## 3 Results and Discussion
### Microbial Communities Diversity in Sediments and on the Surfaces of Electrodes  
The degradation of lignocellulosic biomass in sediment needs the synergistic effect of different microbial populations, including the hydrolytic, cellulolytic, hemicellulose, and syntrophic hydrogen-producing bacteria [35]. In this study, a 16S rRNA gene pyrosequencing approach was used to analyze the bacterial community in sediments and on the surfaces of electrodes after 700 days of incubation. A total of 75,557 bacterial sequences and 7348 OTUs were obtained from the 10 samples (Table 1). Both of the two original sediments had the highest OTU values, whereas only 351 and 254 OTUs were identified on the Site TH1 and Site TH2 cathode samples. Coverage covered 88.5-92.1% of the bacterial communities (Table 1). This means that most of the libraries have been established as a matter of record. Shannon diversity indicates that bacteria diversity decreased in sediments with SMFC compared to the control without SMFC. However, higher diversities and more sequences were found in sediments from site TH1 under all treatments. Figure 5A,B show the relative bacterial community abundance in sediments under different treatments on the phylum level. Proteobacteria (14.5-45.3% from site TH1 and 18.7-75.9% from site TH2), and Bacteroidetes (11.6-38.9% from site TH1, 10.0-32.2% from site TH2) were the most abundant  
Figure 4: Organic carbon contents of humic acid (HA) (**A**) and FA (**B**) in TH1 and TH2 sediments after 700 days of incubation. phyla in all samples. Proteobacteria and Bacteroidetes were the dominated organic matter degradation microorganisms and most frequently implicated in facilitating cellulose degradation in sediment [36]. The abundance of Proteobacteria was relatively more pronounced on the cathode surface in site TH1 and on both electrodes in site TH2. The phylum of Bacteroidetes could degrade waste lignocellulosic and enhance biogas production under the mesophilic condition [37] and the abundance was higher in sediments with added SMFCs, especially for the samples from the macrophyte-dominated area. Figure 5: Relative abundances percentages of bacterial phyla (**A**,**B**) communities in original sediments, bulk sediments and electrode biofilm from the two sampling sites at the end of the experiments. The minor designation represents the sum of bacteria in a sample with a relative abundance \(<3\%\). Besides phylum taxa, the analyses were based on the genus level and were, therefore, more conducive to further verification of the function of the bacterial community. Relative percentages of the top 30 genera in both sites are listed in Figure 6. The sediments from both site TH1 and site TH2, especially with SMFCs in site TH1, _Treponema_ was the dominant genera and belonged to the family of Spirochaetaceae, which could utilize the more recalcitrant carbon pool [38]. Spirochaetaceae has been known to contain digestive enzymes which can decompose carbohydrates of the food and algal cell walls [39]. The second dominated genera of site TH1 and TH2, especially on the surface of electrodes was _Clostridium_ which belongs to Firmicutes. Firmicutes mainly include a large number of species that produces butanol, acetone, lactic acid, and ethanol through fermentation of a variety of organic matter [40]. In fact, _Clostridium_ was also known to utilize a broad range of carbohydrates to ferment, such as monosaccharides, disaccharides, starches, organic acids, and other substrates [41]. The genera of _GOUTA_19 and _LCP_-6 were abundant in all samples, especially in the sediments with SMFCs and electrodes, but their functions are not known. It is also worth noting the obvious enrichment of _Geobacter_ (belonging to Delta-proteobacteria) in the SMFC sediments, obviously on the anode surface in site TH2. Microorganisms in the cathodic biofilm consortium have been found to contribute to the overall SMFC performance [42].