Microbial assemblages on a cold-water coral mound at the SE Rockall bank

The microbial community composition over Haas Mound, one of the most prominent cold-water coral mounds of the Logachev Mound Province (Rockall Bank, NE Atlantic) was analysed by Roche GS-FLX amplicon sequencing targeting both Bacteria and Archaea. Overlaying water was collected from depths of 400 m as well as 5 and 10 m above the bottom using a CTD/Rosette system. Near-bottom water, sediment, and samples of mucus and skeleton of the coral Lophelia pertusa were obtained with a box-corer. We outline patterns of microbial distribution, vertically - from the seafloor to the water column - and laterally - across the mound - and couple these to mound topography and turbidity. A strong link was found between the microbial community composition and the specific biotopes. At all locations, the near-bottom water differed significantly from water at 5 m above the bottom, illustrating that the near-bottom water in between the coral framework represents a separate microbial habitat. Near-bottom water was distinct from other biotopes by outstanding relative abundance of the class Halobacteria (1.2%) and the genera Nitrosopumilus (3.2%), uncultured Xanthomonadales (1,6%), Defluviicoccus (1.3%), Marinicella (1.2%), and Brocadiaceae W4 lineage (1.1%). The Endozoicomonas found in near-bottom water (0.2%) is probably related to the presence of (dissolved) mucus in the water. The genus was not found in sediment, nor in overlaying water at 5 m above the bottom. The overlaying water community (sampled at 400m and at 500-1200m) was structured according to depth and correlated variables i.e. temperature, salinity and density (17% explained). Turbidity of the overlaying water explained an additional 14% and was correlated with sampling year.