# Ammonia-Oxidizing Bacterial Communities in Tilapia Pond Systems and the Influencing Factors
## 3 Results
### Ammonia-Oxidizing Bacterial Communities and Their Influencing Factors  
All the OTUs obtained from all samples could be assigned to five genera, _Nitrosospira_, _Nitrosococcus_, _Nitrosomonas_, _Proteobacteria_unclassified_, and _Nitrosomonas_ (Figure 1), among which _Nitrososira_ and _Nitrosococcus_ were the dominant bacterial groups in water as well as surface sediments. Further, comparisons of the five genera in the two environments showed that all their abundances were significantly different in water versus sediments (\(p<0.05\)). _Nitrosospira_ and _Nitrosococcus_ were more abundant in sediments (the relative abundances of them were 44.13% and 41.79% respectively) than in water (40.64% and 29.05%), while _Nitrosomonas_, _Proteobacteria_unclassified_, and _Nitrosomonasaceae_unclassified_ were more abundant in water (15.93%, 8.60% and 5.77%) than in sediments (6.98%, 4.53% and 1.55%). Further, we performed a one-way ANOVA on the abundances of the five genera between different months and ponds. The results showed that monthly changes, rather than pond differences, could cause significant variances in the abundances of all five genera in water (\(p<0.05\)) (Figure 2A,B). Meanwhile, monthly changes and pond differences could all cause significant variances in the abundances of all five genera in sediment (\(p<0.05\)) (Figure 2C,D). Figure 1: The abundance comparisons ammonia-oxidizing bacterial community compositions in genus level between water and surface sediment. In addition, correlation analyses were performed between the abundances of the three most abundant genera and physio-chemical parameters, and the results are listed in Table 1. The results of the water samples are as follows: only the pH was significantly and positively related to the relative abundance of _Nitrosospira_, while nitrite and nitrate were significantly and negatively related to it. That means that the abundance of _Nitrosospira_ increased with the increase in pH value (when the pH is in the neutral to weakly alkaline range), while, as the ammonia-oxidation products, nitrite and nitrate presented an inhibition effect on the abundance of _Nitrosospira_. Nitrite, sulphate and DO were significantly and positively related to the relative abundance of _Nitrosococcus_, while TP, ammonia, and pH were negatively related to it. This indicates that _Nitrosococcus_ can significantly reduce the concentration of ammonia and increase the concentration of nitrite under aerobic conditions. Figure 2: The abundance variations of ammonia-oxidizing bacteria in genus level between months changes and pond differences. **(A,B)** represent the abundance variations of the five genera in water with month changes and pond differences, respectively; **(C,D)** represent the abundance changes in the five genera in sediment with month changes and pond differences, respectively. The different letters above the columns mean that there are significant differences between groups at 95% level, “ab” represents that this group has no significant differences with either the group of “a” or “b”. TP, ammonia, and pH were significantly and positively related to the relative abundance of _Nitrosomonas_, while only nitrite was significantly and negatively related to it. This indicates that ammonia promoted the ammonia oxidation reaction based on the existence of _Nitrosomonas_ and made the concentration of nitrite increase significantly. In sediments, the relative abundance of _Nitrosospira_ was negatively related to TN; the relative abundance of _Nitrosococcus_ were positively related to TN, TP, and OM; the relative abundance of _Nitrosomonas_ were negatively related to TN, TP, and OM. Moreover, due to the fact that the two most abundant genera, _Nitrosospira_ and _Nitrosococcus_, could be affected by pond differences, correlation analyses were also performed between these genera and stock densities (STD) and feed coefficients (FC). The results showed that (Table 1) the abundance of _Nitrosospira_ was significantly and positively related to STD and negatively related to FC.