At the phylum level, the analysis revealed that Euryarchaeota and Firmicutes were dominant across all enriched biofilm and sump samples, regardless of inoculum source, while Chloroflexi and Halobacterota were present in some samples (Supplementary Figure provides details on the microbial composition for each inoculum). These phyla are commonly associated with biomethanation systems, particularly in TBRs, as noted in previous studies . Within these phyla, 20 abundant genera were identified, representing over 80% of the biofilm community composition on average. In contrast, these genera accounted for less than 1% of the AS and CM inocula and approximately 50% of the AD inoculum. Among the Euryarchaeota, the genus Methanothermobacter showed a notably high relative abundance in biofilm and sump samples across all reactors despite differences in inoculum sources. Methanothermobacter averaged over 63% relative abundance in biofilm samples from R1, compared to approximately 42% and 36% in biofilm samples from R2 and R3, respectively. Methanothermobacter is recognized as a hydrogenotrophic methanogen capable of converting H₂ and CO₂ to methane under thermophilic conditions . Furthermore, specific Methanothermobacter species, such as Methanothermobacter thermautotrophicus, can utilize CO as a sole carbon source to produce methane and CO₂ .