Mechanistic insights into the effect of imidazolium ionic liquids on the lipid production by Geotrichum fermentans

Abstract

Background: Ionic liquid (IL) pretreatment has emerged as a promising technique that enables complete utilization of lignocellulosic biomass for biofuel production. However, imidazolium IL has recently been shown to exhibit inhibitory effect on cell growth and product formation of industrial microbes, such as oleaginous microorganisms. To date, the mechanism of this inhibition remains largely unknown. Results: In this study, the feasibility of [Bmim][OAc]-pretreated rice straw hydrolysate as a substrate for microbial lipid production by Geotrichum fermentans, also known as Trichosporon fermentans, was evaluated. The residual [Bmim][OAc] present in the hydrolysate caused a reduction in biomass and lipid content (43.6 and 28.1%, respectively) of G. fermentans, compared with those of the control (7.8 g/L and 52.6%, respectively). Seven imidazolium ILs, [Emim][DEP], [Emim]Cl, [Amim]Cl, [Bmim]Cl, [Bzmim]Cl, [Emim][OAc], and [Bmim][OAc], capable of efficient pretreatment of lignocellulosic biomass were tested for their effects on the cell growth and lipid accumulation of G. fermentans to better understand the impact of imidazolium IL on the lipid production. All the ILs tested inhibited the cell growth and lipid accumulation. In addition, both the cation and the anion of IL contributed to IL toxicity. The side chain of IL cations showed a clear impact on toxicity. On examining IL anions, [OAc]− was found to be more toxic than those of [DEP]− and Cl−. IL exhibited its toxicity by inhibiting sugar consumption and key enzyme (malic enzyme and ATP-citrate lyase) activities of G. fermentans. Cell membrane permeability was also altered to different extents in the presence of various ILs. Scanning electron microscopy revealed that IL induces fibrous structure on the surface of G. fermentans cell, which might represent an adaptive mechanism of the yeast to IL. Conclusions: This work gives some mechanistic insights into the impact of imidazolium IL on the cell growth and lipid accumulation of oleaginous yeast, which is important for IL integration in lignocellulosic biofuel production, especially for microbial lipid production.