This dissertation investigates the mechanisms of bacterial utilization of cells and cellular debris in the ocean. Bacteria are key members of marine ecosystems. They serve as the predominant biotic force acting on marine biogeochemical cycling of organic matter, and exert a strong influence on marine microbial community dynamics via their cell-cell interactions. While scientists have begun to untangle the web of seemingly intractable mechanisms that marine bacteria use to acquire and consume organic matter, many unknowns still remain.

This research implements a number of techniques including bacterial production measurements, mass spectrometry, single-cell analyses, and genetics to test two hypotheses: (1) that bacteria can readily respond to a natural pulse of cellular debris from coral mass-spawning, and (2) that contact-dependent predatory bacteria can kill and consume their bacterial prey.

Studies conducted off coastal Panama found that coral gametes elevated the organic matter concentration of the surrounding seawater by ~5-fold after a mass spawning event. Bacterial production measurements and 454 pyrosequencing of bromodeoxyuridine-labeled 16S bacterial rRNA genes showed that it was primarily active Rhodobacteraceae, Flavobacteriaceae, and Saprospiraceae taxa that degraded the coral gametes. High resolution mass spectrometry analysis supported this hypothesis, showing trends that indicated microbial alteration of the organic matter pool. This study demonstrated that coastal marine bacteria can readily respond to a large input of autochthonous cellular debris.

Contact-dependent bacterial predation was tested by using bacteria with a type 6 secretion system (T6SS+) as model predators in competition assays. Nanoscale secondary ion mass spectrometry analysis showed that T6SS+ Vibrio cholerae utilized carbon from its bacterial prey. V. cholerae also exhibited the ability to utilize DNA and ribosomes from lysed bacteria as nutrients. Additionally, independent competition assays between T6SS+ V. cholerae and 15 different marine bacterial isolates showed that V. cholerae and/or the challenged isolate were killed in 12 out of the 15 assays. The experiments also revealed that the coral and shellfish pathogen, Vibrio coralliilyticus has a functional T6SS. Collectively, these results demonstrate that bacterial antagonism often ends in carnage for one or both of the competing species, and suggest that the susceptible prey are fresh fodder for the victorious bacterium.

Marine bacterial influence on colloidal dissolved organic matter is a major driver in the biogeochemical cycling of carbon through the marine microbial loop. Interactions of individual bacteria with ambient colloids can affect nutrient availability and acquisition strategies, reshaping their immediate microenvironment. Direct observation of marine bacteria using atomic force microscopy (AFM) and complementary microscopies can yield insight and empirical evidence regarding mechanisms and effects of interactions of individual bacterial with colloidal particles from dissolved organic matter. The discovery of bacterial nanotubes in marine bacteria is reported with descriptive physical properties. Nanotube structures in Pseudoalteromonas sp. TW7 cells and Alteromonas sp. ALTSIO cell were determined to range between 200–600 nm in length and 50–160 nm in width. Individual connections were observed as hollow structures connecting bacterial cytoplasmic spaces. Particle aggregates, ranging 40–200 nm in width, were localized on marine bacteria surfaces, varying in size and quantity in a cell population, from interactions with E. coli ribosomes as model colloids. TW7 cells were observed with altered surface corrugation features and surface pits that can influence surface interaction with individual colloids. Corrugation features were determined to be 50–100 nm in size, and 20 nm deep. Hollow surface pits were measured to be 50–300 nm in width, and 10–50 nm in depth. A method using curvature radius analysis is proposed for analyzing AFM time-lapse image data of ALTSIO exposed to E. coli ribosomes to quantify particle attachment rates and coverage of cells. From observations, bacteria presented an average increase of 6.0 attached ribosomes when exposed to ribosome-amended seawater medium (approximate concentration of 8×1011 particles mL-1). In conclusion, bacterial nanotubes, surface particle clusters and surface pits are various cell features in bacterial interactions with organic colloid particles that are indicative of underlying mechanisms of bacterial transformation of dissolved organic matter throughout the global ocean.