Efficient mixed culture CO2 bioacetogenesis in anaerobic membrane bioreactor: Net volatile fatty acid production, carbon mass balance, and microbial dynamics

Biological CO 2 valorization using renewable H 2 provides a promising route for carbon resource circularity. However, continuous mixed-culture operation remains limited by unstable gas conversion, low volumetric acid productivity, microbial competition, and insufficient reactor-level controllability. Here, we developed a 15 L mesophilic anaerobic membrane bioreactor (AnMBR) as an efficient platform for continuous CO 2 bioacetogenesis. The AnMBR integrated membrane-mediated biomass retention with permeate discharge, helping to retain high concentrations of functional microorganisms while enhancing CO 2 fixation performance. During 96 days of operation, the performance of the AnMBR was evaluated as the carbon loading rate was stepwise increased from 0.5 to 1.4 g-C/L/d. With the influent H 2 :CO 2 ratio adjusted to 2:1, H 2 and CO 2 conversion efficiencies remained at 75–90 %, and the ΔH 2 /ΔCO 2 ratio approached the stoichiometric requirement for homoacetogenesis. The net volatile fatty acid (VFA) production rate increased from 0.7 to 2.2 g-HAc/L/d, while acetate consistently accounted for approximately 88–93 % of total VFAs. Carbon mass balance further showed that VFAs were the dominant carbon recovery products, with up to 79.6 % of input carbon recovered as VFAs, while the estimated biomass-associated carbon accounted for 5.8–6.6 %. The relative abundance of Clostridium sensu stricto 12 increased from 39.6 % to 53.9 % during operation, supporting homoacetogenesis. These findings demonstrate that the AnMBR provides an effective engineering strategy for stable mixed-culture CO 2 valorization and biological Power-to-X applications.