• ammonium;
  • domoic acid;
  • kinetics;
  • nitrate;
  • nitrogen uptake;
  • Pseudo-nitzschia pseudodelicatissima complex;
  • Pseudo-nitzschia cuspidata ;
  • Pseudo-nitzschia fryxelliana ;
  • urea

The toxigenic diatom Pseudo-nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m−2 · s−1) and subsaturating (40 μmol photons · m−2 · s−1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO3), ammonium (NH4+), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO3-supported cells being significantly more toxic than both NH4+- and urea-supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO3 they exhibited maximum specific uptake rates (Vmax) of 22.7, 29.9, 8.98 × 10−3 · h−1, half-saturation constants (Ks) of 1.34, 2.14, 0.28 μg-at N · L−1, and affinity values (α) of 17.0, 14.7, 32.5 × 10−3 · h−1/(μg-at N · L−1) for NO3, NH4+ and urea, respectively. These labo-ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo-cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.