The Role of Acantharia in Southern Ocean Strontium Cycling and Carbon Export: Insights From Dissolved Strontium Concentrations and Seasonal Flux Patterns

Dissolved strontium (Sr) concentrations from Southern Ocean water samples and Sr export fluxes from sediment trap moorings at 1,000 m were used to assess particulate organic carbon (POC) export associated with Acantharia for 2010, 2018, and 2020. The dissolved Sr data revealed a prominent vertical gradient with lower surface Sr concentrations depleted up to 1.5% relative to deep waters. A strong latitudinal surface gradient was observed, ranging from 86.8 mu mol kg-1 near the northern end to 88.0 mu mol kg-1 near the southern end of a transect through the Australian sector of the Southern Ocean. The vertical and latitudinal gradients are associated with celestite (SrSO4) precipitation by Acantharia and subsequent export to depth. Seasonal variability in Sr export fluxes can be large, particularly during intense events in summer, and reaches a maximum of 11.7 mmol Sr m-2 yr-1, contributing up to 7% of the POC export flux. The coincidence of Sr flux with the second peak of POC export flux implies an association of Acantharia biomass with summertime productivity. Acantharia are tiny marine organisms found worldwide. Their skeletons, composed of a heavy mineral called strontium sulfate (SrSO4, or celestite), make Acantharia important in the oceanic strontium cycling. They also form cysts by resorbing their skeletons and main cellular components, allowing them to sink quickly to depths for reproduction. Despite their dense skeletons and cysts, which could act as effective ballasts, Acantharia were previously thought to have little impact on carbon export in the deeper ocean layers due to their high solubility in seawater. Our study focused on understanding how Acantharia influence strontium cycling and contribute to carbon export in the Southern Ocean. We discovered that Acantharia efficiently remove strontium from the ocean's surface and release it at intermediate depths. These organisms exhibit seasonal variations and are particularly abundant in summer, contributing up to 7% to POC export during the summer productive season. This is significant for the Southern Ocean as a major region for carbon sequestration and global climate buffering. Strong vertical and latitudinal gradients of dissolved Sr observed in the Southern Ocean were driven by Acantharia Sr export fluxes exhibited seasonal variations with peaks occurring annually in summer, contributing up to 7% to particulate organic carbon flux Elevated Acantharia biomass is possibly associated with summertime productivity