Oxygen isotopes of calcite precipitated at high ionic strength: CaCO3-DIC fractionation and carbonic anhydrase inhibition

Background electrolytes affect calcite precipitation and dissolution rates and hence have the potential to impact kinetic isotope fractionation between calcite and the dissolved inorganic carbon (DIC) species. We grew inorganic calcite from NaCl-CaCl2 solutions (T = 25 degrees C, pH = 8.3, and crystal growth rate = 10(-6.3 +/- 0.3) mol m(-2) s(-1)) to test for potential ionic strength effects on the oxygen isotope fractionation between calcite and an isotopically equilibrated inorganic carbon pool (alpha(c/EIC)). Calcite was grown in the presence of the enzyme carbonic anhydrase from bovine erythrocytes (bCA) to promote isotopic equilibration of the DIC pool. No evidence of an ionic strength effect on alpha(c/EIC) was found for NaCl concentrations up to 0.35M (1000ln alpha(c/w) = 28.0 +/- 0.1; n = 7). For experiments conducted with [NaCl] > 0.35 M, the DIC pool could not be maintained in isotopic equilibrium with water due to the inhibitory effect of NaCl on bCA. The use of other types of CA may be required to maintain isotopic equilibration of DIC in solution with ionic strength close to or above that of seawater. The oxygen isotope results were modeled successfully with an isotopic box model for a CO2-fed solution that tracks the isotopic composition of each DIC species and CaCO3. The experimental and modeling results suggest that the efficacy of bCA decreases exponentially with increasing [NaCl]. We quantify the salt effect on the quantity k(cat)/K-M, which relates the catalyzed rate constant for CO2 hydration to the concentration of bCA. The salt effect can be implemented in models of biomineralization with potential to extend our knowledge of oxygen isotope vital effects in marine organisms. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Background electrolytes can affect the precipitation and dissolution rates of calcite, potentially impacting the kinetic isotope fractionation between calcite and dissolved inorganic carbon (DIC). In this study, inorganic calcite was grown from NaCl-CaCl2 solutions to examine the effects of ionic strength on the oxygen isotope fractionation. The results showed no significant ionic strength effect on the fractionation. However, at NaCl concentrations above 0.35M, the isotopic equilibrium between DIC and water could not be maintained due to the inhibitory effect of NaCl on the enzyme carbonic anhydrase. The efficacy of the enzyme decreased exponentially with increasing NaCl concentration. The findings provide insights into oxygen isotope vital effects in marine organisms.