Involvement of Acidic Polysaccharide Ph-PS-2 and Protein in Initiation of Coccolith Mineralization, as Demonstrated by In Vitro Calcification on the Base Plate

Abstract

Coccolithophorids, unicellular marine microalgae, have calcified scales with elaborate structures, called coccoliths, on the cell surface. Coccoliths generally comprise a base plate, CaCO₃, and a crystal coat consisting of acidic polysaccharides. In this study, the in vitro calcification conditions on the base plate of Pleurochrysis haptonemofera were examined to determine the functions of the base plate and acidic polysaccharides (Ph-PS-1, -2, and -3). When EDTA-treated coccoliths (acidic polysaccharide-free base plates) or low pH-treated coccoliths (whole acidic polysaccharide-containing base plates) were used, mineralization was not detected on the base plate. In contrast, in the case of coccoliths which were decalcified by lowering of the pH and then treated with urea (Ph-PS-2-containing base plates), distinct aggregates, probably containing CaCO₃, were observed only on the rim of the base plates. Energy dispersive X-ray spectroscopy (EDS) confirmed that the aggregates contained Ca and O, although X-ray diffraction analysis did not reveal any evidence of crystalline materials. Also, in vitro mineralization experiments performed on EDTA-treated coccoliths using isolated acidic polysaccharides demonstrated that the Ca-containing aggregates were markedly formed only in the presence of Ph-PS-2. Furthermore, in vitro mineralization experiments conducted on protein-extracted base plates suggested that the coccolith-associated protein(s) are involved in the Ca deposition. These findings suggest that Ph-PS-2 associated with the protein(s) on the base plate rim initiates Ca²⁺ binding at the beginning of coccolith formation, and some other factors are required for subsequent calcite formation.