Amorphous and Crystalline Carbonate Biomineralization in Cyanobacterial Biofilms Induced by Synechocystis sp. PCC6803 Cultured in CaCl2-MgCl2-SrCl2 Mediums

Ca2+, Mg2+, and Sr2+ are elements with similar ionic and hydrochemical characteristics, yet the microbial mineralization behavior in their coexisting environments is seldomly explored. In this study, the cyanobacterium Synechocystis sp. PCC6803 was used to induce the precipitation of minerals in mediums with various Mg/Ca and Sr/Ca ratios (Ca2+ = 0.01 M). The medium hydrochemistry, including cell density, solution pH, and alkalinity, was recorded periodically. The bio-precipitates were characterized with X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), Fourier Translation Infrared spectroscopy (FT-IR), and Thermal Gravimetric Analyzer (TGA). The results show the growth of Synechocystis sp. PCC6803 was inhibited by increasing ionic strength, although the final alkalinity and pH values of the medium were not affected. Two crystalline minerals, calcite (CaCO3) and strontianite (SrCO3) were precipitated in the mediums with low ionic strength; these were transformed from early amorphous precipitates by a dissolution and re-precipitation mechanism. The morphology of the precipitated cyanobacterial strontianite changed from columnar to dumbbell shape and finally into a spherulite shape. High concentrations of Sr2+, like Mg2+, prolonged the stabilization of amorphous carbonate precipitates. The acidic amino acids (Glu and Asp) in the EPS of Synechocystis sp. PCC6803 cultured in mediums with high Sr2+ and high Mg2+ concentrations increased significantly, compared to those cultured in a medium with no Sr2+ or Mg2+ (p < 0.005) ions. The negative binding energy calculated by Density Functional Theory (DFT) on the binding between the two acidic amino acids (Glu and Asp) and ion-H2O complexes (Ca[H2O](6)(2+), Mg[H2O](6)(2+), and Sr[H2O](6)(2+)), indicating that they are all thermodynamically-favored processes. Consequently, lower energy was needed in their subsequent precipitation and nucleation within the EPS of Synechocystis sp. PCC6803. This inferred process was also supported by the appearance of amorphous particles in the EPS of Synechocystis sp. PCC6803.