Prorocentrum lima is known to having toxins such as okadaic acid, DTX-1 and DTX-2, causing diarrheal shellfish poisoning (DSP). Toxins within these algae can affect organisms at other trophic levels such as shellfish and humans through the food chain. This species has a high growth rate (up to 0.5 d-1) and requires large amounts of nutrients. The expression level of the toxin increases in nutrient deficient environment. T his study is to observe changes in growth, photosynthetic efficiency, pigment (Chl a, b and carotenoids), toxin (OA and DTX), and gene expression under nutrient depletion. The P. lima cells were cultivated at 20 °C and 100 μmol m-2 s-1 in f/2 medium. The first sampling of cell and seawater was conducted approximately 3 hours after replacing the medium. The f/2 addition was stopped when t he cell concentration reached about 200,000 cells/ml . After the nutrient addition was stopped, the sampling was conducted at a 10-day interval for 30 days. During the exponential growth period, P. lima took up more than 90% of the nitrate and nitrite in the medium within 3 hours. O n day 10, nitrate and nitrite concentration was depleted , and phosphate also became limited, < 0.5 μmol/l (Fig. 1). Interestingly, even after the supply of nutrients was stopped , the cell density was continu ously increased for how many days, and the cell concentration reached about 350,000 cells/ml (Fig. 1). During the experiment, P. lima showed the specific growth rate (SGR) of 0.06 d-1 , with the highest value was 0.12 d-1 during the exponential growth phase. Chl a and carotenoids (pg cell-1 ) showed no significant effect of nutrient deficiency. However, C hl b was higher on the 10th day compared to other days. The results of photosynthetic efficiency, toxins and gene expression will also be presented.
This research was supported by a grant (20163MFDS641) from Ministry of Food and Drug Safety in 2023.