The vital effects of Bacillus subtilis during carbon isotope fractionation in the process of carbonate precipitation

Abstract

The carbon isotope composition (δ13C) of biogenic carbonates has been a valuable tool for reconstructing paleoenvironments worldwide. However, the vital effects of microbes during carbon isotope fractionation (CIF) in the process of microbial induced carbonate precipitation (MICP) have not been fully revealed. In this study, calcium carbonate (CaCO3) biomineralization experiments have been performed using the Bacillus subtilis strain 168 under the standing condition. In addition, corresponding shaking experiments and inorganic experiments were also carried out, respectively. We have quantified the temporal change of δ13C values of the carbonate minerals and dissolved inorganic carbon (DIC) as well as the cellular density, Ca2+ and HCO3− concentrations, and the carbonic anhydrase (CA) activity. X-ray diffraction analysis has shown that the precipitates produced by strain 168 were calcite, while no mineral has been formed in abiotic experiments. The values of δ13CCaCO3 fluctuated around −5.2‰ in Bio-standing experiments, whereas δ13CCaCO3 in Ino-standing experiments ranged from −14.1‰ to −13.1‰. The carbon isotope offset (Δ13CCaCO3–DIC) values increased from +0.8‰ to +7.6‰ with incubation and these in Ino-standing experiments increased from −2.3‰ to −0.2‰. These results indicate that bacteria participated in CIF and atmospheric CO2 is the dominant source of carbon in the carbonate minerals. An increase in pH and hydration of CO2 catalyzed by CA played synergetic roles in increasing the saturation index (SI) and accelerating the precipitation rate (R). The values of Δ13CCaCO3-DIC were found to be closely negatively correlated with SI (r = −0.86, P < 0.0001, n = 21), while no clear relationship in inorganic experiments. Moreover, the Δ13CCaCO3-DIC values increased synchronously with the LogR (r = 0.97, P < 0.0001, n = 21), while a negative correlation emerged in inorganic experiments (r = −0.93, P = 0.02, n = 5). Thus, a large CIF could be attributed to the vital effects of bacteria. The shaking condition can affect the CIF during MICP by increasing bacterial activity. These findings emphasize the importance of evaluating vital effects by bacteria during CIF to accurately reconstruct paleoenvironments and highlight the potential of biogenic carbonate minerals for CO2 sequestration.