Whitby, HannahHannahWhitbyPlanquette, HélèneHélènePlanquetteCassar, NicolasNicolasCassarBucciarelli, EvaEvaBucciarelliOsburn, Christopher LChristopher LOsburnJanssen, David JamesDavid JamesJanssen0000-0002-9091-8936Cullen, Jay TJay TCullenGonzalez, Aridane GAridane GGonzalezVoelker, ChristophChristophVoelkerSarthou, GéraldineGéraldineSarthou2024-09-022024-09-022020-04-09https://boris-portal.unibe.ch/handle/20.500.12422/35632Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.en500 - Science::550 - Earth sciences & geologyarticle10.7892/boris.14331010.1038/s41598-020-62266-7