Zn isotope fractionation during uptake into marine phytoplankton: Implications for oceanic zinc isotopes

The extreme scarcity of zinc (Zn) in the euphotic zone, coupled to deep enrichments, is consistent with biological uptake at the surface and regeneration at depth. In the context of a nutrient-type depth profile so clearly shaped by uptake into phytoplankton, the growing dataset for Zn isotopes presents a challenge. These data either show very minor isotope effects associated with extreme depletion, or enrichment of the light isotopes in the upper ocean. In contrast, culturing of eukaryotes in the laboratory suggests that light Zn isotopes are preferentially taken up into diatoms and coccoliths, implying that Zn depletion at the surface should be associated with extremely heavy residual dissolved signals. Here we present the first Zn isotope measurements for cultured marine cyanobacteria and compare these data to those for eukaryotic diatoms grown under identical conditions. Of the four cyanobacteria cultured, belonging to the genera Synechococcus and Prochlorococcus, three preferentially take up light Zn into the cell, with a variability that is not fundamentally different between pro- and eukaryotic phytoplankton. We also observe only very subtle differences between Zn/P and Fe/P uptake ratios for these three cyanobacteria groups relative to diatoms grown under the same conditions. A fourth strain exhibits preferential uptake of heavy Zn isotopes, and very high Zn/P ratios. Overall, we speculate that the observed variability among cyanobacteria may be related to the molecular structure of their photosynthetic light harvesting apparatus, adapted to significantly different light niches. These new and published culture data support the hypothesis that cellular δ66Zn in culture might largely be controlled by the organic ligands that bind Zn in the medium. Given that the Zn-binding ligands in the ocean have thermodynamic stability constants that are orders of magnitude smaller than the EDTA used in culture media, the surprisingly subtle Zn isotope variability in some parts of the surface ocean may be reconciled with culture data by the lesser, near zero, preference of these weaker complexes for heavy Zn isotopes. "This article is part of a special issue entitled: "Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond" - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González."