Ferrous-driven microbial synergy enhances reductive dechlorination under sulfide stress.

Chlorinated ethenes (CEs), such as trichloroethene (TCE), frequently co-occur with elevated sulfate in contaminated aquifers, where sulfide produced by sulfate reduction strongly inhibits Dehalococcoides mccartyi (Dhc), the key organohalide-respiring bacterium. In this study, we demonstrate that sulfide is far more toxic to Dhc-mediated TCE dechlorination than sulfate, with activity nearly abolished at concentrations above 5 mM. To overcome this bottleneck, we employed a synergistic system combining Dhc strain CWV2 and the sulfate-reducing bacterium Nitratidesulfovibrio liaohensis strain KPS with ferrous chloride supplementation. Supplementation with Fe(II) restored dechlorination efficiency by precipitating sulfide as biogenic ferrous sulfide (FeS), with an optimal Fe/S molar ratio maximizing sulfide removal while avoiding pH decline and secondary mineral formation. Transcriptomic analysis revealed that FeS treatment reactivated Dhc reductive dehalogenase and hydrogenase genes and enhanced sulfur metabolism in KPS, indicating coordinated functional recovery. Microbial community analysis further showed that FeS treatment increased Dhc abundance and enriched fermentative partners that provide hydrogen, acetate, and corrinoids. These findings highlight Fe(II)-mediated sulfide detoxification as an effective approach to stabilize microbial interactions and sustain TCE dechlorination, offering a practical strategy for in situ remediation of sulfate-CE co-contaminated groundwater.