The energy transition requires the extraction and use of large amounts of cobalt (Co) leading to its concentration into the environment. It is thus crucial to better assess its impact on ecosystems. Although little used in chemical risk assessment, prokaryotes play an essential role in the proper functioning of rivers. The aim of this study is therefore to examine how Co affects their structural and functional organization in aquatic biofilms. To that end, artificial rivers were contaminated with Co (0.1, 0.5 and 1 µM) and biofilm formation on artificial substrates was monitored for 28 days. After this exposure period, a Recovery period without contamination was observed for 5 weeks in order to evaluate the biofilms resilience. Water samples were collected every 7 days during Co injection to measure its main physico-chemical parameters. Biofilms were also collected to examine Co bioaccumulation and, their structure and functional potential through metabarcoding. Cobalt accumulation in biofilms was mainly intracellular and well correlated with the concentrations tested. The presence of Co at concentrations of 0.5 and 1 µM increased the abundance of prokaryotes from the 21st day of exposure. These Co concentrations also highly impacted their diversity from the very early stages of colonization. Indeed, Cyanobacteria were sensitive and did not colonize the biofilm support from day 7 for Co concentrations above 0.1 µM. In contrast, Bacteroidota appeared resistant all through the biofilm colonization and dominated Co contaminated biofilms. Relative abundance of planctomycetes class was also impacted after 14 days at 0.5 and 1 µM of Co. Analysis of the functional potential using PiCrust is in progress. Initial results revealed that Co at 0.5 and 1 µM can impact proportions of genes involved in cell cycle, biofilm formation and photosynthetic potential. The obtained results will improve our understanding of Co effects on aquatic ecosystems.