Environmental context. It is now well accepted that iron is an essential micronutrient for phytoplankton growth in many areas of the global ocean, even though this element is present in seawater in extremely low abundance. It is also known that most of the iron in seawater is present as complexes formed with ligands of natural organic matter whose nature and origin remain largely unknown. Here we consider how these iron-complexing ligands might have evolved during geological time, what factors may have given rise to their presence and the possible roles that they play in iron biogeochemistry.

Abstract. Current knowledge about the role of iron-binding organic ligands in the ocean and their role in determining the biogeochemistry of this biologically active element has been summarised. Some electrochemical measurements suggest the presence of at least two ligand types, a strong binding ligand L₁ found mainly in the mixed layer and a weaker ligand L₂ found mainly in deep water. Speciation of Fe^III is dominated by L₁ in the mixed layer and L₂ in the deep ocean. There is some evidence that L₁ is siderophore-like and is specifically generated by marine microbes (i.e. heterotropic bacteria and cyanobacteria). We suggest that this is a specific biological mechanism for sequestering iron in the mixed layer that developed early in the ocean’s history (Archaean period, 2500–3500 million years BP), whereas the more ubiquitous L₂ ligand only arose at the close of the Proterozoic (500–2500 million years BP) when eukaryotic organisms evolved to switch on the ocean’s biological pump, allowing L₂ ligands to form from the oxidation of sinking biological particles. This development coincided with the complete oxygenation of the ocean’s interior which removed the iron-binding sulfide ion and allowed maintenance of the ocean’s iron inventory. These speculations are accompanied by various suggestions about avenues for future research to better understand iron biogeochemistry.