A call for refining the role of humic-like substances in the oceanic iron cycle

Whitby, Hannah; Planquette, Hélène; Cassar, Nicolas; Bucciarelli, Eva; Osburn, Christopher L; Janssen, David J.; Cullen, Jay T; Gonzalez, Aridane G; Voelker, Christoph; Sarthou, Géraldine (2020). A call for refining the role of humic-like substances in the oceanic iron cycle. Scientific Reports, 10(1) Nature Publishing Group 10.1038/s41598-020-62266-7

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Primary 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.

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