Removal of calcium and magnesium ions through biomineralization induced by bacteria has been proven to be an effective and environmentally friendly method to improve water quality, but the process and mechanism are far from fully understood. In this study, a newly isolated probiotic Bacillus licheniformis SRB2 (GenBank: KM884945.1) was used to induce the bio-precipitation of calcium and magnesium at various Mg/Ca molar ratios (0, 6, 8, 10, and 12) in medium with 30 g L⁻¹ sodium chloride. Due to the increasing pH and HCO₃⁻ and CO₃²⁻ concentrations caused by NH₃ and carbonic anhydrase, about 98% Ca²⁺ and 50% Mg²⁺ were precipitated in 12 days. The pathways of bio-precipitation include extracellular and intracellular processes. Biominerals with more negative δ¹³C values (−16‰ to −18‰) were formed including calcite, vaterite, monohydrocalcite, and nesquehonite with preferred orientation. The nucleation on extracellular polymeric substances was controlled by the negatively charged amino acids and organic functional groups. The intracellular amorphous inclusions containing calcium and magnesium also contributed to the bio-precipitation. This study reveals the process and mechanism of microbial desalination for the removal of calcium and magnesium, and provides some references to explain the formation of the nesquehonite and other carbonate minerals in a natural and ancient earth surface environment.

Genesis of the Zhaoxian Gold Deposit, Jiaodong Peninsula, China: Insights From in-situ Pyrite Geochemistry and S-He-Ar Isotopes, and Zircon U-Pb Geochronology

The Tietonggou pluton is mainly composed of gabbroic diorite and diorite. The petrology, zircon U-Pb age, and geochemistry of the Tietonggou diorite have been studied to determine its petrogenesis and metallogenic significance. The diorite samples have 56–58 wt% SiO₂ and 11–14 wt% Al₂O₃ and are peraluminous and sodic (Na₂O/K₂O = 1.29–2.07). All the samples are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs; e.g., Rb, Ba, and Sr) but depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs; e.g., Zr, Nb, and Ta), suggesting subduction-related affinities. The rocks have narrow ranges of (²⁰⁶Pb/²⁰⁴Pb)_t (18.5–19.0), (²⁰⁷Pb/²⁰⁴Pb)_t (15.71–15.75), and (²⁰⁸Pb/²⁰⁴Pb)_t (38.4–39.0) ratios, respectively. Zircons from the Tietonggou diorite yielded a weighted average U-Pb age of 132.86 ± 0.92 Ma (MSWD = 0.48), whilst those from the nearby Laowa diorite yielded 129.72 ± 0.61 Ma (MSWD = 1.05). This suggests that the rocks represent Early Cretaceous plutons, coeval with the peak lithospheric thinning in eastern North China Craton (NCC). The magma likely originated from partial melting of the enriched lithospheric mantle and was contaminated by ancient lower NCC crustal materials. Our study clarifies the tectonic background of the Tietonggou pluton and provides support for the study of the genesis of Fe–skarn deposits in western Shandong.