INCOMPLETENESS OF THE COCCOSPHERE AS A POSSIBLE STIMULUS FOR COCCOLITH FORMATION IN PLEUROCHRYSIS CARTERAE(PRYMNESIOPHYCEAE)1

Pleurochrysis carterae is a marine biflagellate that produces calcified structures called coccoliths. The coccoliths are formed inside the cells and released from the latter after formation. The light dependence of calcium incorporation in this species was studied using⁴⁵Ca as a tracer. Cells exposed to a repeating cycle of 16 h of light and 8 h of darkness incorporated calcium in extracellular coccoliths at a more or less constant rate throughout a cycle. The cells divided during the dark periods with a concomitant decrease in size. Their size increased during the light periods Coccolith formation in cells incubated in continuous darkness was greatly reduced and finally ceased. These cells did not divide and did not increase in size. Removal of extracellular coccoliths prior to the calcium incorporation experiments stimulated coccolith formation both in dark-incubated cells and in cells exposed to a repeating light-dark cycle. Cells in the stationary phase of growth ceased producing coccoliths. Calcification could be induced in these cells by removal of the extracellular coccoliths. Based on these findings we suggest that cells of Pleurochrysis carterae tend to produce a complete cover of coccoliths and that the available cell surface is a factor controlling coccolith formation.