The role of Synechococcus and Prochlorococcus in the biogeochemistry of the oligotrophic oceans

The picocyanobacteria Synechococcus and Prochlorococcus dominate primary production in oligotrophic ocean regions, but several lines of evidence show that there are stark differences in their role in ocean biogeochemistry. Among phytoplankton, Synechococcus were found to be overrepresented in particles compared to the water column in the Sargasso Sea, in contrast to Prochlorococcus, which were underrepresented. Additionally, in a global regression-based modelling analysis of metagenomics data collected during the Tara Oceans expedition, Synechococcus were found to be highly correlated with carbon export in the subtropical oligotrophic ocean, in contrast to Prochlorococcus. Synechococcus are also often found in zooplankton fecal pellets, but not Prochlorococcus. We can now show from laboratory studies of representatives of these cyanobacteria that Synechococcus is able to produce transparent exopolymeric particles (TEP) in axenic cultures that aid them in aggregation, but that Prochlorococcus needs accompanying bacteria to produce TEP and to aggregate. We will discuss how these differences in their physiology might affect aggregation in the open ocean as well as prey utilization by zooplankton, and ultimately their contribution to export production.