Carbonic anhydrase enhanced calcite dissolution: atomic scale mechanisms and measurements in the North Pacific

Near-equilibrium calcite dissolution in seawater buffers ocean acidification and contributes significantly to the regulation of atmospheric CO₂ on 1000-yr timescales. Previous work of our group has showed that the enzyme carbonic anhydrase (CA) increases calcite dissolution rates by an order of 2.5 near equilibrium, leaving the estimate of oceanic CaCO₃ dissolution fluxes highly uncertain considering the distribution of this ubiquitous enzyme. We present here a new atomic-scale mechanistic study of CA-promoted dissolution, and field measurement of CA concentration to help guide the estimates of oceanic carbonate cycling and alkalinity budgets. We used Atomic Force Microscope (AFM) to directly observe how CA catalyzed calcite dissolution in seawater. No significant difference in calcite step retreat velocity was observed with CA dissolved in seawater as compared to seawater of the same saturation state without CA. However, CA was found to enhance dissolution when flocculants came in contact with, or were very close (~nm) to, the calcite surface. The possible catalytic mechanism is through the adsorption of CA on the calcite surface, followed by proton transfer from the CA catalytic center to the calcite surface during CO₂ hydration. These results point out caution in estimating oceanic carbonate dissolution fluxes given measurements of bulk CA concentrations in natural environments, because the key to enhanced dissolution is the contact that occurs between CA and the mineral surface. We also present measurements of CA activity along a North Pacific transect, and discuss the catalysis of carbonate dissolution within marine snow particles and pteropod shells, providing observational evidence for CA-catalyzed, respiration-driven CaCO₃ dissolution in the shallow North Pacific.