Metazoans are ubiquitously associated with microbial symbionts, which can provide direct benefits to their hosts, such as nutrients provisioning, protection against environmental stress, or enhancement of host digestive abilities. These microbial mutualists have the ability to adapt rapidly to changing environments, and could potentially play a key role in the local adaptation of their host, especially in the context of rapid environmental changes imposed by human activities. However, despite their potential key role, symbiotic interactions are rarely considered in the study of adaptation. Here, I propose to adopt such innovative approach, using the water flea Daphnia and its microbiota as a model system. This cosmopolitan freshwater crustacean zooplankter has to deal with anthropogenic induced aquatic habitats changes, more particularly with eutrophication and the associated proliferation of cyanobacteria that produce toxic substances. My aim is to elucidate to what extent symbiotic microbial communities (SMCs) are involved in Daphnia adaptation to such stress, and more particular which microbial genes are involved, using a candidate gene approach. In addition, I propose to investigate the consequences of modifying SMCs on host stress responses at the molecular level, by determining whether the expression of particular tolerance genes towards cyanobacteria in Daphnia are dependent on the type of SMC they harbor. If successful, this project will provide some insight into the consequences of human activities for the functionality of SMCs, and into the role of SMC for host adaptation, which might have important implications in different fields, ranging from ecology and evolution to ecotoxicology, conservation biology and medicine.