N:P ratios controlling the growth of the marine dinoflagellate Alexandrium tamarense: Content and composition of paralytic shellfish poison

摘要

The responses of cell growth and paralytic shellfish poison (PSP) content and composition of a strain of the marine dinoflagellate Alexandrium tamarense to the availability of nitrate, ammonium, and phosphate were tested with a gradient of N:P ratios in a semi-continuous batch culture. The N:P ratios ranged from 4 to 64 and, were prepared with either a fixed phosphorus concentration at 1.56 μM-P or a fixed nitrate or ammonium concentrations at 25 μM-N. These ratios were applied to study the responses of cell growth and PSP. Cell growth was correlated with the concentrations of nitrogen and phosphorous but was correlated with not high concentrations of ammonium, which depressed the growth due to ammonium toxicity. When the phosphate concentration was held at 1.56 μM, the growth rate increased hyperbolically as the N: P ratio increased but decreased logarithmically at ammonium concentrations higher than 25 μM. When nitrogen was held at 25 μM, the growth rate decreased hyperbolically with increasing N :P ratio. The PSP consisted of C2 toxin and gonyautoxins, including GTX4, GTX3, and GTX1, in order of decreasing magnitude. The total cellular toxicities of PSP were estimated from the measurements of cellular content and the composition of C2 toxin and gonyautoxins. The cellular contents of PSP increased hyperbolically with the nitrate concentration as a result of saturated growth, and logarithmically with the ammonium concentration as a result of depressed growth (p < 0.001). PSP contents decreased logarithmically with increasing phosphorus concentration (p < 0.001). The cellular contents of PSP increased significantly with increasing N:P ratios regardless of whether nitrate or ammonium was used (p < 0.05). The most dominant toxin was C2 (about 80%), whereas the least dominant was GTX1 (less than 8%). The highest abundance of GTX was observed at an N:P ratio of 16, which usually corresponded to the transition between the initial increase and saturated phases of growth for the nitrogen variable experiment and vice versa for the phosphate variable experiment, or corresponded to the beginning of a decline in growth due to ammonium toxicity. The present study suggests that the cellular toxicities of PSP in the present strain of the dinoflagellate A. tamarense can be estimated from the N:P ratios, ranging from 4 to 64. The high total cellular toxicities of PSP observed at high N:P ratios should be considered in addition to the conventional consideration of P-limitation for further regulation of nutrients by provincial governments to reduce the aquatic damage caused by the toxic dinoflagellates in coastal waters.