Lignin Fractionation Coupling Laccase-Mediated Depolymerization Enhancing Biological Lignin Valorization

Lignin bioconversion holds great promise to enhance the economics and sustainability of biorefining by efficiently converting lignin into value-added chemicals and/or biofuels. However, the inherent structural heterogeneity and complexity of lignin limit its biodegradability. In this study, the impacts and mechanisms of lignin fractionation on its bioconversion performance were investigated by employing <i>Pseudomonas putida</i> KT2440 fermentation. The highest cell amounts, polyhydroxyalkanoate (PHA) yield, and lignin degradation were achieved from fermentation on the lowest molecular weight lignin fraction F3. Notably, the PHA yield in F3 fraction medium was 0.35 g/L after fermentation for 36 h, representing 1.31, 1.36, and 1.19 times improvement compared to those of F0, F1, and F2, respectively. Furthermore, the addition of laccase and mediator 1-hydroxybenzotriazole hydrate (HBT), which can increase the redox potential and expand the substrate range, resulted in a significant increase in the number of viable bacteria across all four lignin media. The highest PHA yield of 0.41 g/L was achieved on F3, which was 18 and 3.3% greater than those of untreated and laccase-treated samples, respectively. This was attributed to the enhanced ability to cleave β-O-4 bonds with laccase-HBT, which was revealed by the heteronuclear single quantum correlation (HSQC) analysis. The study illustrated that the introduction of a laccase and mediator based on lignin fractionation could result in a uniform low molecular weight of lignin, which can increase the efficiency of lignin bioconversion into the desired product (i.e., PHA). Therefore, lignin fractionation coupled with laccase catalysis is an efficient approach for facilitating microbial lignin valorization.