# Ammonia-Oxidizing Bacterial Communities in Tilapia Pond Systems and the Influencing Factors
## 4 Discussion  
The present study firstly described the ammonia-oxidizing bacterial communities of both water and surface sediments of tilapia aquaculture ponds, and investigated their  
\begin{table} \begin{tabular}{c c c c} \hline \hline
**Source** & **Environmental Vectors** & **R2** & **Pr** \\ \hline \multirow{8}{*}{Water} & TP & 0.796 & 0.001 * \\  & TN & 0.876 & 0.001 * \\  & Ammonia & 0.762 & 0.001 * \\  & Nitrite & 0.399 & 0.002 * \\  & Sulfate & 0.794 & 0.001 * \\  & Nitrate & 0.822 & 0.001 * \\  & TOC & 0.296 & 0.003 * \\  & pH & 0.121 & 0.118 \\  & DO & 0.306 & 0.003 * \\  & SD & 0.642 & 0.001 * \\  & ORP & 0.372 & 0.003 * \\ \hline \multirow{4}{*}{Sediment} & sTP & 0.608 & 0.001 * \\  & sTN & 0.231 & 0.019 * \\ \cline{1-1}  & OM & 0.214 & 0.017 * \\ \hline \hline \end{tabular}   
responses to monthly variations, pond differences, and changes in the physico-chemical parameters. Ammonia-Oxidizing Bacterial Community Compositions in Water and Surface Sediments and Influencing Factors  
The ammonia-oxidizing bacterial community compositions at the genus level and the abundant genera between water and surface sediments were all the same. This is similar to that in other natural water ecosystems [20; 21]. This indicates a close connection of the ammonia-oxidizing bacterial communities between the two habitats. This connection is based on the similarity of these two habitats and the finite physical distance. The relationship between the physicochemical characteristics of the habitats, the functional processes therein, and the microbial community are often closely interwoven [22]. In addition, the promoting effect of biological movements can also promote the communication of microbes between two habitats. On the other hand, all five genera showed significantly different abundances between water and surface sediments. This may indicate that the environmental factors of these two habitats caused different degrees of functional redundancy with respect to ammonia oxidization [23]. Further, of all the five genera, _Nitrosomonas, Proteobacteria_unclassified_, and _Nitrosomonasaceae_unclassified_ appeared the least abundant in water, although water may be their main habitat. However, the case for _Nitrosospira_ and _Nitrosococcus_ was the exact reverse. In water, the abundances of all five genera were affected by monthly variations rather than by pond differences. This indicates that different cultivation practices, including differences in stocking densities, differences in feed input, etc., could not affect the five ammonia-oxidizing bacterial genera. However, the monthly variations were the main factor causing the abundance changes. Meanwhile, in sediment, the abundances of the two most abundant genera, _Nitrosospira_ and _Nitrosococcus_, were hardly affected by monthly variations. They were, however, affected by pond differences. Further, the pond differences were the main influencing factors. As such, the pond differences were closely related to the cultivation management (stocking density and feed coefficients). In addition, lower abundance of _Nitrosopira_ and higher abundance of _Nitrosococcus_ seemed to be a characteristic of the lower stocking density of tilapia. In the present study, the abundances of _Nitrosospira_ and _Nitrosomonas_ were all negatively correlated with nitrite in water. This may be related to the fact that these two betaprotebacterial AOB can not only oxidize the ammonia to nitrite, but also carry out nitrifier denitrification under aerobic conditions [24]. In intensive aquaculture ponds, owing to the continuous feed input and fish respiration, the pond water is often in a state of oxygen deficiency. This may supply suitable dissolved oxygen conditions for nitrifier denitrification. Nitrite has also been shown to be toxic to _Nitrosomonas_[25] and to influence the population structure of _Nitrospira_[26]. The abundance of _Nitrosococcus_, to the contrary, was in positive correlation with nitrite.