By the end of the experiment, all reactors developed core biofilm communities dominated by Methanothermobacter, a genus crucial for CO conversion. However, the CM reactor showed lower CO conversion performance and required a lower LRCO, potentially due to metabolic imbalances associated with high initial VFA accumulation. Given that methanogenic activity is sensitive to excessive VFA levels, the observed delay in CO conversion may have resulted from inhibitory effects on methanogen. These findings emphasize the importance of inoculum selection for early biofilm formation and performance in TBRs. The study supports the potential for anaerobic biofilms to selforganize into efficient communities over time, providing insights for optimizing TBR in CO conversion and methane production. Future research should focus on investigating the metabolic roles of dominant genera within TBR biofilms through in-depth biofilm characterization and analysis.