Interactions among mobile genetic elements influence host physiology and fitness in the marine bacterium Sulfitobacter pontiacus

Bacteria are integral to many ecosystem processes in the oceans. They are key players in Earth’s nutrient and energy cycling, can influence climate, and contribute substantially to global primary productivity. Mobile genetic elements (MGEs), through interactions with their bacterial hosts, can have considerable influence over these microbially-mediated processes. MGEs have been implicated as drivers of genome evolution and environmental adaptation and bacteria harbor a diverse assemblage of them, including plasmids, phages, gene transfer agents, genomic islands, and transposons. All these MGEs are prevalent among marine bacteria, yet molecular mechanisms by which they individually, and collectively, influence host physiology and functions remain to be fully elucidated. Development and characterization of model systems involving marine MGEs and their hosts are critical to understanding their intimate and complex interactions as well as their downstream effects on the ecosystem. Here, my goal was to elucidate the ecological effects of MGEs by looking at their influence on host physiology and fitness within marine bacteria. I expand upon development of an emerging model marine system to characterize MGE interactions within a single marine bacterium, Sulfitobacter pontiacus. I primarily focus on the influences of, and interplay between, two temperate phages that infect this strain (φ-A [phi-A] and φ-D [phi-D]) and four naturally occurring plasmids within the S. pontiacus genome. Both phages and plasmids were observed to influence the physiology and fitness of their common host. These fitness effects were found to be niche-specific, and competition differed among cells grown planktonically or within biofilms. Spontaneous prophage induction (SPI) within lysogenized bacterial strains was seen to be crucial to intraspecies competitions between strains differing in their MGE contents. A novel interaction between plasmids and the lysogenic-lytic switch of the temperate phages was also observed. Overall, the work presented in this dissertation provides an enhanced framework for our understanding of how MGEs interact with one another and their hosts. Furthermore, these results will help inform our understanding of how MGE-host interactions contribute to global nutrient cycling and other ecosystem processes within marine systems.