Nitrogen Utilization and Toxin Production by Two Diatoms of the Pseudo‐nitzschia pseudodelicatissima Complex: P. cuspidata and P. fryxelliana

The toxigenic diatom P seudo‐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 ( NO 3 − ), ammonium ( NH 4 + ), 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 NO 3 − ‐supported cells being significantly more toxic than both NH 4 + ‐ 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 NO 3 − they exhibited maximum specific uptake rates ( V max ) of 22.7, 29.9, 8.98 × 10 −3  · h −1 , half‐saturation constants ( K s ) 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 NO 3 − , NH 4 + 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.