Ecological stoichiometry of marine bacteria: relationship to growth rate, protozoan predation, and organic matter degradation

The cycling of carbon between inorganic and organic compounds is an underlying process that drives all life forms. While the rates of production of organic matter have been extensively examined, the degradation rates and kinetics remain poorly understood. Microbial organisms, highly efficient recyclers, play a pivotal role in the degradation process in the ocean. This thesis explores the interactions among microbes (bacteria and protozoa) and how the competitive and predatory interactions affect the rate of organic matter degradation and regeneration. Emphasis is placed both on the structure and dynamics of the particulate and dissolved organic reservoir. Also, prey C:N:P stoichiometry is examined (both experimentally and in a model) to assess the role of elemental ratio relationships in population dynamics and organic matter cycling. It was found that under low growth rates, there is extensive variability of cell C:P and N:P, dependent on bacterial species, but at high growth rates, most species have similar C:P and N:P due to the necessity of P-rich ribosomes. Using clonal species of bacteria tagged with red and green fluorescent proteins, this thesis provides evidence that protozoan predators may prefer slower growing bacterial cells (with higher C:P and N:P), possibly because their cellular stoichiometry closer resembles that of eukaryotic consumers and less energy would need to be expended on processing the excess nutrients. Data from this thesis suggests that the ultimate bulk percentage of carbon remineralized or respired is primarily dependent on predator/prey interactions and trophic inefficiency, regardless of the limiting nutrient. It has long been debated how the addition of protisian predators stimulates the degradation of organic matter. Here, evidence is provided that the trophic inefficiency of converting bacteria cells to protist cells may account for much of this stimulation.Ph.D.Includes bibliographical references (p. 142-146)