Calcium carbonate precipitation mediated by bacterial carbonic anhydrase in a karst cave: crystal morphology and stable isotopic fractionation

Abstract Stable oxygen (δ18O) and carbon (δ13C) isotopes of speleothems are widely used as important proxies for paleoclimatic and paleoenvironment reconstructions. However, the influence of microbial activity on carbon and oxygen isotope fractionation during speleothem precipitation remains unclear. Bacterial carbonic anhydrase (CA) can promote calcium carbonate precipitation to catalyze the mutual transformation between CO2 and HCO3-. CA-producing bacteria (Lysinibacillus sp. strain LHXY2) were separated in the Xueyu Cave, Chongqing, SW China, and used in laboratory and cave in situ models to investigate their influence on the precipitation amount, mineral components, crystal morphology and carbon and oxygen isotope fractionation of CaCO3. A CA activity gradient was applied in the laboratory model by considering various CA inhibitor acetazolamide (AZ) concentrations, which showed that the CA activity could substantially enhance precipitation, alter the mineral components and morphology, and reduce the δ13C and δ18O values of the CaCO3 formed. Most importantly, the laboratory and in situ model results revealed approximately -7‰ and -1.4‰ δ13C shifts, respectively, compared to the bacteria-free model results, which indicated that microbial-driven carbon isotopic fractionation can cause great uncertainties in paleoclimate and paleoenvironment reconstructions.