Particles are important in geochemical fluxes in the shelf margin in the North Western Mediterranean Sea. The microbial population attached to the particles are the major mediators of these biogeochemical processes. In order to understand the biogeochemical processes it is important to study the microbial processes occurring on particles in the natural environment. The overall objective of this project is to investigate the microbial population dynamics on particles and the role of specific microbial processes in the biogeochemisrty of the water column. The spatial and/or temporal evolution of particle-attached bacteria will be measured in terms of budgets and fluxes in the ocean interior. The relative biomasses, the specific and physiological diversity of natural communities, and the autotrophic and heterotrophic bacteria activities in the particulate system will be compared with those of free-living microorganisms. Theoretical studies will be devoted to increasing the knowledge of the microbial mechanisms governing the dynamics of particulate systems, and reciprocally, to the protective role of particles. All data will be used for modelling the impact of attached microorganisms and aggregates on carbon flux in the water column, and on the particle distribution and remineralization.