Ca-mediated alleviation of Cd²⁺ induced toxicity and improved Cd²⁺ biomineralization by Sporosarcina pasteurii

Microbial induced carbonate precipitation has been widely used in the biomineralization of heavy metals Cd²⁺. However, the low Cd-tolerance of ureolytic bacteria limits the applications with only low Cd²⁺concentrations. In this study, we discovered a simple approach to significantly enhance the Cd²⁺ resistance of ureolytic bacteria through an immediate supplement of Ca²⁺. The Ca²⁺ protected the cells by reducing the extracellular and intracellular Cd²⁺ concentration by about 50%. As a result, the Cd²⁺ removal efficiency was notably improved by about 100% (52.72% to 99.43%, Cd = 5 mM) with Ca²⁺ supplement. Moreover, extremely high concentration of Cd²⁺ could be almost completely removed (99.46% at C₀ = 20 mM and 99.60% at C₀ = 50 mM) within 24 h. Microstructure analyses indicated that the mineralized precipitates were rhombohedral-shaped CdCO₃, CaCO₃, and (Ca0.67, Cd0.33)CO₃. Furthermore, Ca²⁺ could also protect ureolytic bacteria against toxicity from other heavy metals.