A Mechanistic Study of Carbonic Anhydrase-Enhanced Calcite Dissolution

Carbonic anhydrase (CA) has been shown to promote calcite dissolution (Liu, 2001, ; Subhas et al., 2017, ), and understanding the catalytic mechanism will facilitate our understanding of the oceanic alkalinity cycle. We use atomic force microscopy (AFM) to directly observe calcite dissolution in CA-bearing solution. CA is found to etch the calcite surface only when in extreme proximity (similar to 1 nm) to the mineral. Subsequently, the CA-induced etch pits create step edges that serve as active dissolution sites. 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 CO(2)hydration. This study shows that the accessibility of CA to particulate inorganic carbon (PIC) in the ocean is critical in properly estimating oceanic CaCO(3)and alkalinity cycles. Plain Language Summary Calcite dissolution in aqueous solution is one of the most biogeochemically important reaction and has a significant impact on atmospheric CO2 regulation and climate change. Carbonic anhydrase (CA) is an enzyme that regulates pH and CO2 balance inside or surrounding organisms in aqueous and terrestrial environments and is shown to promote calcite dissolution. In this study, we investigate why this ubiquitous enzyme enhances calcite dissolution rates in order to facilitate the proper estimate of dissolution fluxes in natural environments. When observed at atomic scale, we find that the enzyme etches the mineral only in extreme proximity. This observation provides important clue for the catalysis mechanism and reveals that the accessibility of CA to CaCO3 in the ocean is critical in properly estimating the natural fluxes.