## 3 Results
### Toxicity of BPA in Activated Sludge
#### 3.1.5 BPA Interferes with the Microbial Population Structure of Activated Sludge  
There were 24 common microorganisms in the sludge of the control group and the BPA group, six unique microorganisms in the activated sludge of stage III of the reactor under BPA stress, and one unique microorganism in the sludge of both stage I and stage II (Figure S2). The results showed that different concentrations of BPA could lead to changes in the indigenous microbial community structure of activated sludge. Although the species  
Figure 4: Effect of bisphenol A (BPA) on the dehydrogenase activity (DHA) of sequencing batch reactor (SBR) activated sludge. composition in domesticated sludge and BPA-stressed sludge was similar, some species differed. Alpha diversity reflects the species abundance and species diversity of a single sample. Cha01 and Ace indices measure species abundance. Shannon and Simpson indices are used to measure species diversity, and are influenced by species abundance and species evenness in the sample community. In the case of the same species abundance, the greater the evenness of each species in the community, the greater the diversity of the community and the Shannon index value, and the smaller the Simpson index value, indicating that the species diversity of the sample is higher. Compared with the domesticated sludge, the Chao1, Ace, and Shannon indices of BPA-stressed activated sludge continued to decrease, while the Simpson index increased (Table 1). The results showed that BPA significantly reduced the species richness and diversity in activated sludge. BPA had an inhibitory effect on some microorganisms that was not conducive to their growth and eventually led to a decrease in microbial diversity, which was consistent with the results obtained from the viable bacteria ratio. The results revealed differences in the microbial community structure of activated sludge before and after BPA addition. Activated sludge is one of the most important components of sewage treatment plants, and is the key to the biodegradation of pollutants. The community composition of activated sludge directly affects the treatment efficiency of sewage treatment plants. Therefore, this study first analyzed the domestication sludge community composition at the bacterial phylum level. As shown in Figure 5A, the phyla with higher abundances in domestic sludge were Proteobacteria (50.17%), Bacteroidetes (39.41%), Patescibacteria (7.19%), and Nitrospirota (0.23%). This finding was similar to the results of [PERSON] et al., who sequenced activated sludge in WWTPs worldwide [52]. Compared with domesticated sludge, the dominant microbial phyla of activated sludge under BPA stress were Proteobacteria (stage I: 71.79%; stage II: 95.06%; and stage III: 99.7%), Bacteroidetes (stage I: 24.7%; stage II: 4.84%; and stage III: 0.25%), and Patescibacteria (stage I: 3.16%; stage II: 0.08%; and stage III: 0%). The results showed that different concentrations of BPA had a significant impact on the activated sludge microorganisms. Some indigenous microorganisms competed, but could not adapt well to BPA, and were eliminated, or decreased in abundance due to the toxicity of BPA. At the bacterial species level, the domesticated sludge had high biodiversity, and the dominant bacterial species were _Flavobacterium succinicans_ (10.87%) and _Ferribacterium limnticeum_ (10.58%). The dominant bacteria in the first stage in the BPA group reactor were _Methyllotenera mobilis_ (36.41%) and _Flavobacterium succinicans_ (11.02%). The dominant bacteria in stages II and III of the BPA group reactor were _Comanomas korensis_ (19.14-53.78%), _Acinetobacter bohemicus_ (26.78-21.80%), and _Brevundomonas bullata_ (21.46-0.7%) (Figure 5B). [PERSON] et al. used bisphenols to domestic activated sludge, and the results of 16S rRNA sequencing also indicated that _Methyllotenera_ was the dominant genus [53]. The abundance of _M.