Abstract
Imidazolium ionic liquids (ILs) are promising solvents for lignocellulosic biomass (LCB) pretreatment and allow the achievement of higher ethanolic yields after enzymatic hydrolysis and ethanolic fermentation. However, residual ILs entrapped in pretreated biomass are often toxic for fermentative microorganisms, but interaction mechanisms between ILs and cells are still unknown. Here we studied the effects of 1-ethyl-3-methylimidazolium acetate [Emim][OAc] and 1-ethyl-3-methylimidazolium methylphosphonate [Emim][MeO(H)PO2] on Kluyveromyces marxianus, a thermotolerant ethanologenic yeast. Morphological impacts induced by ILs on K. marxianus were characterized by Scanning Electron Microscopy analysis and showed wrinkled, softened, and holed shapes. In Yeast-Malt-Dextrose (YMD) medium, K. marxianus tolerated IL additions up to 2% for [Emim][OAc] and 6% for [Emim][MeO(H)PO2]. Below these thresholds, some IL concentrations enhanced ethanolic yields up to +34% by switching the metabolic status from respiratory to fermentative. Finally, K. marxianus fermentation was applied on several substrates pretreated with [Emim][OAc] or [Emim][MeO(H)PO2] and enzymatically hydrolyzed: a model long fiber cellulose and two industrial LCBs, softwood (spruce) and hardwood (oak) sawdusts. The maximum ethanolic yields obtained were 1.8 to 3.9 times higher when substrates were pretreated with imidazolium ILs. Therefore K. marxianus is an interesting fermentative yeast in a second-generation bioethanol process implying IL pretreatment.
Highlights
Lignocellulosic biomass (LCB) is a promising sustainable raw material for second-generation bioethanol production
We show that K. marxianus can tolerate higher Ionic liquids (ILs) concentrations than the model yeast Saccharomyces cerevisiae without previous adaptation to IL [17]
K. marxianus was utilized in a separate hydrolysis and fermentation (SHF) process on LCBs: the model long fiber cellulose and two industrial woody LCBs, spruce sawdust and oak sawdust
Summary
Lignocellulosic biomass (LCB) is a promising sustainable raw material for second-generation bioethanol production. The impact of low concentrations of these two imidazolium-ILs was studied for the first time on Kluyveromyces marxianus, a yeast species standing out in second-generation bioethanol processes because of its extreme thermotolerance, attractive for simultaneous saccharification and fermentation [14,15,16]. K. marxianus was utilized in a separate hydrolysis and fermentation (SHF) process on LCBs: the model long fiber cellulose and two industrial woody LCBs, spruce sawdust (softwood) and oak sawdust (hardwood). Higher ethanolic yields were obtained for all LCBs after a pretreatment with [Emim][OAc] or [Emim][MeO(H)PO2] and an enzymatic hydrolysis. K. marxianus is a suitable candidate for ethanolic fermentation in low residual imidazolium IL concentrations
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