Fig. 1.

Summary of N₂O production pathways during marine archaeal ammonia oxidation. (A) The hybrid pathway, which combines one N atom from NH₂OH and one from nitrite (either ambient or newly produced inside the cell), has been observed previously (10–12). Black arrows represent known pathways for N atoms; dashed pathways represent hypothesized pathways involving NO. (B) Potential N₂O pathways and the N and O atom sources identified in the current study. Five N₂O pathways are identified: 1) NH₂OH oxidation; 2) hybrid pathway a: NH₂OH reaction with NO that was sourced from the reduction of newly produced NO₂⁻; 3) hybrid pathway b: NH₂OH reaction with NO that was sourced from ambient NO₂⁻; 4) reduction of newly produced NO₂⁻; and 5) reduction of ambient NO₂⁻. Color of the N and O atoms depicts the sources: red and black denote N atoms from NH₄⁺ and ambient NO₂⁻, respectively; blue, purple, and gray denote O atoms from O₂, H₂O, and ambient NO₂⁻, respectively; green represents O atoms of newly produced NO₂⁻, which is a mixture of H₂O and O₂. The gray square denotes the membrane and periplasmic space of the AOA cell. The black arrows represent the ammonia oxidation pathway, the red arrows show the potential N₂O production processes, and the dashed arrows indicate potential pathways that are unresolved in our study. (C) Summary of the N, O atom sources of N₂O and the fractional contribution of each pathway during marine archaeal ammonia oxidation with initial NH₄⁺: NO₂⁻ ratio of 1:1.