Soluble Lignin as a Multifunctional Enhancer in Microbial Fermentation

Transforming lignocellulose into high-value biochemicals via microbial fermentation is critical for valorizing agricultural/forestry residues and advancing sustainable biomanufacturing. However, fermentation efficiency is hindered by environmental stressors. Herein, soluble lignin was prepared by alkaline dissolution and dialysis of enzymatic hydrolysis lignin and its effects on the above three fermentations were systematically investigated. Results showed that soluble lignin buffered fermentation pH via carboxyl protonation, scavenged ROS through phenolic hydroxyl groups to alleviate microbial inactivation, and chelated heavy metals (e.g., Pb²⁺, Cd²⁺) via phenolic hydroxyl/carboxyl coordination to reduce toxicity. Notably, it exhibited pH-responsive recyclability: dissolving at neutral pH (initial fermentation) and precipitating at acidic pH (fermentation end), enabling centrifugal recovery and reuse for ≥10 cycles with retained performance. This study demonstrates that soluble lignin universally enhances multiple microbial fermentations via physicochemical synergies, providing a theoretical basis for optimizing lignocellulose-based fermentation and new avenues for high-value lignin utilization in biomanufacturing.