Variability in zinc:phosphorous and zinc:silicon ratios and zinc isotope fractionation in Southern Ocean diatoms: Observations from laboratory and field experiments

We studied the impact of iron limitation on zinc uptake and the zinc isotope (δ⁶⁶Zn) composition for Southern Ocean phytoplankton. We undertook laboratory culture and field incubation experiments, and linked these to in situ depth profiles of dissolved (dZn) and particulate (pZn) zinc collected from three sites in the Southern Ocean. For the laboratory experiments, diatom growth rates, cellular zinc accumulation, and δ⁶⁶Zn all responded to changes in iron and zinc bioavailability. A significant increase in the cellular quota for zinc (expressed as zinc:phosphorous (Zn:P)) occurred upon iron limitation and zinc enrichment. At the same time, δ⁶⁶Zn for organic tissues became isotopically light under high zinc and low iron concentrations. The opposite occurred for frustule δ⁶⁶Zn values. Here δ⁶⁶Zn_{frustule-organic} for cultured phytoplankton became isotopically heavier under high zinc and low iron concentrations. For senescing and dead cells, Zn:P declined and δ⁶⁶Zn increased, indicating a loss of isotopically light zinc from organic matter. For field incubation experiments, δ⁶⁶Zn_frustule was isotopically heavier than seawater, except for added zinc treatments. The percentage of zinc associated with frustule material for laboratory and field incubations encompassed a wide range with values between 1 and 57%. Depth profiles of δ⁶⁶Zn for dZn and pZn varied, with dZn being isotopically lighter than pZn in low dZn concentration subantarctic waters, whereas the opposite occurred in polar waters where dZn was isotopically heavier than pZn at higher dZn concentrations. Our results show that iron and zinc availability regulates the zinc content of phytoplankton and the δ⁶⁶Zn composition of the Southern Ocean, which is propagated to other parts of the world ocean.