Polyanion-mediated mineralization-mineralization in coccolithophore (Pleurochrysis carterae) variants which do not express PS2, the most abundant and acidic mineral-associated polyanion in wild-type cells

Highly acidic macromolecules are postulated intermediates in biomineralization because they sequester large numbers of calcium ions and occur in high concentrations at mineralizing foci in distantly related organisms. The mineral-associated polyanions in the coccolithophore Pleurochrysis carterae are acidic polysaccharides which surround calcite elements on mineralized scales called coccoliths. PS2 is the most abundant and acidic Pleurochrysis polyanion, containing four carboxyl groups per disaccharide repeat. This study examines the mineralizing function of PS2 by analysis of calcium carbonate deposition in PS2-deficient cells. Mineral-deficient Pleurochrysis variants were separated from wild-type cells by density gradient centrifugation and analyzed for PS2 expression. Three independent low-density variants (two spontaneous and one chemically-induced) not expressing PS2 (ps2(-)) were isolated. The mineral content of each ps2(-) variant was less than 5% of the wild-type level. Most ps2(-) coccoliths were unmineralized, but when crystals were present, they occurred on the coccolith rim as in wild-type cells. When ps2(-) rim elements were adequately large and numerous, they displayed the alternating A and B anvil shapes characteristic of wildtype rim elements. Although ps2(-) cells may have defects which affect the expression of cellular components in addition to PS2, correlation of PS2 nonexpression with mineral deficiency in three independent variants supports the hypothesis that PS2 is a functional intermediate in mineral deposition. PS2 may affect the level of Pleurochrysis mineralization by (a) controlling the amount of calcium ions available for mineralization, (b) accelerating the rate of calcite nucleation, or (c) both. However, PS2 does not influence crystal morphology, since the more mature crystals in ps2(-) cells exhibit the anvil-shaped morphology of wild-type crystals.