From lignocellulosic waste streams to bioethanol – an integrated approach utilizing a genetically optimized Saccharomyces cerevisiae strain

2^nd generation biofuel production aims at biotechnological conversion of lignocellulosic biomass into liquid fuels, typically ethanol. Saccharomyces cerevisiaeis the preferred organism of the fermentation industries. Inhibitors and heterogeneous hexose and pentose sugar composition, released by pretreatment and hydrolysis of the feedstock, are still major challenges for efficient lignocellulose conversion.  

Here we present the S. cerevisiaestrain IBB10B05 which was enabled to xylose fermentation by a combination of metabolic engineering (introduction of the xylose reductase and xylitol dehydrogenase) and laboratory evolution. A crucial novelty is the engineered xylose reductase, whose cofactor preference was altered, to create a largely redox neutral xylose assimilation pathway.

Two industrially significant, un-detoxified lignocellulosic waste streams were used as substrates. Firstly, spent sulfite liquor (SSL), a by-product of the paper industry. Secondly, wheat straw hydrolyzate (WS-H) generated by enzymatic hydrolysis. Enzymes were produced in-house by the optimized Trichoderma reesei strain SVG17, allowing a complete mass balance analysis of the separate hydrolysis and co-fermentation (SHCF) process.

Utilizing IBB10B05, it was possible to co-ferment glucose and xylose in WS-H, SSL and a combination thereof with high ethanol yields (~0.45 g_ethanol/g_sugars) and high sugar consumption rates (q_xylose ≤0.7 g/g_CDW/h; q_glucose ~2.9 g/g_CDW/h). In total, 0.12 L ethanol per kg dry mass wheat straw was produced by SHCF. Despite high substrate loadings (15% dry mass), high conversion efficiencies were attained, making IBB10B05 a robust strain for lignocellulose-to-bioethanol processes.