable of degrading BaP (Burkholderiales) in B and BS treatments were detected as identical using DNA-SIP (see Table S1 in the supplemental material). Nevertheless, the similar BaP removal efficiency between the B and BS treatments hinted that BaP degradation was not stimulated by the functional microbial structure change and increasing Burkholderiales-related bacteria, consistent with a previous study by Powell (38). Since small pH variation might significantly affect the biodegradation of xenobiotics and other organic compounds in oligotrophic environments (55), the possible reason was the decreased PAH degradation activity of dominant Burkholderialesrelated bacteria under low-pH conditions (56), like the pH of 6.0 in Mt.…”

“…DNA-SIP has been widely applied to the identification of pollutant degraders in numerous environmental media and with an ever-expanding pool of compounds (22,27,38). In the present study, the coupling of DNA-SIP and TRFLP techniques revealed the bacteria correlated with BaP metabolism in soils from Mt.…”

“…Maoer microcosms (Table 1). Previous studies showed a significant increase in the rate of naphthalene mineralization in soil after enrichment with salicylate spiking (38). The presence of phenanthrene and salicylate also greatly enhanced the initial removal rates of benz[a]anthracene, chrysene, and benzo[a]pyrene by P. saccharophila P15 (16).…”

“…For example, the addition of salicylate had no effect on phenanthrene or pyrene removal in PAH-contaminated soils (60). After enrichment with salicylate, the initial naphthalene mineralization rate, rather than those of phenanthrene and BaP, was enhanced by the microbial community in a bioreactor for a PAH-contaminated soil treatment (38). In uncontaminated soils, salicylate only improved pyrene removal and did not affect BaP (60), which may explain why salicylate stimulated BaP removal in Mt.…”

“…The addition of salicylate to PAH-contaminated soils was shown to increase the quantity of naphthalene-degrading bacteria (57,58) and stimulate the degradation of benzo[a]anthracene, chrysene (16), fluoranthene (16,17), and BaP (8,16). Salicylate was also reported to sustain populations of biological control bacteria with naphthalene-degrading genes in agricultural systems (59), and various salicylate additives (spiked or slow/continuous addition) have been used to select different microbial communities (38).…”