Effects of iron limitation on silicon uptake kinetics and elementalstoichiometry in two Southern Ocean diatoms, Eucampia antarcticaand Proboscia inermis, and the temperate diatom Thalassiosira pseudonana

Abstract

We investigated the effects of iron (Fe) limitation on the elemental stoichiometry, silicic acid (Si(OH)4) uptake kinetics and cell morphology in two Southern Ocean diatoms Eucampia antarctica and Proboscia inermis and the temperate diatom Thalassiosira pseudonana. An increase in Fe‐stress resulted in reductions in specific growth rate and decreases in cellular nitrogen (N) and carbon (C) content relative to cellular biogenic silica (BSi) in both Southern Ocean diatoms and a reduction in growth rate only for T. pseudonana. Both E. antarctica and P. inermis exhibited an increase in cell volume in response to Fe‐limitation resulting in a decrease in the cell surface to volume ratio, while normalization of BSi content to cell surface area suggests these diatoms do not become more heavily silicified under Fe limitation. Kinetic Si(OH)4 uptake experiments performed on all three diatom species show that Si(OH)4 uptake is reduced under Fe‐limited conditions. For Southern Ocean diatoms, this was manifested through a decrease in the maximum specific uptake rate of Si(OH)4 (VSi‐max), along with a decrease in the half‐saturation constant for Si(OH)4 uptake (KSi) under Fe stress for E. antarctica. Our data also show that when normalized to cell surface area, VSi‐max of the three diatoms species exhibited a linear relationship with cellular growth rate, and was independent of cell morphological variations. Our results suggest that the morphological adaptations of Southern Ocean diatoms in response to Fe‐stress have the potential to affect phytoplankton community dynamics and Si(OH)4 : NO3 uptake and export ratios in Southern Ocean waters.