Abstract
BackgroundIonic liquids (ILs) are considered as suitable candidates for lignocellulosic biomass pretreatment prior enzymatic saccharification and, obviously, for second-generation bioethanol production. However, several reports showed toxic or inhibitory effects of residual ILs on microorganisms, plants, and animal cells which could affect a subsequent enzymatic saccharification and fermentation process.ResultsIn this context, the impact of two hydrophilic imidazolium-based ILs already used in lignocellulosic biomass pretreatment was investigated: 1-ethyl-3-methylimidazolium acetate [Emim][OAc] and 1-ethyl-3-methylimidazolium methylphosphonate [Emim][MeO(H)PO2]. Their effects were assessed on the model yeast for ethanolic fermentation, Saccharomyces cerevisiae, grown in a culture medium containing glucose as carbon source and various IL concentrations. Classical fermentation parameters were followed: growth, glucose consumption and ethanol production, and two original factors: the respiratory status with the oxygen transfer rate (OTR) and carbon dioxide transfer rate (CTR) of yeasts which were monitored online by respiratory activity monitoring systems (RAMOS). In addition, yeast morphology was characterized by environmental scanning electron microscope (ESEM).The addition of ILs to the growth medium inhibited the OTR and switched the metabolism from respiration (conversion of glucose into biomass) to fermentation (conversion of glucose to ethanol). This behavior could be observed at low IL concentrations (≤5% IL) while above there is no significant growth or ethanol production. The presence of IL in the growth medium also induced changes of yeast morphology, which exhibited wrinkled, softened, and holed shapes. Both ILs showed the same effects, but [Emim][MeO(H)PO2] was more biocompatible than [Emim][OAc] and could be better tolerated by S. cerevisiae.ConclusionsThese two imidazolium-derived ILs were appropriate candidates for useful pretreatment of lignocellulosic biomass in the context of second-generation bioethanol production. This fundamental study provides additional information about the toxic effects of ILs. Indeed, the investigations highlighted the better tolerance by S. cerevisiae of [Emim][MeO(H)PO2] than [Emim][OAc].
Highlights
Ionic liquids (ILs) are considered as suitable candidates for lignocellulosic biomass pretreatment prior enzymatic saccharification and, obviously, for second-generation bioethanol production
S. cerevisiae growth in the presence of [Emim][OAc] or [Emim][MeO(H)PO2] without previous adaptation The impact of [Emim][OAc] and [Emim][MeO(H)PO2] ILs on yeast growth was first assessed by biomass measurements, expressed as cell dry weight (CDW), in YMD culture medium (Figure 1)
The decrease in ethanol concentration observed without IL after glucose depletion was less pronounced with [Emim][MeO(H)PO2] addition and the ethanol was reduced from 47% at 1% IL, 23% at 2% IL, and 17% at 4% IL between the glucose depletion and the end of the culture at 72 h. These results showed that both imidazolium-based ILs, [Emim][OAc] and [Emim][MeO(H)PO2], increased significantly the measured maximum ethanol yield with 9.4 g/L for an addition of 1% [Emim][OAc] and 8.0 g/L for 4% [Emim][MeO(H)PO2] instead of 5.8 g/L ethanol for the control without IL
Summary
Ionic liquids (ILs) are considered as suitable candidates for lignocellulosic biomass pretreatment prior enzymatic saccharification and, obviously, for second-generation bioethanol production. Several reports showed toxic or inhibitory effects of residual ILs on microorganisms, plants, and animal cells which could affect a subsequent enzymatic saccharification and fermentation process. Lignocellulosic plant biomass is generally considered as the most promising renewable feedstock for bioproduction of transportation fuels and commodity chemicals, without alimentary competition [2,3]. The bioethanol production from lignocellulosic biomass begins with a pretreatment step to increase cellulose accessibility and digestibility by the cellulolytic enzymes into glucose monomers (improving the saccharification step), followed by an ethanolic fermentation and ends by a distillation step of bioethanol [4]. Chemical pretreatments are the most commonly used for lignocellulosic biomass and include alkali or acid pretreatments, ozonolysis, and organosolv process [1,4]
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