Effects of nitrogen (N) and phosphorus (P) limitation on calcification and coccolith production in Emiliania huxleyi were investigated in batch and chemostat cultures by means of chemical analyses of calcium (Ca) and organic carbon (C), counts of coccoliths and cells, and scanning electron microscopy. In a normally calcifying (C-cell) clone growing in the absence of nutrient limitation in batch or semi-continuous cultures, c. 36 attached plus detached coccoliths were present per cell. In batch cultures as well as in chemostats, the number rose to 70–120 when either N or P became strongly limiting. The molar Ca/C ratio, measured in chemostats, showed a corresponding increase from 1·07 to c. 1·38. The two types of limitation had opposing effects on the calcification of individual coccoliths: the mean Ca content of a coccolith decreased from a normal value of 0·60 pg to 0·46–0·49 pg under N limitation, and increased to 0·67–0·73 pg under P limitation. These effects were accompanied by corresponding modifications of coccolith morphology. In batch cultures of one naked (N-cell) clone grown to the stationary phase, P limitation but not N limitation induced the formation of c. 40 coccoliths per cell; these coccoliths were of aberrant shape. In another N-cell clone, P limitation had no such effect. The results suggest on one hand that coccolith formation is generally less dependent on N and P nutrients than is cell replication, and on the other hand that stages exist in the growth and calcification of a coccolith that are specifically influenced by either N- or P-containing cell constituents.

Single coccoliths of Emiliania huxleyi grown in mesocosm enclosures (60°16′N, 05°14′E, May–June 1991) under different N[ratio ]P regimes were analysed in a scanning electron microscope. The results indicate that only E. huxleyi with Type A coccoliths was present in the enclosures. Approximately 80–90% of the total coccolith assemblages had developed normally, whilst the remainder were malformed, incompletely grown or dissolved. Severely under-calcified specimens were rare and dissolution and breakage less than 5%. The coccoliths were of larger size than normal, as has been found previously in fjords of southwestern Norway, supporting the conclusion that a local population of E. huxleyi has developed, specific to these waters. Both phosphorus and nitrogen stress caused significant changes in coccolith size and evidence of malformation was clear, particularly in the low-phosphate enclosure. Although the observations presented here concern only Type A coccoliths and it is not known how nutrient stress may affect the coccoliths of the other types of E. huxleyi, they do serve to stress the fact that environmental conditions may possibly obscure genetically determined features. Following our observations on coccolith morphology in relation to nutrient status in enclosures, it will be of interest to test whether a similar correlation can be detected in the natural environment.