Using the diatom Cylindrotheca closterium, several tests have been carried out, adding the Zeolite A and polycarboxylic acid (PCA) system, at various concentrations, fractions and ratios, to aged filtered seawater. Statistical analysis has revealed an homogenous response of this microalgal species. No evidence of a causal relationship between the presence of Zeolite A - PCA system and the formation of mucous aggregates was revealed.