**Uncovering the Core Microbiome and Distribution of Palmerolide in** *Synoicum adareanum* **Across the Anvers Island Archipelago, Antarctica**

**Alison E. Murray 1,\*, Nicole E. Avalon 2, Lucas Bishop 1, Karen W. Davenport 3, Erwan Delage 4, Armand E.K. Dichosa 3, Damien Eveillard 4, Mary L. Higham 1, Sofia Kokkaliari 2, Chien-Chi Lo 3, Christian S. Riesenfeld 1, Ryan M. Young 2, Patrick S.G. Chain 3,\* and Bill J. Baker 2,\***


Received: 16 April 2020; Accepted: 27 May 2020; Published: 2 June 2020

**Abstract:** Polar marine ecosystems hold the potential for bioactive compound biodiscovery, based on their untapped macro- and microorganism diversity. Characterization of polar benthic marine invertebrate-associated microbiomes is limited to few studies. This study was motivated by our interest in better understanding the microbiome structure and composition of the ascidian, *Synoicum adareanum*, in which palmerolide A (PalA), a bioactive macrolide with specificity against melanoma, was isolated. PalA bears structural resemblance to a hybrid nonribosomal peptide-polyketide that has similarities to microbially-produced macrolides. We conducted a spatial survey to assess both PalA levels and microbiome composition in *S. adareanum* in a region of the Antarctic Peninsula near Anvers Island (64◦46- S, 64◦03- W). PalA was ubiquitous and abundant across a collection of 21 ascidians (3 subsamples each) sampled from seven sites across the Anvers Island Archipelago. The microbiome composition (V3–V4 16S rRNA gene sequence variants) of these 63 samples revealed a core suite of 21 bacterial amplicon sequence variants (ASVs)—20 of which were distinct from regional bacterioplankton. ASV co-occurrence analysis across all 63 samples yielded subgroups of taxa that may be interacting biologically (interacting subsystems) and, although the levels of PalA detected were not found to correlate with specific sequence variants, the core members appeared to occur in a preferred optimum and tolerance range of PalA levels. These results, together with an analysis of the biosynthetic potential of related microbiome taxa, describe a conserved, high-latitude core microbiome with unique composition and substantial promise for natural product biosynthesis that likely influences the ecology of the holobiont.

**Keywords:** Antarctica; ascidian; microbiome; microbial diversity; palmerolide A; co-occurrence
