Diversity of deep-water coral-associated bacteria and comparison across depth gradients.

Environmental conditions influence species composition, including the microbial communities that associate with benthic organisms such as corals. In this study we identified and compared bacteria that associate with three common deep-water corals, Lophelia pertusa, Madrepora oculata and Paragorgia arborea, from a reef habitat on the mid-Norwegian shelf. The 16S rRNA gene amplicon sequencing data obtained revealed that >50% of sequences were represented by only five operational taxonomic units. Three were host-specific and unclassified below class level, belonging to Alphaproteobacteria with affiliation to members of the Rhizobiales order (L. pertusa), Flavobacteria affiliated with members of the Elisabethkingia genus (M. oculata) and Mollicutes sequences affiliated with the Mycoplasma genus (P. arborea). In addition, gammaproteobacterial sequences within the genera Sulfitobacter and Oleispira were found across all three deep-water coral taxa. Although highly abundant in the coral microbiomes, these sequences accounted for <0.1% of the surrounding bacterioplankton, supporting specific relationships. We combined this information with previous studies, undertaking a meta-data analysis of 165 widespread samples across coral hosts and habitats. Patterns in bacterial diversity indicated enrichment of distinct uncultured species in coral microbiomes that differed among deep (>200 m), mesophotic (30-200 m) and shallow (<30 m) reefs.