| It is inherently difficult to study the direct interactions between marine microbes due to the size of the organisms and microscale environments in which they exist. Recent evidence suggests that the direct associations between marine phytoplankton and particle-colonizing bacteria can influence marine microbial community succession, microalgal primary production, toxin production by marine phytoplankton, and have potential implications for human health. The goal of this dissertation was to elucidate some of the physical and chemical mechanisms that regulate microbial interactions, particularly the colonization of phytoplankton and other marine particles by bacteria. To accomplish these goals, I used a creative and multi-disciplinary approach, incorporating aspects of microbiology, natural products chemistry, multiple microscopy techniques, and image analysis and particle tracking to explore microbial relationships in the environment and in cultures.;This research elucidated some of the physical and chemical interactions between marine microbes. The results of field and laboratory experiments indicate that phytoplankton are rarely colonized by bacteria when they are healthy, autotrophic cells. When colonization does occur, it is a function of diatom morphology and bacteria may attach to specific regions of phytoplankton chains or colonies. Marine bacteria heavily colonize older senescent phytoplankton that are physiologically or physically compromised and preferentially attach to particles created from the polymers excreted by phytoplankton. Data from microchannel chemotaxis experiments with the human pathogen Vibrio cholerae, a particle and phytoplankton colonizing bacteria, show that an antibiotic from a competing bacterium influenced the motile behaviors of V. cholerae and would repel the pathogen from colonizing particles. These results are consistent with previous research that demonstrated this antagonistic relationship, but could not provide a mechanism for this relationship. Vibrio cholerae also exhibited enhanced chemotaxis towards a source of exudates from exponentially growing phytoplankton cultures. The bacteria, however, did not physically associate with visually healthy and intact diatom cells in cultures. These results suggest that the phycosphere or cell surface of healthy, autotrophic phytoplankton are uninhabitable or undesired locations for particle-attaching marine bacteria. |
