Discovery of potential pathways for biological conversion of poplar wood into lipids by co-fermentation of Rhodococci strains

BackgroundBiological routes for utilizing both carbohydrates and lignin are important to reach the ultimate goal of bioconversion of full carbon in biomass into biofuels and biochemicals.Recent biotechnology advances have shown promises toward facilitating biological transformation of lignin into lipids. Natural and engineeredRhodococcus strains(e.g.,R. opacus PD630, R. jostiiRHA1, and R. jostiiRHA1 VanA(-)) have been demonstrated to utilize lignin for lipid production, and co-culture of them can promote lipid production from lignin.ResultsIn this study, a co-fermentation module of natural and engineeredRhodococcus strainswith significant improved lignin degradation and/or lipid biosynthesis capacities was established, which enabled simultaneous conversion of glucose, lignin, and its derivatives into lipids. AlthoughRhodococcisp. showed preference to glucose over lignin, nearly half of the lignin was quickly depolymerized to monomers by these strains for cell growth and lipid synthesis after glucose was nearly consumed up. Profiles of metabolites produced by Rhodococcusstrainsgrowing on different carbon sources (e.g., glucose, alkali lignin, and dilute acid flowthrough-pretreated poplar wood slurry)confirmed lignin conversion during co-fermentation, and indicated novel metabolic capacities and unexplored metabolic pathways in these organisms.Proteome profiles suggested that lignin depolymerization byRhodococci sp. involved multiple peroxidases with accessory oxidases. Besides the -ketoadipate pathway, thephenylacetic acid (PAA) pathway was another potential route for thein vivoring cleavage activity. In addition,deficiency of reducing power and cellular oxidative stress probably led to lower lipid production using lignin as the sole carbon source than that using glucose.ConclusionsThis work demonstrated a potential strategy for efficient bioconversion of both lignin and glucose into lipids by co-culture of multiple natural and engineered Rhodococcus strains. In addition, the involvement of PAA pathway in lignin degradation can help to further improve lignin utilization, and the combinatory proteomics and bioinformatics strategies used in this study can also be applied into other systems to reveal the metabolic and regulatory pathways for balanced cellular metabolism and to select genetic targets for efficient conversionof both lignin and carbohydrates into biofuels.