Kluyveromyces marxianus, an Attractive Yeast for Ethanolic Fermentation in the Presence of Imidazolium Ionic Liquids

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 <i>Kluyveromyces marxianus</i>, a thermotolerant ethanologenic yeast. Morphological impacts induced by ILs on <i>K. marxianus</i> were characterized by Scanning Electron Microscopy analysis and showed wrinkled, softened, and holed shapes. In Yeast-Malt-Dextrose (YMD) medium, <i>K. marxianus</i> 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, <i>K. marxianus</i> 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 <i>K. marxianus</i> is an interesting fermentative yeast in a second-generation bioethanol process implying IL pretreatment.