Reduced coccolith calcite production under light-limited growth: a comparative study of three clones of Emiliania huxleyi (Prymnesiophyceae)

Three clonal isolates of Emiliania huxleyi (Lohmann) Hay et Mohler of diffe rent geographical origins were grown in semicontinuous culture under irradi ances ranging from 8 to 330 mu mol quanta m(-2) s(-1), resulting in growth rates between < 5% and 100% of the maximum rate. Coccolith calcium and cell carbon were determined by chemical analysis. Counts of cells and of attach ed plus detached coccoliths were obtained by microscopy. The molar ratio of calcium in coccoliths to organic carbon in cells (the Ca : org.C ratio) de clined by 30-40% when irradiance was reduced from 100 to 18 mu mol m(-2) s( -1). This decline was due to two separate effects: in two of the clones, fe wer coccoliths were produced by each cell; and in all three clones, less ca lcium carbonate was deposited in each coccolith. Electron microscopy showed that the latter effect was associated with a debilitation of the distal sh ield elements. No naked cells appeared in these cultures. Calcification in a coccolith-forming culture and in nature is probably regulated in such a w ay that a complete layer of 12-15 coccoliths on the cell surface is secured even under adverse light conditions. Significant interclonal differences a ppear to exist in the Ca:org.C ratio. possibly also in the mean calcium con tent of individual coccoliths.