Rhizarians and diatoms: their relative impact in the silicon cycle in the Southern Ocean (Ross Sea)

Silicifiers are a critical component of the global marine silicon (Si) cycle. Until recently, diatoms were considered the main contributors to this cycle. However, novel studies on radiolarians and phaeodarians, which are also typified by siliceous skeletons, have highlighted their potentially substantial impact on the biogeochemical Si cycle. The Southern Ocean, a silica hub characterised by high silicic acid concentrations and contributing around 30% of the world's production of biogenic silica in its surface waters, is an ideal habitat for silicifiers.

During the Tangaroa summer cruise (January and February 2019), in the Ross Sea (eastern flank of the Iselin Bank and Ross gyre), we examined the respective contribution of diatom and Rhizaria to the overall biogenic silica production in this region. We used ³²Si radioisotope to measure the uptake rate of silicic acid by these two groups of silicifiers. Diatom uptake rates were estimated from small volume incubation experiments, which preclude the contribution of large radiolarians, while the radiolarian uptake rates were estimated from specimens manually sorted under the microscope and pooled together in filtered seawater.

First results evidence that rhizarians show very high production rates per cell as compared to diatoms (up to 63 nmol Si cell^-1d^-1). By studying these two groups of siliceous organisms in parallel, and taking into account their abundance in the water column, we calculate their relative contribution to the silica fluxes in Antarctic waters. Moreover, to establish their ecological significance and their role within the planktonic community, we discuss species abundances and biogenic silica production rates in relation to environmental drivers.