Genomic personalities of Dehalococcoides subspecies and Dehalogenimonas illuminate complete trichloroethene dechlorination in high-salt conditions.

Global salinization increasingly threatens ecosystem integrity and the regulation of biogeochemical cycles. Our study reveals novel insights into the microbial contributions to the organohalide decomposition in saline environments, demonstrating the unprecedented ability of organohalide-respiring bacteria Dehalococcoides and Dehalogenimonas to completely dechlorinate trichloroethene to non-toxic ethene under hypersaline conditions (up to 31.3 g/L) in long-term operations. Using gradient salinity reactors and metagenomic analyses, we identified the evolved genomic features associated with high-salt tolerance. The Cornell subgroup of Dehalococcoides and Dehalogenimonas exhibit significantly lower average protein isoelectric points and retain the ribosomal protein L33p gene, unlike the Victoria and Pinellas subgroups. Dehalococcoides shows subspecies-level genomic divergence and unique codon usage biases. Intriguingly, the L33p gene is found in diverse bacterial phyla from saline environments, suggesting it may provide a growth advantage under salt stress. These genomic traits, hypothesized to enhance salt tolerance and dechlorination efficiency under salt stress, correlate with performance at elevated salinity. Our findings advance the understanding of microbial salt adaptation mechanisms and support the development of bioremediation strategies tailored for saline environments.