Abstract The elemental content of diatoms determines in part their productivity and their influence on biogeochemical cycles. We used a cell-specific technique, synchrotron based x-ray fluorescence microscopy (SXRF), to study for the first time how the cellular Fe, Si, P and S contents of natural diatoms respond to additions of Fe and Si in the eastern equatorial Pacific (EEP), a major natural source of CO 2 to the atmosphere. We then compare these measurements to ratios of silicic acid, nitrate and phosphate drawdown in the experiments and to measurements of dissolved silicic acid and iron in the EEP. Addition of silicic acid (20 μM) resulted in increases of 25-50% in cell volume and cellular silica content in two experiments, but there was no significant change in either variable during a third experiment. No other effects of Si addition on cellular stoichiometry were observed. Cellular Fe content and molar ratios of Fe:P and Fe:S increased by 24- 14- and 17- fold 48 hours after addition of 2 nM Fe, but then declined to 7-, 4- and 6-fold higher than those in Control after 96 hours. Cellular P and S (a proxy for cell protein) both increased by up to 2-fold in response to Fe addition, and cell volume increased by 50-80%. Cellular Si content was not affected by Fe addition, but Si:P and Si:S ratios declined due to the effects of Fe on P and S. Our results suggest that Fe supply affects the ratios at which silicic acid and nitrate are utilized by blooming diatoms in the EEP. However, the production of biogenic silica per cell will not be affected by Fe supply. Consequently, in the EEP Fe availability is unlikely to influence the density and vertical transport of cellular material by affecting the amount of silica mineral ballast in cells. The relationship between silica content of diatoms and supply of silicic acid may help dampen variability in dissolved silicic acid concentrations in the EEP.