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Reef sitesFig. 1 Hydrothermal vent field and Acroporid corals. a Overview of the vent field. b Appearance of the coral aggregation found on the outskirts. c Methane bubbles released directly below the Acroporid colonies. d Microbial mat. e Mid-part of the colonies with well-pigmented appearance. f Reduced pigmentation around branch tips Acroporid corals growing over a methane-bubbling hydrothermal vent, Southern Ryukyu Archipelago

Coral skeletons harbor diverse assemblages of endolithic microorganisms that often have a profound influence on the ecology and physiology of the coral host.While the cyanobacterial and eukaryotic microalgal com ponents of these endolithic assemblages are well char acterized, information on the other components remains scarce. Here, we characterize the endolithic microbial assemblages in the skeleton of the massive coral Gonia strea aspera in both oligotrohic and eutrophic environ ments. In addition to the green bands typically found in massive coral skeletons, multiple black bands were observed in skeletons from oligotrophic waters. Sequence analysis of the dsrB (dissimilatroy sulfite reductase beta subunit) gene revealed that colonies with black bands harbor a diverse assemblage of sulfatereducing bacteria (SRB). In contrast, SRB were not detected in skeletons from eutrophic waters. We conclude that increases in nutrients may have an important effect on the microbial assemblages residing within coral skeletons.