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Background: Iron storage by ferritin enables diatom bloom upon iron input. Results: Ferroxidase center variants show faster iron mineralization and rate of post-oxidation reorganization of iron. Conclusion: Glu-130 and Glu-44 regulate the flux of iron through the ferroxidase center. Significance: Optimization of ferritin for iron oxidation but not mineralization suggests an iron buffering function in addition to long-term iron storage., Ferritin from the marine pennate diatom Pseudo-nitzschia multiseries (PmFTN) plays a key role in sustaining growth in iron-limited ocean environments. The di-iron catalytic ferroxidase center of PmFTN (sites A and B) has a nearby third iron site (site C) in an arrangement typically observed in prokaryotic ferritins. Here we demonstrate that Glu-44, a site C ligand, and Glu-130, a residue that bridges iron bound at sites B and C, limit the rate of post-oxidation reorganization of iron coordination and the rate at which Fe3+ exits the ferroxidase center for storage within the mineral core. The latter, in particular, severely limits the overall rate of iron mineralization. Thus, the diatom ferritin is optimized for initial Fe2+ oxidation but not for mineralization, pointing to a role for this protein in buffering iron availability and facilitating iron-sparing rather than only long-term iron storage.