Bacterial community composition and functional potential in the highly toxic ribbon worm Cephalothrix cf. simula (Nemertea: Palaeonemertea)

Ribbon worms of the Cephalothrix simula s.l. species complex are known for their exceptional toxicity due to high concentrations of tetrodotoxin (TTX). However, the microbial basis of the TTX accumulation in these nemerteans, as well as in other TTX-bearing animals, remains poorly understood. We used high-throughput 16S rRNA gene sequencing on 32 Cephalothrix cf. simula individuals with varying TTX levels from Spokoynaya Bay (Sea of Japan, Russian Far East) to characterize their microbiome composition, diversity, and predicted metabolic functions. The microbiota of C. cf. simula included 21 phyla, with Bacteroidota and Pseudomonadota being the dominant phyla, and Pseudomonas and Vibrio among the most abundant genera. Microbial diversity did not differ significantly between high- and low-TTX individuals, indicating that overall community structure is not a primary determinant of toxin content. Nevertheless, amplicon sequence variants corresponding to known TTX-producing bacterial strains were detected in all individuals, including a highly abundant amplicon sequence variant related to Pseudomonas, which accounted for up to 29.5% of reads in some samples. Predicted functional profiles did not differ between toxin groups and indicated enrichment in pathways related to amino acid and carbohydrate metabolism, energy production, and the biosynthesis of secondary metabolites (including polyketide-, peptide-, and alkaloid-related pathways), suggesting potential microbial contributions to the complex toxin repertoire of the nemertean.