Effect of different nitrogen/phosphorus nutrient ratios on the toxin content in Alexandrium minutum

Alexandrium minutum (strain AM89BM) was grown in semi-continuous culture (0.2 volume d(-1)) in N-limiting (NO3:PO4 = 1.6 and 3.16), in P-limiting (N:P = 160 and 80), and in N- and P-balanced (N:P = 16) media. Daily supplies of limiting nutrients were taken up until exhaustion; most residual concentrations before renewal were near the detection limits. Samples were taken during 2 separate periods: Days 5 to 12 and 26 to 36. During both periods, cells grew at the real rates of 0.23 division d(-1) in the N:P balanced medium, while the growth rate was decreased in all N:P <16 or >16 media during the first period, and only in the N:P = 1.6 medium during the second period. Cells grown in N-limiting media contained 1/3 less PON and 1/2 less chlorophyll a than cells grown in the N:P balanced condition. The POP content per cell did not vary significantly. In contrast, PON:POP ratios in cells grown in P-Limiting conditions were over 50. Thus, cells grown in the 2 N:P <16 conditions were nitrogen-deficient, while cells grown in N:P = 80 and 160 conditions were nitrogen-surfeit. The toxin content in cells greatly changed according to the N:P regime. Cells grown at N:P balanced conditions showed an average total paralytic shellfish poisoning (PSP) content of 1.24 fmol cell(-1). In N-limiting conditions, cells contained ca 3 times less toxin, with mean values of 0.41 to 0.45 fmol cell(-1). In contrast, cells grown in P-limiting conditions contained on average 3.5 and 7 times more toxins than under balanced N:P conditions; 4.31 fmol cell(-1) in the N:P = 160 medium and 8.01 fmol cell(-1) in the N:P = 80 medium.