Cell density, chemical composition and toxicity of Chrysochromulina polylepis (haptophyta) in relation to different N:P supply ratios

The influence of different N:P supply ratios on cell accumulation, chemical composition and toxicity of the marine haptophyte Chrysochromulina polylepis was examined in semi-continuous cultures. A non-axenic strain of C. polylepis was exposed to five different N:P supply ratios (N:P = 1:1, 4.1, 16.1, 80:1 and 160.1, by atoms), in order to create a range of N- and P-limited conditions. The toxicity per cell in C. polylepis was determined on four occasions at steady state cell density using the haemolytic activity of the cells expressed as saponin nanoequivalents. Haemolytic activity was demonstrated in all treatments, and increased in the algae when cell growth was nutrient limited (N:P = 1:1, 4.1, 80:1 and 160.1), compared to cells grown under nonlimiting conditions (N:P = 16.1). This occurred regardless of the growth-limiting nutrient (N or P) and became more pronounced as nutrient limitation increased. In P-limited cultures the haemolytic activity per cell increased linearly with the cellular N:P ratio, whereas the N-limited cultures showed an opposite trend. The haemolytic activity per cell showed an inverse relationship with both cellular N and cellular P content. Cells limited by P showed a higher haemolytic activity than cells limited by N. The results suggest that toxicity in C. polylepis is strongly influenced by the physiological state of the algae. This may partially explain the large variability previously observed in the toxicity of C. polylepis blooms. The potential ecological significance of our findings is also discussed.