The search for a better understanding of why cyanobacteria often dominate phytoplankton communities in eutrophic freshwater ecosystems has led to a growing interest in the interactions between cyanobacteria and bacteria. Against this background, we studied the location of bacteria within Microcystis colonies, and compared the structural and phylogenetic diversity of Microcystis-attached and free-living bacterial communities living in the same French lake, the Villerest reservoir. Using transmission electron microscopy, we show that most of the bacteria inside the colonies were located close to detrital materials that probably resulted from lysis of Microcystis cells. The 16S rRNA sequencing approach revealed a clear distinction between the attached and free-living communities at the levels of both their general structure and their operational taxonomic unit (OTU) composition. In particular, Microcystis colonies appeared to be depleted of Actinobacteria, but conversely enriched in Gammaproteobacteria, in particular when the bloom was declining. At the OTU level, a clear distinction was also found between attached and free-living bacteria, and new clades were identified among our sequences. All these findings suggest that Microcystis colonies constitute a distinct habitat for bacteria living in freshwater ecosystems, and that direct and indirect interactions (cell lysis, nutrient recycling, etc.) may occur between them inside these colonies.
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