Nitrous oxide-sink capability of denitrifying bacteria impacted by nitrite and pH

Nitrous oxide (N2O) is a highly potent greenhouse gas and ozone-depleting substance, produced and consumed during denitrification. Evaluation of the N2O production and consumption activities of complete denitrifying bacteria is essential for understanding their capacity to act as N2O sinks in engineered systems for cost-effective nitrogen removal via nitrite (NO2−). However, the physiologies of these N2O-reducing bacteria (N2ORB) are poorly understood. This study aimed to evaluate the physiologies of two N2ORB, Azospira sp. strain I13 and Alicycliphilus denitrificans strain I51. A 15N tracer method was applied to determine N2O production and consumption activities in the co-presence of NO2− and N2O. Both N2ORB displayed a higher N2O consumption rate (RN2O by Azospira sp. strain I13 and Alicycliphilus denitrificans strain I51, 23.85 and 7.60 μmol-N mg-biomass−1h−1, respectively) than N2O production rate (PN2O, 5.88 and 1.32 μmol-N mg-biomass−1h−1, respectively) at an initial NO2− concentration of 2.14 mmol-N L−1 with exogenous addition of N2O, indicating that these N2ORB acted as N2O sinks. On increasing the NO2− concentration from 0.36 to 7.14 mmol-N L−1, the net N2O consumption rate RO_N2O (=RN2O – PN2O) decreased for both N2ORB; the magnitude of the decrease was greater for Azospira sp. strain I13 than for Alicycliphilus denitrificans strain I51. The formation of free nitrous acid (FNA) from NO2− in acidic conditions noticeably affected the N2O sink activities of the N2ORB. A higher FNA concentration decreased RO_N2O for both N2ORB, creating the risk of N2O emission at pH 6 and high NO2− concentration. Our results show the ranges of pH and NO2− concentration where Azospira sp. strain I13 is promising for use as an N2O sink in shortcut nitrogen removal via NO2−.