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

Biological CO2 valorization using renewable H2 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 CO2 bioacetogenesis. The AnMBR integrated membrane-mediated biomass retention with permeate discharge, helping to retain high concentrations of functional microorganisms while enhancing CO2 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 H2:CO2 ratio adjusted to 2:1, H2 and CO2 conversion efficiencies remained at 75-90 %, and the ΔH2/ΔCO2 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 CO2 valorization and biological Power-to-X applications.