Given the importance of marine invertebrates and their microbial communities in marine ecosystems, we sought to characterize the largely unknown microbial associates, metal sequestration, and metabolite production of the marine colonial tunicate, Botryllus schlosseri, a model organism for cellular and developmental studies. Using an integrated multidisciplinary approach, we identified significant correlations between metals, metabolites, and bacterial taxa. B. schlosseri tissue was highly enriched in metals compared to seawater, and B. schlosseri microbiome beta-diversity was significantly different from seawater. We also introduced the concept of the pan-metabolome to classify metabolites based on their presence or absence across complex samples and found microbial metabolites in both the core and flexible metabolome. These findings offer insights into B. schlosseri’s biological and chemical interactions with microorganisms and their environment, bridging the knowledge gap of host-microbiome-environment interactions and establishing a foundation for continuing research on the ecological effects of trace metals in these biological systems.