# Structure of Microbial Communities When Complementary
## 3 Results and Discussion
### Molecular Characterization of Microbial Communities
Other phyla were not considered in this discussion due to their low presence (0.01–1.2%): Armatimonadetes; Atribacteria; BRC1; Chlamydia; Chlorobi; Chloroflexi; Cloacimonetes; Cyanobacteria; Deinococcus-Thermus; Fibrobactenes; Fusobacteria; Hydrogenedentes; Lentisphaerae; Microgenonates; Nitrospinae; Parcubacteria; Planctomycetes; Saccharomyces; Verrucomicrobia; WS6; unidentified bacteria. high metabolic versatility of Firmicutes and Bacteroidetes suggests that cellulose residues from PE and other complex organic compounds, present in OMW, were degraded to form the acids [49; 54; 55; 56; 57; 58]. Some Firmicutes are known to be syntrophic bacteria capable of degrading VFAs, such as butyrate and its analogs [52]. [PERSON] et al. [57] already mentioned that Bacteroidetes and Firmicutes have been identified as the main bacterial phyla present within digesters. Figure 4: _Cont._  
Regarding Archaea domain, considerable changes were also observed during anaerobic digestion. Of most archaeal sequences, 99% were classified within the phylum Eur-yarchaeota, and only nine sequences were assigned to the phylum Woesearchaeota, which were later not detected after anaerobic digestion. _Methanobrevisbacter_, which was initially the predominant genus on PE sample with a relative abundance of 72%, decreased its population to 10% (Figure 5). _Methanospirillum_, which represents only 6 and 9% of the archaea sequences assigned to the genus in PE and 70P + 30O samples, respectively, at the beginning of these assays, was not detected at the end of the anaerobic digestion since these microorganisms preferably utilize H\({}_{2}\)/CO\({}_{2}\) over organic acids as substrate. After anaerobic digestion, _Methanosarcina_ became the predominant populations, increasing from 7% to 90%, in PE samples, while in 70P + 30O, the content of _Methanobrevisbacter_  
Figure 4: Relative abundances of orders (**a**) Firmicutes (**b**) Bacteroidetes, (**c**) \(\alpha\)-Proteobacteria, \(\beta\)-Proteobacteria, \(\delta\)-Proteobacteria, and \(\gamma\)-Proteobacteria, and (**d**) Actinobacteria composition in samples from anaerobic digestion experiments. The abundance is presented in terms of a percentage of the total number of sequences in a sample. Figure 5: Relative abundance of Archaea genera. Other genera were not considered in this discussion due to their low presence (0.79–1.77%) in samples: _Methanogenium_; unidentified archaea. ter_ and _Mehanosarcina_ became predominant after the experiment, with relative abundances of 50% and 48%, respectively. The presence of _Methanosarcina_ in the two mixtures suggests methane formation mainly from acetate [56; 59; 60], and these results are accordingly with the VFA removals (Table 7), indicating that methane production occurred mainly through acetoclastic methanogenesis. Unlike _Methanosarcina_, _Methanosaeta_ is an obligately acetoclastic methanogen, reported in the literature as predominant in anaerobic digestion systems [49], however, it was detected only at the beginning of the tests, in the PE and 70P + 30O samples, with 2.5% of assigned sequences, and was not detected at the end of the anaerobic digestion. According to [PERSON] et al. [59], this situation can be explained by the difference in maximum specific growth rate (\(\mu_{\text{max}}\)) and half-saturation constant (Ks) between these two genera. While _Methanosaeta_ spp. have a low \(\mu_{\text{max}}\) of 0.20 d\({}^{-1}\) and a Ks of 10-50 mg COD/L, _Methanosarcina_ spp. are characterized by a high \(\mu_{\text{max}}\) of 0.60 d\({}^{-1}\) and a Ks of 200-280 mg COD/L. As a result, _Methanosaeta_ spp. are known to be dominant at acetate concentrations not exceeding 100-150 mg COD/L, whereas _Methanosarcina_ spp. became dominant at acetate concentrations above 250-500 mg COD/L [61]. Thus, higher concentrations of VFA (especially acetate) seemed to create favorable conditions for _Methanosarcina_ spp. to be dominant.