Carbonate chemistry and calcification data of the substrate-inhibitor interaction experiment with E. huxleyi. The percentage of 200 cells possessing/producing coccoliths was investigated by means of cross-polarized light microscopy which allows detection of internal coccoliths. These birefringence measurements were backed up with scanning electron microscopy (SEM) investigations when the presence of coccoliths could not be unequivocally determined. In case of discrepancies among the two methods, we relied on SEM investigations since this is the most reliable approach.