Kinetic control on Zn isotope signatures recorded in marine diatoms

Abstract

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.

The oceanic cycles of the transition metals and their isotopes

“…Subsequent culturing experiments of multiple diatom strains, as well as cyanobacteria and coccolithophorids, over a wider range of environmental variables (e.g. Köbberich and Vance 2017) has led us to the view that fractionation of Zn isotopes upon uptake is a very minor effect, consistent with the data from the real oceans in Fig. 1.…”

Zinc Geochemical Cycling in a Phosphorus‐Rich Ocean During the Early Ediacaran

“…This is confirmed by Köbberich and Vance (), who found that extremely low Zn uptake rate could result in the enrichment of isotopically heavy Zn in diatom, but more rapid Zn uptake rate did not produce Zn isotope fractionation between diatom and the external free Zn pool. Recently, the GEOTRACES investigation demonstrated either invariant or decreasing δ 66 Zn values toward the surface in modern ocean (Conway & John, , ; Zhao et al, ), which may also suggest no net Zn isotope fractionation when Zn is fixed into a diatom opal (Archer et al, ; Zhao et al, ), or the adsorption of heavy Zn isotope on organic particle surface (John & Conway, ; Köbberich & Vance, ). Other experiments have suggested that lighter Zn isotopes are preferred in diatom cell (Δ 66 Zn cell‐solution = −0.80‰ to −0.2‰), but heavier Zn isotopes are preferred on cell or organic particle surface during adsorption (Gélabert et al, ; John et al, ; John & Conway, ).…”

“…Early experimental study showed that diatoms in freshwater and seawater were preferentially enriched in isotopically heavy Zn (Δ 66 Zn cell-solution = 0.27‰ to 0.35‰), no matter what Zn was fixed in cells or adsorbed on the surface (Gélabert et al, 2006). This is confirmed by Köbberich and Vance (2017), who found that extremely low Zn uptake rate could result in the enrichment of isotopically heavy Zn in diatom, but more rapid Zn uptake rate did not produce Zn isotope fractionation between diatom and the external free Zn pool. Recently, the GEOTRACES investigation demonstrated either invariant or decreasing δ 66 Zn values toward the surface in modern ocean , 2015Zhao et al, 2014), which may also suggest no net Zn isotope fractionation when Zn is fixed into a diatom opal Zhao et al, 2014), or the adsorption of heavy Zn isotope on organic particle surface Köbberich & Vance, 2017).…”

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes

“…Profiles for δ 66 Zn in modern marine water columns do not typically increase toward the surface despite the export of light organic Zn from seawater (Conway & John, ; John & Conway, ; Zhao, Vance, Abouchami, & De Baar, ). This apparent lack of an expressed biological fractionation in near‐surface waters may reflect a role for Zn adsorption in controlling water column δ 66 Zn (John & Conway, ; Köbberich & Vance, ) (Figure S1). Zinc is predominantly complexed by organic ligands, many with unknown structures, or sorbed to biological or mineralogical surfaces (Bruland, ; Jakuba et al., ; John & Conway, ; Lohan, Statham, & Crawford, ).…”

“…Zinc is predominantly complexed by organic ligands, many with unknown structures, or sorbed to biological or mineralogical surfaces (Bruland, ; Jakuba et al., ; John & Conway, ; Lohan, Statham, & Crawford, ). Ligands preferentially complex the heavy isotope (Jouvin, Louvat, Juillot, Maréchal, & Benedetti, ; Köbberich & Vance, ; Marković et al., ), leaving the residual dissolved Zn 2+ pool isotopically light (John & Conway, ). It has been proposed that natural phytoplankton communities also have the ability to regulate ambient Zn concentrations in surface waters via rapid ligand production to reduce Zn toxicity, particularly where Zn concentrations are high (Lohan et al., ).…”