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 polylep is was examined in semi-continuous cultures. A non-axenic strain of C. poly lepis 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 o ccasions at steady state cell density using the haemolytic activity of the cells expressed as saponin nanoequivalents. Haemolytic activity was demonst rated in all treatments, and increased in the algae when cell growth was nu trient limited (N:P = 1:1, 4.1, 80:1 and 160.1), compared to cells grown un der nonlimiting conditions (N:P = 16.1). This occurred regardless of the gr owth-limiting nutrient (N or P) and became more pronounced as nutrient limi tation increased. In P-limited cultures the haemolytic activity per cell in creased linearly with the cellular N:P ratio, whereas the N-limited culture s showed an opposite trend. The haemolytic activity per cell showed an inve rse relationship with both cellular N and cellular P content. Cells limited by P showed a higher haemolytic activity than cells limited by N. The resu lts suggest that toxicity in C. polylepis is strongly influenced by the phy siological state of the algae. This may partially explain the large variabi lity previously observed in the toxicity of C. polylepis blooms. The potent ial ecological significance of our findings is also discussed.