## 3 Results and Analysis
### The Impact of Sponge Iron on the Microbial Community Structure
#### 3.3.2 Analysis of the Microbial Community Structure
However, the addition of SFe  
Figure 6: Microbial analysis at the phylum level for each sample (top 10%): (a) community structure analysis and (b) hierarchical clustering analysis. reduced the relative abundance of _Bacteroidetes_, indicating that the growth and reproduction process of _Bacteroidetes_ is sensitive to iron ion concentration, and the higher the iron ion concentration, the worse its activity. _Chloroflexi_ is a facultative anaerobic bacterium that can strengthen the biofilm structure [43] and has a strong effect on organic matter degradation [44,45], with relative abundances of 4.73%, 3.11%, 3.64%, and 4.63%, respectively. This indicates that with the operation of the reactor, a stable anaerobic zone is gradually formed in the BSIS system, providing a good living environment for facultative anaerobic bacteria. _Acidobacteria_ has a high relative abundance in the BSIS system, with 2.43%, 4.72%, and 4.52% in the initial-operation phase, stable-operation phase, and final-operation phase of the BSIS sludge, respectively, all higher than the 0.89% in the conventional activated sludge system. _Acidobacteria_ is a typical heterotrophic microorganism that has the ability of nitrification and aerobic denitrification and can carry out these two processes simultaneously [46,47]. The increase in the relative abundance of _Acidobacteria_ in the BSIS system indicates that SFe can promote its growth and reproduction, and the \"anaerobic-anoxic-aerobic\" microenvironment created by the addition of SFe SPBF is beneficial for the interaction between heterotrophic nitrifying aerobic denitrifying microorganisms and various nitrogen substances. _Actinobacteria_ have been proven to have a denitrification effect under anaerobic conditions [48], and their relative abundance in the BSIS system (3.04%, 1.72%, and 1.68%) is lower than that in the conventional activated sludge system (4.88%), indicating that the addition of SFe inhibits the growth and reproduction of _Actinobacteria_. _Nitrospine_ contains many nitrifying bacteria with good nitrification ability [49] and has the highest relative abundance in the stable-operation phase of the BSIS system, at 2.30%, indicating that SFe promotes the role of _Nitrospine_. The aggregation of these phyla in the reactor with an SFe dosage of 90 g/L is beneficial to the transformation of nitrogen and the removal of organic pollutants in the BSIS system, which is also consistent with the results shown in Figures 2 and 3. At the genus level (Figure 7), a notable shift was observed in the microbial community from L1 to L4. The reddish-hued groups experienced significant fluctuations, and the relative abundance of certain genera decreased substantially. The most abundant dominant species in the four samples is _Saccharimonadales_. In the stable conventional activated sludge system and the BSIS system sludge, during the initial-operation phase, stable-operation phase, and final-operation phase, they account for 21.68%, 16.24%, 33.63%, and 38.55%, respectively. _Saccharimonadales_ are known to promote the formation of aerobic granular sludge [50]. There is a positive correlation between the denitrification genes within _Saccharimonadales_ and the removal efficiency of NO\({}_{3}\)\({}^{-}\)-N [51], making them common denitrifiers [52]. Within optimal iron concentration ranges, _Saccharimonadales_ are capable of synthesizing requisite iron-dependent enzymes, thereby sustaining efficient metabolic processes and achieving maximal growth rates as well as cellular yields. [PERSON] [53] uncovered a significant positive correlation between the relative abundance of _Saccharimonadales_ bacteria in the rhizosphere soil and the available iron content, suggesting that _Saccharimonadales_ may play a role in the iron cycling within the soil and potentially assist plants in acquiring this essential nutrient. The increase in iron ion concentration substantially promotes the growth and reproduction of _Saccharimonadales_, which aligns with the trends in the physicochemical properties of the samples in this study.