Our main objective is to understand whether the interplay of the Pacific oyster C. gigas with its microbiota facilitates the survival of this invasive species in the intertidal environment of the German Wadden Sea. We will use a two-stage approach combining laboratory and field research (Fig. 1). First, we will conduct laboratory studies to determine the potential role of host-microbiome in holobiont responses to environmental stressors. This will be done by measuring stress-induced changes in the physiology, immune functions and transcriptomic profiles of oysters that have intact microbiome compared with microbiome-depleted (i.e. treated with antibiotics) conspecifics. Taxonomic composition of microbiota (based on 16S RNA metabarcoding) will be assessed in different tissues (hemocytes, gills, and digestive gland) of the stress-exposed oysters with intact and depleted microbiomes. The holobionts samples that show the strongest correlated change in the host response and microbiome composition will be selected for metatranscriptomic analysis to identify the potential microbial pathways that might contribute to the host fitness under different stress conditions. Secondly, we will conduct field transplant experiments to assess whether the molecular stress signatures of the host-microbiome interactions can be tracked in the natural habitats with the different degree of abiotic stress. For this, the long-term succession of C. gigas-associated microbial assemblages and the correlated host-microbiome transcriptional response to stress will be investigated in a transplant experiment off the island of Sylt (Germany). Oysters from the high intertidal zone will be transplanted to the subtidal zone and vice versa (Fig. 1). After 12 months, oysters will be collected, and the analysis of physiological and molecular stress responses (using immune, bioenergetic and transcriptomic markers) will be combined with the analysis of microbiome diversity to identify holobionts with the lowest and highest levels of physiological stress in each habitat. These samples will be used for metatranscriptomic analyses to assess the potential functional interactions of hosts with microbial communities during long-term acclimatization to different tidal regimes. This integrative approach will deliver novel information on functions and pathways involved in host-microbiome interactions modulating holobiont performance under environmental stress conditions in the laboratory and the field.