CHARACTERIZATION OF PHAGE INFECTION AND ALGAL INTERACTIONS OF A MODEL ROSEOBACTER GENUS

Mobile genetic elements (MGEs) are crucial contributors to bacterial fitness and symbiotic interactions spanning different domains of life. In addition to directly altering host behavior, MGEs, which include plasmids, bacteriophages, and transposases, also influence one another, demonstrated through plasmid replication systems mediating plasmid-incompatibility and plasmid-encoded toxin-antitoxin systems influencing phage resistance. While these interactions and their impacts on host fitness have been well characterized in industrially and medically relevant bacteria, their influence on various environmentally relevant bacteria has only recently been explored. Here, I aim to further our understanding of MGEs on host physiology, fitness, and MGE maintenance in an environmentally relevant system. Previous work on the marine bacterium, Sulfitobacter pontiacus CB2047, demonstrates it encodes four large, low copy plasmids and is readily infected by a suite of isolated bacteriophage. My research identifies interplay between, and characterization of, MGEs while expanding the system to address broader ecological questions. Specifically, this work characterizes several plasmid-mediated phenotypes (e.g., motility, pigment production, LPS modification, sulfite tolerance) that come at the cost of enhanced phage susceptibility, as a temperate phage (φCB2047-A) requires the plasmid-associated LPS profile for successful infection. Additionally, we characterize viral life traits of a lytic phage (φCB2047-B), identifying a phage adsorption target, extended host range, and putative phage defense systems. Lastly, we expanded this model system to include the eukaryotic algae Gephyrocapsa huxleyi. While CB2047 was isolated from an induced G. huxleyi bloom, its potential as an algal symbiont had yet to be assessed. We demonstrate that CB2047, like other S. pontiacus strains, may protect G. huxleyi from specific environmental pathogens. However, as similar algal-roseobacter interactions appear fragile and easily influenced by tertiary microbial parties, it begs the question of how relevant these interactions are in the environment. Collectively, this dissertation characterizes several symbiotic interactions utilizing, primarily, a single roseobacter, setting the foundation for future work. Expansion of these bipartite components to a mesocosm format will assist in determining the relevance and validity of extrapolating laboratory findings to the marine environment.