Molekularbiologische Charakterisierung und vergleichende Genomik von ausgewählten Vertretern mariner Roseobacter-Stämme

Abstract

The genome analyses presented in this thesis extend the current knowledge of the genomic potential of the Roseobacter clade and identify potential adaptations to ecological niches within marine habitats. New features were identified in the polar sea ice organisms Octadecabacter arcticus 238 and O. antarcticus 307, which have not been previously reported in members of the Roseobacter clade and are most likely adaptations to polar or sea ice associated habitats. A special highlight of these analyses is the characterization of a new subgroup of xanthorhodopsins in the polar Octadecabacter strains. This new xanthorhodopsin subgroup differs from previously described xanthorhodopsins not only phylogenetically, but also in their inability to bind 4-keto-carotenoids and in their preferred occurrence in ice-associated organisms. Both polar Octadecabacter strains were shown to possess an unusually high genome plasticity. This seems to be linked to the unique sea ice habitat of these organisms, which is assumed to be a hot spot for horizontal gene transfer (HGT). Furthermore it could explain the discrepancy between the low genome synteny and high 16S rRNA gene sequence similarity between both strains. Despite these differences, O. arcticus and O. antarcticus share remarkable features which indicate a common exclusive gene pool of Octadecabacter members in both polar regions. This view is supported by 16S rRNA-based phylogenetic analyses of Octadecabacter strains from various habitats. Therefore the bacterial communities of both polar regions seem to linked via a direct connection, possibly in the form of deep sea currents which span both hemispheres. The currently available Genomesequences were used for in-depth comparative analyses, enabling new insights into niche adaptations within the Roseobacter clade and the high importance of horizontal gene transfer for this group. Furthermore, this information represents a basis for the classification and analysis of new Roseobacter-associated genome and metagenome sequences.