Phoronids and their tubes harbor distinct microbiomes compared to surrounding sediment

Phoronids are a phylum of animals with only ∼12 described species, all of which are marine filter feeders that build external tubes for shelter and produce chemical deterrents against predators. Many tube-building invertebrates host distinct microbial communities and even have obligate symbionts for survival in sulfur-rich marine sediments. However, the microbiome of phoronids has only recently begun to be described. To address this, we surveyed the composition of the microbiome of the phoronid, Phoronopsis harmeri , using 16S rRNA gene amplicon and metagenomic sequencing. We found that the phoronid microbiome was dominated by members of the orders Campylobacterales, Desulfobulbales, and Desulfobacterales. We also found that the microbiomes of tubes and phoronids were less diverse than that of surrounding sediment, and that the microbiomes of phoronids, tubes and surrounding sediment were all distinctly structured. Based on analysis of metagenomic data, and even though we were only able to recover low quality MAGs of abundant taxa, we found preliminary evidence that taxa associated with phoronids and their tubes likely participate in sulfur cycling pathways. Future work should perform more robust metagenomic sequencing and chemical analysis to assess if there is a link between known phoronid chemical defenses and microorganisms. Overall, this study provides foundational insight into the microbial communities associated with phoronids and these initial findings suggest that these communities may play an important role in sulfur cycling in marine sediments.