The Roseobacter clade constitutes one of the most widely distributed groups of microbes in the oceans in near surface regions and in benthic habitats. It comprises prominent members in many marine ecosystems and members of this clade are important mediators in global elemental cycles. On the basis of 8 complete genome sequences of representative members of the Roseobacter clade a comparative genome analysis will be carried out. Seven were granted by the Gordon & Betty Moore Foundation as part of their Marine Microbial Genome Initiative and one by the US-Department of Energy (DOE) and the Joint Genome Institute (JGI) to several of the applicants in an international competitive call for grant applications. The proposed project will close the sequencing gaps of the 7 incomplete genomes (“finishing”) and manually annotate the genomes for a comprehensive analysis of the genetic properties of the isolates. Further, the experimental physiological and genetic basis for the thorough biological investigation of this group of organism will be established, leading to detailed post genomic gene expression analyses. In a final part a comparative genome analysis will be carried out. Data of the annotated genome, the transciptome and proteome and deduced regulatory and metabolic networks will be stored in the novel publicly accessible Roseobacter Systems biology database and tool box “RoSy”. The proposed project will allow for the first time to explore and assess comprehensively the genetic properties of prominent members of a major clade of marine bacteria, the Roseobacter clade, an essential prerequisite to explore it for possible biotechnological applications. The project contributes to better understand and to explain the occurrence in and adaptation of this important clade to very different marine environments and initiates its systems biological development.