Cd distribution and phenanthrene degradation pathways in Cd-tolerant and phenanthrene-degrading bacteria under...

Microbial remediation of soils co-contaminated with heavy metals and polycyclic aromatic hydrocarbons is promising, yet understanding of bacterial adaptive mechanisms remains limited. This study investigated the performance of two isolated Cd-tolerant, phenanthrene (Phe)-degrading bacterial strains (Paenibacillus sp. WQ1 and Rhodococcus sp. WQ2) under Cd-Phe co-contamination. After 5-day of incubation, the survival rates of strains WQ1 and WQ2 reached 94.5 % and 86.1 % at a Cd^2 + concentration of 1.5 mg/L and 89.7 % and 76.9 % at a Cd²⁺ concentration of 10 mg/L, respectively. After 7-day of incubation, strain WQ1 achieved Cd²⁺ removal rates of 39.0 % and 20.8 % and Phe (100 mg/L) degradation rates of 70.1 % and 62.6 % at 1.5 and 10 mg/L Cd²⁺, respectively. Correspondingly, strain WQ2 achieved Cd²⁺ removal rates of 31.7 % and 10.9 % and Phe degradation rates of 86.0 % and 74.2 % at the same Cd²⁺ concentrations of 1.5 and 10 mg/L, respectively. Crucially, a "low-toxicity convergence strategy" was adopted to mitigate toxicity by both strains under elevated Cd²⁺ stress (10 mg/L) through prioritizing extracellular adsorption (notably, strain WQ2 shifted from intracellular accumulation) and utilizing the lower-toxicity benzoate degradation pathway (where strain WQ1 shifted from the salicylate pathway). Overall, this study provided valuable microbial resources and theoretical insights for the remediation of Cd-Phe co-contamination.