Dissolution of Phosphate and Precipitation of Carbonate in the Biomineralization of the Bivalve Shell Limnoperna fortunei

The mantle of bivalves plays a fundamental role in shell formation and maintenance through biomineralization. Experiments performed on mantle–shell preparations using different experimental techniques revealed the presence of phosphorus (P)-containing compounds in the shell as the first mineral phase formed at the growing edge of the periostracum in the freshwater bivalve Limnoperna fortunei. The low P concentration in the shells suggests that phosphate is restricted to the growth regions and occurs at concentrations too low to be detected by techniques such as X-ray diffraction (XRD). Nevertheless, a crystal morphology closely resembling that of hydroxyapatite (HAp) was identified at the shell growth front, and a Ca/P ratio of 1.67, consistent with HAp, was determined in two of these regions. Fourier transform infrared (FTIR) spectroscopy also revealed the principal and most intense absorption band of the phosphate group (PO43−) at 1024 cm−1. Enzymes such as carbonic anhydrase (CA), or proteins with equivalent functions, are likely involved in phosphate dissolution and the subsequent precipitation of calcium carbonate. In addition, the occurrence of calcium phosphate was confirmed in the shells of the marine bivalve Perna perna. Phosphate dissolution and carbonate precipitation during biomineralization suggest two important evolutionary advantages: first, the release and availability of phosphate, an essential nutrient for energy production and other metabolic functions; and second, the formation of a calcium carbonate shell, a structure indispensable for the protection, mechanical support, and survival of mollusks.