Domoic Acid Production is not linked to silicate limitation in natural populations of Pseudo-nitzschia

W.P. Cochlan¹, M.L. Wells², V.L. Trainer³, C.G. Trick⁴, E.J. Lessard⁵, and B.M. Hickey<sup>5

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
³ NOAA Fisheries, Northwest Fisheries Science Center, Seattle, WA 98112, USA
⁴ Schulich School of Medicine, University of Western Ontario, London, Ontario N6A 5B7, Canada
⁵ School of Oceanography, University of Washington, Seattle, WA 98195, USA

A central paradigm in the study of toxigenic diatoms is that silicate limitation induces intense cellular production of domoic acid. We tested this hypothesis during a 2004 ECOHAB-PNW cruise in the coastal waters off Washington State and British Columbia where toxigenic Pseudo-nitzschia typically occur. Our results show that the highest levels of cellular toxin (5-64 pg/cell) correlate poorly with ambient silicate concentrations and occur where dissolved silicate concentrations are 5-50 µM. None of the ~400 particulate analyses showed elevated cellular toxin concentrations at < 4 µM Si. These concentrations are well above that considered limiting for Si uptake and growth of most neritic diatoms. Cellular toxin levels also did not correlate with N or P concentrations, indicating that toxin production in Pseudo-nitzschia spp. is not governed by macronutrient availability. A similar result was found during the intense toxic Pseudo-nitzschia bloom in Monterey Bay, 1998. The most established correlate for elevated domoic acid levels in 2004 was the distribution of low dissolved iron concentrations; a finding consistent with laboratory culture experiments and our field incubation studies in this region. These findings provide perhaps the most detailed insight to date into the triggers for toxin production in natural populations of Pseudo-nitzschia.