## 3 Results and Discussion
### Factors Affecting Nitrogen Removal by Strain F19
#### 3.2.2 Effects of pH and Temperature  
The effect of pH on the nitrogen removal efficiency capacity of F19 was investigated between pH 5.0 and 11.0. F19 was found to be an alkaline-tolerant bacterium with a maximal NH4+-N removal efficiency of 93.99% and maximal cell growth at pH 10.0 (Figure 2C). The strain was remarkably stable over a wide pH range. After incubating at pH values ranging from 5.0 to 11.0 for 36 h, strain F19 retained residual nitrogen removal activity of more than 50% and 90% at 5.0-7.0 and 8.0-11.0, respectively, compared with controls. However, acidic conditions were tremendously harmful to cell growth; at pH 5, the cell density was only 25% that of the maximal absorbance at 600 nm (OD\({}_{600}\) value), resulting in a residual nitrogen removal rate of only 50.07%.  
This result suggested that strain F19 had strong alkaline resistance, which was beneficial for cell growth under alkaline conditions. The efficient removal of ammonium under alkaline conditions  
Figure 2: Analysis of factors affecting the nitrogen removal capacity and cell growth of strain F19. (**A**,**B**) C/N ratio. (**C**,**D**) pH. (**E**,**F**) Temperature and time.  
may be due to the presence of greater quantities of free NH\({}_{3}\) in alkaline medium, which is especially conducive to nitrification [30]. This result is consistent with some other heterotrophic bacteria [25] and suggests that the pH tolerance of nitrogen removal by F19 may be superior to that of most previously characterized aerobic denitrifrifiers [19; 31]. This indicates that strain F19 may have potential for use in alkaline wastewater treatment, especially for the efficient bioremediation of alkaline soils [32; 33]  
Figure 2E shows that the optimal temperature was 30 \({}^{\circ}\)C, with a maximal NH\({}_{4}\)\({}^{+}\)-N removal rate of 95.91% after 36 h, and a similar removal rate was measured at temperatures between 25 and 35 \({}^{\circ}\)C. At lower temperatures, strain F19 exhibited relatively high removal efficiency; even at 10-20 \({}^{\circ}\)C, the enzyme maintained almost 60% of its maximal activity. When the temperature was lower than 5 \({}^{\circ}\)C, the removal rate with 40 mg/L NH\({}_{4}\)\({}^{+}\)-N was almost 40% of maximal activity, with a NH\({}_{4}\)\({}^{+}\)-N removal rate of 0.44 mg/L/h. Notably, the maximum OD\({}_{600}\) value at 5 \({}^{\circ}\)C was 0.58 after 48 h, with a maximum specific growth rate of 0.01-0.02 OD\({}_{600}\)/h at a C/N ratio of 2:1. This growth rate was much higher than the rate of 0.0039 OD\({}_{600}\)/h achieved by _Janthinobacterium sp_. M-11 at 2 \({}^{\circ}\)C [27] and will have greater potential application. The cell growth rate is of great relevance to cell colonization during nitrogen metabolism [34]. The number of species employed in these previous reports for water treatment is not suitable for the actual conditions needed for domestic sewage treatment [35]. By contrast, the relatively fast growth rate of strain F19 at low temperatures indicates potential for use in existing mainstream wastewater treatment processes, making it favorable for most areas of China during winter and spring.