Coupled changes in the cell morphology and elemental (C, N, and Si) composition of the pennate diatom Pseudo‐nitzschia due to iron deficiency

Abstract

We investigated the changes in cell morphology and elemental composition with varying iron nutritional status in four oceanic and two coastal isolates of the marine pennate diatom Pseudo‐nitzschia. When iron‐deficient, most isolates exhibited slower specific growth rates (m), reduced maximum photochemical yields (WM), and lower chlorophyll a (Chl a) contents than iron‐replete cultures. Iron‐deficient cells exhibited reduced cell volumes, primarily as the result of decreased valve transapical widths, which increased the valve aspect ratios. The increase in aspect ratios varied among isolates and was not correlated with the degree of growth limitation due to irondeficiency. In all Pseudo‐nitzschia isolates, the mean carbon (Ccell), nitrogen (Ncell), and silicon (Sicell) cell quotas of iron‐deficient cells decreased when compared to iron‐replete cells. Similarly, in five out of the six isolates, the mean C and N quotas normalized per unit cell volume (Cvol and Nvol) of iron‐deficient cells also decreased, but by a lesser extent. In contrast, there were no clear differences in the changes in the mean Si quotas normalized per unit cell surface area (Sisa). The Sicell:Ncell and Sicell : Ccell ratios (quotas normalized per cell) of iron‐deficient cells increased compared to iron‐replete conditions due to greater reductions in C and N cell quotas compared to Si cell quotas. This increase in the Sicell:Ncell ratio was proportional to the changes in the valve aspect ratios. Our results show that iron‐deficient Pseudo‐nitzschia cells acclimate to low iron concentrations by increasing their valve aspect ratios, thus reducing their cell volumes and increasing their surface area‐to‐volume (SA : V) ratios. These changes in cell morphology increase the Si‐containing valve surface area relative to the volume of the internal components, which may then influence the cellular elemental composition, in particular, the Si :N and Si :C ratios.