Tuning The Mg Content Of Mg Calcite By Shell Matrix Macromolecules Of Pinctada Fucata

Calcite is the most widespread and hitherto most extensively studied biomineral. Since seawater contains a considerable amount of Mg²⁺, calcites mineralized by halobios are all Mg calcites. Magnesium is the principle impurity of calcite. The stability of calcite may significantly be affected by incorporated Mg atoms in its lattice. Previous researches have demonstrated a stringent physiological control on biogenic calcite Mg content. The origin of such control was generally attributed to a direct physiological modification of the composition of the parent crystallizing solution. Based on this work, a novel possible mechanism was suggested that organisms may utilize solute biomacromolecules to control calcite Mg content.The prismatic layer of pearl oyster Pinctada fucata is composed of Mg calcite. The physiological control on shell mineralization basically originates from the biomacromolecules in the crystallizing solution, which later becomes shell matrix. Therefore, this work sought to verify if the shell EDTA-soluble matrix (ESM) of P.fucata may affect calcite Mg content in in vivo crystallizing systems. Energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) results showed that additive prismatic ESM significantly reduced bulk phase Mg content of the precipitated calcite, while nacreous ESM exhibited negligible effects. X-ray photoelectron spectroscopy (XPS) analysis showed that calcite surface Mg content was also significantly reduced by additive prismatic ESM, but significantly increased by nacreous ESM. In addition, the Mg reduction effect of prismatic ESM corresponds with the observed low Mg content characteristic of prismatic calcite. These results demonstrated that additive natural biomacromolecules are able to regulate crystal impurity content. Such mechanism may be utilized by organisms to ensure the accuracy of biomineralization control.Preliminary mechanism study showed that the crystallization inhibition of prismatic ESM and nacreous ESM were similar, notwithstanding their distinct effects on Mg content. In addition, ESM was demonstrated not to significantly alter the solution Ca/Mg ratio at the experiment concentration. Hence, the identified ESM effect was realized neither by altering crystal precipitation rate nor by regulating the solution relative ion activity.