Facilitated Mechanism of Biological Vaterite Stability Mediated by Bacillus velezensis and Its Secretions

Bacteria from various natural habitats have been found to be able to induce the formation of acid-resistant and heat-resistant vaterite; however, the mechanism underpinning their stability remains ambiguous. In this study, Bacillus velezensis (B. velezensis) was selected as a model bacterium to induce the formation of biological vaterite (BV) and evaluate the mechanism underpinning its stability under water, acid/base, or high-temperature environments. Combined with the analysis of thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy/energy-dispersive X-ray spectrometry, it was found that the incorporation of organic matter secreted by bacteria into the BV crystal structure accounted for higher relative amounts (c. 21%) and facilitated the stability of BV. Fourier transform ion cyclotron resonance mass spectrometry and X-ray photoelectron spectroscopy were further used to analyze the more detailed compositions of the organic matter incorporated in the crystal structure of BV. The results showed that the classes of organic matter present in vaterite were mainly the oxygen-containing molecules C20-28O9-14 containing special functional groups (−OH, –COOH, −CONH–, etc.) with DBE > 6. Such organic matter could interact with Ca2+ or carbonate ions to form an organic–inorganic hybridization force, which improved the stability of BV. Biomimetic mineralization experiments and molecular dynamics (MD) simulation further confirmed the interaction of the organic matter and calcium carbonate in BV, which facilitated the stability of BV.