Effects Of Nitrogen And Phosphorus On Growth And Toxin Production Of Alexandrium Catenella (ACDH)

Blooms of Alexandrium catenella were frequently observed in sea areas adjacent to the estuary of the Changjiang River in spring during the harmful algal bloom (HAB) studies in the last several years. It was considered that the increasing intensity of these dinoflagellate blooms was related to the incremental nutrient influx from the Changjiang River over the last 40 years. Based on the nutrient status in situ in sea areas adjacent to the estuary of the Changjiang River, the effects of major nutrients on growth and toxin production of Alexandrium catenella (strain ACDH) was studied to understand the relationship between eutrophication and the population dynamics, as well as potential impacts, of A. catenella blooms.The results showed that strain ACDH reached the maximum specific growth rate when nitrate concentration was above 28μM. Nitrate concentration in situ could support strain ACDH to grow at a rate close to the maximum specific growth rate. Strain ACDH reached the highest specific growth rate when phosphate concentration was above 2.5μM, suggesting that the phosphate concentration in situ would be a limiting factor for the growth of strain ACDH. The optimal N:P ratio for the growth of A. catenella is about 9-11, which is much lower than the ratio of nitrate to phosphate in sea areas adjacent to the Changjiang River estuary. The maximum uptake rate (Vm) and half saturation concentration (Ks) of strain ACDH for phosphate uptake are 0.22 pmol/cell.h and 0.75μM, respectively. ACDH is not competitive in utilization of low-concentration phosphate in seawater. ACDH could use nitrate, ammonium and urea as sole N source to grow, with similar specific growth rate and maximum cell density. The results, in association with the published references, indicate that the nutrient uptake and utilization characteristics, as well as the specific life history and behavior features of A. catenella, are the major reason accounting for the formation of A. catenella blooms. However, the high nutrient concentrations in seawater should be the major reason for the increasing intensity of A. catenella blooms.The addition of both nitrate and ammonium to N-starved cells of strain ACDH will lead to rapid and dramatic increases of cellular paralytic shellfish poisoning (PSP) toxin content, however, only a slight increase of cellular toxin content can be observed after the addition of urea. And the utilization of urea as a sole nitrogen source by strain ACDH will lead to decreased cellular toxin content, even lower than the cells cultured under nitrogen-limitation condition. The cellular toxin contents of A. catenella cells collected from the field are much higher than those growing on sufficient nutrients in the lab, but comparable to those cultured under phosphorus limitation condition. It is indicated that the high N:P ratio and phosphate limitation in the East China Seal might lead to high cellular toxin content.In summary, our results suggest that eutrophication characteristics in the Changjiang River estuary have direct effects on the bloom dynamics and potential impacts of A. catenella blooms. The adaptive strategies of A. catenella, such as the capability in utilizing organic phosphorus compounds, are critical for the bloom formation in this region. More importantly, the nutrient structure of high N:P ratio and phosphate limitation in this area might lead to elevated cellular toxin content, and potential impacts, of the A. catenella algal blooms.