Positive effects of continuous low nitrate levels on growth and photosynthesis of Alexandrium tamarense (Gonyaulacales, Dinophyceae)

The growth and photosynthesis of Alexandrium tamarense (Lebour) Balech in different nutrient conditions were investigated. Low nitrate level (0.0882 mmol/L) resulted in the highest average growth rate from day 0 to day 10 (4.58 ¥ 10 cells mL d), but the lowest cell yield (5420 cells mL) in three nitrate level cultures. High nitrate-grown cells showed lower levels of chlorophyll a-specific and cell-specific light-saturated photosynthetic rate (Pm a and Pm), dark respiration rate (Rd a and Rd) and chlorophyll a-specific apparent photosynthetic efficiency (a ) than was seen for low nitrate-grown cells; whereas the cells became light saturated at higher irradiance at low nitrate condition. When cultures at low nitrate were supplemented with nitrate at 0.7938 mmol/L in late exponential growth phase, or with nitrate at 0.7938 mmol/L and phosphate at 0.072 mmol/L in stationary growth phase, the cell yield was drastically enhanced, a 7–9 times increase compared with non-supplemented control culture, achieving 43 540 cells mL and 52 300 cells mL, respectively; however, supplementation with nitrate in the stationary growth phase or with nitrate and phosphate in the late exponential growth phase increased the cell yield by no more than 2 times. The results suggested that continuous low level of nitrate with sufficient supply of phosphate may facilitate the growth of A. tamarense.