The effect of Fe and Cu on growth and domoic acid production by Pseudo-nitzschia multiseries and Pseudo-nitzschia australis

Toxigenic pennate diatoms of the genus Pseudo-mitzschia produce domoic acid (DA), the neurotoxin linked to amnesic shellfish poisoning. We investigated how Fe and Cu affect growth and DA production by P. multiseries and P. australis clones isolated from Monterey Bay, California. Growth rates of these species could be inhibited by both Fe limitation (pFe 20.5, mu = 50% mu(max)) and Cu toxicity (pCu 10.5, mu = 30-50% mu(max)). The rate of DA production during the exponential phase was a significant inverse function of cellular growth rates (P < 0.02). Voltammetric measurements of dissolved DA-equivalents in short-term experiments indicated that the cellular production of DA increased from similar to5 amol DA cell(-1) h(-1) under optimal growth conditions to similar to40 amol DA cell I h(-1) under Fe-deficient conditions and similar to105 amol DA cell(-1) h(-1) under Cu-stressed conditions. The DA was released to the medium under metal stress conditions, with intracellular DA concentrations decreasing relative to nonstressed cells. Fe uptake rates by P. multiseries were slow compared to other marine diatoms in the absence of dissolved DA but were enhanced threefold (P < 0.03) by adding DA to the medium. DA addition also partially alleviated toxic Cu conditions. Our findings suggest that DA production during exponential growth of these two toxigenic Pseudo-mitzschia species is directly induced by Fe-deficient or Cu stress conditions and that 95% of this DA is actively released into the medium. Changing trace metal conditions in coastal waters therefore may have a profound effect upon intracellular DA concentrations and thereby influence the toxic effect of these harmful bloom events.