Diversity and function of phages modulating North Sea bacteria

Abstract

The investigation of phages, their diversity, and their modulating role of bacterial populations during spring blooms in the North Sea was the major goal of this thesis. The research was conducted close to the island of Helgoland, in the German Bight, at the long-term ecological research station Kabeltonne. While factors inducing phytoplankton blooms and succeeding bacterioplankton blooms are intensively studied and fairly well understood, the mortality factors responsible for the subsequent decrease in microbial abundance are not yet well explored. To explore the relevance of phage lysis as a mortality factor modulating the bacterial population, lytic phages were isolated from the coastal waters off Helgoland during the spring phytoplankton blooms of two subsequent years. Most bacterial host strains were selected from bacterial taxa previously identified to be abundant in spring blooms. Phages infecting Flavobacteriia were the focus of this thesis, because their hosts degrade high molecular weight substrates and are thus central catalysts of algal biomass remineralization. Phages were enriched by enrichment cultures and tangential flow filtration, and subsequently characterized by transmission electron microscopy, and comparative genome sequence analysis. In total, seventeen new phage species were isolated, including phages for the three major bacterial classes responding to phytoplankton blooms, Flavobacteriia, Alphaproteobacteria, and Gammaproteobacteria. The new species were grouped into thirteen new phage genera, belonging to eleven new families in three realms. For the official acceptance of this classification taxonomic proposals have been formulated in this thesis, which will be evaluated by the International Committee on Taxonomy of Viruses (ICTV). Overall, the huge phage diversity down to the strain level, the detection of active infections, and the phage recurrence, together suggest that phages are highly relevant mortality factors of heterotrophic bacteria during spring phytoplankton blooms.