COPPER-UPTAKE KINETICS OF COASTAL AND OCEANIC DIATOMS

We investigated copper (Cu) acquisition mechanisms and uptake kinetics of the marine diatoms Thalassiosira oceanica Hasle, an oceanic strain, and Thalassiosira pseudonana Hasle et Heimdal, a coastal strain, grown under replete and limiting iron (Fe) and Cu availabilities. The Cu-uptake kinetics of these two diatoms followed classical Michaelis-Menten kinetics. Biphasic uptake kinetics as a function of Cu concentration were observed, suggesting the presence of both high- and low-affinity Cu-transport systems. The half-saturation constants (K-m) and the maximum Cu-uptake rates (V-max) of the high-affinity Cu-transport systems (similar to 7-350 nM and 1.5-17 zmol center dot mu m-2 center dot h-1, respectively) were significantly lower than those of the low-affinity systems (> 800 nM and 30-250 zmol center dot mu m-2 center dot h-1, respectively). The two Cu-transport systems were controlled differently by low Fe and/or Cu. The high-affinity Cu-transport system of both diatoms was down-regulated under Fe limitation. Under optimal-Fe and low-Cu growth conditions, the K-m of the high-affinity transport system of T. oceanica was lower (7.3 nM) than that of T. pseudonana (373 nM), indicating that T. oceanica had a better ability to acquire Cu at subsaturating concentrations. When Fe was sufficient, the low-affinity Cu-transport system of T. oceanica saturated at 2,000 nM Cu, while that of T. pseudonana did not saturate, indicating different Cu-transport regulation by these two diatoms. Using CuEDTA as a model organic complex, our results also suggest that diatoms might be able to access Cu bound within organic Cu complexes.