Impact of lignin depolymerization on aerobic and anaerobic bioconversion of alkaline liquor from sugarcane bagasse

Abstract

Introduction Lignin recalcitrance, together with its complex interactions with cellulose and hemicelluloses within the plant cell wall, remains a major barrier to efficient lignocellulosic biomass valorization. Mild alkaline pretreatment partially addresses this challenge by solubilizing lignin and acetate into a lignin-rich alkaline liquor (AL). However, the recovered lignin is largely oligomeric, limiting its direct microbial conversion. Methods To evaluate strategies for improving AL bioconversion, alkaline liquor was subjected to thermochemical depolymerization under different severities, generating liquors with compositions ranging from oligomer-rich to monomer-rich profiles. The resulting streams were evaluated through two bioconversion routes: aerobic metabolism by Pseudomonas putida KT2440 and anaerobic digestion for biomethane production. Results Mild to moderate depolymerization conditions (≤240 °C) improved the growth of P. putida, whereas the highest depolymerization severity strongly inhibited bacterial growth. This inhibition was associated with increased concentrations of aromatic monomers that are poorly metabolized by P. putida, including phenol and alkyl-substituted phenols. In anaerobic digestion assays, mild to moderate depolymerization yielded the highest specific methane productions, corresponding to a 10%–20% increase relative to non-depolymerized AL, while severe depolymerization conditions negatively affected methane production. Discussion These findings demonstrate that controlled lignin depolymerization can enhance both aerobic and anaerobic bioconversion by balancing chemical accessibility with biological compatibility. This approach provides a promising strategy for improving lignin utilization within integrated biorefinery concepts.