Effects of Iron limitation on Silicon Metabolism and Silicon Isotopic Discrimination in Southern Ocean Diatoms
Creators: name: Meyerink, Scott William
Publisher: name: The Australian National University
Year: 2016
Subjects: Southern Ocean
Diatom
Silicon isotopes
Silicon
Silicon uptake kinetics
E. antarctica
P. inermis

description: The marine biogeochemical cycle of Silicon (Si) in the Southern Ocean is effectively ‘decoupled’ from that of Carbon (C) and Nitrogen (N). This is because the lack of bio- available iron (Fe) in the region prevents the complete utilisation of Nitrate (NO3-) relative to silicic acid (Si(OH)4) by diatoms in Antarctic surface waters. Consequently, higher Si:N and Si:C ratios in Antarctic diatoms results in the preferential export of Si(OH)4 from surface waters relative to NO3-, leading to surface waters north of the Antarctic Polar Frontal Zone becoming depleted with respect to Si(OH)4. Regional variations in the export flux of Si and C in the Southern Ocean has the potential to alter global atmospheric CO2 concentrations and marine Si(OH)4 inventories. However, the ability to make predictions on how these fluxes will vary in response to future climate change is confounded by the general lack of knowledge on the mechanisms behind how Si and C metabolisms in Southern Ocean diatoms change in response to variations in the physico-chemical environment. This study attempts to determine how Southern Ocean diatom physiology changes in response to chronic Fe-limitation. Specifically, attempts are made to elucidate mechanisms driving variations in cell morphology, elemental stoichiometry, Si(OH)4 uptake kinetics and Si-isotope fractionation in two Southern Ocean diatoms (Probocia inermis and Eucampia antarctica) and the coastal isolate, Thalassiosira pseudonana. All diatoms cultured under Fe-limiting conditions exhibited respective decreases in cellular growth rate, C and N content and maximal Si(OH)4 uptake (VSi-max), whilst responses varied in cell morphology and biogenic silica (BSi) content. T. pseudonana exhibited little variation in cell volume and BSi content under varying Fe- concentrations, while both Southern Ocean diatoms increased their respective cell surface area and cell volume in response to Fe-limiting conditions. BSi content on a cell-surface area basis in both Southern Ocean diatoms either did not change (P. inermis) or decreased (E. antarctica), while the half saturation constant for Si(OH)4 uptake (KSi) in E. antarctica decreased in response to reductions in VSi-max. Although there were no visible relationships between variations in VSi-max and KSi with variations in cell volume or cell surface area; reductions in VSi-max were observed to scale linearly with reductions in cellular growth rate in all species. Si-isotope fractionation appeared independent of variations in extra-cellular Fe-concentrations in T. pseudonana, however, both Southern Ocean species exhibited variations in the Si-isotope fractionation factor (ε) in response to Fe-limitation. Potential mechanisms as to why this occurs could be related to the specific affinity for Si(OH)4 in Southern Ocean diatoms compared to coastal diatoms such as T. pseudonana, and it is likely that Si-isotope fractionation in Southern Ocean diatoms, which are endemic to high-Si environments; is more sensitive to variations in Fe-supply. In addition, physiological variations in diatoms as a result of Fe-limitation are likely to have multiple effects on Antarctic food webs and Si:N/Si:C export ratios in the Southern Ocean. Results from these in-vitro studies suggest that growth rate, rather than the surface to volume (S/V) ratio should be related to kinetic parameters relating to Si(OH)4 uptake in biogeochemical models of planktonic ecosystems. Finally, a mesocosm experiment was conducted in subtropical waters east of New Zealand to put findings from in-vitro studies into context. Si-isotope fractionation of the resident phytoplankton community in the mesocosm exhibited classical Rayleigh-style closed-system fractionation kinetics, and Si-isotope fractionation factor (ε) of -1.13 ‰ was calculated. Comparisons between results from the mesocosm and observations from the phytoplankton community in surface waters suggest microbial control of the Fe inventory and diatom community likely limits the Si-isotope composition (δ30Si) for diatom BSi in surface waters.