## 3 Results and Discussion
### Response of Bacterial and Archaeal Communities
#### 3.4.1 Effects of Mixing Ratio on Bacterial Community  
The phylum-level composition of the bacterial community displayed remarkable heterogeneities among various mixing ratio groups, which indicated that the great change in the microbial community distribution was caused by the co-digestion of FW and MA compared with that found in mono-digestion systems. The dominant phyla were Chloroflexi, Bacteroidetes, Thermotogae, Synergistetes, Proteobacteria, Patescibacteria, and Firmicutes, which had \(>\)5% average relative abundance across sequenced samples (Figure 4a). The abundance of Chloroflexi increased with FW addition, and maximum relative abundance was identified in FW/MA = 1:0 (20.24%). Previous studies have revealed that Chloroflexi is a strict anaerobic multicellular filamentous and electroactive bacteria that can participate in glucose degradation and interspecies electron transfer [44]. The large quantity of easily biodegradated matter in FW would be beneficial for Chloroflexi proliferation. The abundances of Bacteroidetes, Firmicutes, and Synergistetes reached their maximums at FW/MA = 1:1 (10.80%, 6.37%, and 7.14%, respectively). Bacteroidetes and Firmicutes, two commonly typical acidogenic bacteria, are typically found as the dominant phyla in anaerobic digestion systems [33]; they can hydrolyze carbohydrates and proteins to produce VFAs [44]. The FW/MA = 1:1 group had a higher macromolecule degradation activity, which was consistent with the highest \(k\) value of 0.45/d that was also obtained in this co-digestion group (Figure 2). The remarkable enrichment of the aforementioned acidogenic bacteria could enhance the efficiency of acidogenesis in the anaerobic digestion process, especially during co-digestion of FW and MA.  
To further illustrate the synergistic effect of FW and MA, the bacterial community structure at the genus level was compared. The genera Mesotoga, Longilinea, Syntrophobacter, Sulfurovum, Syner-01, Anaerolinea, and Flexiline had \(>\)1% of average abundance in different mixing ratio systems (Figure 4b). A previous study reported that Mesotoga could efficiently degrade carbohydrates and short-chain fatty acids to acetic acid [34]. Inconsistently, the abundance of Mesotoga (7.30%) in FW/MA = 1:1 was markedly higher than in other digestion systems. With increasing added MA, the relative abundance of Proteiniphlum in the FW/MA = 0:1 group was 2.5 times higher than that of the FW/MA = 1:0 group. The genus Proteiniphlum was revealed to be responsible for the transformation of cellobiose and crude protein to acetic acid [3]. Overall, co-digestion of FW and MA could facilitate positive co-metabolism of mixtures of substrates (cellobiose, glucose, and starch). The apparent changes in dominant genera further confirmed that co-digestion of FW and MA reshaped the structure of the bacterial community. This may be an important reason explaining the improved biogas production performance seen with co-digestion.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **[PERSON]** & **[PERSON]** & **[PERSON]** & **[PERSON]** & **Goods Coverage** \\ \hline FW/MA = 0:1 & 415 & 557.73 & 3.27 & 0.77 & 0.9987 \\ FW/MA = 1:3 & 409 & 530.78 & 3.58 & 0.84 & 0.9988 \\ FW/MA = 1:1 & 473 & 618.43 & 3.66 & 0.83 & 0.9987 \\ FW/MA = 3:1 & 468 & 617.15 & 3.83 & 0.86 & 0.9986 \\ FW/MA = 1:0 & 450 & 599.47 & 3.94 & 0.88 & 0.9986 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The alpha diversity and coverage in each digestion system.