Bacterial Influence on the bloom dynamics of the dinoflagellate Lingulodinium polyedrum

Algal blooms, particularly those dominated by dinoflagellates, are widespread in marine ecosystems and can have devastating ocean and human health impacts. The factors that lead to their demise are not well characterized but are thought to include a combination of physical, chemical, and biological factors. Previous studies have isolated bacteria that kill dinoflagellates in laboratory experiments, but little is known about the impact that such algicidal bacteria exert on natural blooms. In this dissertation, I investigated the possibility that algicidal bacteria can impact the population dynamics of the dinoflagellate Lingulodinium polyedrum, a species that causes large and long-lived blooms off the coast of Southern California. I used a combination of laboratory experiments and field sampling to try to link knowledge about model systems in controlled experiments with the complex world of natural blooms in the environment. Chapter one is a review of the current knowledge about the impact of algicidal bacteria in the sea. In chapter two, I report the isolation of algicidal bacteria that induced the formation of temporary cysts in L. polyedrum laboratory cultures as a probable defense mechanism against bacterial colonization. In chapter three, I report bacterial colonization intensities in various L. polyedrum cultures and in two natural blooms, suggesting a relationship between algal population dynamics and bacterial colonization intensity. In chapter four, I report a novel interaction of algicidal bacteria on L. polyedrum motility mediated by excreted putative proteases, and describe decreased motility at the end of an L. polyedrum bloom, suggesting algal cells were unhealthy before bloom decline. In chapter five, I report the novel cultivation of a globally widespread and abundant marine bacterium from the Roseobacter group that killed L. polyedrum cultures by attachment. Using species-specific DNA probes, I studied the colonization of this bacterium on L. polyedrum cells in a natural bloom and found increased colonization during the bloom decline phase, suggesting a direct effect of this algicidal bacterium on bloom dynamic