Impact of copper and iron on growth and silicic acid uptake in two Southern Ocean diatoms

Copper (Cu) and iron (Fe) are vital micronutrients that influence diatom physiology, growth rate, and nutrient stoichiometry. This study examines the interactive effects of Cu and Fe on two Southern Ocean diatoms, Proboscia inermis and Chaetoceros flexuosus, cultured under varying Fe and Cu conditions. Results show that Cu and Fe interactions lead to an overall decline in growth rates with decreasing Fe and Cu concentrations. Cu deficiency inhibits growth rate, suggesting a constitutive role of Cu in biochemical pathways, especially under Fe‐limiting conditions. Changes in Fe and Cu also influenced cell volume (CV) and surface area (SA), with P. inermis showing an increased CV and SA under Fe stress. For C. flexuosus, changes in CV and SA were more variable. Changes in Fe and Cu bioavailability significantly affected silicic acid (Si(OH)4) uptake, as reflected in the maximum Si(OH)4 uptake rate and the half‐saturation constant for Si(OH)4 uptake. Both diatoms also displayed a wide range of Si isotope fractionation values, ranging from −0.38‰ ± 0.03‰ to −1.43‰ ± 0.11‰ in P. inermis and −1.33‰ ± 0.10‰ to −2.18‰ ± 0.05‰ in C. flexuosus under changing Fe and Cu concentrations. Our findings confirm that Cu and Fe interactions significantly affect morphological and physiological parameters in diatoms, particularly Si(OH)4 affinity and Si isotope fractionation. This work highlights the need for further investigation into these synergistic effects, as they can influence diatom production, which may, in turn, impact community composition dynamics in the Southern Ocean.