Sinking Cells in the Twilight Zone and Their Contribution to Carbon Export

The settling of individual phytoplankton cells is one presumed pathway of carbon export, but its relative importance remains unquantified. The assumption based on Stokes law of settling is that individual plankton cells (10-1000 micrometers), which originate in the euphotic zone, should be incapable of settling to the depths of the mesopelagic before being consumed or degraded. Nevertheless, small solo cells (e.g. diatoms, coccolithophores, heterotrophic protists) arrived in gel layers placed in 30 sediment traps placed throughout the upper 500m during the NASA EXPORTS field campaign in the North Pacific. Cells may have arrived in the traps after disaggregation of larger sinking particles, through their own ballast, or as a result of vertical advective processes and we assessed which of these mechanisms was the most likely at this location. We identified principal diatoms-- large heavy Rhizosolenia and small ubiquitous Thalassiosira--as key exporters of organic carbon. While phytoplankton directly exported unprocessed carbon from the surface, mesopelagic protists, such as large phaeodarians, reprocessed a portion of surface-derived detritus, produced minipellets, and also sank as individual cells. Several recent studies suggest that large rhizaria are important intermediaries in the vertical flux of material from the euphotic zone to the deep ocean. Here, we explored the role of these heterotrophic protists by offering direct observations of rhizaria contributions to carbon export. We also quantified the relative contribution of different cell types to total particulate organic carbon flux. Each cell type attenuated carbon differently in the mesopelagic; here we identify species-specific patterns of flux. If these cells are settling by themselves, they would represent a direct and highly efficient export pathway, as they effectively bypass carbon losses associated with intermediary trophic interactions.