Continuous and long-term production of single-cell protein from CO₂ in an electrolytic gas-lift reactor

Electro-driven hydrogen-oxidizing bacteria (HOB) fermentation represents a promising strategy for sustainable protein production, renewable energy storage, and efficient CO₂ utilization. However, current electrolytic gas-lift reactors for single-cell protein (SCP) production operating in batch mode face significant challenges, including product inhibition, low hydrogen-to-protein yield, and the inability to sustain long-term stable operation at high current densities. These limitations lead to inefficient hydrogen utilization, energy waste, and limited scalability. To address these challenges, this study established a continuous-flow operation strategy and systematically evaluated reactor performance under different hydraulic retention times (HRTs) and applied currents. Under optimal conditions (6.0 A applied current (133 A·m⁻²) and 1.55 days HRT), the reactor achieved a 7.4-fold increase in HOB-SCP productivity (0.89 vs. 0.12 g CDW∙L⁻¹∙d⁻¹ in batch mode) and a 14% rise in protein content (72% vs. 58%). Furthermore, the utilization efficiencies of H₂, O₂, and CO₂ reached 100%, 99%, and 93.5%, respectively, with a nitrogen recovery rate of 68.6%. This work provides both theoretical evidence and experimental validation for the scalability and practical potential of electrolytic gas-lift reactors in sustainable protein biomanufacturing.