Using Silicon Isotopes to Trace the Biogenic Silica in the Southern Ocean

Abstract

The diatoms, a unique group of primary producers, are inextricably linked to the marine silicon (Si) and carbon (C) cycles. Diatoms are regarded as unicellular, eukaryotic organisms that form biogenic silica (bSi) frustules. The use of stable isotopes and physicochemical factors (temperature, light, and trace metal) provides a promising method for understanding how Si and C fluxes will change in response to future climate change. The Si cycle in the Southern Ocean was investigated using an isotopic approach in incubation experiments and the natural environment. This study provides new measurements of the Si isotope composition of biogenic silica in a natural phytoplankton community and for multiple species isolated from the Southern Ocean and grown within a laboratory setting. During the annual phytoplankton bloom in the austral spring of 2018, a field study was conducted. This study compared high productivity during an austral spring bloom in the East Australian Current (EAC) to low productivity in the subantarctic region. A clear trend of Si isotope fractionation toward heavier isotope values at the surface with a decrease in in-depth was observed in the EAC with an estimated fractionation factor of -1.8 +/- 0.5per mil.. These findings support the use of bSi isotopic composition as a tool for characterising and improving our understanding of the Si biogeochemical cycle during a spring bloom. In addition, an 8-day shipboard incubation experiment investigated the role of light intensity (Low light, UV-Filtered, High light, and Dark) on a mixed plankton community in the EAC to better understand Si biogeochemical cycling and the effect of light intensity. The light had a clear interaction with the growth, stoichiometric composition, photosynthetic parameters, and increased chlorophyll a (Chl a) and bSi compared to the Dark treatment. A slight increase in the Si isotope composition of bSi for low light and UV-filtered treatments (Day 8) was observed. Due to the low utilisation of dissolved Si by the phytoplankton community in the different light treatments, a closed-system Rayleigh fractionation kinetics model could not be applied. We also investigated how Fe limitation and if predicted ocean warming (from 3 to 5 degrees) affects phytoplankton physiology, nutrient stoichiometry, Si uptake kinetics, and Si isotope fractionation in model diatoms. An increase in temperature resulted in an increase in growth rate for all the diatoms under Fe-replete conditions while Fe limitation resulted in a decrease in growth rate. Both bSi and chlorophyll concentration increased per cell volume for all the cultures when the temperature was increased from 3 to 5 degrees in the Fe-replete conditions. Temperature also influenced affected Si kinetic uptake by decreasing the maximal Si uptake rate (Vmax) for both the Fe replete and Fe limited Chaetoceros flexuosus. For Thalasiossira antarctica temperature had the opposite effect on Vmax. Silicon isotope fractionation for all the species became more negative when the temperature was increased. The effects of Fe, Zn, and Co co-limitation (-Fe, -Zn and -Co) on C. flexuosus and C. neogracilis, and any influence on the silicon isotope fractionation factor was investigated. C. neogracilis was able to replace the Co with Zn. Trace metal limitation increased the Si content for C. flexuosus, while Fe and Co limitation increased the Si content for C. neogracilis. Under trace metal deficiency and ocean warming scenario, it appears that cells will become enriched with bSi thus increasing ballast, potentially leading to increased C export to the deep ocean while simultaneously lowering the Si concentration in the surface waters driving towards Si limitation. These findings demonstrate that light intensity, warming of the ocean and deficiency of trace metals can affect cellular physiology and silicon isotope fractionation of Southern Ocean diatoms. Show more