Postdoctoral Fellowship: OPP-PRF: Understanding the Role of Specific Iron-binding Organic Ligands in Governing Iron Biogeochemistry in the Southern Ocean

Iron is an essential nutrient in the global ocean, required by phytoplankton to perform several essential cellular functions. In the Southern Ocean, iron is especially scarce and exerts a primary control on primary production and resulting CO2 uptake by the surface ocean. Most dissolved iron in the Southern Ocean is bound to a complex mixture of organic iron-binding compounds, called ligands. The concentrations and identities of these ligands control iron bioavailability, reactivity, and transport. Exopolysaccharides (EPS) are a particularly important group of ligands that are produced in high concentrations by local microbial communities for a variety of functions, including as cryoprotectants. Despite the known importance of EPS, most iron-binding ligand studies focus on total ligand concentrations or siderophore identification. Siderophores, small iron-binding molecules produced as a microbial iron acquisition strategy, are thought to play a disproportionate role in iron cycling due to their unusually high iron-binding affinities. This project adds essential EPS concentration measurements to a subset of samples from the 2023 U.S. GEOTRACES GP17-ANT expedition to the Amundsen Sea. This project integrates EPS, siderophores and bulk ligand measurements for the first time, thereby offering unprecedented insights into how the composition of organic ligands governs the supply and fate of iron to the Southern Ocean. The research plan involves a combination of field and experimental approaches. Field samples for electrochemical EPS analysis will be collected on a GP17-ANT transect spanning from the ice edge to open water. Previously funded paired samples of bulk ligand concentrations and siderophore identification will also be taken along the same transect. Results from the three analyses will be combined to assess the relative contributions of EPS and siderophores to the total ligand pool and interpret their impact on iron biogeochemical cycling in the region. Lab experiments will be performed to evaluate whether EPS alters the size fractionation of siderophores and iron in seawater, shuttling both species into the particulate phase. Experimental results will then be used to interpret the integrated field data and improve understanding of the impact of ligands on iron transport and bioavailability in the Amundsen Sea. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.