Lignin
valorization offers significant potential to enhance the
economic viability of lignocellulosic biorefineries. However, because
of its heterogeneous and recalcitrant nature, conversion of lignin
to value-added coproducts remains a considerable technical challenge.
In this study, we employ base-catalyzed depolymerization (BCD) using
a process-relevant solid lignin stream produced via deacetylation,
mechanical refining, and enzymatic hydrolysis to enable biological
lignin conversion. BCD was conducted with the solid lignin substrate
over a range of temperatures at two NaOH concentrations, and the results
demonstrate that the lignin can be partially extracted and saponified
at temperatures as low as 60 °C. At 120 °C and 2% NaOH,
the high extent of lignin solubility was accompanied by a considerable
decrease in the lignin average molecular weight and the release of
lignin-derived monomers including hydroxycinnamic acids. BCD liquors were tested for
microbial growth using seven aromatic-catabolizing bacteria and two
yeasts. Three organisms (Pseudomonas putida KT2440, Rhodotorula mucilaginosa, and Corynebacterium glutamicum) tolerate high BCD liquor concentrations (up to 90% v/v) and rapidly
consume the main lignin-derived monomers, resulting in lignin conversion
of up to 15%. Furthermore, as a proof of concept, muconic acid production
from a representative lignin BCD liquor was demonstrated with an engineered P. putida KT2440 strain. These results highlight the potential
for a mild lignin depolymerization process to enhance the microbial
conversion of solid lignin-rich biorefinery streams.