Effects of iron limitation on silicon uptake kinetics and elemental stoichiometry in two Southern Ocean diatoms, Eucampia antarctica and 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.

Fe and Nutrients in Coastal Antarctic Streams: Implications for Primary Production in the Ross Sea

“…It was noted, however, that such fertilization experiments only yielded a temporary biological response to increased Fe and that transport of fixed carbon to depth is required for long‐term carbon sequestration from this enhanced primary productivity (Watson et al, ). Several studies have confirmed the earlier investigations on the importance of Fe in helping to drive primary production in Antarctic waters (Arrigo & van Dijken, ; Bertrand et al, ; Cassar et al, ; De Jong et al, ; Fennel et al, ; Smetacek et al, ; Tagliabue & Arrigo, ), as well as having an impact on the structure of the pelagic ecosystem (Hare et al, ; Meyerink et al, ). More recently, a review of metal uptake by phytoplankton throughout the global oceans showed that diatoms from the Southern Ocean have a lower Fe quota than some diatoms isolated from coastal and oligotrophic environments, which may be a strategy that has developed to cope with low Fe concentrations (Twining & Baines, ).…”

Iron Availability Influences Silicon Isotope Fractionation in Two Southern Ocean Diatoms (Proboscia inermis and Eucampia antarctica) and a Coastal Diatom (Thalassiosira pseudonana)

“…This is the first study to specifically investigate the effects of Felimitation on Si-isotope fractionation in diatoms. Previous work has demonstrated that Fe-limitation can alter Si(OH) 4 uptake kinetics, cell morphology and nutrient stoichiometry in diatoms (Hutchins and Bruland, 1998;Takeda, 1998;Brzezinski et al, 2002;Leynaert et al, 2004;Meyerink et al, 2017). Here, the processes by which Si isotopes are fractionated during diatom Si uptake and cell wall synthesis are explored and related back to mechanisms involving Si(OH) 4 uptake.…”

“…Diatoms in the Southern Ocean experience rapid fluctuations in both physical and chemical conditions associated with the variations in sea ice extent, deep winter mixing and alterations in nutrient supply (Sackett et al, 2013). In the short term, these extremes in physico-chemical conditions can potentially influence Si isotope fractionation in diatoms through variations in cell morphology, growth rate and nutrient uptake rate (Hutchins and Bruland, 1998;Takeda, 1998;Brzezinski et al, 2002;Leynaert et al, 2004;Meyerink et al, 2017). Field studies show that Si isotopic fractionation by siliceous phytoplankton varies across the Antarctic Polar Frontal Zone (APFZ), and coincides with the presence of a strong northward gradient in Si(OH) 4 concentrations (Cardinal et al, 2007;Fripiat et al, 2011).…”

“…Iron (Fe) availability affects the growth rate, Si uptake, cell morphology and BSi content in diatoms (Leynaert et al, 2004;Hoffmann et al, 2008;Marchetti and Cassar, 2009;Boutorh et al, 2016;Meyerink et al, 2017). These microscale effects can have far reaching consequences on the physico-chemical environment, particularly in the Southern Ocean.…”

Effects of iron and light availability on phytoplankton photosynthetic properties in the Ross Sea

“…Using measurements of Fe-limited and Fe-replete growth together with active fluorescence techniques, this photoacclimation strategy has been documented using phytoplankton cultures from throughout the Southern Ocean , Luxem et al 2017, Meyerink et al 2017, Moreno et al 2018) and from mixed assemblages sampled in the Ross Sea (Ryan-Keogh et al 2017a) and Weddell Sea (Hoppe et al 2013). Strzepek et al (2012) was the first to show that the ratio of Fe-limited to Fe-replete growth (μ −Fe /μ +Fe ) in Southern Ocean phytoplankton cultures did not decrease at low light, as would be expected if their photoacclimation strategy increased their Fe demand.…”