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Disease is increasingly viewed as a major factor in marine ecology and its impact is expected to increase with environmental change such as global warming. This thesis focuses on understanding a "bleaching" disease, which affects the temperate-water red macroalga Delisea pulchra, particularly in summer when sea water temperatures are elevated. The bleaching disease could be reproduced in vitro by a combination of increased temperature and the presence of bacterial pathogens, which are phylogenetically affiliated with the genera Microbulbifer, Alteromonas, Cellulophaga and the marine Roseobacter lineage. Since only a fraction of environmental bacteria are cultivable by standard laboratory techniques, a culture-independent metagenomics approach was used to characterize the phylogenetic and functional shifts induced by bleaching in the surface-associated, bacterial community. Phylogenetic shifts mainly reflected relative changes in the genera Thalassomonas, Thioclava and Parvularcula, while changes in gene composition were mainly due to functions associated with transcriptional regulation, cation/multidrug efflux and non-ribosomal peptide synthesis. To obtain an enhanced understanding of the molecular mechanisms of pathogenicity, the genome sequence of strain R11, a previously identified Rosebacter-clade pathogen of D. pulchra, was analyzed. In addition to the metabolic versatility that is characteristic of the Roseobacter lineage, the R11 genome revealed specific adaptations that may allow for switching between free-living, surface-associated and pathogenic lifestyles. Of particular interest to algal athogenicity is the synthesis by strain R11 of plant hormone indole acetic acid, a known effector of virulence in phyto-pathogens. Comparative genomics including pathogenic and non-pathogenic Roseobacter strains, led to the identification of a unique quorum sensing (QS) dependent transcriptional regulator that may co-ordinate expression of virulence genes. This virulence regulator also provides a direct link to the furanone-based chemical defense system of D. pulchra, which is known to suppress QS and hence would prevent strain R11 from expressing virulence genes.The outcomes of this thesis support the model that indigenous members of the alga's epiphytic microbial community can become opportunistic pathogens under environmental conditions, when innate host defense mechanisms are compromised and virulence genes are expressed. |
