Fourquez, MarionJanssen, David J.Conway, Tim M.Cabanes, DamienEllwood, Michael J.Sieber, MatthiasTrimborn, ScarlettHassler, Christel2025-03-162025-03-16researchoutputwizard:a383154xPUB42314Scopus:85163643453https://dspace-test.anu.edu.au/handle/1885/733719301Dissolved iron (dFe) availability limits the uptake of atmospheric CO2 by the Southern Ocean (SO) biological pump. Hence, any change in bioavailable dFe in this region can directly influence climate. On the basis of Fe uptake experiments with Phaeocystis antarctica, we show that the range of dFe bioavailability in natural samples is wider (<1 to ~200% compared to free inorganic Fe′) than previously thought, with higher bioavailability found near glacial sources. The degree of bioavailability varied regardless of in situ dFe concentration and depth, challenging the consensus that sole dFe concentrations can be used to predict Fe uptake in modeling studies. Further, our data suggest a disproportionately major role of biologically mediated ligands and encourage revisiting the role of humic substances in influencing marine Fe biogeochemical cycling in the SO. Last, we describe a linkage between in situ dFe bioavailability and isotopic signatures that, we anticipate, will stimulate future research.We are grateful to all scientists and support staff involved in the collection and analysis of samples, both at sea and on shore, especially C. Völker and T. Brenneis (AWI, Germany) for eHS measurements as well as K. Bergin for TPZT measurements. We also thank S. Fawcett (University of Cape Town, South Africa) and R. Simó (Institut de Ciències del Mar, Spain) who shared data on in situ biomass cell abundance and N. Cassar (Duke University, USA) and S. Bonnet (MIO, France) who provided valuable advices in the writing of this manuscript. We also thank M.-N. Houssais (LOCEAN, France) for the time devoted to the deployment and analysis of the CTD profiles. This study was supported by Project 16 of the Antarctic Circumnavigation Expedition (ACE). ACE was a scientific expedition that carried out under the auspices of the Swiss Polar Institute (SPI), supported by funding from the ACE Foundation and Ferring Pharmaceuticals. M.F. and C.H. were supported by the Swiss National Science Foundation (PP00P2_138955 and PP00P2_166197), and M.F. was further funded by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 894264 (BULLE-project) and by Grant ERC no. 101044637 (HOPE-project). In addition, publications fees for this article were covered by the Mediterranean Institute of Oceanography.EnglishPublisher Copyright: Copyright © 2023 The Authors, some rights reserved.202310.1126/sciadv.adf9696http://www.scopus.com/inward/record.url?scp=85163643453&partnerID=8YFLogxK