The role of plankton biodiversity in the South Brazil Bight’s biological carbon pump

In the ocean biological pump marine phytoplankton use dissolved CO2 to build up biomass. A small portion of this biomass sinks to deeper layers and is eventually remineralized, storing carbon for extended periods in deeper depths. Besides phytoplankton, other planktonic organisms contribute to the biological pump, with some of them producing rigid structures mostly made of silica or calcium carbonate, which do not undergo bacterial degradation as they sink. Sediment traps, used since the 1980s to capture sinking matter and study the carbon flux, have only recently been applied in the South Brazil Bight, a region of economic and environmental importance.The diversity of planktonic hard structures collected by sediment traps in the South Brazil Bight was investigated for the first time. Diatoms were a highly significant component, contributing to peaks in the vertical flux. Calcareous dinoflagellate cysts were also relevant, especially during low diatom fluxes. Other structures, such as foraminifera shells, radiolarian skeletons and fecal pellets, were diverse but less abundant. The diversity found in the samples reveals the complexity of organisms maintaining the biological pump. Early biogeochemical models classified plankton as primary producers or consumers. As our understanding develops, models need to describe roles such as mixotrophy, ballast particle production and symbiotic relationships. Identifying these organisms is crucial for developing more refined biogeochemical models.