Adaptive restructuring of organohalide-respiring microbial communities under salinity stress maintains reductive dechlorination capacity.

Elevated salinity constrains microbial reductive dechlorination in groundwater contaminated with chlorinated aliphatic hydrocarbons, and stepwise salinity acclimation offers a strategy to sustain dechlorination activity. In this study, an organohalide-respiring enrichment consortium supplied with both 1,2-dichloroethane and trichloroethylene was subjected to stepwise salinity acclimation and subsequently evaluated through batch dechlorination assays and microbial community analyses. The acclimated consortium achieved complete dechlorination at 40 g/L NaCl after extended incubation, whereas conversion at 50 g/L remained incomplete, with persistent vinyl chloride accumulation. Community richness declined with increasing salinity, whereas Dehalococcoides remained detectable across all salinity treatments. Proteiniphilum and Sphaerochaeta were identified as candidate salt-tolerant taxa. Community restructuring was further characterized by an increased relative contribution of homogeneous selection, reduced co-occurrence network connectivity, and increased modularity. Functional prediction analyses also revealed stage-dependent shifts in predicted metabolic and biosynthetic potential. These findings demonstrate that stepwise salinity acclimation can improve and sustain dechlorination performance under saline conditions, while terminal conversion remains a key functional constraint at high salinity. The results provide a basis for further development of salinity-acclimated consortia for groundwater remediation and highlight the importance of minimizing chlorinated intermediate accumulation and improving terminal dechlorination efficiency.