Doctoral Dissertations

Date of Award

12-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Microbiology

Major Professor

Frank Löffler

Committee Members

Gary Sayler, Alison Buchan, Qiang He

Abstract

Nitrous oxide (N2O) is a potent ozone-depleting greenhouse gas that is primarily produced by microbial denitrification and nitrification in terrestrial and aquatic systems. N2O interferes with cobalt(I)-containing corrinoids, an essential enzyme cofactor in biology facilitating carbon skeleton rearrangements, methyl group transfer and reductive dehalogenation. With increasing N2O emissions and elevated concentrations in air-soil-water systems due to human perturbations of the N cycle, the impacts of N2O on microbial processes that depend on corrinoid-dependent enzyme systems must be elucidated. Methanogenic archaea (methanogens) and organohalide-respiring bacteria (OHRB) play key roles in carbon cycling and both microbial groups rely on the Co(I) supernucleophile in their energy metabolism. This dissertation work investigates the impact of N2O on these two groups of organisms and explores potential strategies to relieve N2O toxicity. Experimental evidences presented demonstrate that micromolar levels of N2O inhibit CH4 production by methanogens as well as reductive dechlorination of chlorinated compounds by OHRB. Detailed kinetic analyses determined the inhibitory constants (KI) of N2O for CH4 production from different methanogenic substrates (i.e., acetate, hydrogen and CO2, methanol) and key steps of bacterial tetrachloroethene (PCE) reductive dechlorination, corroborated that N2O serves as a potent inhibitor of corrinoid-dependent microbial processes. Further 16S rRNA gene-based community composition analyses, as well as qPCR and RT-qPCR assays revealed that N2O affects growth yields and community compositions of methanogen populations. Metabolomic analyses using dechlorinating consortium SDC-9 demonstrated that N2O affects corrinoids and metabolic profiles of dechlorinating communities. The introduction of N2O reducers to N2O-inhibited OHRB cultures restored dechlorination activity, suggesting reversibility of N2O inhibition and highlighting a potentially unrecognized detoxification role for phylogenetically diverse bacteria reducing N2O to dinitrogen gas (N2). Together, the findings of this work reveal an overlooked negative feedback of N2O on corrinoid-dependent microbial processes, including methanogenesis and reductive dechlorination, and provide new information for implementing successful contaminated-site cleanup and enriching Earth System Models with more accurate predictions of greenhouse gas emissions.

Comments

Portions of this document were previously published in journal "Environmental sciences & technology"

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