A membrane-mediated algal-bacterial coupling strategy for energy - efficient and low-carbon PHA production

Polyhydroxyalkanoates (PHAs) are promising substitutes for petroleum-based plastics, but their production is constrained by the high energy demand and CO₂ emissions of mechanical aeration. We developed a membrane-mediated, photosynthetically coupled system in which a hydrophobic polytetrafluoroethylene (PTFE) membrane enables gas exchange between spatially separated microalgal and PHA-storing mixed-culture chambers, establishing an internal O₂-CO₂ cycle. The effects of microbial-to-algal biomass ratio (Mp/Ma), substrate-to-microbe ratio (F/M), membrane area-to-volume ratio (θ) and initial inorganic carbon concentration (IC_ini) were evaluated. Optimal performance was achieved at Mp/Ma = 3:1 and F/M = 1:1; increasing θ to 0.012 m² L^-1 allowed microalgal oxygen to sustain a PHA content of 51% (VSS), comparable to mechanical aeration. Under these conditions, specific energy consumption and process-related CO₂ emissions per unit PHA were reduced by 90% and 38%, respectively, with microalgal CO₂ fixation contributing 11%, rising to 16.7% at 30 mmol L^-1 inorganic carbon. This configuration offers a promising route toward low-energy, low-carbon PHA production.