Effects of different nitrate and phosphate concentrations on the growth and toxin production of an Alexandrium tamarense strain collected from Drake Passage

Nitrate (N) and phosphate (P) are believed to be two of the most important nutrients for the growth and toxin production for Alexandrium species. The study of the growth and toxicity characteristics of the Alexandrium spp. under the change of N and P can help us to understand the dynamics of algal bloom and toxification events in natural environments. A strain of Alexandrium tamarense (designated as Kci) was successfully isolated from the Drake Passage in 2001 and the clonal culture has been kept in our laboratory (Ho et al., 2003, 2012). In order to extend our understanding on the growth physiology and toxicity of this A. tamarense strain, growth and cellular toxin content were examined in unialgal batch cultures under different concentrations of N and P. The effects of variable N, P concentrations on growth, cellular toxicity (fg STXeq. cell^-1), and toxin composition (% molar) were determined in both exponential and stationary growth phases. The toxin profile, determined by high-performance liquid chromatography with fluorescence detection (HPLC-FD), was found to be remained relatively stable and was consistently dominated by the N-sulfocarbamoyl C-toxins (>90%) under different conditions and growth phases. There were also trace amounts of other carbamate gonyautoxins consistently expressed. The cellular toxicity varied under different N and P concentrations, as well as different growth stages. A positive correlation was observed between cellular toxicity and N concentrations, but the toxicity was enhanced when P was depleted. Both cell densities and growth rate of the cells were severely suppressed under N- or P-depletion. However, the biovolume of the cells tended to be larger at N- or P-depleted cultures. Results from the present study provide valuable insight for the ecophysiology of Alexandrium species in the coastal ecosystem of Drake Passage.