LAUSR

ABSTRACT Bioremediation treatment (e.g. biostimulation) can decrease groundwater pH with consequences for Dehalococcoides mccartyi (Dhc) reductive dechlorination activity. To explore the pH resilience of Dhc, the Dhc‐containing consortium BDI was exposed to pH 5.5 for up to 40 days. Following 8‐ and 16‐day exposure periods to pH 5.5, dechlorination activity and growth recovered when returned to pH 7.2; however, the ability of the culture to dechlorinate vinyl chloride (VC) to ethene was impaired (i.e. decreased rate of VC transformation). Dhc cells exposed to pH 5.5 for 40 days did not recover the ethene‐producing phenotype upon transfer to pH 7.2 even after 200 days of incubation. When returned to pH 7.2 conditions after an 8‐, a 16‐ and a 40‐day low pH exposure, tceA and vcrA genes showed distinct fold increases, suggesting Dhc strain‐specific responses to low pH exposure. Furthermore, a survey of Dhc biomarker genes in groundwater samples revealed the average abundances of Dhc 16S rRNA, tceA and vcrA genes in pH 4.5‐6 groundwater were significantly lower (P‐value < 0.05) than in pH 6‐8.3 groundwater. Overall, the results of the laboratory study and the assessment of field data demonstrate that sustained Dhc activity should not be expected in low pH groundwater, and the duration of low pH exposure affects the ability of Dhc to recover activity at circumneutral pH. &NA; Graphical Abstract Figure. Dehalococcoides can survive low pH stress and recover at least partial dechlorination activity at circumneutral pH, although the duration of low pH exposure affects detoxification potential.