First report on iron demand by Antarctic sea-ice algae and under-ice phytoplankton

Antarctic sea ice is a seasonal source of iron (Fe) to the Southern Ocean (SO), where surface waters Fe levels are otherwise generally low. The effectiveness of Fe released from melting sea ice does not only depends on the magnitude of the supply, but also on the biological Fe demand. Here, we hypothesize that Fe uptake rates by sea-ice algae and under-ice phytoplankton are higher than the rates reported for open ocean phytoplankton in the SO. We performed 55 Fe and carbon ( 14 C) short-term uptake field measurements in, on and under Antarctic sea ice. Our results show that over 90% of Fe taken up by sea-ice algae and under-ice phytoplankton was found on the outside of the algal cells. The intracellular Fe (Fe intra ) uptake rates were high and reached up 68, 194 and 203 pmol Fe L − 1 d − 1 in seawater, bottom ice and over-ice respectively. Overall C uptake in bottom sea ice was low, ranging between 0.03 and 3.2 µmol C L − 1 d − 1 , but similar to previous sea-ice studies. The Fe intra C ratios in sea ice also increased over the 26 days times series, from 5.8 to 230 µmol Fe mol C − 1 . This is well above the ranges reported for other naturally Fe-fertilized areas of the SO. As the Antarctic icescape and associated ecosystems are changing, this dataset will undoubtedly inform the parameterisation of sea-ice biogeochemical and ecological models in ice-covered regions.