**4. Conclusions**

This work has advanced our understanding of the Antarctic ascidian *S. adareanum*, PalA distributions, and microbiome in several ways. First, we found PalA to be a dominant product across all 63 samples, with some variation but no coherent trends with the site, sample, or microbiome ASV. Second, the results point to a conserved, core, microbiome represented by 21 ASVs—20 of which appear to be distinct from the Antarctic bacterioplankton. The phylogenetic distribution of these taxa was diverse and distinct from other ascidian microbiomes in which organisms with both heterotrophic and chemosynthetic lifestyles are predicted. Third, the co-occurrence analysis suggested the potential for ecologically interacting microbial networks that may improve our understanding of this ascidian–microbiome–natural product system. Likewise, based on the occurrence and likelihood of natural product biosynthesis, there are a number of taxa that may bear biosynthetic capabilities, including that of PalA. These results advance the long-term goal of palmerolide-producing *Synoicum adareanum* and host-associated microbiome research, which is compelled by the fact that by identifying the producer, genome sequencing could then provide information on PalA biosynthesis, and then lead to the development of a potential therapeutic agen<sup>t</sup> to fight melanoma.

### **5. Materials and Methods**
