Title: Structural and Functional Microbial Ecology of Denitrifying Bacteria Using Different Organic Carbon Sources
Description: This dissertation research represents one of the first attempts to investigate the structural and functional microbial ecology of methanol, ethanol and glycerol fostered denitrification. The overarching goal of this research was to elucidate the link between the structure and function of denitrifying microbial populations grown on different carbon sources. Specific objectives were to: 1) diagnose bacteria specifically assimilating methanol and ethanol and determine denitrification kinetics induced by the two carbon sources; 2) investigate factors leading to nitrous oxide (N2O) and nitric oxide (NO) emissions from methanol and ethanol feeding denitrification reactors; 3) characterize glycerol assimilating populations that perform suspended- and biofilm-growth denitrification; 4) examine the potential of using alcohol dehydrogenase gene as a biomarker for methanol and glycerol induced denitrification activity; 5) evaluate the impact of different carbon sources (methanol and ethanol) on the transcript and proteome of a model facultative methylotroph, Methyloversatilis universalis FAM5.

First, the technique of DNA stable isotope probing and quantitative polymerase chain reaction were adapted to diagnose and track methylotrophic denitrifying bacteria in activated sludge. Methanol assimilating populations in the methanol fed denitrifying sequencing batch reactor (SBR) were Methyloversatilis spp. and Hyphomicrobium spp. related species. Upon switching to ethanol, only Methyloversatilis spp. was sustained pointing to their metabolic versatility at least with respect to carbon assimilation. This study represents one of the first investigations of the existence and utilization of facultative methylotrophy in activated sludge. Second, the potential of N2O and NO emitted from methanol and ethanol fed denitrifying SBRs was studied during different transient shocks, including organic carbon limitation, nitrite inhibition and oxygen inhibition. Organic carbon limitation and exposure to nitrite did not result in statistically significant emissions over the control. However, statistically higher N2O emissions were observed during exposure to oxygen on the ethanol fed biomass and coincided with sustained denitrification activity in the presence of oxygen.

Therefore, the results suggest that the dosage of ethanol to anoxic zones needs to be strictly controlled to minimize N2O emissions in the downstream aerobic zones. Third, the structure-function analysis of denitrification was extended to glycerol (the main component of biodiesel waste and a potential replacement for methanol) in both suspended and biofilm phases of a hybrid integrated fixed-film bioreactor. During long-term operation on glycerol, the biofilm community had a higher phylogenetic diversity (dominated by Comamonas spp., Bradyrhizobium spp., and Tessaracoccus spp.), and lower denitrification kinetics than the suspended community (dominated by Comamonas spp. and Diaphorobacter spp.). Distinct identities of glycerol assimilating populations due to the different substrate availability in the suspended and biofilm phases were shown for the first time. Fourth, carbon source-specific biomarkers of denitrification activity based on gene expression were developed. Based on short-term batch denitrification activity assays as well as long-term bioreactor operation, the applicability of alcohol dehydrogenase gene expression as quantitative descriptors of denitrification activity on methanol and glycerol in mixed cultures was demonstrated.

Finally, Methyloversatilis universalis was selected as model organism to study the effects of varying electron donors (from methanol to ethanol) on its gene and protein expression profiles. Genes encoding essential enzymes that involve carbon oxidation, C1 assimilation and central metabolism were found to be differentially expressed during growth on methanol and ethanol. Several physiological and metabolic responses by M. universalis pointed to a well-defined strategy to overcome carbon limitation for surviving in engineered or natural denitrifying environments. In sum, the structural and functional ecology and the metabolism of heterotrophic denitrification on methanol, ethanol and glycerol as applicable to engineered denitrifying bioreactors was investigated in detail. From an engineering perspective, the knowledge gained can help to guide the selection and application of potential organic carbon sources for denitrification in biological nitrogen removal systems. It is expected that such judicious selection can also eventually result in better design, operation and control of engineered nitrogen removal processes and thus help attain ever more stringent nitrogen standards.