Zinc, cadmium, and cobalt interreplacement and relative use efficiencies in the coccolithophore Emiliania huxleyi

We investigate the interreplacement of Zn, Cd, and Co in the cosmopolitan coccolithophore Emiliania huxleyi by examining its growth rate, cellular metal quotas, and cellular metal uptake rates under different combinations of Zn, Cd, and Co concentrations in the medium. Co and Zn can fully replace each other, except perhaps for small individual requirements that are met by minute metal concentrations supplied as contaminants. Zn is used at 75% efficiency by E. huxleyi compared with Co. In contrast, Cd can only partially replace Zn or Co and is used at 66% efficiency compared with Co. Up to 50% of the cellular Zn or Co can be replaced by Cd, once the minimum cellular Zn or Co requirement is fulfilled. The limitation by Zn, Cd, and Co and the interreplacement of the three metals occurs over a range of unchelated metal concentrations, 0.2-5 pmol L-1, that is relevant to surface oceanic waters. Based on our laboratory results and the published metal concentrations in surface oceanic waters, we calculate that interreplacement of Zn, Cd, and Co should occur in regions where Zn is highly depleted.