Effects of Ca and Mg on growth and calcification of the coccolithophorid Pleurochrysis haptonemofera: Ca requirement for cell division in coccolith-bearing cells and for normal coccolith formation with acidic polysaccharides.

The effects of Ca2+ and Mg2+ on cellular growth and calcification in Pleurochrysis haptonemofera were investigated. In the presence of a normal concentration of Mg2+, coccolith-bearing cells (C-cells) required more than 0.5 mM Ca2+ for growth, while naked cells could grow even with 0.5 mM Ca2+. The calcification rate of C-cells, which was determined using decalcified cells, was significantly repressed with less than or equal to 0.5 mM Ca2+. Although the calcification rate did not change so much with 5-30 mM Ca2+, it decreased with higher concentrations of Ca2+, as well as C-cell-specific growth repression. Under these conditions, Ca2+ affected the rate of coccolith formation, but neither the coccolith morphology nor total amounts and ratios of divalent cations and acidic polysaccharides (Ph-PS-1, -2, and -3) were included in coccoliths. These findings suggest that sufficient calcification is required for the division of C-cells. Under low Ca2+ and high Mg2+ conditions, coccoliths with an abnormal morphology, having immature shield elements, were synthesized. Composition analysis of the coccoliths revealed high Mg/Ca and low Ph-PS-2/(Ph-PS-1 and -3) ratios, as compared with those under low Ca2+ and normal Mg2+ conditions, suggesting that the abnormal morphology is due to a change in the crystal type and/or acidic polysaccharide composition.