Phylosymbiosis and co-phylogeny in the holobionts of octocorals from the Mediterranean Sea and Red Sea

Background Corals are the foundational species of coral reefs and coralligenous ecosystems.Their success is linked to their symbioses with microorganisms,therefore, the coral host and its symbionts are considered a single entity called the holobiont. But whether there is an evolutionary link between corals and their microbiomes has not been fully investigated. While there is evidence of phylosymbiosis in Scleractinian hexacorals, little is known about the holobionts of Alcyonacean octocorals. Results We used 16S rRNA gene amplicon sequencing to explore the composition of the bacterial communities associated with 14 Alcyonacean species (belonging to 10 genera from 5 (sub)orders) collected from the mesophotic zones of the Mediterranean and Red Seas. The low level of disturbance in mesophotic ecosystems allowed us to study potential evolutionary links (e.g., phylosymbiosis and cophylogeny) between these corals and their microbiota. Clear differences were observed between corals from the Mediterranean Sea and the Red Sea. The low diversity and consistent dominance of Endozoicomonadaceae and/or Spirochaetaceae in the bacterial communities of Mediterranean octocorals suggested that these corals may have evolved in parallel with their microbiota. Phylosymbiotic signals were indeed detected and cophylogeny in associations between several bacterial strains belonging to Endozoicomonadaceae or Spirochaetaceae and coral species were identified. Conversely, phylosymbiotic patterns were not evident in Red Sea octocorals, likely due to the high bacterial taxonomic diversity in their microbiota, but cophylogeny in associations between certain coral and bacterial species was observed. Noteworthy were the associations with Endozoicomonadaceae , suggesting a plausible evolutionary link that warrants further investigations to uncover potential underlying patterns. Conclusions Overall, our findings emphasize the importance of Endozoicomonadaceae and Spirochaetaceae in coral symbiosis and the significance of exploring host-microbiome interactions in mesophotic ecosystems for a comprehensive understanding of coral-microbiome coevolution.