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Bacterial colonization of transparent exopolymeric particles in mesocosms under different turbulence intensities and nutrient conditions

We examined bacterial colonization of transparent exopolymeric particles (TEP) in nutrient-manipulated mesocosms subjected to different turbulence intensities. We quantified hetero- trophic bacterial abundance, production and diversity using fluorescence in situ hybridization (FISH). The abundance of attached bacteria increased significantly with TEP size in all treatments. The slope values of the number of bacteria scaled with TEP size varied from 1.15 on Day 1 to 2.04 on Day 11 in still and turbulent tanks, respectively. Comparison between these slopes indicated that the number of attached bacteria for a given TEP diameter was higher under turbulence. The addition of nutrients and turbulence in mesocosms neither supported higher bacterial abundance nor resulted in a corre- lation between bacterial concentrations and TEP. However, in enriched turbulent tanks, the number of attached bacteria was higher and was linked to rapid TEP formation after the diatom bloom. This was accompanied by higher bacterial production (1.21 µmol C l -1 d -1 at the intermediate level of tur- bulence on Day 9), which was positively related to TEP. These correlations highlight the finding that higher bacterial productivity was due to a larger fraction of attached bacteria (up to 56.4%). Changes in bacterial production occurred with simultaneous changes in the community structure, i.e. the per- centages of the Cytophaga-Flavobacteria-Bacteroides cluster increased significantly. By Day 9, this cluster became the dominant group of bacteria attached to TEP (85 and 90%, respectively at interme- diate and high levels of turbulence). Among the marine heterotrophic bacterioplankton, this phylum is known to be common as attached bacteria. Therefore, it seems that the added nutrients and subse- quent TEP production can be used by bacteria more efficiently under turbulent conditions through a shift in group-specific composition towards more attached bacteria with higher production rates.