Environmental Context. Several trace elements are essential to the growth of microorganisms, iron being arguably the most important. Marine microorganisms, which affect the global carbon cycle and consequently indirectly influence the world’s climate, are therefore sensitive to the presence of iron. This link means iron-related oceanic processes are a significant ecological and political issue.

Abstract. The importance of the role of iron as a limiting micronutrient for primary production in the World Ocean has become increasingly clear following large-scale in situ iron fertilization experiments in high-nutrient, low-chlorophyll (HNLC) regions.^[1] This has led to intensive international research with the aim of understanding the marine biogeochemistry of iron and quantifying the spatial distribution and transport of the element in the oceans. Recent studies have benefited from improved trace metal handling protocols and sensitive analytical techniques, but uncertainties remain concerning fundamental processes such as redox transfer, solubility, adsorption, biological uptake, and remineralization.

This review summarizes our present knowledge of iron biogeochemistry. It begins with a discussion of the effects of the physicochemical speciation of iron in seawater from a thermodynamic perspective, including important topics such as inorganic and organic complexation and redox chemistry. This is followed by an overview of the fluxes of iron to the ocean interface and a description of iron cycling within the open ocean water column. Current uncertainties of iron biogeochemistry are highlighted and suggestions of future work provided.