Distribution and ecological characteristics of members of the Roseobacter group
by Florian Lenk
Date of Examination:2020-07-09
Date of issue:2020-08-27
Advisor:Prof. Dr. Rolf Daniel
Referee:Prof. Dr. Rolf Daniel
Referee:PD Dr. Michael Hoppert
Persistent Address:
http://dx.doi.org/10.53846/goediss-8176
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Abstract
English
Marine Rhodobacteraceae are collectively referred to as the Roseobacter group, and due to their functional versatility and high abundance in diverse marine habitats, serve as model organisms in the study of microbial interactions, evolutionary processes, and metabolic pathways. Two species within this group, which belong to the Octadecabacter genus, are of particular interest, as they were isolated from sea ice at both poles, an extreme habitat with regard to temperature, oxidative stress, and nutrient availability. Initial analyses demonstrated their unique position among roseobacters, since they contained an unusually large number of transposable elements, inferred gene duplications, and genome rearrangements. However, a lack of comparable genome sequences from closely related strains left unclear, to what extent these observations are actual evidence of environmental adaptations, and how their genomic features compare to those of temperate strains in the same genus. This thesis expanded the pool of available Octadecabacter genome sequences, and utilised the additional data to examine general genomic properties and the nature of extreme adaptation in this group. Using phylogenomic methods, the phylogenetic history of the Octadecabacter-associated strains was reconstructed and evaluated in the wider context of the Roseobacter group. Comparative gene content analyses were applied to illustrate which aspects of cellular metabolism and biochemistry are altered in polar Octadecabacters, and were put into an evolutionary perspective utilising a model of functional gene content evolution. In addition, the global distribution of individual subgroups within the Octadecabacters was examined by means of a metagenomic mapping approach. The analyses presented here demonstrate that genomes of polar Octadecabacters encode more complex metabolic networks, consistent with a broader spectrum of available nutrients and more diverse microbial interactions in sea ice. Genome flexibility, and evolvability in general, constitute important prerequisites for efficient adaptation to this extreme habitat, and are both more pronounced in polar than in temperate Octadecabacter genomes. Detection patterns of Octadecabacters in metagenome sequences suggest that the currently available polar isolates are members of a cosmopolitan genus that also features non-polar species. The presented results add to our knowledge of the nature of extreme adaptation and its potential underlying processes in roseobacters, and are a contribution to our goal of understanding the biogeography of this important marine group.
Keywords: Octadecabacter; Roseobacter group; marine Rhodobacteraceae; cold adaptation; psychrophile; comparative genomics; polar adaptation