Iron binding ligands in the ocean are similar to humic substances

Organic complexation plays a key role in the geochemistry and bioavailability of iron but surprisingly the composition of the organic ligands responsible for the binding of Fe is largely unknown. Likely candidates for the identity of Fe-binding ligands include siderophores (specific Fe-binding compounds produced by bacteria), exopolysaccharides (released by bacteria) and humic substances. Here we demonstrate that iron-binding humic substances account for 98 % of the unknown ligands in the ocean and on the shelf. This revolutionary finding challenges the paradigm that iron-binding ligands largely originate from in-situ production by microorganisms. It is more likely that iron is retained in solution by humics from terrestrial origin (explaining the island effects seen in primary productivity) although it is possible that these are augmented by humic substances resulting from in-situ breakdown of organic matter.