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

Abstract

Mexandrium minutum (strain AM89BM) was grown in semi-continuous culture (0.2 volume d-l) in N-limiting (N03:P0, = 1.6 and 3.16), in P-Luniting (N P = 160 and 80), and in N-and P-balanced (N:P = 16) me&a. 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-' in the N:P balanced medium, while the growth rate was decreased in all N:P < 16 or > l 6 media during the first period, and only in the N:P = 1.6 medium during the second period. Cells grown in Nlimiting 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, P0N:POP ratios in cells grown in P-Limiting conditions were over 50. Thus, cells grown in the 2 N:P < l 6 conditions were nitrogen-deficient, whlle 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 ceY-' In N-limiting conations, cells contained ca 3 times less toxin, with mean values of 0.41 to 0.45 fmol cell-'. 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-' in the N:P = 160 medium and 8.01 fmol cell-' in the N:P = 80 medium.