Microbial degradation of organic macromolecules in Arctic fjords and in the Gulf of Mexico

Polysaccharides represent a labile, abundant class of marine dissolved organic matter (DOM), which must be hydrolyzed by extracellular enzymes prior to uptake by heterotrophic microbes. Pelagic microbial communities differ in their ability to access polysaccharides: some communities are completely incapable of accessing certain polysaccharides, which are rapidly hydrolyzed elsewhere. This dissertation approaches the questions of how, when, and why microbial communities express specific sets of extracellular polysaccharide hydrolases using four independent but complimentary approaches. First, degradation rates in seawater of several enzymes are measured, and the factors that control those rates are investigated. These rates will help to clarify when and why production of extracellular enzymes can be a viable strategy for heterotrophic marine microbes. Second, hydrolysis rates of six polysaccharides are measured along horizontal and depth gradients in the Gulf of Mexico and compared to glucose turnover rate constants in order to evaluate the potential for spatial variability in the capabilities of microbial communities with respect to polysaccharide degradation and metabolism. Third, microbial communities in a Svalbard fjord were probed in order to investigate why those communities apparently lack the ability to metabolize pullulan, a soluble, linear glucose polymer, even when incubated in the presence of relatively high concentrations of pullulan for weeks. Finally, a new assay for polysaccharide hydrolysis rates in environmental aquatic samples is presented, which raises the possibility for novel experiments which would not be feasible using previous methodology. The results presented here indicate that polysaccharide lability in the ocean is not purely a function of intrinsic factors relating to polysaccharides, but is also strongly dependant on the composition and activity of in situ microbial communities. These characteristics must therefore also be accounted for in models of the marine carbon cycle.