Calcium carbonate deposition in the motile (Crystallolithus) phase ofCoccolithus pelagicus(Prymnesiophyceae)

Abstract

Non-motile heterococcolith-bearing cells of Coccolithus pelagicus grown in clonal culture eventually give rise to motile cells of the “Crystallolithus” phase. Such motile cells have a complex periplast comprising layers of organic scales, holococcoliths of the crystallolith type and an outer investment referred to here as the envelope. Scales, crystallolith baseplates and discrete units of envelope are formed in Golgi-associated cisternae. The deposition of calcium carbonate does not appear to take place intracellularly as it does in the non-motile heterococcolithophorid phase. In order to locate the site of crystallolith synthesis, cells were treated with CO₂ to remove the existing CaCO₃. After resuspending decalcified cells in fresh medium, CaCO₃ first appeared on crystallolith baseplates outside the plasmalemma, external to the mature cisternae of the Golgi apparatus and near to the flagellar bases. Crystalloliths apparently undergoing calcification are always closely associated with the envelope. The calcite crystals of crystalloliths appear to be covered by a thin layer of organic material which may act as a matrix during calcium carbonate deposition.