Octocoral microbiomes vary substantially across environmental gradients in deep waters

Coral-associated microbiomes vary greatly between colonies and localities with functional consequences on the host. However, the full extent of variability across the ranges of most coral species remains unknown, especially in corals living in deep waters. Here we characterized the microbiomes of four octocoral species from mesophotic and deep-sea habitats in the northern Gulf of Mexico, Muricea pendula, Swiftia exserta, Callogorgia delta, and Paramuricea biscaya using 16S metabarcoding. We tested for microbiome differentiation between and within species, examining the influence of the corals genotype and environmental factors that vary with depth (53-2224 m) and geographic location (over 680 m). Coral microbiomes were often dominated by amplicon sequence variants whose abundances varied across hosts ranges including corallicolids, Endozoicomonas, members of the Mollicutes, and the BD1-7 clade. Coral species, depth, and geographic location significantly affected diversity, microbial community composition, and the abundance of individual microbes. Differences in host genotype, bottom temperature, and surface primary productivity could explain part of the variation associated with depth and geographic location. Altogether, this work demonstrates that the microbiomes of corals vary substantially across their ranges with potential functional consequences, identifies important ecological drivers in mesophotic and deep-sea corals, and can inform restoration efforts.