Many essential biological processes rely on the special catalytic properties of iron and thus this element is a required mineral nutrient for most organisms from bacteria to mammals. In key environments, growth of microorganisms is limited by the availability of iron. For example, humans have several systems that sequester iron to limit its availability to invading pathogens that are referred to as nutritional immunity. Many regions of the open ocean are iron limited receiving periodic inputs from terrestrial dust or deep water upwelling. A central focus my research is the investigation of the mechanism of iron uptake and storage systems in bacteria, yeast and diatoms employed to succeed in iron poor environments. We are combining structure determination by x-ray crystallography with biochemical and microbial approaches to define the molecular basis of iron uptake and storage.