The effects of iron on domoic acid production by Pseudo-nitzschia multiseries

Domoic acid (DA), the responsible agent for Amnesic Shellfish Poisoning, contains three carboxylic acid residues that could potentially bind trace metals such as iron. To investigate if DA production was affected by the iron status of the cell, replicate cultures of the diatom Pseudo-nitzschia multiseries were grown under trace-metal clean conditions containing 0, 0.12 mu M and 11. 7 mu M added iron. All three iron treatments showed similar initial growth rates and resulted in stationary phase densities of > 10 super(5) cells mL super(-1) after 10 days in culture. In contrast, DCMU-enhanced fluorescence indicated that the cultures without added iron were iron-limited by day 18, as evidenced by an Fv/Fm ratio of 0.2 as compared to the iron-replete cultures (11.7 mu M Fe) with a ratio of 0.5 throughout the culture period. Cultures without added iron showed a marked drop in the cellular chlorophyll a content in stationary phase and never produced more than 5 pg DA cell super(-1) <1 000 ng mL super(-1)). In contrast, iron-replete cells contained 5-10 times more chlorophyll alpha per cell, and DA production increased steadily throughout stationary phase to nearly 50 pg cell super(-1) (5500 ng mL super(-1)). The intermediate level of added iron (0.12 mu M) showed an intermediate response. These results indicate that the lack of available iron strongly inhibits the ability of P. multiseries to produce DA. The decrease in DA production, chlorophyll a synthesis, and photosynthetic efficiency suggest that DA is not produced as a chelator to increase the availability of iron to the cells. The cause of this decrease is unknown, but the biosynthesis of DA requires both nitrogen and energy, two resources that are likely to be limiting under iron-deficient conditions.