Dissolution of Phosphate and Precipitation of Carbonate in the Bio-mineralization of the Bivalve Shell Limnoperna fortunei

The mantle of bivalves plays a crucial role in the formation and maintenance of their shells through biomineralization. Detailed studies using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis have revealed the presence of phosphorus (P) compounds as the primary phase during biomineralization at the growing edge of the periostracum of the bivalve shell Limnoperna fortunei (Dunker, 1857). The presence of a crystal morphology like hydroxyapatite (HAp) at the growing edge of the shell has also been identified, and the Ca/P ratio compatible with HAp. Carbonic anhydrase (CA), whose presence in the shell was investigated in this work, and/or bivalve proteins with identical capability are likely responsible for the dissolution phosphate and calcium carbonate precipitation. Other experimental techniques (ICP-OES, WDXRF) were used to quantify the main chemical elements in the shell of L. fortunei and the marine bivalve P. perna. The concentration of P in the shells suggests that phosphate is confined to the growing regions. FTIR and FTIR-ATR spectroscopies indicate aragonite as the main phase at the shell edges but also show the presence of phosphate absorption bands. X-ray diffraction (XRD) analyses revealed aragonite and calcite phases at the shell edges, with the presence of one of the main peaks of crystalline calcium phosphate both in L. fortunei and P. perna. The presence of phosphate as the primary phase in the biomineralization process of L. fortunei rekindles the discussion about the importance of the co-occurrence of phosphate and carbonate in the bivalve biomineralization dynamics and suggests an important evolutionary advantage in acquiring phosphate compounds essential for energy production and organism function.