Gaillard The overall objective of this project is to determine the influence of microbial processes on the chemical speciation of metals in sediments, i.e., to evaluate how bacterial defense mechanisms, in response to a metal stress, affect the chemistry of metals in sediments. The project will be conducted in two phases. The first phase will consist in investigating the chemistry and microbiology of contaminated freshwater sediments. Along a contamination gradient, the microbial ecology of three sites, as well as their complete geochemical characterization, will be performed using molecular tools. The second phase will consist in laboratory experiments where natural sedimentary microbial populations will be submitted to Zn stress. The pore waters and the microbial communities will be monitored as a function of exposure time to determine if metal stress results in community changes and the liberation of specific ligands that complex the metal. Concurrently, the changes in the local molecular environment of the metal, in the sediment particles, will be probed using X-Ray Absorption Fine Structure. The expected result of this research is a better understanding of the processes that are controlling the physical and chemical form of metals in contaminated sediments. In addition, it should pro-vide new evidence on the key role played by microorganisms in the biogeochemical cycles of metals in aquatic environments.