The interaction between metals and microscopic plant-like organisms called phytoplankton is a key link to global carbon balance. More than a half of atmospheric CO2 on earth is taken up by phytoplankton, but iron (Fe) limits their growth in large regions of the oceans. Ongoing ocean acidification and global warming will influence Fe-stress in marine phytoplankton and hence the biological carbon fixation. Key existing knowledge gaps are the pathways by which phytoplankton take up Fe, and influences of chemical conditions in the microenvironment surrounding algal cells (i.e., phycosphere) on Fe speciation and bioavailability. This knowledge represents an impediment to understanding the complex effects of climate change on Fe uptake and oceanic carbon fixation. The project ‘Phycosphere Fe’ will determine chemical conditions and Fe speciation in the phycosphere of model phytoplankton species, quantify the role of phycosphere Fe speciation in Fe bioavailability, and investigate influences of climate change (i.e., warming and increased CO2) on Fe-algae interfacial processes. The project is key to the assessment of Fe bioavailability, growth and CO2 fixation of phytoplankton in current and future oceans, which make key contributions to global carbon sequestration. The project will improve our ability to model phytoplankton dynamics and predict biological carbon fixation in a changing ocean.