Iron is a limiting nutrient across approximately one third of the surface ocean. The critical role of iron for ocean primary productivity means we need to constrain its biogeochemistry in order to project how iron bioavailability will change across the present and future ocean. Iron bioavailability is known to depend on the chemical speciation of iron, nevertheless, aquatic iron chemistry is a complex and many-faceted subject that incorporates multiple oxidation states, mysterious “ligands”, microbially produced siderophores, a high affinity for hydroxide ions and a preference for the particulate rather than the dissolved phase, making it challenging to constrain.
Here I will outline our recent progress in unravelling the ligand component of aquatic iron chemistry ^1–3. I will show how representing the intrinsic chemical properties of dissolved organic matter as a function of its heterogeneity allows us to assess the impact of pH and temperature on iron chemistry in the surface ocean. This new way of conceptualising iron-organic matter interactions in the ocean provides a mechanistic pathway to project changes in iron availability across the surface ocean and assess the impact of changing iron chemistry for end of century phytoplankton growth. 
1.            Gledhill, M. et al. Trace metal stoichiometry of dissolved organic matter in the Amazon plume. Science Advances 8, eabm2249 (2022).
2.            Lodeiro, P., Rey-Castro, C., David, C., Humphreys, M. P. & Gledhill, M. Proton Binding Characteristics of Dissolved Organic Matter Extracted from the North Atlantic. Environ. Sci. Technol. 57, 21136–21144 (2023).
3.            Zhu, K. et al. Influence of changes in pH and temperature on the distribution of apparent iron solubility in the oceans. Global Biogeochemical Cycles 37, e2022GB007617 (2023).
 

ocean acidification, iron, productivity, diatom