Short-term effects of different nitrogen substrates on growth and toxin production of dinoflagellate Alexandrium catenella Balech (strain ACDH)

Abstract

Blooms of Alexandrium catenella were frequently found in sea areas adjacent to the estuary of the Changjiang River in spring of the last several years, often in association with large-scale blooms of Prorocentrum donghaiense. 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, especially the “excess” nitrogen (N) input. A nutrient-addition experiment was then designed to study the responses of a strain (ACDH) of A. catenella, isolated from the East China Sea, to the addition of different N substrates, including nitrate, ammonium and urea. It was found that N-starved cells of strain ACDH could use all the three N substrates as the sole N source to grow, with similar specific growth rate and maximum cell density. All the three N substrates could elevate cellular toxin content shortly after the addition of nutrients, but cellular toxin content of the urea group was much lower than the nitrate group and the ammonium group. In the following days, the cellular toxin content of the urea group was even lower than the N-limited control group, but cellular toxin contents of the nitrate group and ammonium group were still comparable to the control. It was presumed, based on these findings, that a relatively slower N uptake and assimilation rate occurred in the urea group, and urea–N adsorbed was preferentially utilized for growth rather than toxin synthesis. Therefore, urea in natural seawater could promote the formation of algal blooms, but decrease the toxicity, of strain ACDH.