Recent progress and challenges in biological degradation and biotechnological valorization of lignin as an emerging source of bioenergy: A state-of-the-art review

Due to concerns over climate change and the depletion of fossil fuels, recent studies have focused on biorefineries utilizing lignocellulosic biomass as a renewable feedstock for producing value-added fuels and chemicals. Among the components of lignocellulosic biomass, cellulosic/hemicellulosic components have mainly been utilized to obtain fermentable sugars in biorefineries, whereas lignin has commonly been treated as a byproduct thus far. Nonetheless, lignin is the most widely available macromolecule in nature; thus, recent biorefineries have paid special attention to lignin as an attractive source for renewable energy. Given that natural lignin is highly recalcitrant to breakdown, lignin degradation is the most significant issue for further valorization. Accordingly, biological approaches involving enzymes and microorganisms for lignin degradation are comprehensively reviewed in this study. Additionally, this review intensively addresses the recent progress in biotechnological lignin valorization including not only metabolic engineering for producing useful fuels and chemicals but also the use of lignin-based hybrid materials for biosensor and biomedical applications. Furthermore, future studies on challenging issues are suggested as an aid to developing feasible lignin-based biorefineries. The results discussed in this review might provide insights for zero-waste refineries utilizing all components of lignocellulosic biomass.