Several studies have shown that domoic acid production by Pseudo-nitzschia species varies as a function of the limiting nutrient. Generally there is a requirement for nitrogen replete conditions for domoic acid production, with both phosphate and silicate limitation resulting in production of the toxin. Most studies have focused on P. multiseries. The present study focuses on a comparison of growth rates and toxin production between both P. multiseries and P. australis, the species responsible for two major toxic blooms in Monterey Bay within the last nine years. Clones of P. multiseries and P. australis were isolated from Pacific coastal waters during the fall of 1998. P. multiseries and P. australis were grown in batch culture under continuous saturating light (120microEinsteins per meter squared per second) at 14°C. Guillard\'s f/2 formulation was modified to produce nitrate-, silicate- and phosphate-limited media. Standard f/2 was used as an additional treatment for more direct comparison with previous studies using highly enriched media. Experiments were conducted with four replicates for each treatment and the control. Samples were taken daily for cell counts, particulate and dissolved domoic acid (DA), and dissolved nutrients. Upon cessation of exponential growth, two replicates for each treatment were spiked with the limiting nutrient in an amount equal to the initial limiting nutrient concentration. The stationary growth phase was reached between days 9 and 13 depending on treatment and species. Experiments were maintained for an additional 15 to 20 days after the nutrient spike. Domoic acid was analyzed using FMOC derivitization followed by HPLC analysis (i.e., Pocklington et al. 1990).

Preliminary results suggest that although P. multiseries generally grows more rapidly than P. australis, when compared by volume their toxin contents are not significantly different. Comparative data for toxin production concomitant with nutrient depletion will be shown for both species after analyses of domoic acid and nutrients are completed.