Fig. 1. NH₄⁺ oxidation coupled with extracellular electron transfer.

a Photographs of serum vials after 216 h of incubation with different species of anammox bacteria, ¹⁵NH₄⁺, and graphene oxide (GO). The presence of black precipitates indicates the formation of reduced GO (rGO). No obvious change in color was observed in the abiotic control vials after the same period of incubation with ¹⁵NH₄⁺ and GO. b Raman spectra of the vials after 216 h of incubation. Peaks in bands of 2D and D + D′ located at ∼2700 and ∼2900 cm⁻¹, respectively, indicate the formation of rGO. c ³⁰N₂ production by different anammox bacteria from ¹⁵NH₄⁺ and GO as the sole electron acceptor. Anammox cells were incubated with 4 mM ¹⁵NH₄⁺ and GO to a final concentration of 200 mg L⁻¹. There was no ²⁹N₂ formation throughout the experiment. NO and N₂O were not detected throughout the experiment. Results from triplicate serum vial experiments are represented as mean ± SD.