Enhanced Synechococcus export associated with diatom sinking

Phytoplankton sinking represents a major component of oceanic biological carbon sequestration, commonly referred to as the biological carbon pump (BCP). Despite their small size and lack of mineral ballast, pico-sized cyanobacteria are transported to the deep ocean and contribute to carbon sequestration. However, the mechanisms governing their sinking remain poorly understood. To address this knowledge gap, we investigated sinking eukaryotic and cyanobacterial communities and their DNA-based fluxes in zooplankton fecal pellets and non-fecal particles using metabarcoding and qPCR analyses targeting the 16S and 18S rRNA genes. Cyanobacteria, mainly Synechococcus subcluster 5.1 clade I, accounted for 3.11%-46.2% of phytoplankton reads in sinking particles, with their contribution and flux peaking during the spring diatom bloom. Diatoms were the largest contributors among phytoplankton and a dominant component of the eukaryotic assemblages in sinking particles. A substantial proportion of the sinking cyanobacterial flux was observed in the non-fecal particle fraction. The cyanobacterial sinking flux was not associated with satellite-derived surface cyanobacterial biomass but was significantly and positively correlated with the diatom sinking flux. These results suggest that the incorporation of cyanobacteria into diatom-containing aggregates represents a major pathway by which they contribute to the BCP, especially during the spring bloom. Our findings demonstrate positive feedback of diatom biomass on the BCP and highlight that modulation of interactions among phytoplankton groups regulates cyanobacterial sinking.