Photosynthesis-fermentation hybrid system to produce lipid feedstock for algal biofuel

To avoid bacterial contamination due to medium replacement in the expanded application of a photosynthesis–fermentation model, an integrated photosynthesis-fermentation hybrid system was set up and evaluated for algal lipid production using Chlorella protothecoides. In this system, the CO₂-rich off-gas from the fermentation process was recycled to agitate medium in the photobioreactor, which could provide initial cells for the heterotrophic fermentation. The cell concentration reached 1.03±0.07 g/L during photoautotrophic growth and then the concentrated green cells were switched to heterotrophic fermentation after removing over 99.5% of nitrogen in the medium by a nitrogen removal device. At the end of fermentation in the system, the cell concentration could reach as high as 100.51±2.03 g/L, and 60.05±1.38% lipid content was achieved simultaneously. The lipid yield (60.36±2.63 g/L) in the hybrid system was over 700 times higher than that in a photobioreactor and exceeded that by fermentation alone (47.56±7.31 g/L). The developed photosynthesis-fermentation hybrid system in this study was not only a feasible option to enhance microalgal lipid production, but also an environment-friendly approach to produce biofuel feedstock through concurrent utilization of ammonia nitrogen, CO₂, and organic carbons.