Selection and propagation of IncP conjugative plasmids following long-term anthropogenic metal pollution in river sediments

For a century, the MetalEurop foundry released metals into the river ``La Deule''. Previous work revealed higher microbial diversity in metal impacted sediments, and horizontal gene transfer mediated by conjugative plasmids was suggested to drive the community adaptation to metals. We used an integrative state-of-the-art molecular approach coupling quantitative PCR, conjugation assays, flow cytometry, fluorescence activated cell sorting and 16S rRNA gene amplicon sequencing to investigate the presence of conjugative plasmids and their propagation patterns in sediment microbiomes. We highlighted the existence of a native broad-host range IncP conjugative plasmid population in polluted sediments, confirming their ecological importance for microbial adaptation. However, despite incompatibilities and decreased transfer frequencies with our own alien IncP plasmid, we evidenced that a wide diversity of bacterial members was still prone to uptake the plasmid, indicating that sediment microbial communities are still inclined to receive conjugative plasmids from the same group. We observed that metal pollution favoured exogenous plasmid transfer to specific metal-selected bacteria, which are likely coming from upstream sources (e.g. wastewater treatment plant, farms...). Altogether, our results suggest that MetalEurop sediments are hotspots for gene transfer via plasmids, acting as an ``environmental reservoir'' for microbes and mobile elements released by human activities.