Machanism of iron regulation on amnesic neurontoxin production by coastal diatoms

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Batch culture experiments were performed to investigate iron requirement for growth of a coastal diatom Pseudo-nitzschia multiseries, which id known to produce domoic acid, a causative toxin of amnestic shellfish poisoning. Specific growth rates of P.multiseries showed a 50% decrease when dissolved inorganic iron concentration [Fe'] in the media was about 0/1 nM. P.multiseriesi required relatively high dissolved inorganic iron concentration for the maximum growth, that is similar to the other coastal phytoplankton. However, P.multiseriesi was still able to grow at [Fe']<0.02 nM, which limit the growth of most coastal species completely. Under iron-stressed condition, specific growth rates of P.multiseriesi were not affected by the addition of 2μM domoic acid, suggesting domoic acid itself does stimulate iron transport to support high growth rates. When iron availability was greatly reduced by the addition of a fungal siderophore desferrioxamine B(DFB), growth rates of P.multiseriesi significantly increased. Even under these strict iron stressed conditions, domoic acid concentrations in the cells showed little change compared with control without DFB. In addition, domoic acid concentrations in the media were always below the detection limit. These findings suggest that P.multiseriesi may utilize organically complexed iron under severe iron limitation and this iron transport system becomes active without production or release of domoic acid. Such iron uptake mechanism may allow this species to grow at the extremely low iron conditions.