Holistic valorization of microalgal slurry into biofuels via integrated simultaneous saccharification and fermentation using co-immobilized tri-enzymes

Abstract

The commercial viability of microalgal biorefineries is currently constrained by high energy demands during pretreatment and incomplete biomass valorization arising from cell wall recalcitrance. To overcome these challenges, this study developed a high-efficiency simultaneous saccharification and fermentation (SSF) strategy using an immobilized tri-enzyme (ITE) system to convert high-solids microalgal slurries (100–120 g/L) into biofuels. A strategic low-energy microwave pretreatment (MP, 9.9–10.6 MJ/kg) was optimized to permeabilize the cell wall while preserving proteins. Subsequently, the ITE-mediated SSF facilitated rapid hydrolysis, drastically reducing the lag phase from 24 h to a negligible < 0.01 h during carbohydrate fermentation (CF). This integrated process achieved high production titers and yields for bioethanol (28.4–36.4 g/L; 0.47–0.48 g/g), higher alcohols (6.5–7.9 g/L; 0.43–0.44 g/g), and biodiesel (15.4–61.5 g/L; 0.90 g/g). Overall, approximately 90% of the biomass was utilized, corresponding to a total conversion efficiency of 50–69%. Collectively, these results demonstrate the feasibility of comprehensive microalgal valorization into multiple energy carriers with minimal waste generation, thereby providing a highly efficient and potentially sustainable conceptual framework for advanced microalgal biorefineries.