Removal of Prymnesium parvum (Haptophyceae) cells under different nutrient conditions by clay

In this study, we have shown that the ichthyotoxic Prymnesium parvum (haptophyte) can be successfully removed by spraying the surface with a phosphatic clay/polyaluminium chloride (PAC) mix (final concentration, 4 g l−1 and 5 ppm, respectively). The alga was grown in non-axenic batch cultures with nitrogen deficient, phosphorus deficient and nutrient sufficient media. Sub-samples of the nutrient sufficient culture were diluted to obtain cell abundances equal to those in the nutrient deficient cultures. Clay/PAC removed up to 100% of the cells in the low cell nutrient sufficient treatment after 72 h, but removal in the other treatments was lower (up to 84%). The nitrogen deficient cells were found to be the most toxic, measured as haemolytic activity of the cells (HE50), just prior to the start of the experiment. However, the toxicity of the cells in all treatments was found to fluctuate during the incubation time with a general increase in toxicity towards the end, suggesting that the cells became stressed during sedimentation and/or when trapped in the clay. But the amount of released toxins was always below the detection limit of the haemolytic assay, and the abundance of free-living bacteria, derived from flow cytometer counts, increased throughout the experiment. This suggests that released toxins were either trapped by the clay particles or effectively degraded by the bacterial community. The study shows that the clay method can be efficient in mitigation of P. parvum blooms, but further studies have to be conducted where optimum clay concentrations are determined to ensure that the efficiency is high against nutrient deficient cells and at high cell abundances, i.e. conditions found during blooms in eutrophic coastal waters, and to determine the fate of the cells and their toxins during and after sedimentation.