THE OVERLOOKED MODULATING ROLE OF NITROUS OXIDE FOR CORRINOID-DEPENDENT MICROBIAL PROCESSES

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 (K₁) 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.

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