Domoic acid binds iron and copper: a possible role for the toxin produced by the marine diatom Pseudo-nitzschia

Toxigenic species of the pennate diatom Pseudo-nitzschia can produce domoic acid (DA), an analog of the excitatory neurotransmitter glutamate and know n to cause the human illness amnesic shellfish poisoning (ASP). Although th e trophic transfer of this phycotoxin resulting in mass marine bird and mam mal mortality has recently been demonstrated, the physiological role of dom oic acid to the causative organism is still unknown. Domoic acid is a small tricarboxylate amino acid whose structure resembles that of known iron-com plexing agents produced by terrestrial plants, such as mugineic acid. This similarity in chemical structure of domoic acid to other phytosiderophores suggests a role for domoic acid as a trace metal chelator. Using a highly s ensitive adsorptive cathodic stripping voltammetric technique, we investiga ted the iron and copper-binding characteristics of domoic acid revealing it does form chelates with both iron and copper and with the following condit ional stability constants: K-FeDA,Fe(III)'(cond) = 10(8.7 +/- 0.5) M-1 and K-CuDA,Cu(II)'(cond) = 10(9.0 +/- 0/2) M-1 (K-FeDA,Fe3+(cond) = 10(18.7 +/- 0.5) M-1 and K-CuDA, Cu2+(cond) = 10(10.3) +/- M-1). Certain Pseudo-nitzsc hia species may therefore produce domoic acid selectively bind trace metals in order to either increase the availability of an essential micronutrient , such as in the case of iron, or to decrease the availability of a potenti ally toxic trace metal, such as in the case of copper. The strength with wh ich domoic acid binds iron and copper in seawater combined with concentrati ons of dissolved domoic acid potentially produced and released during toxic bloom conditions render dissolved iron and copper sufficiently bound to do moic acid in seawater so as to affect their chemical speciation and thus th eir biological availability. In addition, domoic acid may be particularly i mportant in solubilizing particulate iron suspended in these coastal waters where Pseudo-nitzschia blooms tend to occur. Thus, possible physiological roles for domoic acid with respect to the harmful algal species Pseudo-nitz schia may include the acquisition (iron) or detoxification (copper) of trac e metals in seawater. (C) 2001 Elsevier Science B.V. All rights reserved.