## 3 Results and Discussion
### Microbial Interaction and Methane-Producing Pathways of the Dominant Species in the PSD and AnMBR  
To better understand the predicted function carried out by each MAG of the draft genome, genes were annotated through BlastX analyses against the KEGG and CAZy databases. The number of predicted ORFs in MAGs ranged from 1658 to 4635 with an average of 2648 (Table 2). On average, 50.8% (ranged from 36.4% to 63.7%) and 2.44% (ranged from 0.74% to 8.66%) of the ORFs were annotated in the KEGG and CAZy databases (Table S7). Genes associated with carbohydrate-active and methanogenic metabolism were recruited from each MAG, benefiting the prediction of the functions carried out by the predominant species during methane generation. The abundance of CAZyme-encoding genes varied across the MAGs. MAG59 (classified as _Prolixibucteraceae_ bacterium), MAG158 (_Bacteroidetes_ bacterium) and MAG127 (_Bacteroidetes_ bacterium) had the most abundant CAZy genes, with total numbers of  
Figure 3: Taxonomic assignment of the curated MAGs reconstructed from the PSD and AnMBR metagenomes. The lineages of the MAGs and selected related organisms are shown in different colors. 269, 232, and 103, respectively. In carbohydrate-active modules, MAG59 had the highest abundance of genes encoding GHs, GTs, and PLs, with gene numbers of 191, 46, and 10 (Figure 4), respectively, providing genetic evidence for the high cellulolytic and transglycosid abilities of _Prolixibacteraceae_ bacterium. The most abundant genes of CEs and CBMs were identified in MAG158. The above results suggest that polysaccharides within the DWTS habitat are hydrolyzed by the predominant _Bacteroidetes_ bacterium and _Prolixibacteraceae_ bacterium. Methanogenic archaea can be classified into seven well-established orders: _Methanobacteriales, Methanococcus, Methanomicrobiales, Methanosarcinales, Methanocelles, Methanopyrales,_ and _Methanomassilicoccales_[66; 67]. In this study, MAGs associated with _Methanobacterium, Methanomicrobiales, Methanomassilicoccales_, and _Methanosaeta_ were recruited, indicating the metabolic versatility of the methanogenesis pathway in the PSD and AnMBR of the DWTS. The annotated results of the methanogenesis genes in archaeal MGAs indicated three types of methanogenic pathways: (1) hydrogenotrophic (Figure 5), (2) acetonitrile (Figure S4), and (3) methylotrophic (Figure S5). For hydrogenotrophic methanogenesis, carbon dioxide is successively reduced to methane through a series of intermediates with a methyl group. The methyl group is then transferred to Coenzyme M, forming methyl-CoM. Thus, methyl-CoM is reduced to methane through methyl coenzyme M reductase (Mcr) at the final step. Genes encoding enzymes that participate in hydrogenotrophic methanogenesis had the highest abundance in the archaeal MAGs, followed by methylotrophic and acetonitrile methanogenesis (Figure 5, Figures S4 and S5). Generally, the genes involved in the hydrogenotrophic methanogenesis pathway had higher abundance in archaea MAGs (Figure 5), especially for formylmethanofuran dehydrogenase (K00200-K00203) and tetrahydroromethanopter S-methyltransferase transferase (K00577-K00584). The genes of MAG14 and MAG16 (both classified as _Methanobacterium_ sp.) included almost all the enzymes in the completely hydrogenotrophic pathway, indicating _Methanobacterium_-associated archaea tended to convert carbon dioxide to methane using hydrogen, which has been proven by genomic and physiological data [68; 69]. It was interesting that the genes for complete typical hydrogenotrophic pathway were not detected in MAG215 (classified as _Methanomassilicoccales_ archaea), while MAG215 harbored a high abundance of genes involved in methylotrophic methanogenesis; both of these results suggest that _Methanomassilicoccales_ archaea  
Figure 4: Numbers of genes associated with CAzyme in bacterial and archaeal MAGs. produced methane from methanol, in agreement with genomic insights into the methylotrophic lifestyle of _Methanomassilicocales_[70].