A cooperation mechanism between Bacillus thuringiensis and Citrobacter freundii that enhances cadmium biomineralization

Abstract

The viability and tolerance of individual ureolytic bacteria are a bottleneck in the remediation of cadmium (Cd) by microbially induced carbonate precipitation (MICP) technology. To solve this issue, strains of Bacillus thuringiensis (B. thuringiensis, BT) and Citrobacter freundii (C. freundii, CF) were isolated from soil and studied for their growth characteristics and metabolism. A cooperation system (BT+CF, 1:1, v/v) was constructed and exposed to 20 mg/kg Cd²⁺ for 7 days, compared with individual bacteria. The synergistic mechanism of strains that immobilize Cd²⁺ was explored using characterization techniques. Results showed that the main metabolic pathways leading to urea up-regulation were pyrimidine metabolism, urea cycle, and lysine degradation by metabolomic analysis. The cooperation system can effectively remove Cd²⁺ with an efficiency of 97.68%, which is higher than BT (66.66%) and CF (88.61%). The SEM-EDS, TEM, and XPS results revealed that the calcium carbonate polycrystals (vaterite and calcite) were formed during the MICP process, and the XRD and FTIR confirmed that the BT+CF produces more stable carbonate crystals. The BT+CF cooperation system was efficient at immobilizing Cd²⁺ by synergizing the molecular mechanisms of ureolytic bacteria. These results provide a novel perspective for the application of MICP.