Seagrass meadows are among the most productive marine ecosystems, playing essential structural and functional roles, and are recognized as the most efficient blue carbon sinks globally. The plant-associated microbiome, a diverse array of microbial taxa forming intimate symbiotic relationships with seagrasses, is critical for maintaining ecosystem stability and facilitating plant adaptation to environmental stressors. In the Mediterranean Sea, however, the endemic seagrass Posidonia oceanica is in decline due to climate change-driven stressors, particularly thermal stress. The sponge Chondrilla nucula is commonly found in association with P. oceanica, and it is hypothesized that this sponge-plant symbiosis enhances the metabolic capabilities of both organisms, mediated by key microbial communities, especially under thermal stress. Despite this potential, the precise dynamics of nutrient exchange and interactions between seagrasses and sponges, facilitated by their microbiomes, remain poorly understood. This knowledge gap limits our ability to fully comprehend the ecological and metabolic functions of these associations and hinders the development of nature-based solutions to mitigate the impacts of climate change. MicroSymb aims to address this gap by studying the P. oceanica–C. nucula association through their microbiomes, utilizing field sampling, mesocosm and stable isotope probing experiments, metabarcoding, and meta-omics approaches. This project will evaluate the capacity of this association to adapt and remain resilient in the face of ocean warming. Throughout MicroSymb, I will acquire advanced skills in metagenomics, metaproteomics, and nutrient cycling within holobionts, while contributing my expertise in marine ecology, host-microbiome interactions, holobiont functioning, molecular biology, and bioinformatics. MicroSymb will position me at the forefront of host-microbiome research and the study of adaptive capacities in marine organisms amid climate change.