We thank the officers, crew, and shipboard scientific party for excellent support during RV SONNE cruises SO143 and SO148. Tina Treude helped with sampling on board, Julia Polansky is acknowledged for assistance with FISH, Sabine Schäfer for total cell counts, Tina Lösekann and Heiko Löbner for sulfate reduction rates, Doris Setzkorn for thymidine incorporation, and Dirk Rickert for sulfate and porosity data. We thank Beth Orcutt and the two reviewers for their extremely helpful comments on the manuscript. This study was part of the programs MUMM (Mikrobielle UMsatzraten von Methan in gashydrathaltigen Sedimenten, 03G0554A) and TECFLUX I and II (TECtonically induced FLUXes, 03G0148A) supported by the Bundesministerium für Bildung und Forschung (BMBF, Germany). Further support was provided from the Max-Planck Society, Germany. This is publication GEOTECH-17 of the program GEOTECHNOLOGIEN of the BMBF and the Deutsche Forschungsgemeinschaft (Germany).

The pseudocolonial coral Lophelia pertusa (Scleractinia, Caryophylliidae) is a eurybathic, stenothermal cosmopolitan cold-water species. It occurs in two color varieties, white and red. L. pertusa builds vast cold-water coral reefs along the continental margins, which are among the most diverse deep-sea habitats. Microbiology of L. pertusa has been in scientific focus for only a few years, but the question of whether the coral holds a host-specific bacterial community has not been finally answered. Bacteria on coral samples from the Trondheimsfjord (Norway) were characterized by the culture-independent 16S rRNA gene-based techniques terminal restriction fragment length polymorphism and sequence analysis. L. pertusa revealed a high microbial richness. Clone sequences were dominated by members of the Alpha-and Gammaproteobacteria. Other abundant taxa were Bacteroidetes, Actinobacteria, Verrucomicrobia, Firmicutes, and Planctomycetes. The bacterial community of L. pertusa not only differed conspicuously from that of the environment but also varied with both the location and color variety of its host. Therefore, the microbial colonization cannot be termed "specific" sensu stricto. However, similarities to other coral-bacterium associations suggest the existence of "cold-water coral-specific" bacterial groups sensu lato. L. pertusa-associated bacteria appear to play a significant role in the nutrition of their host by degradation of sulfur compounds, cellulose, chitin, and end products of the coral's anaerobic metabolism. Some coral-associated microbes were regarded as opportunistic pathogens. Dominance of mixotrophic members of the Rhodobacteraceae in white L. pertusa could explain the wider dispersal of this phenotype by supplementary nutrition.

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