Carbon isotope fractionation and its tracer significance to carbon source during precipitation of calcium carbonate in the presence of Bacillus cereus LV-1

Abstract

Microbial-induced carbonate precipitation (MICP) has the potential to immobilize carbon durably. The carbon source of carbonate minerals is the crucial issue for understanding the fixation mechanism of CO2 by MICP. However, the carbon source and its temporal changes in microbial-induced carbonate minerals remain little explored. In this study, calcium carbonate (CaCO3) biomineralization experiments have been carried out using Bacillus cereus in the medium without additional dissolved inorganic carbon (DIC). X-ray diffraction (XRD), attenuated total reflection-infrared spectroscopy (ATR-IR) and scanning electron microscopy (SEM) indicated that the precipitate produced mainly consisted of rhombohedral and irregular calcite. The δ13C values of DIC, dissolved organic carbon (DOC), and calcite were the main parameters studied. Carbon isotope fractionation was characterized by carbon isotope offset (Δ13Ccalcite-DIC or Δ13Ccalcite-fluid). The Δ13Ccalcite-DIC values ranged from +8.2‰ to +21.5‰, indicating that strain LV-1 induced the accumulation of 13C in calcite. The Δ13Ccalcite-fluid values indicated that the calcite was up to +15.0‰ (on average) 13C-enriched relative to the fluid. Calculated chemical mass balance data showed that the proportion of CO2(g) derived from DOC on days 16 and 20 was negative, but it became positive after day 20. Meanwhile, the δ13CCO2(g) value calculated by isotope mass balance was −8.5‰, near to δ13C for air (−8.0‰) on day 16, and then shifted to −16.4‰, similar to δ13C for tryptone (−17.2‰) after day 20. These results suggested that the amount of CO2 arising from organic matter through bacterial respiration and the action of enzymes might determine whether the carbon in calcite was derived from organic carbon or atmospheric CO2. Our findings corroborate the potential utility of MICP for immobilization of CO2 to reduce net soil CO2 emissions and to mitigate the greenhouse effect. Microbial-driven carbon isotope fractionation can also cause great carbon isotope shifts in calcite. Thus, carbon isotope value might be used as a marker to identify whether carbonate minerals/rocks are of microbial origin.