Comparison of three inoculum sources for acetate production and microbial succession in H₂/CO₂-fed anaerobic system

Introducing a synthetic H₂/CO₂ gas mixture into anaerobic systems is a promising strategy for C1 gas valorization, renewable energy storage, and sustainable chemical synthesis. However, systematic comparisons of inoculum sources under identical operational conditions remain limited. In this study, three inoculum sources, fermentation sludge (S1), lake sediment (S2), and brewery anaerobic sludge (S3), were compared for acetate production from CO₂ and H₂, with particular attention to microbial community succession and reactor stability. S3 showed the highest CO₂ consumption rate (331 ± 4 mg CO₂/(L·d)), cumulative CO₂ fixation (7.8 ± 0.3 g/L), and acetate production efficiency (5.3% electron allocation to acetate), along with stable pH conditions (7.1 ± 0.3). These outcomes were associated with the rapid enrichment of genera that include known homoacetogenic species, mainly Clostridium and Acetobacterium, which together reached approximately 40% relative abundance in S3 after acclimation. The findings suggest that initial inoculum composition can influence the extent of homoacetogen enrichment and subsequent acetate production performance. In addition, the post-peak increase in the acetate-oxidizing genus Geobacter (an acetate consumer rather than an acetogen) in S3 points to a potential challenge for sustained product recovery in mixed-culture systems. Overall, these results provide insights for inoculum selection and process optimization in H₂/CO₂ fermentation systems for acetate production.