## 3 Results and Discussion
### Shifts in Microbial Community Structures with Seasonal Temperature
Although the relative abundance of the hydrogenotrophic genera was much higher than the acetoclastic genera in winter, COD removal and biogas production in winter were lower compared with in summer. The low temperature in winter may affect more the H\({}_{2}\)/CO\({}_{2}\) methanogenesis than H\({}_{2}\)/CO\({}_{2}\)-dependent acetate production, and decrease the conversion rate of H\({}_{2}\)/CO\({}_{2}\) to methane and increase the acetate accumulation [41; 44]. In contrast to the archaeal community, the bacterial community in the AWFR system showed high diversity: 33 taxonomic categories at phyla level were identified (Table S1). This result was consistent with the previous studies [45; 46; 47]. This high diversity might suggest that a variety of bacterial communities participated in multiple metabolic pathways of organic matter degradation under seasonal temperature variation. The predominant bacteria in the AWFR system during the whole operation period were grouped into four phyla affiliated with _Firmicutes_, _Proteobacteria_, _Bacteroides_, and _Chloroflexi_, which occupied 89.3 \(\pm\) 4.08% of the total phyla (Figure 9). These dominant phyla were also detected in the full-scale biogas digesesters [47; 48]. During the start-up period, the predominant microorganisms in the AWFR system belonged to the phylum _Proteobacteria_, _Firmicutes_, _Bacteroides_ and _Chloroflexi_, with a relative abundance of 33.01%, 22.19%, 19.94% and 11.29%, respectively. With an increase of temperatures from spring to summer, the bacterial community structures remarkably changed. The abundance of the phylum _Firmicutes_ increased from 37.26% to 64.97%, whereas the relative abundances of the phyla _Proteobacteria_, _Bacteroides_, and _Chloroflexi_ decreased from 28.02% to 14.96%, 16.03% to 10.06%, and 6.00% to 3.87%, respectively. Interestingly, with a decrease of seasonal temperatures from summer to winter, the relative abundance of _Firmicutes_ drastically decreased from 64.97% to 29.39%. In contrast, _Proteobacteria_, and _Bacteroides_ obviously increased from 14.96% to 31.50% and 10.06% to 20.66%, respectively. Each phylum in our system likely possessed different tolerance and adaptation mechanisms to seasonal temperature variations. This was consistent with previous studies, stressing that temperature was the vital factor affecting the structures of the bacterial community in anaerobic digestion [48]. To gain further insight into the bacterial community structure in the AWFR system, some main bacteria at class level are shown in Figure 9b. During the start-up period, the dominant class was _Gammaproteobacteria_ affiliated with the phylum _Proteobacteria_, with a relative abundance of 13.67%, followed by _Bacteroidia_, _Clostridia_, _Anaerolineae_ and _Bacilli_, with a relative abundance of 12.87%, 11.88%, 10.96% and 9.89%, respectively. Among these predominant classes, _Bacteroidia_ and _Clostridia_ played significant roles in hydrolysis metabolism [47; 48; 49]. _Gammaproteobacteria_ was the most representative class during the start-up period, which was also the main contributor to the phylum _Proteobacteria_. However, with an increased temperature during the summer period, _Bacilli_ affiliated with the phylum _Firmicutes_ was the most representative class, with a relative abundance of 57.17%, while the relative abundances of _Bacteroidia_ and _Clostridia_ were 6.61% and 7.43%, respectively. These results probably indicate that seasonal temperature in summer benefited the growth of _Bacilli_ bacteria. _Bacteroidales_ affiliated with the class _Bacteroidia_ had been identified as hydrolys and fermenter that participated in the conversation of cellulose and polysaccharide into VFAs [49]. _Clostridiales_ affiliated with the class _Clostridia_ was correlated with methane production, due to their diverse metabolism that utilized polysaccharide fermentation to produce VFAs for methanogenesis [47; 48].