Four large, open‐ocean diatoms from the Southern Ocean ( Actinocyclus sp., Thalassiosira sp., Fragilariopsis kerguelensis , and Corethron pennatum ) were grown in natural (low iron) Southern ocean seawater with increasing Fe concentrations. With increasing dissolved iron (Fe diss ) concentrations, the growth rates increased three‐ to sixfold. The species with the smallest cells had the highest growth rates. The half‐saturation...