Low-energy dialysis enables removal of inhibitory metabolites and recovery of inorganic salts for stable CO2-to-protein production by sulfur-oxidizing bacteria in a membrane bioreactor

Escalating CO 2 emissions and global protein shortages have driven interest in microbial CO 2 fixation coupled with single cell protein (SCP) production, but long-term chemoautotrophic cultivation is severely constrained by accumulated extracellular organic metabolites (EOMs) that inhibit carbon assimilation. Herein, a low-energy, fouling-resistant flat-sheet dialysis module driven by transmembrane concentration gradients was fabricated for spent medium recycling in Halothiobacillus neapolitanus cultivation. Screening of four membranes with distinct molecular weight cutoffs identified the 3000 Da specification as optimal, delivering 34% inorganic salt permeation, 76% EOM retention, and a maximum salt-to-EOM separation selectivity of 2.54—achieved through a diffusion-driven size-exclusion mechanism that avoids the hydraulic compaction and fouling inherent to pressure-driven processes. The dialysis-pretreated recycled medium sustains stable chemoautotrophic growth, preserving over 65% of fresh medium's carbon fixation capacity at a 40% reuse ratio, while maintaining favorable SCP nutritional quality with sulfur-containing amino acid levels superior to soybean. This strategy effectively resolves the resource consumption bottleneck of conventional continuous cultivation, laying a foundation for low-carbon circular production of CO 2 -derived microbial protein.