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

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.