Contrasting microbiomes in deep-sea scleractinian and octocoral species from the South Atlantic Ocean

Deep-sea corals (DSC) are ecologically important, yet their microbiomes’ composition, structure, and host specificity remain largely unresolved in the South Atlantic, especially for octocorals. Here, we provide a comprehensive characterisation of DSC microbiomes, spanning Hexacorallia (Desmophyllum pertusum, Solenosmilia variabilis) and Octocorallia (Candidella imbricata, three Malacalcyonacea species), collected at ∼710 m depth in the Campos Basin, an active oil- and gas-producing region. Abiotic characterisation revealed total hydrocarbon concentrations. Seawater and sediment samples provided further environmental context. Amplicon sequencing (16S rRNA gene), combined with alpha and beta diversity analyses, differential abundance analysis, and indicator species analysis, revealed that each coral species harboured a microbial community structure distinct from the surrounding environment, with host specificity varying along a continuum. C. imbricata was the individually resolved coral species with the most specialised microbiome, dominated by Endozoicomonadaceae, the lowest Shannon diversity, the highest intragroup homogeneity, and the largest number of indicator amplicon sequencing variants (ASVs). D. pertusum showed no exclusive indicators, overlapping broadly with environmental communities, but selectively enriched the potentially hydrocarbon-degrading family Halieaceae. To our knowledge, these findings establish the first microbial baseline for DSC holobionts in this region, documenting contrasting host-specificity across scleractinian and octocoral taxa.