Differential toxin response of Pseudo-nitzschia multiseries as a function of nitrogen speciation in batch and continuous cultures, and during a natural assemblage experiment

Shipboard N enrichment experiments using natural phytoplankton assemblages were conducted off the west coast of Washington in an area characterized by elevated concentrations of macronutrients and iron. All N (NO3−, NH4+ and urea) treatments showed significant increases in biomass (as measured by total and size-fractionated chlorophyll a) and the abundance of Pseudo-nitzschia species over the 6-d experiment. As with the unialgal laboratory experiments, cellular toxicity varied as a function of the N source supporting growth, and the planktonic assemblages enriched with either NH4+ or urea demonstrated greater cellular toxicity than the assemblages supported solely by NO3−. These laboratory and field results demonstrate that N substrate can regulate the toxicity of Pseudo-nitzschia species, and that N source should be considered when evaluating the potential effects of cultural eutrophication on the growth of toxigenic diatoms.