Domoic acid: The synergy of iron, copper, and the toxicity of diatoms

Abstract

Diatom blooms generated by the alleviation of iron limitation in high nitrate–low chlorophyll (HNLC) regions of the oceans often are composed of pennate diatoms of the genus Pseudo-nitzschia, many of which periodically produce the potent neurotoxin domoic acid. We show that toxigenic diatoms have an inducible high-affinity iron uptake capability that enables them to grow efficiently on iron complexed by strong organic ligands in seawater. This low-iron adaptive strategy requires copper and domoic acid, a copper chelator whose production increases sharply when both iron and copper are limiting. Addition of either domoic acid or copper to seawater improves the growth of Pseudo-nitzschia spp. on strongly complexed iron during deck incubation experiments with natural phytoplankton. Our findings indicate that domoic acid is a functional component of the unusual high-affinity iron acquisition system of these organisms. This system may help explain why Pseudo-nitzschia spp. are persistent seed populations in oceanic HNLC regions, as well as in some neritic regions. Our findings also indicate that in the absence of an adequate copper supply, iron-limited natural populations of Pseudo-nitzschia will become increasingly toxic.