Differential Immobilization of Pb2+ and Cd2+ by Marine Bacillus velezensis Hao 2023: Mechanisms and Fermentation Optimization for Enhanced Exopolysaccharide Production

Soil contamination with lead (Pb) and cadmium (Cd) poses a severe threat to agricultural safety. This study explored the marine bacterium <i>Bacillus velezensis</i> Hao 2023 for bioremediation potential and EPS yield enhancement. Soil filtrate tests under metal stress revealed high tolerance to Pb²⁺ (250 mg/L) and Cd²⁺ (2.5 mg/L), with distinct mechanisms: Cd²⁺ removal was strongly correlated with significant pH increase (up to 8.10), suggesting that immobilization likely occurred through precipitation, while Pb²⁺ was sequestered via EPS synthesis, achieving a yield of 1.62 g/L under stress. To decouple production from metal stress, fermentation was optimized using single-factor and response surface methodology. Key conditions (sucrose, ammonium sulfate, 45 g/L sea salt, 35 °C, pH 6.0, 8% inoculum, 150 rpm) achieved 1.081 g/L EPS under metal-free conditions. These results demonstrate strain Hao 2023's metal-specific resistance and provide a scalable process for soil remediation agent development.