Growth dynamics and toxicity of Alexandrium fundyense (Dinophyceae):: the effect of changing N:P supply ratios on internal toxin and nutrient levels

The effect of N- and P-limitation on the growth and paralytic shellfish toxin content of Alexandrium fundyense was studied in a series of batch cultures in which the N:P supply mass ratio was varied from less than I up to 160 (cf. Redfield = 7.2). N- and P-limitation of growth was observed at the lower and higher ratios respectively, with dual limitation between. Cellular parameters were similar during the exponential phase of growth, regardless of external N:P ratio. Upon nutrient exhaustion cell yields and internal pools were affected, although these also varied according to the N-source (nitrate vs. ammonium). Generally N-limitation alone led to a higher C:N ratio, lower C:P and N:P ratios and a decreased toxin content in stationary-phase cells. P-limitation led to increased cell size and cell quotas, lower C:N ratios, and higher C:P and N:P ratios (to maxima of 350 and 35 respectively). Complex internal P-fractionation schemes were found unnecessary to determine the P-status of cells and extraction using potassium persulphate, cold water and cold HCl alone were adequate. Severe P-limitation led to increased toxin content but only when N was also limiting, suggesting a synergistic effect of N and P availability on toxin synthesis and turnover. A positive relationship was found between toxin content and the intracellular concentration of arginine, but this varied with the nutrient status tin particular P-status) of the cells.