PNNL

Abstract

Lignin represents the largest renewable source of aromatic carbon but remains underutilized due to its structural heterogeneity and recalcitrance. In this study, Aspergillus niger was engineered to enable the bioconversion of lignin derived aromatic acids and real base catalyzed depolymerized (BCD) lignin streams into malic acid, a value added C4 dicarboxylic acid with broad industrial relevance. Overexpression of the C4 dicarboxylate transporter C4T318 facilitated efficient malic acid secretion, while medium optimization further enhanced production under buffered conditions. The engineered strain exhibited robust assimilation of representative lignin derived aromatics, including 4-hydroxybenzoic acid and p-coumaric acid, and maintained consistent malic acid synthesis across both individual and mixed substrate fermentations. Conversion of BCD lignin liquors from poplar and sorghum confirmed effective utilization of heterogeneous aromatic mixtures and stable fermentation performance. Comparable carbon conversion efficiencies were observed between hardwood and grass lignin streams, indicating the strain’s adaptability to diverse feedstocks. This work represents the first demonstration of direct fungal conversion of real lignin streams into malic acid and establishes A. niger as a robust microbial platform for sustainable lignin valorization. The integration of transporter engineering with process optimization provides a promising strategy for advancing biological lignin conversion and expanding the role of filamentous fungi in renewable bioproduct synthesis within future biorefineries.