Zinc isotope fractionation during high-affinity and low-affinity zinc transport by the marine diatom Thalassiosira oceanica

John, Seth G., Robert W. Geis, Mak A. Saito, Edward A. Boyle

Limnol. Oceanogr., 52(6), 2007, 2710-2714 | DOI: 10.4319/lo.2007.52.6.2710

ABSTRACT: We have measured the isotopic fractionation of zinc (Zn) during uptake by the marine diatom Thalassiosira oceanica cultured at a range of free Zn2+ concentrations representative of the natural range from coastal and oligotrophic regions of the ocean. Harvested cells were rinsed with either plain seawater or a wash designed to remove adsorbed extracellular metals. Unwashed cells had much higher levels of Zn and were isotopically heavier than the media, indicating a positive isotope effect for extracellular Zn adsorption. Internalized Zn, measured in washed cells, was isotopically lighter than the media. The magnitude of Zn isotope fractionation changed with free Zn2+ concentration, corresponding to a switch on the part of T. oceanica between the predominance of high- and low-affinity Zn transport pathways. The total isotope effect for uptake (Δ66Zn) was -0.2% for high-affinity uptake at low Zn concentrations and -0.8% at the highest Zn concentrations, where low-affinity uptake is dominant. To our knowledge, this is the first study to describe a physiological basis for biological metal isotope fractionation during transport across the cell membrane. Similar high- and low-affinity Zn transport pathways are common among marine phytoplankton, suggesting that the processes described here are an important factor in natural marine Zn isotope variations.

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