Beneficial factors for biomineralization by ureolytic bacterium Sporosarcina pasteurii.

BACKGROUND: The ureolytic bacterium Sporosarcina pasteurii is well-known for its capability of microbially induced calcite precipitation (MICP), representing a great potential in constructional engineering and material applications. However, the molecular mechanism for its biomineralization remains unresolved, as few studies were carried out. RESULTS: The addition of urea into the culture medium provided an alkaline environment that is suitable for S. pasteurii. As compared to S. pasteurii cultivated without urea, S. pasteurii grown with urea showed faster growth and urease production, better shape, more negative surface charge and higher biomineralization ability. To survive the unfavorable growth environment due to the absence of urea, S. pasteurii up-regulated the expression of genes involved in urease production, ATPase synthesis and flagella, possibly occupying resources that can be deployed for MICP. As compared to non-mineralizing bacteria, S. pasteurii exhibited more negative cell surface charge for binding calcium ions and more robust cell structure as nucleation sites. During MICP process, the genes for ATPase synthesis in S. pasteurii was up-regulated while genes for urease production were unchanged. Interestingly, genes involved in flagella were down-regulated during MICP, which might lead to poor mobility of S. pasteurii. Meanwhile, genes in fatty acid degradation pathway were inhibited to maintain the intact cell structure found in calcite precipitation. Both weak mobility and intact cell structure are advantageous for S. pasteurii to serve as nucleation sites during MICP. CONCLUSIONS: Four factors are demonstrated to benefit the super performance of S. pasteurii in MICP. First, the good correlation of biomass growth and urease production of S. pasteurii provides sufficient biomass and urease simultaneously for improved biomineralization. Second, the highly negative cell surface charge of S. pasteurii is good for binding calcium ions. Third, the robust cell structure and fourth, the weak mobility, are key for S. pasteurii to be nucleation sites during MICP.

Sporosarcina pasteurii can form nanoscale calcium carbonate crystals on cell surface.

The bacterium Sporosarcina pasteurii (SP) is known for its ability to cause the phenomenon of microbially induced calcium carbonate precipitation (MICP). We explored bacterial participation in the initial stages of the MICP process at the cellular length scale under two different growth environments (a) liquid culture (b) MICP in a soft agar (0.5%) column. In the liquid culture, ex-situ imaging of the cellular environment indicated that S. pasteurii was facilitating nucleation of nanoscale crystals of calcium carbonate on bacterial cell surface and its growth via ureolysis. During the same period, the meso-scale environment (bulk medium) was found to have overgrown calcium carbonate crystals. The effect of media components (urea, CaCl2), presence of live and dead in the growth medium were explored. The agar column method allows for in-situ visualization of the phenomena, and using this platform, we found conclusive evidence of the bacterial cell surface facilitating formation of nanoscale crystals in the microenvironment. Here also the bulk environment or the meso-scale environment was found to possess overgrown calcium carbonate crystals. Extensive elemental analysis using Energy dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRD), confirmed that the crystals to be calcium carbonate, and two different polymorphs (calcite and vaterite) were identified. Active participation of S. pasteurii cell surface as the site of calcium carbonate precipitation has been shown using EDS elemental mapping with Scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM).