Lignin/sodium alginate composite hydrogel enhances microbial stress resistance during fermentation

Fermenting lignocellulose-derived sugars into bulk chemicals and fuels is a pivotal route for valorizing renewable plant biomass. Microbial immobilization enables stable continuous production, yet traditional hydrogel carriers fail to protect microbes from harsh fermentation conditions and inhibitors due to structural and functional limitations. Here, a dual-crosslinked lignin/sodium alginate (L-SA) composite hydrogel was fabricated, with adhesion and chewiness enhanced by 55.8 % and 95.2 % compared to pure sodium alginate (SA) hydrogel. Lignin's weak acidic groups buffered fermentation pH via protonation, while its phenolic hydroxyls mitigated oxidative stress by scavenging radicals. Endowed with lignin-derived hydrophobic microdomains, the L-SA hydrogel adsorbed 96.9 ± 3.3 % of steroidal saponin (3.0-fold that of the SA hydrogel) and efficiently sequestered furfural, p-hydroxybenzoic acid, and heavy metals. Under inhibitory stress, the concentrations of ethanol (40 mg/L dioscin), citric acid (5 g/L furfural), and glutamate (35 mg/L Cu²⁺) were 34.0 ± 0.5 g/L, 40.5 ± 0.6 g/L, and 21.2 ± 0.7 g/L, which were 46.5 %, 28.6 %, and 40.8 % higher than those of the SA group. After 15 fermentation cycles, the L-SA hydrogel retained 91.1 % of its initial ethanol concentration (vs. 68.6 % for the SA hydrogel).