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

Xiaolu Li,Yongfu Yang,Shihui Yang,Bin Yang,Haoxi Ben,Matthew J Gaffrey,Libing Zhang,Joshua S Yuan,Wei-Jun Qian,Yucai He,Zhangyang Xu

doi:10.1186/s13068-019-1395-x

Abstract

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 engineered Rhodococcus strains (e.g., R. opacus PD630, R. jostii RHA1, and R. jostii RHA1 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 engineered Rhodococcus strains with significant improved lignin degradation and/or lipid biosynthesis capacities was established, which enabled simultaneous conversion of glucose, lignin, and its derivatives into lipids. Although Rhodococci sp. 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 Rhodococcus strains growing 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 by Rhodococci sp. involved multiple peroxidases with accessory oxidases. Besides the β-ketoadipate pathway, the phenylacetic acid (PAA) pathway was another potential route for the in vivo ring 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 conversion of both lignin and carbohydrates into biofuels.

Highlights

Biological 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
Lignin degradation by singlestrain culture or co-culture of R. jostii RHA1, R. jostii RHA1 VanA−, or R. opacus PD630 was first examined to select the best combination of candidates for fermentation
Conversion of mixed carbon sources of carbohydrate and lignin for lipid production To investigate viability of the fermentation using the supplemented whole flowthrough-pretreated slurry followed by enzymatic hydrolysis as substrate of carbohydrates and lignin by Rhodococci, 6-day fermentation of mixed carbon sources of glucose and lignin model compounds, or alkali-pretreated corn stover lignin was examined as well

Summary

Introduction

Biological 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. Current biological processing platforms only convert plant polysaccharides into biofuels, resulting in the formation of a significant process stream rich in lignin It is utilized as an energy resource for power/electrical generation, partially due to the lack of efficient chemical conversion processes to convert both sugars and lignin into transportation biofuels or high-value chemicals [4,5,6,7,8,9,10]. Triacylglycerides (TAGs) are synthesized by bioconversion of organic compounds (e.g., sugars and organic acids) derived from the lignocellulosic biomass These TAGs of monoalkyl esters of long-chain fatty acids combined with glycerol can be converted into fatty acid short-chain alcohol esters in the form of FAME (methanol) and FAEE (ethanol) for biodiesel production, which is well established on a commercial scale [1, 2, 14, 18], but the cost associated with the development of biofuels remains challenging.

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