# Ammonia-Oxidizing Bacterial Communities in Tilapia Pond Systems and the Influencing Factors
## 4 Discussion
Either way, the results mentioned above could not reflect their decisive roles in nitrite concentrations since ammonia-oxidizing archaea (AOA) also played important roles in the process of ammonia oxidization. In sediment, the abundance of _Nitrosococcus_ was positively correlative with TP, TN, and OM. This suggests that _Nitrosococcus_uncultured_bacterium_ (the most abundant AOB species in sediment) may be the facultative autotrophic bacteria, which implicates that it could also utilize organic matters as a carbon source. Meanwhile, the abundance of _Nitrosospira_ was negatively correlative with TN. Thus, TN in sediment may be a potential factor to be used for adjusting the ratio of these two dominant genera. Diversity of Ammonia-Oxidizing Bacterial Community in Water and Surface Sediments and Influencing Factors  
Studies have shown that spatial and temporal factors could all affect the diversity of ammonia-oxidizing bacteria in aquatic ecosystems [27; 28; 29; 30; 31]. The present study is not an exception. Moreover, ammonia-oxidizing bacterial diversity in water and sediment of tilapia ponds showed different response patterns to spatial and temporal variations. As mentioned above, pond differences could only affect the diversity of ammonia-oxidizing bacterial community in sediments rather than that in water, which may suggest that the ammonia-oxidizing bacterial community in sediments was more sensitive to the differences of the culture operations. As far as we know, the ammonia in the aquaculture pond was generated mainly from sediments and the concentrations of ammonia increased with the sediment depth [32]. Further, the ammonia-oxidizing bacteria was mainly distributed in surface sediments, rather than in deep layers of the sediment [8]. Thus, concentrations of ammonia in pore water increased over time as fish grew and feeding rates increased, which may explain why the ammonia-oxidizing bacteria in surface sediments responded first. Meanwhile, concentrations of ammonia in water that diffused from the sediments were not high enough to make ammonia-oxidizing bacteria in water respond in time. The physico-chemical conditions in the water, by contrast, were more easily affected by seasonal variances, which may be the reason why diversity of the ammonia-oxidizing bacteria in water could be affected by monthly variances. Beta diversity reflected by an NMDS analysis further supported the findings that the ammonia-oxidizing bacterial community in water and sediments was mainly affected by monthly variances and pond differences, respectively. In addition to spatial and temporal factors, many environmental parameters, such as salinity [33], DO [34], temperature [35], pH [36], etc., have been proven to be able to influence the ammonia-oxidizing bacterial diversity. The present study shows that, in water, most of the environmental parameters were related to the variations of the beta-diversity of the ammonia-oxidizing bacterial diversity, which may indicate that the variations of the ammonia-oxidizing bacterial communities with the monthly changes were all related to these environmental parameters. TN, nitrate, TP, sulphate, and ammonia were more related. Meanwhile, only nitrite, sulphate, nitrate, and SD were significantly related to changes in the alpha-diversity (Shannon index). Among them, nitrate was positively correlated with the values of the Shannon indices, and nitrite, sulphate, and SD were negatively correlated with the values of the Shannon indices. Nitrate was negatively correlated with _Nitrosospira_ abundance, which may also be attributed to the nitrite identifications carried out by _Nitrosospira_ under aerobic conditions. Further, this may be the reason that nitrate was positively correlated with the Shannon indices of ammonia-oxidizing bacteria. The abundance reduction of the most abundant genus, _Nitrosospira_, together with the abundance increase in the less abundant genera, made the Shannon indices of ammonia-oxidizing bacteria increase.