Toxic species of the pennate diatom Pseudo-nitzschia can produce domoic acid, an analog of the excitatory neurotransmitter glutamate and a known causative agent of the human illness amnesic shellfish poisoning. Domoic acid is a small tricarboxylate amino acid whose algal metabolic role is presently unknown. The chemical structure of domoic acid resembles that of iron-complexing agents, suggesting a role for domoic acid as an iron chelator. Certain Pseudo-nitzschia species may therefore produce domoic acid to bind iron, possibly increasing the availability of this essential micronutrient. There is increasing evidence that marine phytoplankton can influence ocean productivity by releasing strong, trace metal-binding dissolved organic compounds. Dissolved iron in surface seawater has been found to be >99% bound organically.Using a highly sensitive adsorptive cathodic stripping voltammetric technique, we investigated the iron-binding characteristics of domoic acid (certified reference material, NRC) and found it does chelate iron to such a degree as to significantly affect its chemical speciation (log K with respect to Fe\' = 8.7 (+/ 0.5)).Cultures of Pseudo-nitzschia australis were grown for the first time in a chemically defined, trace metal clean media. Results show that under iron-stressed conditions ([Fe(III)\'] = 10e-14 M) these organisms produced 7.6 nM of iron-binding chelator with the same conditional stability constant as that measured for the domoic acid standard (log K\' = 8.2). This dissolved domoic acid concentration is similar to concentrations produced by Pseudo-nitzschia species previously published and is also consistent with that expected during a recent bloom along the California coast in May-June, 1998. The concentration of domoic acid measured in toxic bloom conditions, taken together with the complexing binding strength we have measured will render iron significantly bound to domoic acid. Thus, our findings show that domoic acid may affect the availability of iron. Possible biological roles for domoic acid include acquisition and/or detoxification of trace metals in seawater.