Marine diatoms dominate the oceanic cycle of the essential micronutrient zinc (Zn). The stable isotopes of zinc and other metals are increasingly used to understand trace metal micronutrient cycling in the oceans. One clear feature of the early isotope data is the heavy Zn isotope signature of the average oceanic dissolved pool relative to the inputs, potentially driven by uptake of light isotopes into phytoplankton cells and export to sediments. However, despite the fact that diatoms strip Zn from surface waters across the Antarctic polar front in the Southern Ocean, the local upper ocean is not isotopically heavy. Here we use culturing experiments to quantify the extent of Zn isotope fractionation by diatoms and to elucidate the mechanisms driving it. We have cultured two different open-ocean diatom species (T. oceanica and Chaetoceros sp.) in a series of experiments at constant medium Zn concentration but at bioavailable medium Fe ranging from limiting to replete. We find that T. oceanica can maintain high growth rates and Zn uptake rates over the full range of bioavailable iron (Fe) investigated, and that the Zn taken up has a δ 66 Zn that is unfractionated relative to that of the bioavailable free Zn in the medium. The studied representative of the genus Chaetoceros, on the other hand, shows more significantly reduced Zn uptake rates at low Fe and records more variable biomass δ 66 Zn signatures, of up to 0.85 ‰ heavier than the medium. We interpret the preferential uptake of heavy isotopes at extremely low Zn uptake rates as potentially due to either of the following two mechanisms. First, the release of extracellular polymeric substances (EPS), at low Fe levels, may preferentially scavenge heavy Zn isotopes. Second, the Zn uptake rate may be slow enough to establish pseudoequilibrium conditions at the transporter site, with heavy Zn isotopes forming more stable surface complexes. Thus we find that, in our experiments, Fe-limitation exerts a key control that not only limits diatom growth, but also affects the Zn uptake physiology of diatoms. Uptake of heavy isotopes occurs under Fe-limiting conditions that drive extremely low Zn uptake rates. On the other hand, more rapid Zn uptake rates result in biomass that is indistinguishable from the external bioavailable free Zn pool. These experimental results can, in principle, explain the range of Zn isotopic compositions found in the real surface ocean, given the geographically variable interplay 47 between Fe-limitation, Zn uptake rates, and the degree of organic complexation of oceanic Zn.

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The stable isotope systems of the transition metals potentially provide constraints on the current and past operation of the biological pump, and on the state of ocean redox in Earth history. Here we focus on two exemplar metals, nickel (Ni) and zinc (Zn). The oceanic dissolved pool of both elements is isotopically heavier than the known inputs, implying an output with light isotope compositions. The modern oceanic cycle of both these elements is dominated by biological uptake into photosynthesised organic matter and output to sediment. It is increasingly clear, however, that such uptake is associated with only very minor isotope fractionation. We suggest that the isotopic balance is instead closed by the sequestration of light isotopes to sulphide in anoxic and organic-rich sediments, so that it is ocean chemistry that controls these isotope systems, and suggesting a different but equally interesting array of questions in Earth history that can be addressed with these systems.

Uptake of trace metals by marine phytoplankton for metabolic use exerts a fundamental control on their marine geochemical distributions. Moreover, such trace metals limit primary productivity over large areas of the surface ocean. As such, an understanding of the mechanisms and extent of phytoplankton uptake are essential components of oceanic trace metal chemistry. Efforts to quantify intra-cellular quotas of phytoplankton are complicated by the presence of metals adsorbed to external surfaces, including surface-bound Fe-hydroxides, both in nature and in culturing experiments. In the relatively new discipline focused on oceanic metal isotopes, these surface-bound metal hydroxides may be of particular importance in that they could result in isotope signatures that complicate studies that seek to understand intracellular signatures related to metabolic uptake. In this contribution, we assess the extent to which heavy Zn isotopes are preferentially adsorbed to surface-bound Fe-hydroxides on marine diatoms. For this purpose, the marine diatom Thalassiosira oceanica has been cultured at low versus high inorganic Fe concentrations in the medium, while two further diatoms strains have been compared at elevated Fe levels. The formation of surface bound Fe-hydroxides at elevated Fe was further stimulated by reducing the trace metal buffering capacity of the experimental medium, lowering the concentration of the used organic chelator. We also investigate an alternative procedure for quantifying intracellular metal quotas, the analysis of the contents of deliberately lysed cells. In good agreement with previous work, we find that biomass associated Fe/P ratios represent a good proxy for the absolute quantity of Fe-precipitates on diatom surfaces. Zn sorption to these surface-bound Fe-hydroxides can drive bulk biomass δ Zn compositions up. On the other hand, the loss of heavy Zn from the experimental medium causes the biomass Δ 66 Zn, if referred to the starting medium, to be biased in the other direction, towards more negative values. To avoid any such complications, likely to occur at high Fe or low buffer capacities, we conclude that diatoms cultured at low Fe are most likely to record the Δ 66 Zn signatures of Zn uptake into the phytoplankton cell.