## 3 Results
### Microbial Community Analysis  
Four samples taken from the main biocathode and auxiliary bioachode were analyzed by pyrosequencing. They yielded 14706 (CFB\({}_{\text{up}}\)), 18726 (CFB\({}_{\text{bottom}}\)), 34133 (CAC\({}_{\text{up}}\)) and 32657 (CAC\({}_{\text{bottom}}\)) qualified sequencing reads, with each sample clustered to 1120, 1011, 1808 and 1775 operational taxonomic units (OTUs) based on a threshold of 97%. The Shannon index indicated the CAC\({}_{\text{up}}\) had the highest diversity (Shannon = 4.78), being slightly larger than that of the CAC\({}_{\text{bottom}}\) (Shannon = 4.32), and the CFB\({}_{\text{bottom}}\) showed the lowest diversity (Shannon = 3.02) (Table 1). The coverage value of each sample was more than 0.95, showing sufficient sampling for the assessment of microbial community composition. Qualified reads retrieved from CFB\({}_{\text{up}}\), CFB\({}_{\text{bottom}}\), CAC\({}_{\text{up}}\), and CAC\({}_{\text{up}}\) were assigned to phyla and genera (Figure 6). The four samples mainly belonged to nine phyla, including Proteobacteria, Bacteroidetes, Firmicutes, Chlorofoflexi and so on. Proteobacteria was the most abundant phylum in each sample, with a relative abundance of 52.5%, 69.5%, 32.3% and 33.1% in CFB\({}_{\text{up}}\), CFB\({}_{\text{bottom}}\), CAC\({}_{\text{up}}\), and CAC\({}_{\text{bottom}}\). This was consistent with previous studies, since a large number of nitrifving bacteria and denitrifying bacteria belonged to the Proteobacteria and played an important part in the nitrogen transformation process of the water environment [40]. The second abundant phylum varied among each sample, demonstrating by the fact that CFB\({}_{\text{up}}\) was affiliated with Bacteroidetes(23.6%), while the CFB\({}_{\text{bottom}}\) was affiliated with Chloroflexi(12.3%), and the CAC\({}_{\text{up}}\) and CAC\({}_{\text{bottom}}\) were affiliated with Firmicutes (22.2% and 30.5%, respectively) (Figure 6a) The reason for larger fraction of Bacteroidetes in CFB\({}_{\text{up}}\) than in other samples was likely due to the relatively high DO concentration in the upper part of carbon fiber brush bioachode (\(2.3\pm 0.7\) mg L\({}^{-1}\)), since Bacteroidetes was a common existing phylum in the aerobic condition [41]. Genus level classification was conducted to further investigate the microbial community composition. The dominant genera identified in CFB\({}_{\text{up}}\) were affiliated with _Nitrosomonas_ (20.4%), _Sphingobacterium_ (8.7%), _Xanthomonas_ (7.2%) and _Flavobacterium_ (5.4%) (Figure 6b) It is well known that _Nitrosomonas_ is a main nitrifving bacterium in wastewater treatment plants, where it get rid of excess ammonia by converting it to nitrite [42]. _Nitrosomonas_ was also detected in CFB\({}_{\text{bottom}}\), CAC\({}_{\text{up}}\), and CAC\({}_{\text{bottom}}\) with a relative abundance of 3.7%, 5.4%, and 5.0%, which was much lower than that in CFB\({}_{\text{up}}\), indicating the upper portion of carbon fiber brush biocathode was the main place for ammonia oxidation. The genus _Nitrospira_, constituting a diverse group of nitrite oxidizing bacteria, was also enriched in CFB\({}_{\text{up}}\) (2.9%). Additionally, it was present in CFB\({}_{\text{bottom}}\), CAC\({}_{\text{up}}\), and CAC\({}_{\text{bottom}}\), which was associated with the oxidation of nitrite to nitrate. _Xanthomonas_, identified as dominant in mixed-community biocathodes in a previous study [43], was likely to play a key role in cathodic electron transfer in the TB-MES. The most frequently identified sequence in CFB\({}_{\text{bottom}}\) were assigned to _Azospira_ (13.1%), _Azorcus_ (12.6%), _Anaeroline_ (8.7%), and _Xanthomonas_ (5.4%). The genus _Azorcus_ is a well-known denitrifying species that can anaerobically degrade both organic ethylbenzene and other aromatic hydrocarbons [44].