Characterization and transcriptome analysis of mercury-resistant Pseudomonas canadensis isolated from a chlor-alkali contaminated soil revealed mer-independent detoxification pathways

Human activities have caused severe soil pollution through the discharge of pollutants, reshaping microbial communities. Among soil microbiota, the genus Pseudomonas exhibited remarkable tolerance to multiple contaminants, including mercury (Hg). This study compared the Hg resistance mechanisms of two Pseudomonas canadensis strains isolated from contrasting environments: a resistant isolate from the rhizosphere of Hg-contaminated soil and a control strain from unpolluted soil. The strain originating from a Hg-enriched soil showed significant biovolatilisation capacity and 20-fold higher Hg resistance compared to the strain isolated from an unpolluted environment. Biomass and necromass responses indicated that the control strain primarily relied on passive resistance, while the resistant strain exhibited active detoxification. Transcriptomic analysis revealed a rapid upregulation of genes associated with metal detoxification and efflux pathways in the resistant strain, independently of the mer operon, suggesting alternative resistance mechanisms. In contrast, the control strain exhibited transcriptional signatures of cellular stress, with increased thiol metabolism, DNA repair, and oxidative stress responses. These findings demonstrate that long-term Hg exposure selects for functional adaptations that sustain resistance in microbial populations. This work sheds new lights on Hg tolerance in P. canadensis and highlights the potential of naturally adapted strains for bioremediation strategies.