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Nitrous Oxide Is a Potent Inhibitor of Bacterial Reductive Dechlorination

Yin, Yongchao, Yan, Jun, Chen, Gao, Murdoch, Fadime Kara, Pfisterer, Nina, Löffler, Frank E.
Environmental science & technology 2018 v.53 no.2 pp. 692-701
Dehalococcoides mccartyi, Geobacter, agricultural runoff, bacteria, bioremediation, cobalt, dechlorination, enzymes, ethylene, groundwater, models, nitrates, nitrous oxide, organochlorine compounds, tetrachloroethylene, vinyl chloride, watersheds
Organohalide-respiring bacteria are key players for the turnover of organohalogens. At sites impacted with chlorinated ethenes, bioremediation promotes reductive dechlorination; however, stoichiometric conversion to environmentally benign ethene is not always achieved. We demonstrate that nitrous oxide (N₂O), a compound commonly present in groundwater, inhibits organohalide respiration. N₂O concentrations in the low micromolar range decreased dechlorination rates and resulted in incomplete dechlorination of tetrachloroethene (PCE) in Geobacter lovleyi strain SZ and of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) in Dehalococcoides mccartyi strain BAV1 axenic cultures. Presumably, N₂O interferes with reductive dechlorination by reacting with super-reduced Co(I)–corrinoids of reductive dehalogenases, which is supported by the finding that N₂O did not inhibit corrinoid-independent fumarate-to-succinate reduction in strain SZ. Kinetic analyses revealed a best fit to the noncompetitive Michaelis–Menten inhibition model and determined N₂O inhibitory constants, KI, for PCE and cDCE dechlorination of 40.8 ± 3.8 and 21.2 ± 3.5 μM in strain SZ and strain BAV1, respectively. The lowest KI value of 9.6 ± 0.4 μM was determined for VC to ethene reductive dechlorination in strain BAV1, suggesting that this crucial dechlorination step for achieving detoxification is most susceptible to N₂O inhibition. Groundwater N₂O concentrations exceeding 100 μM are not uncommon, especially in watersheds impacted by nitrate runoff from agricultural sources. Thus, dissolved N₂O measurements can inform about cDCE and VC stalls at sites impacted with chlorinated ethenes.