Metal contamination poses biotoxicity and bioaccumulation issues, affecting both abiotic conditions and biological activity in ecosystems. For a century, the MetalEurop foundry released zinc, copper, cadmium and lead directly into the river "la Deûle", resulting in a 30-fold increase in metal concentrations in downstream sediments compared to upstream. We used an integrative approach coupling in situ sequencing of both DNA and RNA, Horizontal Gene Transfer analysis (HGT), and microcosm monitoring in order to fully understand the mechanisms driving community adaptation to metal pollution. We applied the ecological concept of Functional Response Groups (FRGs) to decipher the adaptive tolerance range of the sediment communities through characterization of microbial strategists, revealing in situ differences in diversity and composition. Deeper HGT analysis using qPCR and in vitro conjugation assays coupled to Fluorescence Activated Cell Sorting (FACS) revealed the presence of an enriched native pool of conjugative plasmids in the polluted sediments, highlighting their importance for longterm adaptation of the community facing metal contamination. Furthermore, in vitro microcosms analysis with upstream non-polluted sediments challenged with metals and daily supplied of fresh river water allowed to monitor the short-term impact of metal pollution on the microbial community over 6 months in controlled condition using 16S rDNA amplicon sequencing and the ecological concept of Time Response Groups (TRGs). Altogether, our integrative study reinforced the notion that microbial strategies were established to face the me