Deep eutectic solvent assists Bacillus australimaris to transform alkali lignin waste into small aromatic compounds

Abstract

To improve lignin valorization, a combination of chemical depolymerization by choline chloride (ChCl)-based deep eutectic solvent (DES) and bioconversion by Bacillus australimaris was explored. Lignin depolymerization by glycerin/ChCl released more soluble small molecules, predominantly comprising acids, aromatics, and alcohols, when compared with that by water/ChCl or only water. The most diverse depolymerization products were generated by glycerin/ChCl under the conditions of 170 °C and 50 min, releasing 17 acids with relative abundance of 80.66%, five small molecular alcohols, and nine aromatic compounds. In contrast, the maximum number of products species released by water/ChCl or only water was 12 and 21, respectively, at 100 °C. The number of lignin depolymerization products species decreased after bioconversion by B. australimaris, the bacterial strain with advantages of DES tolerance and lignin utilization. Acids presented the most significant decline during fermentation at 130 °C for 30 min, with the number of acids species decreasing from 12 with relative abundance of 73.2% (before fermentation) to six with relative abundance of 7.65% (day 3 and 7). In contrast, the relative abundance of aromatic compounds increased from 7.01% to 92.35% after 7-day bioconversion, although the number of species decreased with the fermentation duration. These results indicated that B. australimaris assimilated diverse products of DES depolymerization for cellular reproduction and further selectively degraded soluble short lignin fragments to aromatic monomers, thus confirming improvement in lignin valorization via the combination of chemical and biological methods.