Growth Rates, Half-Saturation Constants, and Silicate, Nitrate, and Phosphate Depletion in Relation to Iron Availability of Four Large, Open-Ocean Diatoms from the Southern Ocean

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 $(\text{Fe}_{\text{diss}})$ concentrations, the growth rates increased three- to sixfold. The species with the smallest cells had the highest growth rates. The half-saturation constants (Km) for growth were low (0.19-1.14 nmol L-1 $\text{Fe}_{\text{diss}}$), and close to the ambient $\text{Fe}_{\text{diss}}$ concentrations of 0.2 nmol L-1. The range in Km with respect to $\text{Fe}_{\text{diss}}$ also varied with the size of the diatoms: the smallest species had the lowest Km and the largest species had the highest Km As $\text{Fe}_{\text{diss}}$ concentrations decreased, silicate consumption per cell increased, but nitrate consumption per cell decreased. Phosphate consumption per cell varied without clear relation to the dissolved iron concentrations. The differences in nutrient consumption per cell resulted in marked differences in elemental depletion ratios in relation to $\text{Fe}_{\text{diss}}$ concentrations, with the depletion ratios being most affected by iron limitation in the largest cells. These experimental findings are in agreement with previous laboratory and field studies, showing the relatively high requirements of large diatoms for Fe. The size-dependent response of the diatoms with respect to nutrient depletion is a good illustration of the effects of Fe on silicate, nitrate, and phosphate metabolism.