Assessing the consequences of chemical contamination on the structure and funtioning of aquatic ecosystems under urban pressure is a challenging task. Indeed, the link between a biological response, for instance measured at the scale of an organism exposed and collected in the field, and chemical contamination remains difficult to establish. In particular, numerous confounding factors and adaptation processes are likely to modify the biological responses registered. Adaptation to a toxic pressure is characterized by physiological acclimatation and/or genetic modifications which lead to an increase of tolerance or resistance of the exposed organism. At the scale of the community, adaptation is also often related to the disappearance of sensitive species. Understanding and evaluating the impacts of the chemical pressure exerted on living organisms in urban areas thus requires the investigation of the biological processes of adaptation at low concentrations of contaminant which are typical to urban areas. The SequAdapt project aims at exploring both the mechanisms and the consequences of adaptation at the scale of the organism (using gammarids as a biological model) and of the community (using river biofilms). The study will focus on metals (in particular Cu, Cd, Ni, Pb et Zn), which are non-biodegradable pollutants and are dispersed from several sources in the Seine basin (wastewater, erosion processes, atmospheric fallouts, etc.). The study is divided in two parts: - part 1 focuses on investigating the mechanisms of adaptation. At the scale of the organism, we aim at investigating the variability of bioaccumulation toxico-kinetics by pre-exposed or control gammarids. At the scale of the community, we aim at investigating the link between community tolerance acquisition using a PICT (Pollution-Induced Community Tolerance ) approach and the expression of metal-resistance genes using RT-qPCR - part 2 focuses on the ecological cost of adaptation. At the scale of the organism, enzymatic and proteomic approaches will be undertaken to evaluate modifications of functional responses of gammarids that are adapted to metals. As far as biofilms are concerned, community tolerance acquisition evaluated on heterotrophic communities, will be interpreted in relation to measurements of bacterial genetic diversity by microbiome profiling using 16S-rDNA sequencing and to community-level physiological profiles based on the respiration of different substrates in microplaques.