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The genus Methanosaeta represented the more predominant acetoclastic methanogen than the genus Methanosarcina in the 10th-round culture, possibly due to the concentration of acetate as the substrate. The MI-773 (SAR405838) Methanosaeta and Methanosarcina become predominant at acetate concentrations of below 100–150 mg COD/L, corresponding to 1.7–2.5 mM, and above 250–500 mg COD/L, corresponding to 4.2–8.4 mM, respectively ( De Vrieze et al., 2012). In this study, the period of exposure to the latter high concentrations of acetate changed during the cultivation as follows: above 7.4 mM between 2 and 17 days during the 20-day cultivation period in the first round of cultivation ( Fig. 2a); above 4.5 mM for all of the 12-day cultivation period in the third round of cultivation ( Fig. 2b), and temporary exposure to above 17 mM at 8 and 16 h in the 10th round of cultivation ( Fig. 2c). These results indicate that the period of exposure to high concentrations of acetate became shortened. The shortened period of exposure to high concentrations of acetate may facilitate the cultivation of an acetoclastic methanogenic community with Methanosaeta as the predominant genus.