First report on biological iron uptake in the Antarctic sea‑ice environment

Melting sea ice is a seasonal source of iron (Fe) to the Southern Ocean (SO), where Fe levels in surface waters are otherwise generally too low to support phytoplankton growth. However, the effectiveness of sea-ice Fe fertilization in stimulating SO primary production is unknown since no data exist on Fe uptake by microorganisms in the sea-ice environment. This study reports a unique dataset on Fe uptake rates, Fe-to-carbon (C) uptake ratio (Fe uptake normalized to C uptake) and Fe:C uptake rate (Fe uptake normalized to biomass) by in situ microbial communities inhabiting sea ice and the underlying seawater. Radioisotopes ⁵⁵Fe and ¹⁴C were used in short-term uptake experiments during the 32-day Ice Station POLarstern (ISPOL) time series to evaluate the contributions of small (0.8–10 µm) and large (> 10 µm) microbes to Fe uptake. Overall, results show that over 90% of Fe was bound to the outside of the cells. Intracellular Fe (Fe_intra) uptake rates reached up to 68, 194, and 203 pmol Fe L⁻¹d^{− 1} in under-ice seawater, bottom ice, and top ice, respectively. Inorganic carbon uptake ranged between 0.03 and 3.2 µmol C L⁻¹ d⁻¹, with the lowest rate observed in under-ice seawater. Importantly, between the start and end of ISPOL, we observed a 30-fold increase in Fe_intra normalized to carbon biomass in bottom sea ice. This trend was likely due to changes in the microbial community from a dominance of large diatoms at the start of the survey to small diatoms later in the season. As the Antarctic icescape and associated ecosystems are changing, this dataset will help inform the parameterisation of sea-ice biogeochemical and ecological models in ice-covered regions.