Nitrogen dynamics of Pseudo-nitzschia cuspidata from the U.S. Pacific Northwest

M.E. Auro¹, W.P. Cochlan¹, and V.L. Trainer<sup>2

1</sup> Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920-1205, USA
² School of Marine Sciences, University of Maine, Orono, ME 04469, USA

The growth capabilities of the toxigenic diatom species Pseudo-nitzschia cuspidata, isolated from the Juan de Fuca Eddy region during Fall surveys of the ECOHAB-PNW project in 2004 and 2005, were examined in non-axenic, semi-continuous, batch cultures enriched with 40 µM nitrate, 40 µM ammonium or 20 µM of urea as the sole nitrogen source. Experiments, conducted at both high (120 µEm-2s-1) and low (40 µEm-2s-1) photosynthetic photon flux densities (PPFDs), demonstrate that P. cuspidata grew significantly faster at the higher PPFD, but shows no preference for one nitrogen source over the other at either PPFDs. Exponential growth rates (determined using cell abundance over time) and particulate domoic acid content (using ELISA) did not significantly differ as a function of the nitrogen growth substrate. In contrast to most previous studies using other cultured Pseudo-nitzschia species, the particulate DA per cell averaged 60% greater during exponential growth compared to stationary growth of P. cuspidata, regardless of nitrogen source or PPFD. These results demonstrate the capability of this diatom to grow equally well on both oxidized and reduced forms of nitrogen, although our field observations have generally found greatest cell abundances in the nitrate-rich upwelled waters of the Juan de Fuca Eddy.