Reductive dechlorination resilience to oxygen intrusion: dehalogenase-dependent differential recovery and...

Oxygen intrusion in chlorinated solvent-contaminated aquifers frequently compromises organohalide-respiring bacteria (OHRB)-driven in-situ bioremediation. This study examined the dechlorination dynamics of a tetrachloroethene (PCE)-respiring consortium for 72 days after exposure to 0-4 mg/L dissolved oxygen (DO). Although DO fell below 0.2 mg/L within 36 h, recovery differed among dechlorination steps. At an initial DO of 4 mg/L, PCE-to-dichloroethene (DCE) dechlorination was retained at up to 3.8 μmol Cl^-·L^-1·d^-1, whereas vinyl chloride (VC) detoxification ceased above 2 mg/L. By day 72, the 4 mg/L treatment contained 39.6% DCE and 9.4% VC, with less than 1% of the supplied PCE dechlorinated to ethene. Integrated metagenomic and metatranscriptomic analyses attributed this differential resilience to functional redundancy among diverse PceA-carrying OHRB (Dehalobacter, Dehalococcoides, and other taxa) for upstream steps, versus the confinement of vcrA to a single oxygen-sensitive Dehalococcoides population, whose depletion coincided with delayed VC-to-ethene recovery. Facultative aerobes, particularly Pseudomonas, rapidly depleted O₂ through aerobic respiration. These findings identify the terminal VC-to-ethene step as the most vulnerable link in PCE attenuation. Oxygen intrusion therefore shifts attenuation end products from ethene towards the more mobile DCE and VC, prolonging contaminant persistence at oxygen-disturbed sites.