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Keywords:

  • Ca2+-stimulated ATPase;
  • calcification;
  • coccolithogenesis;
  • coccolithophorids;
  • Coccolithus pelagicus;
  • Golgi apparatus;
  • organelle separation;
  • Prymnesiophyceae

ABSTRACT

Cells of the marine alga Coccolithus pelagicus (Wal-lich)J. Schiller grown in axenic cultures were homogenized and fractionated. The distribution of organelle markers was assessed enzymatically after centrifugation through zonal, density, and flotation gradients made with sucrose, sorbitol, or Percoll. Mitochondria (1.19 g·cm-3) and chloroplasts (1.15 g·cm-3) were recovered in sucrose gradients at densities similar to those observed for higher plants and most algae. The position of endoplasmic reticulum and plasma membrane in the gradients was monitored by NADPH cytochrome c reductase and vanadate-sensitive Mg2+-ATPase, respectively. Higher plant Golgi markers, latent undine diphosphatase (UDPase) and glucan synthase I, were colocalized at a density range including two peaks of activity at 1.13–1.15 g·cm-3.

Bound calcium was associated with high density (1.15 g·cm-3) membranes. Ca2+-stimulated ATPase was found at high levels on membranes that did not coisolate with the latent UDPase-containing membranes. The Ca2+-stimulated ATPase, a possible participant during calcification, was associated with a chloroplast-enriched fraction in all the organelle separation systems. However, about 30% of the total activity was separated from both the chloroplasts and Golgi on 0–70% Percoll gradients containing 0.4 M sucrose. The possible relationship of the Golgi and the high-density organelle exhibiting Ca2+-stimulated ATPase to coccolithogenesis and the process of calcification and crystal formation is discussed.