| Summary: | Trabajo presentado en la IPY 2012 Conference (From Knowledge to Action), celebrada en montreal del 22 al 27 de abril de 2012. A comparative analysis of heterotrophic bacteria, phytoplankton and nanograzers distribution in Drake passage (a region where data are scarce) and Bransfield Strait (Antarctic Ocean) was made by using flow citometry during COUPLING cruise (January 2010). This cruise was focused on the analysis of physical-biological coupling at the mesoscale range, studying hydrodynamics, water attributes and plankton structure around the South Shetland Islands (SSI). An improvement of the understanding of microbial system in the context of the carbon flux in polar regions is crucial, as climate chan ge may have the fastest impact in these areas. In the present work, we describe the spatial distribution of the different assemblages that make up the microbial community. We grouped the st ations based on the depth integrated abundances using cluster anal ysis. Physical features and nutr ients concentrations were related to the clusters found. Two bacterial subpopulations were discriminated after SYTO-13 staining based on their different green fluorescence (FL1) signal: low nucleic acid content (LNA) and high nucleic acid content (HNA). Total bacterial numbers varied from 0.21 to 4.21 x10 5 Cells/mL, HNA comprised 56.8 % (± 7.6) of all bacteria. Four groups of eukaryotic autotrophic cells and a pool of nanoheterotrophs were detected using the wide angle light scatter signal (SSC) and orange and red fluorescence (FL2 and FL3, respectively). These groups had different distributions in the Drake Passage, around the SSI platform, in the Bransfield Strait and in the Antarctic Sound. In this latter area the microplankton (>20 m) size fraction predominated, reaching also the highest Chl a concentrations. The only phytoplankton group present in all stations was “ Nano medium ” (5.2 μm mean Equivalent Spherical Diameter (ESD)) being also the most abundant (5.3 x10 11 ( Cells/m 2 )). This group represented 75.9% ...
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