• centric diatoms;
  • element mapping;
  • intrapopulation variability;
  • iron localization;
  • single-cell analysis;
  • synchrotron X-ray fluorescence;
  • vacuole

The cellular iron (Fe) quota of centric diatoms has been shown to vary in response to the ambient dissolved Fe concentration; however, it is not known how centric diatoms store excess intracellular Fe. Here, we use synchrotron X-ray fluorescence (SXRF) element mapping to identify Fe storage features in cells of Thalassiosira pseudonana Hasle et Heimdal and Thalassiosira weissflogii G. A. Fryxell et Hasle grown at low and high Fe concentrations. Localized intracellular Fe storage features, defined as anomalously high Fe concentrations in regions of relatively low phosphorus (P), sulfur (S), silicon (Si), and zinc (Zn), were twice as common in T. weissflogii cells grown at high Fe compared to low-Fe cells. Cellular Fe quotas of this strain increased 2.9-fold, the spatial extent of the features increased 4.6-fold, and the Fe content of the features increased 14-fold under high-Fe conditions, consistent with a vacuole storage mechanism. The element stoichiometry of the Fe features is consistent with polyphosphate-bound Fe as a potential vacuolar Fe storage pool. Iron quotas increased 2.5-fold in T. pseudonana grown at high Fe, but storage features contained only 2-fold more Fe and did not increase in size compared to low-Fe cells. The differences in Fe storage observed between T. pseudonana and T. weissflogii may have been due to differences in the growth states of the cultures.