Abstract
The fractionation of silicon (Si) isotopes was measured in two Southern Ocean diatoms (Proboscia inermis and Eucampia Antarctica) and a coastal diatom (Thalassiosira pseudonana) that were grown under varying iron (Fe) concentrations. Varying Fe concentrations had no effect on the Si isotope enrichment factor (e) in T. pseudonana, whilst E. antarctica and P. inermis exhibited significant variations in the value of e between Fe-replete and Fe-limited conditions. Mean e values in P. inermis and E. antarctica decreased from (± 1SD) -1.11 ± 0.15‰ and -1.42 ± 0.41‰ (respectively) under Fe-replete conditions, to -1.38 ± 0.27‰ and -1.57 ± 0.5‰ (respectively) under Fe-limiting conditions. These variations likely arise from adaptations in diatoms arising from the nutrient status of their environment. T. pseudonana is a coastal clone typically accustomed to low Si but high Fe conditions whereas E. antarctica and P. inermis are typically accustomed to High Si, High nitrate low Fe conditions. Growth induced variations in silicic acid (Si(OH)4) uptake arising from Fe-limitation is the likely mechanism leading to Si-isotope variability in E. antarctica and P. inermis. The multiplicative effects of species diversity and resource limitation (e.g. Fe) on Si-isotope fractionation in diatoms can potentially alter the Si-isotope composition of diatom opal in diatamaceous sediments and sea surface Si(OH)4. This work highlights the need for further in-vitro studies into intracellular mechanisms involved in Si(OH)4 uptake, and the associated pathways for Si-isotope fractionation in diatoms.
Highlights
Diatoms play a vital role in the biogeochemical cycles of carbon (C) and silicon (Si)
Antarctica grown under Fe-replete conditions were not statistically different (p > 0.05) from the mean value of −1.1 ‰ obtained by De La Rocha et al (1997); they were statistically different compared to the mean ε value for T. pseudonana (p < 0.05)
Whilst little effect in the variability of the Si-isotope fractionation factor was observed in T. pseudonana in response to Festress (Figure 2B), both Southern Ocean species exhibited a fractionation factor that was more negative under Fe-limitation (Figure 2A)
Summary
Diatoms play a vital role in the biogeochemical cycles of carbon (C) and silicon (Si) They dominate the production of biogenic silica (BSi) in the ocean, and have a controlling influence on the marine Si cycle through the utilization of dissolved silicon [silicic acid, Si(OH)4], which they use in the formation of their cell wall, or frustule (Nelson et al, 1995; Trèguer et al, 1995; Tréguer and De La Rocha, 2013). During Si uptake, diatoms discriminate against heavier isotopes, which changes the isotopic composition of both the diatom opal and the residual Si(OH) pool (De La Rocha et al, 1997; Sutton et al, 2013) This results in a lighter isotope composition (28Si or 29Si vs 30Si) in the frustule, relative to its dissolved Si source.
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