Chlorinated ethene biodegradation and associated bacterial taxa in multi-polluted groundwater: Insights from biomolecular markers and stable isotope analysis.

Chlorinated ethenes (CEs) are most problematic pollutants in groundwater. Dehalogenating bacteria, and in particular organohalide-respiring bacteria (OHRB), can transform PCE to ethene under anaerobic conditions, and thus contribute to bioremediation of contaminated sites. Current approaches to characterize in situ biodegradation of CEs include hydrochemical analyses, quantification of the abundance of key species (e.g. Dehalococcoides mccartyi) and dehalogenase genes (pceA, vcrA, bvcA and tceA) involved in different steps of organohalide respiration (OHR) by qPCR, and compound-specific isotope analysis (CSIA) of CEs. Here we combined these approaches with sequencing of 16S rRNA gene amplicons to consider both OHRB and bacterial taxa involved in CE transformation at a multi-contaminated site. Integrated analysis of hydrogeochemical characteristics, gene abundances and bacterial diversity shows that bacterial diversity and OHRB mainly correlated with hydrogeochemical conditions, suggesting that pollutant exposure acts as a central driver of bacterial diversity. CSIA, abundances of four reductive dehalogenase encoding genes and the prevalence of Dehalococcoides highlighted sustained PCE, DCE and VC degradation in several wells of the polluted plume. These results suggest that bacterial taxa associated with OHR play an essential role in natural attenuation of CEs, and that representatives of taxa including Dehalobacterium and Desulfosporosinus co-occur with Dehalococcoides. Overall, our study emphasizes the benefits of combining several approaches to evaluate the interplay between the dynamics of bacterial diversity in CE-polluted plumes and in situ degradation of CEs, and to contribute to a more robust assessment of natural attenuation at multi-polluted sites.