PNNL

Abstract

Biological routes for utilizing both carbohydrates and lignin are important to reach the ultimate goal of bioconversion of full carbon in biomass to biofuels and biochemicals. Recent biotechnology advances have shown promises toward facilitating biological transformation of lignin to lipids. In this study, a series of natural and engineered Rhodococcus strains (e.g. R. opacus PD630, R. jostii RHA1, and R. jostii RHA1 VanA-) with lignin degradation and/or lipid biosynthesis capacities were selected to establish a co-fermentation module that enabled a platform for fundamental understanding of bioconversion pathways of glucose and lignin to lipids. Profiles of metabolites produced by Rhodococcus strains following growth on different carbon sources (e.g. alkali lignin, flowthrough pretreated poplar slurry) revealed several unexpected fermentation products, suggesting novel metabolic capacities and unexplored metabolic pathways in these organisms. Although Rhodococci showed preference to glucose over lignin, nearly half of the lignin was quickly depolymerized to monomers by these strains for cell growth and lipid accumulation after glucose was nearly exhausted. Proteomic profiles showed that lignin depolymerization by Rhodococci involved multiple peroxidases with accessory oxidases. Besides the ß-ketoadipate pathway, the phenylacetic acid (PAA) pathway played a predominant role in the in vivo ring cleavage activity. Deficiency of reducing power and cellular oxidative stress led to lower lipid production while using lignin as the sole carbon source compared with that of using glucose. This work thus suggests that synthetic reconstruction and balanced modification of key regulators and enzymes in lignin depolymerization, aromatic compound metabolism, lipid biosynthesis, and other relevant processes will enable efficient conversion of both lignin and carbohydrates to lipids by Rhodococci.