Increasing evidence suggests that positive interactions with invertebrates and the microbial community are vital to sustain seagrass ecosystems functioning and survival. Nonetheless, such species interactions received sparse attention in these systems. In this project, we will focus on two study systems: i) Posidonia oceanica and the sponge Chondrilla nucula and ii) Cymodocea nodosa and the lucinid bivalve Loripes orbiculatus. In each of these model associations, we will assess whether the organisms alone vs the association can reduce levels of local/regional stressors, alone or in combination with global stressors, using in situ and mesocosm experiments. We will simultaneously investigate diversity and function of host-associated and free-living microbes, to evaluate their role in promoting the success of the association.