Environmental and nutritional factors which regulate population dynamics and toxin production in the dinoflagellate Alexandrium catenella

Abstract

The effects of environmental and nutritional factors on population dynamics and toxin production were examined in Alexandrium catenella, maintained in enriched K media in laboratory cultures. Starting with a density of 50 cell ml⁻¹, the dinoflagellate population typically showed a lag phase and an exponential growth phase which lasted 14 days each, and then entered the stationary phase, with a maximal capacity of 12–18,000 cell ml⁻¹-. Population densities showed distinct diurnal patterns, with population growth beginning 2–4 hours in darkness. The optimal physical conditions for growth were pH 8.5,salinity of 30–35‰, temperature of 20–25°C, and photoperiod of 14//10D to 16L/8D. The cell cycle was determined by flow cytometry on synchronized batch cultures maintained at optimal pH, salinity, temperature and under 5 different photoperiod regimes. It was found that the G₁ phase was timed to end at approximately 3 h after onset of darkness, and the G₂/M phase had begun at 4 hours. Nutrient supply markedly affected population growth. Under optimal physical conditions, the optimal concentrations for macronutrients and micronutrients were: NH⁺⁻⁴- 0.025–0.2 mM,NO⁻³ 0.22–8.83 mM, glycerophosphate0.04–0.06 mM, silicate 0.1–0.54 mM; FeEDTA 0.07–0.11 mM;Co 0.1 µM, Cu 0.005–0.04 µM; Mn 0.22–7.2 µM;Mo 0.03–0.6 µM; Se 0.02–0.1 µM; Zn 0.04–1.6µM; thiamin 0.075–6 µM; vitamin B₁₂0.0004–0.004 µM; biotin 0.007–0.015 µM; EDTA5–40 µM.

The toxin profile of A. catenella was determined by HPLC and found to include in descending order: GTX-4, GTX-3, GTX-1, B2, neosaxitoxin, saxitoxin. Toxin content per cell was highest in cell populations in the early exponential phase. The highest toxin per litre medium was recorded at 20°C at the beginning of the stationary phase,when cell density was highest and toxin/cell was still relatively high. At10°C, the cell density was low while the amount of toxin/cell was high;while at 30°C, the population at full capacity was low and the toxin/cell was also low. The population and toxin data thus provided an explanation for the peak level of PSP contamination in shellfish during the months of March–April around the eastern and southern side of Hong Kong and a minor peak extending to the western side in September–October, when the physical conditions of the seawater provided the right environment for toxin accumulation.

Toxin content in the dinoflagellate reached its maximum during the S-phase of the cell cycle. Nitrogen restriction in the medium reduced population growth and toxin production, while phosphorus restriction reduced only population growth but enhanced toxin accumulation in the cells.