Impact of Mg2+ and pH on Amorphous Calcium Carbonate Nanoparticle Formation: Implications for Biomineralization and Ocean Acidification

Calcium carbonate biomineralization, crucial for many marine organisms, often proceeds via amorphous calcium carbonate (ACC) as an intermediate. Using a novel in situ small-angle X-ray scattering (SAXS) setup with sub-20 ms resolution, we reveal how magnesium and pH work together to shift ACC nanoparticle formation to the spinodal line, resulting in an exceptionally narrow size distribution. This narrow distribution is essential for forming well-ordered crystals from ACC particles, critical in biomineralization. We also show that small pH changes, such as those from ocean acidification, dramatically impact particle size distribution. These findings highlight the roles of magnesium and pH in controlling ACC crystallization and underscore the vulnerability of marine calcifiers to environmental changes.