Weak Organic Ligands Enhance Zinc Uptake in Marine Phytoplankton

A recent study of the effect of pH on Zn and Cd bioavailability shows that binding to weak organic ligands can increase the pool of metals available to phytoplankton in the presence of strong chelating agents. We explore the underlying mechanism in laboratory experiments with the model species Emiliania huxleyi and Thalassiosira weissflogii. Additions of l- and d- isomers of cysteine (Cys) result in similar increases in Zn uptake rates in the presence of the strong chelator ethylenediaminetetraacetic acid (EDTA) but decrease it in the absence of EDTA, ruling out uptake by a specific Zn–Cys transporter. The effect of Cys does not result from alleviating diffusion limitation of inorganic Zn. The enhancement of Zn uptake kinetics by weak ligands is consistent with a mechanism involving formation of a transient ternary complex with uptake molecules: (1) the enhancement is most dramatic in Zn limited cells whose high affinity transporters should be most effective at extracting Zn from weak ligands; (2) the enhancement occurs with a variety of weak ligands, demonstrating that the underlying mechanism has little chemical specificity; and (3) no enhancement of uptake is seen when Zn is bound in complexes that would make formation of multiligand complexes with uptake molecules difficult. Weak complexing agents which have received heretofore little attention may play a key role in the bioavailability of metals in natural waters.