In a sea of microbes, eddy events trigger diatom export in the Sargasso Sea

Sinking particles are important conduits of organic carbon from the euphotic zone to the deep ocean, but their origin and community composition are still a matter of investigation. Events in the northwestern Sargasso Sea, such as winter convective mixing, summer stratification, and mesoscale eddies, affect the vertical and temporal composition and abundance of pelagic and particle-attached microorganisms. We sampled the euphotic zone and collected sinking particles using shallow traps near the Bermuda Atlantic Time-series Study site during the spring and summer of 2012 to assess eddy-driven impact on microbial communities. In the spring, we sampled a cyclonic eddy, while in the summer, we targeted both the center and edge of an anticyclonic eddy. Prokaryotic and photoautotrophic (plastid and cyanobacteria) communities were analyzed using V4-V5 amplicons of the 16S rRNA gene. Community and clustering analysis of prokaryotes revealed a clear separation between seawater and particles samples. However, the same was not observed for photoautotrophs. Indicator species analysis showed that small phytoplankton taxa dominated particle communities. Interestingly, differential abundance analyses revealed that the large centric diatom, Rhizosolenia, generally rare in the oligotrophic Sargasso Sea, was enriched in the photoautotrophic communities of sinking particles collected in the center of the anticyclonic eddy with unusual upwelling due to eddy-wind interactions. We hypothesize that the steady contribution of small-celled phytoplankton to particle flux is punctuated by pulses of production and flux of larger-sized phytoplankton in response to episodic eddy upwelling events and can lead to higher export of particulate organic matter during the summer.