LAUSR

The toxigenic diatom Pseudo-nitzschia multiseries Hasle, isolated from the U.S. Pacific Northwest, was examined in unialgal laboratory cultures and in natural assemblages during shipboard experiments, to examine cellular growth and domoic acid (DA) production as a function of nitrogen (N) substrate and availability expected during bloom development and decline. Laboratory experiments utilizing batch cultures conducted at saturating (120 μmol photons m-2 s-1) photosynthetic photon flux density (PPFD), demonstrated that P. multiseries (strain NWFSC-245) grows equally well on the three N substrates tested (nitrate [NO3-], ammonium [NH4+] and urea), and achieved an average specific growth rate of 0.83 d-1. Despite equivalent growth rates, cellular toxicity (particulate DA concentration normalized to cell abundance) varied as a function of N substrate, with urea-grown cells demonstrating 1.3- and 3.4-fold more toxicity than both NH4+- and NO3--grown cells. Cellular toxicity of the N-limited chemostat cultures, grown at a dilution rate of 0.48 d-1, were less than the cellular toxicity measured for the N-replete batch cultures for all three N substrates, but again cellular toxicity varied as a function of N substrate and the urea-supported cells were 3.5- and 4.3-fold more toxic than the respective NH4+- and NO3--supported cells. Starved cultures of P. multiseries showed no decline in cellular toxicity or change in the order of toxicity as a function of N substrate, and cells previously supported by urea were 13- and 5-fold more toxic than NH4+- and NO3--supported cells. At all three levels of N-sufficiency, the urea-grown cells consistently produced the highest concentration of particulate DA per cell compared to cells grown on either NO3- or NH4+. 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.