| Summary: | Coral reef ecosystems, often called "rainforests of the sea", represent structurally complex and dynamic environments in both shallow and deep ocean realms, which harbor a unique plethora of organisms. However, only little is known about the role these biodiversity hotspots play as habitat for the "unseen majority" in the oceans: microbial communities. The general aim of this thesis was therefore to advance hitherto existing knowledge of microbial diversity in warm and, particularly, cold-water coral reef ecosystems by exploring bacterial community variation and potential environmental drivers across a combination of different ecological (organizational, temporal, spatial) scales - with explicit focus on the importance of microbial habitat differentiation. For this purpose, molecular techniques, such as high-throughput DNA fingerprinting, and a suite of multivariate statistical tools were applied. The first study focused on principle patterns of bacterial community structure associated with the cold-water coral Lophelia pertusa. The coral and environment-specific structuring of bacterial communities was assessed under both natural (reef) and controlled (aquarium) conditions by targeting four distinct microbial habitats: coral skeleton surface, coral mucus, ambient seawater and proximal sediment. In the second study, intra- and inter-reef variations in bacterial community structure of four cold-water coral reef ecosystems were investigated in a multi-scale survey spanning five levels of ecological reef organization (microbial habitat type, coral species and color, reef zoning, reef boundary, reef site) by focusing on the cold-water corals Lophelia pertusa and Madrepora oculata. In the third study, the importance of permeable carbonate and silicate reef sands as specific microbial habitats in a warm-water coral reef ecosystem was assessed. The two sand types were compared for their potential to promote differences in bacterial diversity and biomass, while also taking into account the influence of season and space. ...
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