Cadmium toxicity and Zn Limitation were examined in centric diatoms to lest the hypothesis that resistance to high concentrations of toxic metals was related to the assimilation of essential ones. Nine species of the genus Thalassiosira were cultured in artificial seawater media under Zn deficiency and in trace-metal-replete media with Cd additions ranging from 10 nM to 10 mu M Cadmium sensitivity measured as [Cd2+] required to inhibit growth by 50% (pCd(50)), varied significantly among species (p < 0.001). Clones isolated from offshore oligotrophic environments were significantly more resistant to Cd toxicity and Zn deficiency than those indigenous to coastal regions (p < 0.01). Differences in Cd resistance could not be explained by cellular exclusion since all species accumulated remarkably similar amounts of Cd (Cd quota similar to 30 pmol Cd:mol C) at the highest concentrations tested. pCd(50)s were positively correlated to QCd(50)s (Cd quotas required to inhibit growth by 50%). Tolerance of the diatoms to intra- and extracellular Cd was positively correlated to their growth rates under Zn-limiting conditions so that species most resistant to Cd toxicity were those least affected by Zn limitation. Cadmium concentrations that were not toxic under trace-metal-saturating conditions inhibited growth rate of Zn-limited T. weissflogii cultures by approximately 70%, but had no effect on Fe-limited cultures. The results thus suggest that Cd toxicity is mediated in part through the impairment of Zn assimilation in diatoms.
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