Bioconversion of wheat straw lignocellulosic sugars to ethanol by recombinant Escherichia coli

Abstract

Recombinant microorganisms are a promising alternative for production of bioethanol from sugars produced from lignocellulosic materials. In the present work, recombinant Escherichia coli FBR16 has been utilized to produce bioethanol from simulated glucose-xylose mixtures and wheat straw hydrolysates. Hydrolysates were produced by sequential treatment of dilute acid pretreatment at 180 °C for 7 min using 0.5% (v/v) H2SO4 and enzymatic saccharification using cellulase from Trichoderma reesei and β-glucosidase from Aspergillus niger. With increased concentration of glucose-xylose sugar mixtures, ethanol yield and volumetric ethanol productivity decreased. At 22 g/l, ethanol yield of 0.34 g/g and volumetric ethanol productivity of 0.36 g/l·h were obtained which reduced to only 0.19 g/g and 0.17 g/l·h, respectively, at 160 g/l glucose-xylose sugar mixture. Fermentation kinetic parameters were also estimated and it was found that values of parameters were highly dependent on initial sugar concentration. Furthermore, it was observed that E. coli FBR16 is capable of producing bioethanol from almost all lignocellulosic monomeric sugars, especially glucose and xylose. At 16.4 g/l lignocellulosic hydrolysate concentration, ethanol yield of 0.32 g/g and productivity of 0.24 g/l·h were obtained. In order to see the effect of lignocellulosic sugar concentration on ethanol production, hydrolysates were concentrated to 50 g/l from the original concentration of 16.4 g/l. E. coli FBR16 was able to ferment the increased sugar concentration as well; however decreased ethanol yield of 0.29 g/g and volumetric ethanol productivity of 0.17 g/l·h were obtained.