Mechanisms in bacterial utilization of cellular debris in the ocean
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.