Toxigenic Pseudo-nitzschia sp. have been linked to amnesic shellfish poisoning events along the California coast. These diatoms can produce the toxin domoic acid (DA), a secondary metabolite whose algal physiological role is presently unknown. Factors triggering Pseudo-nitzschia blooms and enhancing the production of DA in nature are poorly understood. We are investigating the potential role of trace metals, both as limiting and/or toxic nutrients, on Pseudo-nitzschia population dynamics and toxicity. To date, lab studies have only demonstrated a link between cellular DA production and macronutrient limitation. But standard culturing techniques may inadvertently contaminate the media with trace metals, and thus are inadequate for assessing the role of trace metals on toxigenic Pseudo-nitzschia blooms. The chemically well-defined synthetic seawater medium Aquil was modified to optimally culture two Pseudo-nitzschia sp., australis and multiseries, under trace metal clean conditions. The growth rates of these species in Aquil (1.04 ± 0.16 d-1) were comparable to those achieved previously in amended natural seawater (1.25 ± 0.2 d-1). Concentrations of Fe, Mn, and Cu in Aquil media were then altered to determine optimal cellular trace metal requirements. Our preliminary results suggest that these Pseudo-nitzschia sp. have uniquely high Fe and Mn requirements, and that Fe and Mn interact in a synergistic manner. We also examined whether the production of DA by these diatoms is enhanced under trace metal limiting (Fe and Mn) or toxic (Cu) conditions. These findings will potentially allow us to predict the occurrence of toxic bloom events based on trace metal concentrations and their speciation in coastal waters.