Abstract

"Among the environmental factors that control the formation of toxic blooms in phytoplanktonic species, nutrients are considered fundamental. Alexandrium catenella strain AY2 was cultured during one month, in order to determine the specific growth rate, abundance and toxicity in different nutrients ratios. The three conditions studied were: nitrogen-saturated medium (N: P = 24:1), Nitrogen-balanced (N: P = 12:1) and Nitrogen limited medium (N: P = 3:1). Phosphorus was considered limiting in all three scenarios. Two parallel experiments were conducted, the frst one to assess cellular counts every three days and the other to determine the cell toxicity, which were measured every six days. Our results showed that the fastest specific growth rate was in the nitrogen-saturated medium with 0.49 μ, but the highest abundance (in the exponential phase) occurred in the in the nitrogen-balanced medium with a value of 5,266 cells ml-1 at day 21, while the lowest abundance was found in the nitrogen-limited medium, on day 12, with 4,072 cells ml-1. Regarding toxicity, the highest value was observed on day 12 with 24.56 fmol cell-1 in the nitrogen-limited treatment and the lowest showed values of 8.85 fmol cell-1 in the nitrogen-balanced medium. The GTX3 toxin was dominant, being found in the 3 treatments. The results in this study suggest that the nitrogen/phosphorus ratios infuenced in a non-synergistic way in the growth and toxin production of A. catenella, resulting into a stimulation of growth by nitrogen (ratios 24:1 and 3:1) and toxins increased by limitation of this nutrient."