Energy-efficient treatment of sugar-rich wastewater under low-light conditions enabled by a high-respiration...

Low-light operation offers clear advantages for energy-efficient treatment of sugar-rich wastewater by alleviating photoinhibition and reducing electricity demand; however, carbon removal efficiency of conventional microalgae remains insufficient under such conditions. Here, we isolated Chlorella sorokiniana NS4-2 (C. sorokiniana NS4-2) from natural waters, a fast-growing strain characterized by a high-respiration phenotype that sustains rapid carbon turnover and growth under low-light conditions. In low-light photobioreactor conditions using simulated glucose-containing wastewater, C. sorokiniana NS4-2 exhibited simultaneous glucose and acetate utilization, consistent with its enhanced respiratory metabolism. Acetate supplementation further increased glucose removal to 2.2 g L^-1 d^-1, among the highest reported under comparable low-light conditions. Glucose feeding also enhanced lipid accumulation, with eicosapentaenoic acid content increasing nearly tenfold, indicating the potential for value-added lipid co-production during wastewater treatment. When applied to real sugar-rich wastewater under sustained low-light conditions, the system achieved 99% total sugar removal, and the treated effluent met applicable discharge standards. Collectively, these results demonstrate that C. sorokiniana NS4-2 enables efficient carbon removal under low-light operation, with respiratory capacity emerging as a key determinant of this performance, revealing a metabolism-driven mechanism underlying carbon removal under light-limited conditions and providing a rational criterion for selecting low-light-adapted microalgal strains, thereby supporting the development of energy-efficient wastewater treatment strategies with integrated resource recovery.