Mass producing microalgae uncontaminated by bacteria is a tough assignment whatever the type of algoculture used, and once achieved, maintaining a bacteria-free installation is extremely difficult. While these prokaryotic micro-organisms have negligible biomass, their presence affects algal metabolism, and can provoke variations in yields obtained. Controlling associated microbial populations is therefore vital to the durability and improvement of cultures and will usually be undertaken in the context of a quality directive or plan. The Symbiose project financed by the ANR has enabled the laboratory to embark on research in the field of microalgal-bacterial relationships. The PBA lab now has a bacterial library, specific to microalgae, and characterized by phylogenetic approaches. The development of a microplate-based culture screening system has enabled selection of bacterial strains that improve microalgal growth yields. Interaction mechanisms between algae and bacteria are studied using ecophysiological approaches.