A Gulf Stream cold-core eddy harbors a distinct microbiome compared to environmentally-similar adjacent waters

Marine microbiome community assembly and consequently biogeochemical cycling are shaped by past and current environments, as well as stochasticity, dispersal etc. To better understand the processes that structure marine microbial communities, we examine a cyclonic, Gulf Stream frontal eddy, whose microbiome potentially contains signatures of both the shelf community entrained during formation as well as current environmental conditions. In spite of the potential for eddy-driven upwelling to introduce nutrients and stimulate primary production, the eddy surface waters exhibit lower chlorophyll a along with a distinct and less even microbial community relative to the neighboring Gulf Stream waters. Although the eddy microbiome is distinct from the Gulf Stream's, especially in cyanobacteria (e.g. lower Trichodesmium and higher Prochlorococcus), it is most similar to the Gulf Stream, suggesting eddy microbiome assembly favors environmental filtering over historical contingencies. However, better delineation of the relative roles of ecological processes will likely require Lagrangian sampling throughout the photic zone to clarify the contribution of these mesoscale features to primary production and export as well as identifying the key processes driving microbiome assembly in the oceans. ### Competing Interest Statement The authors have declared no competing interest.