Clade and strain specific contributions of Synechococcus and Prochlorococcus to carbon export in the Sargasso Sea

The marine picocyanobacteria Prochlorococcus and Synechococcus dominate primary production in oligotrophic oceans, but their role in the particulate organic carbon (POC) flux into the deeper ocean remains unresolved. Here, we present for the first-time data on the absolute clade and strain-specific contribution to POC flux of two of the most abundant clades of Synechococcus (II and III) and strains of Prochlorococcus (high-light strain MIT 9312 and low-light strain MIT 9313) in the Sargasso Sea, by targeting the 23S-16S rDNA internally transcribed spacer region using quantitative Polymerase Chain Reaction. We collected seawater samples and sinking particles using shallow sediment traps during winter and summer 2012 around the Bermuda Atlantic Time Series station. Both clades of Synechococcus had a higher contribution to the total POC flux during winter compared to the summer, with both clades contributing up to 2.7% to the total winter POC flux. Prochlorococcus was more abundant during the summer, but both strains of Prochlorococcus contributed less than 0.2% to the POC flux in both seasons. We found these differences in the contribution to flux to be mainly due to the smaller size of the Prochlorococcus. In addition, the flux of Synechococcus clade III, and Prochlorococcus MIT9312 was enhanced relative to their standing stock in a cyclonic eddy sampled during winter 2012. Using quantification of genetic markers, this study establishes distinct clade-and strain specific patterns in POC flux among the most abundant picophytoplankton groups in the Atlantic Ocean.