Ribbon worms (Nemertea) microbiomes from the Sea of Japan

Nemerteans, or ribbon worms, are mostly carnivorous marine worms using a variety of low molecular weight neurotoxins for hunting and defense. One of the toxins found in all major nemertean groups is tetrodotoxin. It is a low molecular weight non-proteinaceous neurotoxin that causes seafood poisoning around the world. TTX-producing bacteria were found in the cultivable microflora of many TTX-bearing hosts, including some nemertean species, however metagenomic studies of TTX-bearing animals addressing the whole microbial composition and estimating the contribution of TTX-producing bacteria in the overall toxicity of the host were not conducted. To date, only one study described the metagenome of two samples of toxic ribbon worm. For this study, a total of 24 samples of the eight nemertean species were collected in the Spokoynaya and Vostok Bays of the Peter the Great Bay (Sea of Japan, Russian Far East) in July and August 2019. DNA was extracted and bacterial communities were sequenced on an Illumina Miseq platform. Primers targeted the V3-V4 region of the 16S rRNA gene. 24 studied samples comprised of four TTX-bearing (Kulikovia alborostrata, Cephalothrix simula, Tubulanus punctatus, and Quasitetrastemma stimpsoni) and four non-TTX-bearing (Micrura cf. bella, Parahubrechtia sp., Hubrechtella juliae, and Cerebratulus cf. marginatus) nemertean species. Analysis of microbiomes composition showed that different nemertean species harbor distinct bacterial communities, while representatives of the same species mostly have more similar microbiomes. The bacteria historically associated with TTX production were found in all studied samples, but predominated in TTX-bearing nemertean species.