Nitrous Oxide Is a Potent Inhibitor of Bacterial Reductive Dechlorination.

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 (NO), a compound commonly present in groundwater, inhibits organohalide respiration. NO 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, NO interferes with reductive dechlorination by reacting with super-reduced Co(I)-corrinoids of reductive dehalogenases, which is supported by the finding that NO 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 NO inhibitory constants, K, 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 K 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 NO inhibition. Groundwater NO concentrations exceeding 100 μM are not uncommon, especially in watersheds impacted by nitrate runoff from agricultural sources. Thus, dissolved NO measurements can inform about cDCE and VC stalls at sites impacted with chlorinated ethenes.