LAUSR

Abstract 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 intra-cellular 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 intra-cellular 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 δ66Zn compositions up. On the other hand, the loss of heavy Zn from the experimental medium causes the biomass Δ66Zn, 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 Δ66Zn signatures of Zn uptake into the phytoplankton cell.