Exogenous vitamin B12 alleviated inhibition of salinity on anaerobic dichloromethane degradation by reducing cofactor-related constraints and reshaping community functional potential.

Dichloromethane (DCM) frequently co-occurs with high salinity in industrial wastewater, imposing dual stress on anaerobic treatment. However, how anaerobic DCM degraders respond to salt stress and whether exogenous vitamin B12 (VB12, a key cofactor in DCM transformation) can facilitate DCM degradation remain poorly understood. Here, we established long-term enrichments (>800 days) of DCM-degrading consortia under non-saline and salt-stressed conditions (10 g/L NaCl) to investigate how VB12 affected degradation performance, community assembly, and functional potential. Salt stress significantly inhibited DCM degradation, reducing the maximum degradation rate by 71.5%, whereas VB12 substantially alleviated this inhibition and increased the degradation rate to 55.9% of the non-saline control. Metagenomic and co-occurrence network analyses indicated that salinity drove community reassembly and niche differentiation, linking DCM degraders, methanogens/homoacetogens, and fermenters within an inferred producer-cooperator-cross-feeder framework that maintained community stability under salt stress. Functional analyses showed that VB12 was associated with shifts in community functional potential toward hydrogenotrophic/acetoclastic methanogenesis and osmoadaptive metabolism, supporting stress adaptation under saline conditions. Further analysis of the mec (methylene chloride catabolism) cassette suggested that VB12 likely reduced cofactor-related constraints and reinforced downstream product-consuming functions, thereby contributing to the enhanced degradation performance. Notably, a previously uncharacterized Dehalobacteriaceae MAG, D_MAG.168, emerged as a dominant candidate DCM degrader under salt stress. Overall, these findings provide insight into the functional responses of DCM-degrading consortia to VB12 supplementation under salt stress and support the further development of VB12-assisted bioaugmentation strategies for DCM-contaminated saline industrial wastewater.