## 3 Results and Discussion
### Analysis of Nitrification and Denitrification Capacity  
To characterize the heterotrophic nitrification and denitrification capacities of strain F19, ammonium nitrate, sodium nitrite, and hydroxylamine hydrochloride were used as nitrogen sources at a C/N ratio of 2, an initial pH of 10.0, a culturing temperature of 25 \({}^{\circ}\)C, and with sodium acetate as the carbon source. The cell growth and ammonium nitrogen removal performance are shown in  
Figure 3: Effects of different carbon sources on the nitrogen removal capacity of strain F19. (**A**) Cell growth. (**B**) Ammonia removal. Figure 4. A lag period was observed initially, and the OD\({}_{600}\) value of strain F19 increased slowly from 0.01 to 0.52 after 24 h. Cell growth then entered the logarithmic phase and increased significantly from 0.52 to 1.26 at 48 h. NH\({}_{4}\)+-N at an initial concentration of 46.41 mg/L NH\({}_{4}\)+-N was rapidly reduced to 1.65 mg/L, and the corresponding removal efficiency reached 96.44% after 72 h of cultivation. The initial 46.35 mg/L nitrate was decreased to 3.56 mg/L after 36 h, the removal efficiency reached 92.32%, and the removal rate was 1.19 mg/L/h (Figure 4A). Moreover, accumulation of the nitrite intermediate was not detected. These experimental results indicate that strain F19 could conduct heterotrophic nitrification and aerobic denitrification simultaneously under aerobic conditions and efficiently carry out ammonia removal without nitrite accumulation during nitrogen conversion. To investigate the denitrification capacity of strain F19, NO\({}_{2}\)-N at an initial concentration of 9.84 mg/L and NH\({}_{2}\)OH at an initial concentration of 43.35 mg/L were used as nitrogen sources in DM2 and DM3, respectively. Using nitrite as a nitrogen source, excellent nitrite nitrogen removal efficiency was observed for strain F19. NO\({}_{2}\)-N was decreased from 9.84 to 0.25 mg/L within 36 h  
Figure 4: Cell growth and nitrogen removal characterization of strain F19, and demonstration of simultaneous heterotrophic nitrification and aerobic denitrification. (**A**) Nitrification ability of F19 in ammonium nitrate medium. (**B**) Denitrification ability of F19 in nitrite and (**C**) hydroxylamine medium. (**D**) PCR amplification of periplasmic nitrate reductase (_napA_), hydroxylamine oxidoreductase(_hao_), and nitrite reductase (_nitS_) genes. (**E**) Predicted nitrogen utilization pathway in _Janthinobacterium swallowensis_ F19. of incubation (Figure 4B). Approximately 97.46% of nitrite was removed, and the corresponding maximum removal rate was 0.27 mg/L/h, higher than observed previously for _Pseudomonas stutzeri_ and _Paracoccus denitrificans_[36]. Unexpectedly, significant nitrate accumulation was observed during the denitrification of nitrite, but this was removed gradually with decreasing nitrite. Previous reports have indicated that large amounts of nitrite nitrogen can inhibit bacterial growth and thereby repress their nitrification and denitrification activities [19]. However, our results showed that the bacterial growth and denitrification capacity of strain F19 were not affected by the concentration of nitrite. This could indicate the countercurrent production of nitrate in the nitrite denitrification process via a unique pathway for nitrite detoxification. When hydroxylamine was used as the sole nitrogen source, strain F19 achieved a similarly high removal efficiency of 96.69%, and NH\({}_{2}\)OH was decreased from the initial 43.35 to 1.43 mg/L within 24 h (Figure 4C). Taken together, these results suggest that strain F19 possesses broad substrate utilization capabilities and efficient aerobic denitrification capacity. To further investigate the possible pathway of simultaneous nitrification and aerobic denitrification, and the enzymes involved, hydroxylamine oxidoreductase (HAO), periplasmic nitrate reductase (NAP), and nitrite reductase (NirS), were used as functional biomarkers that are related to the physiology of nitrifying and denittifying bacteria [37].