With the aim of reducing anthropogenic CO2 emission in the atmosphere and subsequent climatic changes and ocean acidification, carbon capture and storage (CCS) in the deep subsurface has been envisaged as one of the mitigating solution. Its efficiency is currently evaluated at the Hellisheidi CO2 injection site in Iceland. This pilot site is implemented in the Hengill area (SW Iceland). This field scale injection of CO2-charged waters is designed to study the feasibility of storing permanently CO2 in mafic rocks and to optimize associated industrial techniques. Prior to the injection, the microbiological initial state was characterized through regular sampling of groundwaters at various seasons since October 2008. The microbial communities have been followed all along 2 CO2 injections using cloning and pyrosequencing of the16S RNA gene. These results show a high reactivity of the microbial communities after the injection, especially involving microorganisms of the C, S, N and Fe cycles. However, the implication of the nutrient cycles has not been determined yet. Our objectives for this study are:1. To understand the microbial communities function in relation to C, S, Fe and N cycles through a metagenomics analysis.2. To study the community structure and dynamics of the biofilms adhered to the basaltic substrate, using cloning, pyrosequencing and qPCR.3. To describe the microbial-mineral interactions in relation to potential CO2 precipitation and biological C fixation. FISH, single cell genomics and SEM analyses will be performed.Besides the relevant scientific contribution of the project, the obtained results will contribute to the development of the CO2 fixation technology. The contribution of microorganisms to carbon storage has not been integrated into geochemists predictive models, nor understood. We will provide valuable information on the microbial potential for direct or indirect CO2 fixation that will be integrated into further projects.