Microbial associations of shallow-water Mediterranean marine cave Solenogastres (Mollusca)

The first cave-dwelling Solenogastres-marine shell-less worm-like mollusks-were sampled from Mediterranean marine caves floor silt in the Marseille area. The mollusks were 1.5 mm in length, had a transparent body with shiny spicules and appear to represent a new Tegulaherpia species. Electron microscopy revealed a high number of microbial cells, located on the surface of the spicules as well as in the cuticle of Tegulaherpia sp. The observed microbial cells varied in morphology and were unequally distributed through the cuticle, reaching a highest density on the dorsal and lateral sides and being practically absent on the ventral side. Next Generation Sequencing (NGS) of V4 region of 16S rRNA gene amplicons, obtained from the DNA samples of whole bodies of Tegulaherpia sp. revealed three dominating microorganisms, two of which were bacteria of Bacteroidetes and Nitrospirae phyla, while the third one represented archaea of Thaumarchaeota phylum. The Operational Taxonomic Unit (OTU), affiliated with Bacteroidetes was an uncultured bacteria of the family Saprospiraceae (93-95% of Bacteroidetes and 25-44% of the total community, depending on sample), OTU, affiliated with Nitrospirae belonged to the genus Nitrospira (8-30% of the community), while the thaumarchaeal OTU was classified as Candidatus Nitrosopumilus (11-15% of the community). Members of these three microbial taxa are known to form associations with various marine animals such as sponges or snails where they contribute to nitrogen metabolism or the decomposition of biopolymers. A similar role is assumed to be played by the microorganisms associated with Tegulaherpia sp.