Zinc association with surface-bound iron-hydroxides on cultured marine diatoms: A zinc stable isotope perspective

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 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.

Zn Isotope Composition in the Water Column of the Northwestern Pacific Ocean: The Importance of External Sources

“…Dissolved δ 66 Zn observed in this study also possess isotopically light signature in the surface water (Figures and a). In addition to the seminal culture study (John et al, ), a recent culture study also indicated that Zn adsorption onto the surface‐bound Fe‐hydroxides may cause bulk δ 66 Zn to be isotopically heavy (Köbberich & Vance, ). Scavenging seems to be a major process causing the surface isotopically light Zn signature in the oceans.…”

“…The low dissolved δ 66 Zn observations are apparently at odds with the expectation that isotopically light Zn would be preferentially taken up by phytoplankton, leaving behind heavy Zn in dissolved pool. Actually, culture experiments have observed that phytoplankton take up isotopically light Zn relative to the culture medium, with 0.2 to 0.4‰ difference between phytoplankton and the medium (John et al, ; Köbberich & Vance, , ; Samanta et al, ). However, recent studies have argued that Zn fractionation of biological uptake can be insignificant due to the complexation effect of organic ligands in culture medium.…”

“…On the other hand, some studies have demonstrated that adsorption of Zn onto degrading phytoplankton or biofilms tends to remove relatively heavy Zn from seawater (Coutaud et al, ; John et al, ; John & Conway, ). Köbberich and Vance () found that bulk biomass δ 66 Zn were isotopically heavy under high Fe availability, which can be attributed to partial Zn adsorption onto surface‐bound Fe‐hydroxides. We also found that Zn to P ratios in suspended particles were an order of magnitude higher than the intracellular range for phytoplankton in oceanic regions strongly influenced by anthropogenic aerosol deposition, which reinforces the notion of scavenging of anthropogenic aerosol Zn onto plankton cell surfaces (Ho et al, ; Liao & Ho, ).…”

Isotope fractionation of zinc in the paddy rice soil-water environment and the role of 2’deoxymugineic acid (DMA) as zincophore under Zn limiting conditions

“…J o u r n a l P r e -p r o o f change in the coordination environment of dissolved and of adsorbed Zn could, however, be responsible for the isotopic fractionation based on the findings of other studies (Aucour et al, 2015;Aucour et al, 2017;Guinoiseau et al, 2016;Juillot et al, 2008;Köbberich and Vance, 2018;Little et al, 2014;Pokrovsky et al, 2005). Indeed, Aucour and co-workers studied the biogeochemical cycle of Zn isotopes at an infiltration basin receiving urban stormwater, in which Phalaris arundinacea (reed canary grass) developed spontaneously, and identified tetrahedral Zn species sorbed on iron oxides (Aucour et al, 2015).…”

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

“…Of the two groups of organisms cultured here for Zn isotopes, the data for cyanobacteria are the 246 most novel. Previous studies have presented data for diatoms Köbberich and 247 Vance, 2017;Köbberich and Vance, 2018), while Samanta et al (2017) have published Zn 248 isotope data for another eukaryote group, the coccoliths. Thus, we first discuss the variation 249 within the cyanobacteria strains cultured, before moving on to compare these new data with the 250 new and published data for eukaryotes.…”