Effects of nutrient depletion duration on growth, photosynthesis and toxins (OA and DTX) in the dinoflagellate Prorocentrum lima

Prorocentrum lima is a marine benthic dinoflagellate known for producing toxins such as okadaic acid (OA) and dinophysistoxin (DTX), which cause diarrheal shellfish poisoning (DSP). P. lima is known to increase toxin production under low nutrient concentrations, but there have been few studies examining the effect of prolonged nutrient depletion. This study investigates changes in growth, photosynthetic efficiency, pigments (Chl a and carotenoids) and toxin levels (OA and DTX) during the period of nutrient depletion. Nutrient addition was stopped when the cell concentration reached the stationary phase at approximately 200,000 cells ml⁻¹. After stopping nutrient addition, sampling was conducted at 10-day intervals for 30 days. During the exponential growth phase, P. lima took up more than 90 % of nitrate and nitrite from the medium within 3 hours. Even after the nutrient supply was stopped, cell density continued to increase, reaching about 340,000 cells ml⁻¹. Chl a and carotenoids did not show significant differences, but photosynthetic parameters, such as relative electron transport rate (rETR), ETRmax and Ik decreased. The levels of OA and DTX-1 were also significantly higher on day 30 compared to day 0. These results suggest that, while the cell density can be maintained during nutrient depletion, the toxin content per cell increases significantly, and photosynthetic efficiency decreases.