Since unlabeled CO2 was periodically supplied in to the RI, the production of a higher proportion of unlabeled acetate indicates that CO2 was likely reduced to acetate at high H2 concentration through homo-acetogenesis. On the other hand, TTR values of acetate M0+1 were lower in the RUI because the acetate produced through homo-acetogenesis pathway was simultaneously degraded through acetoclastic methanogenesis.