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Article, The capability of different bacterial populations to degrade abundant polymers, such as algal-derived polysaccharides, or to utilize prefentially polymers over monomers, remains largely unknown. In this study, microautoradiaography was combined with fluorescence in situ hybridization (MAR-FISH) to evaluate the ability of Bacterioidetes, SAR11, Roseobacter spp, Gammaproteobacteria and SAR86 cells to use bicarbonate, leucine and starch under natural light conditions at two locations in shelf surface waters off NW Spain. The percentage of cells incorporating bicarbonate was relatively high (mean 32% +- 4%) and was positively correlated with the intensity of solar radiation. The proportion of cells using starch (mean 56% +- 4%) or leucine (mean 47% +- 4%) was significantly higher than that using bicarbonate. On average, SAR11, Roseobacter spp and Gammaproteobacteria showed a similarly high percentage of cells using leucine (47%-65% of hybridized cells) than using starch (51%-64% of hybridized cells) while Bacteroidetes and SAR86 cells preferentially used starch (53% of hybridized cells) over leucine (34%-40% of hybridized cells). We suggest that the great percentage of bacteria using starch is related to a high ambient availability of polymers associated to algal cell lysis, which, in turn, weakens the short-term coupling between phytoplankton release and bacterial production., 5,843