Fig. 6.
Model of coral skeleton formation combining nanoparticles and ion-by-ion growth. The values for calcium and carbonate concentrations ([Ca], [CO32-]), pH, and supersaturation with respect to aragonite (Ωarag) in calcifying fluid and seawater were measured by Sevilgen et al. (26). The CF is endocytosed into a calicoblastic cell from its apical side (bottom) by macropinocytosis (25), enriched in Ca, HCO3−, other ions, organics; protons are removed, so ACC-H2O nanoparticles can form (red). These are then exocytosed into the CF and attach to the growing skeleton. Gradually the solid aggregate of nanoparticles and ions dehydrates (green), crystallizes (blue), and ion attachment fills interstitial spaces. Crystalline fibers radiate from CoCs, and form spherulites (21), as each crystal fiber grows at the expense of the amorphous precursor phases. All three phases persist in CoCs.