GROWTH, TOXICITY AND NITROGEN UPTAKE CAPABILITIES OF THE TOXIGENIC DIATOM PSEUDO-NITZSCHIA CUSPIDATA FROM THE PACIFIC NORTHWEST

Maureen E. Auro¹, William P. Cochlan¹, and Vera L. Trainer²

¹1Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA, 94920-1205
²NOAA Northwest Fisheries Science Center, Seattle, WA, 98112

The toxigenic diatom, Pseudo-nitzschia cuspidata (Hasle) has recently been found in bloom concentrations in the Juan de Fuca Eddy region off the coasts of Washington State, U.S.A. and British Columbia, Canada. Isolates collected during the September surveys of the ECOHAB-PNW project during 2004 and 2005 were examined for their growth, toxicity and nitrogen uptake response to inorganic (nitrate, ammonium) and organic (urea) nitrogen. The kinetics of N uptake as a function of substrate concentration were estimated from short (20-min) incubations using the ¹⁵N-tracer technique, and are compared to the long-term exponential growth rates of P. cuspidata determined in semi-continuous, batch cultures grown on the various nitrogen substrates. Based on the estimated maximum specific uptakes rates (V_max), nitrogen preference follows the order: ammonium > nitrate > urea, whereas the nutrient affinity indices - half saturation constants (K_s) and alpha parameter (alpha = V_max/K_s), indicate that N affinity follows the order: urea > nitrate > ammonium. Long-term growth experiments conducted at saturating (120 µE•m^-2•s^-1) and sub-saturating (40 µE•m^-2•s^-1) photosynthetic photon flux densities (PPFDs), demonstrate that P. cuspidata grows significantly faster at the higher PPFD for all N substrates, but there are substantial differences in the growth rates achieved on the various N substrates. The exponential growth rate (determined using raw fluorescence units) at high PPFD is slower for cells grown on urea (0.84 ± 0.03 d^-1) compared to the cells maintained on nitrate (0.88 ± 0.01 d^-1) or ammonium (0.91 ± 0.02 d^-1), whereas at low PPFD, urea supported faster growth (0.65 ± 0.003 d^-1) than either nitrate (0.55 ± 0.01 d^-1) or ammonium (0.51 ± 0.02 d^-1).

Recently, Armstrong-Howard et al. (2007) showed that both field assemblages and laboratory cultures of P. australis produce more domoic acid (both cellular and dissolved DA) when grown on urea versus nitrate or ammonium. In our study, the cellular DA content (determined using cELISA) for P. cuspidata did not significantly differ as a function of the nitrogen substrate used for growth at saturating PPFD, but at sub-saturating PPFD, nitrate-grown cells produced 74% and 78% more pDA per cell than ammonium- and urea-grown cells. In contrast to other Pseudo-nitzschia species where cellular domoic acid is generally enhanced during stationary phase, the cellular DA concentrations for P. cuspidata were always greater during exponential growth compared to stationary growth, regardless of the N substrate or PPFD used for growth. These laboratory results demonstrate the capability of this small, toxigenic diatom to grow and produce DA on both oxidized and reduced N substrates supporting field observations that Pseudo-nitzschia species bloom during both upwelling and non-upwelling conditions off the west coast of North America where substantial differences in the nitrogenous nutrition of P. cuspidata can be expected.

ARMSTRONG-HOWARD, M. D, COCHLAN, W. P., LADIZINSKY, N. L. AND KUDELA, R. M. 2007. Nitrogenous preference of toxigenic Pseudo-nitzschia australis (Bacillariophyceae) from field and laboratory experiments. Harmful Algae 6: 206-217.