Mesoscale eddies: Hotspots of prokaryotic activity and differential community structure in the ocean

Abstract

To investigate the effects of mesoscale eddies on prokaryotic assemblage structure and activity, we\nsampled two cyclonic eddies (CEs) and two anticyclonic eddies (AEs) in the permanent eddy-field\ndownstream the Canary Islands. The eddy stations were compared with two far-field (FF) stations\nlocated also in the Canary Current, but outside the influence of the eddy field. The distribution of\nprokaryotic abundance (PA), bulk prokaryotic heterotrophic activity (PHA), various indicators of\nsingle-cell activity (such as nucleic acid content, proportion of live cells, and fraction of cells\nactively incorporating leucine), as well as bacterial and archaeal community structure were\ndetermined from the surface to 2000m depth. In the upper epipelagic layer (0-200 m), the effect of\neddies on the prokaryotic community was more apparent, as indicated by the higher PA, PHA,\nfraction of living cells, and percentage of active cells incorporating leucine within eddies than at FF\nstations. Prokaryotic community composition differed also between eddy and FF stations in the\nepipelagic layer. In the mesopelagic layer (200-1000 m), there were also significant differences in PA\nand PHA between eddy and FF stations, although in general, there were no clear differences in\ncommunity composition or single-cell activity. The effects on prokaryotic activity and community\nstructure were stronger in AE than CE, decreasing with depth in both types of eddies. Overall, both\ntypes of eddies show distinct community compositions (as compared with FF in the epipelagic), and\nrepresent oceanic ?hotspots? of prokaryotic activity (in the epi- and mesopelagic realms).