Kinetics of nitrous oxide (N₂O) formation and reduction by Paracoccus pantotrophus

Nitrous oxide (N₂O) is a powerful greenhouse gas emitted from wastewater treatment, as well as natural systems, as a result of biological nitrification and denitrification. While denitrifying bacteria can be a significant source of N₂O, they can also reduce N₂O to N₂. More information on the kinetics of N₂O formation and reduction by denitrifying bacteria is needed to predict and quantify their impact on N₂O emissions. In this study, kinetic parameters were determined for Paracoccus pantotrophus, a common denitrifying bacterium. Parameters included the maximum specific reduction rates, q^, growth rates, μ ^, and yields, Y, for reduction of NO₃ ⁻ (nitrate) to nitrite (NO₂ ⁻), NO₂ ⁻ to N₂O, and N₂O to N₂, with acetate as the electron donor. The q^ values were 2.9 gN gCOD⁻¹ d⁻¹ for NO₃ ⁻ to NO₂ ⁻, 1.4 gN gCOD⁻¹ d⁻¹ for NO₂ ⁻ to N₂O, and 5.3 gN gCOD⁻¹ d⁻¹ for N₂O to N₂. The μ ^ values were 2.7, 0.93, and 1.5 d⁻¹, respectively. When N₂O and NO₃ ⁻ were added concurrently, the apparent (extant) kinetics, q^ _app, assuming reduction to N₂, were 6.3 gCOD gCOD⁻¹ d⁻¹, compared to 5.4 gCOD gCOD⁻¹ d⁻¹ for NO₃ ⁻ as the sole added acceptor. The μ ^ _app was 1.6 d⁻¹, compared to 2.5 d⁻¹ for NO₃ ⁻ alone. These results suggest that NO₃ ⁻ and N₂O were reduced concurrently. Based on this research, denitrifying bacteria like P. pantotrophus may serve as a significant sink for N₂O. With careful design and operation, treatment plants can use denitrifying bacteria to minimize N₂O emissions.