Mesophotic coral ecosystems (MCEs) are light-dependent communities that extend from approximately 30 to 150 m depth. Unlike their shallow-water counterparts, they cannot be reached within the limits of basic SCUBA training. Therefore, they remain largely unmapped, under-explored, and consequently poorly understood. Leptoseris spp. are zooxanthellate stony corals that thrive in MCEs worldwide, often forming large fields which provide habitat for a diversity of reef fishes and invertebrates. As in shallow-water corals, the healthy microbiomes of Leptoseris spp. are likely fundamental to their hosts’ ecological dominance. The objectives of the project were to identify and characterize the tissue-associated microbial symbionts of the mesophotic coral Leptoseris.