THE total zinc concentration in unpolluted marine waters has been reported to be in the 10-10 M range1. Such low concentration of an essential micronutrient suggests that the growth of some phytoplankton may be zinc limited. Of the trace metals necessary for phytoplankton growth, only iron has been considered a potential limiting micronutrient in the marine environment2,3. On the basis of laboratory work which focused mostly on copper, it is well known that the toxicity of a trace metal depends on its chemical speciation and can be related uniquely to its free ion activity 4,5. However, it has not been established unequivocally that the availability of some metals may also be controlled by their free ion activities and may thus be depressed by organic complexation. Here we report laboratory experiments demonstrating that the zinc ion activity (rather than the total zinc concentration) can limit the growth rate of a coastal diatom Thalassiosira weissflogii (Grun. = T. fluviatalis Hust.) and that the limitation occurs at zinc ion activities which would be present in unpolluted seawater if any organic complexation of zinc were taking place.
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