From Grazer Control to Carbon Export: Contrasting the Role of Synechococcus and Prochlorococcus in the Sargasso Sea.

The picocyanobacteria Synechococcus and Prochlorococcus dominate the primary production in the Sargasso Sea, with Synechococcus being more abundant in the winter, and Prochlorococcus more abundant in the summer. With the ultimate goal of finding a link between the trophic dynamics and the role in carbon export of these cyanobacteria, we determined their growth and grazing rates using dilution experiments and their relative and absolute contribution to the particulate organic carbon (POC) export using DNA-based techniques (454 pyrosequencing targeting 16S rDNA V4 region, and quantitative Polymerase Chain Reaction targeting 23-16 rDNA internally transcribed spacer region) in winter/spring and summer of 2012 in the Sargasso Sea. Our main hypothesis was that in a system where cyanobacteria are not tightly controlled by micro-grazers (e.g., growth exceeding grazing), their carbon export would be higher compared to a scenario where they are tightly controlled by micro-grazers, because more of their primary production would be available for direct mesozooplankton grazing and/or export via aggregate formation and sinking. We found indeed that during summer when Synechococcus was tightly controlled by microzoooplankton grazing, its contribution to the total carbon export was lower (2%) compared to spring 2012 when its growth far exceeded grazing rates and Synechococcus contributed between 10 and 14% to the total POC export. On the other hand, Prochlorococcus’ growth and grazing rates were closely matched during both seasons, and these cyanobacteria contributed to the export of carbon less than 1%. The results of our study establish for the first time a clear link between the microbial trophic dynamics in the euphotic zone and the export of these cyanobacteria in oligotrophic oceans.