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Abstract
Bacteria are dominant organisms in the marine environment, providing the foundation for food webs that are essential for ecosystem structure and function. The ubiquity of bacteria in the marine environment is a result of their capacity to survive in many different niches. This is largely due to the evolution of a range of adaptive responses that allow bacteria to survive stressors such as nutrient deprivation, fluctuations in salinity and temperature and predation by bacterivorous bacteria and heterotrophic protists. The environmental persistence of marine bacteria can be attributed to multiple intra- and inter-specific strategies such as biofilm formation on biotic and abiotic surfaces, as well as interactions with other organisms, e.g. algae and heterotrophic protists. This thesis examines factors that influence resistance to protozoan grazing, specifically, the role of habitat of origin, such as association with algal surfaces, plays in resistance to predation by the ciliate Tetrahymena pyriformis. Results from this study found that bacteria derived from algal surfaces had superior biofilm growth performance when compared to seawater-derived bacterial isolates. However, seawater-derived bacterial isolates demonstrated higher resistance to grazing than surface-attached bacteria. A variety of protozoan grazing resistance mechanisms were detected, including toxicity, biofilm formation and intracellular survival. Intracellular survival was employed by Shewanella spp. as T. pyriformis feeding on these organisms led to expulsion of Shewanella-filled vacuoles into the external milieu. The role of intracellular survival as an environmental persistence mechanism for Shewanella sp. cp20 was investigated. Bottom-up environmental controls, specifically phosphorus starvation, played a role in the production of bacteria-filled expelled vacuoles. Long-term survival of Shewanella sp. cp20 within expelled vacuoles suggests that residence within vacuoles may provide a vehicle for environmental dispersal. Shewanella sp. cp20 genome analysis identified genes required for tolerance of the intracellular niche and subversion of T. pyriformis processes, e.g. secretion systems, detoxification enzymes, effector proteins and metal efflux systems, all of which have been shown to play a role in intracellular survival. This was the first study to describe intracellular survival for a member of the Shewanella genus and to identify virulence traits within this common aquatic organism.