Doctoral Dissertations

Date of Award

8-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Microbiology

Major Professor

Frank E. Löffler

Committee Members

Alison Buchan, Qiang He, Karen Lloyd

Abstract

Nitrous oxide (N2O) is a potent greenhouse gas and ozone-depleting substance produced by many different pathways in the nitrogen cycle, including nitrification, denitrification, and chemodenitrification. Abiotic sources of N2O such as the chemical reaction between nitrite (NO2-) and ferrous ion (Fe[II]) are generally neglected in studies of N turnover in soils. Abiotic controls containing nitrate (NO3-) and ferric iron (Fe[III]) fail to capture potential reactions between intermediates of N cycle pathways (e.g., NO2- as an intermediate in NO3 - reduction to ammonium [NH4+] or nitrogen gas) or replenishment of reactants by iron cycling. Recent studies suggest that Fe(II) plays an important role in N turnover through combined biotic and abiotic reactions. Anaeromyxobacter dehalogenans strain 2CP-C is a common soil bacterium with a versatile metabolism, including microaerobic and anaerobic respiration. A. dehalogenans reduces Fe(III) to Fe(II) and reduces NO3- to NH4+ via respiratory ammonification. The present work investigates the mechanisms by which A. dehalogenans utilizes O2 and the synergistic effects of Fe(III) and NO3- reduction. Evidence for respiration of 21% O2 via a low-affinity cytochrome c oxidase is presented, further expanding the respiratory versatility of A. dehalogenans. A previously unrecognized ecophysiological role is revealed in which A. dehalogenans reduces NO3- to approximately 50% N2 and 50% NH4+ via coupled biotic-abiotic reactions, revealing a mechanism for denitrification in the absence of nitric oxide (NO)-forming NO2- reductases. Further analysis of publicly available sequenced genomes reveal other microorganisms with the potential to couple biotic-abiotic reactions for reduction of NO3- to N2 in the absence of NO-forming NO2- reductases. Finally, the effects of sulfide and molybdenum are tested, demonstrating that A. dehalogenans reduces NO3- to NH4+ and NO2- to N2O in the presence of Fe(II) and sulfide, due to the inhibition of NO3- and N2O reductases by sulfide sequestration of trace metals. Collectively, this work demonstrates an underestimated mechanism for denitrification and expands our knowledge of the role of A. dehalogenans in O2 respiration and Fe and N cycling. Future efforts assessing the fate of N in agricultural soils should take into account the iron, molybdenum, and sulfide content of soils in addition to molecular information.

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