Cellulolytic enzyme expression and simultaneous conversion of lignocellulosic sugars into ethanol and xylitol by a new Candida tropicalis strain

Abstract

Background: Lignocellulosic ethanol production involves major steps such as thermochemical pretreatment of biomass, enzymatic hydrolysis of pre-treated biomass and the fermentation of released sugars into ethanol. At least two different organisms are conventionally utilized for producing cellulolytic enzymes and for ethanol production through fermentation, whereas in the present study a single yeast isolate with the capacity to simultaneously produce cellulases and xylanases and ferment the released sugars into ethanol and xylitol has been described.

Results: A yeast strain isolated from soil samples and identified as Candida tropicalis MTCC 25057 expressed cellulases and xylanases over a wide range of temperatures (32 and 42 °C) and in the presence of different cellulosic substrates [carboxymethylcellulose and wheat straw (WS)]. The studies indicated that the cultivation of yeast at 42 °C in pre-treated hydrolysate containing 0.5 % WS resulted in proportional expression of cellulases (exoglucanases and endoglucanases) at concentrations of 114.1 and 97.8 U g(-1) ds, respectively. A high xylanase activity (689.3 U g(-1) ds) was also exhibited by the yeast under similar growth conditions. Maximum expression of cellulolytic enzymes by the yeast occurred within 24 h of incubation. Of the sugars released from biomass after pretreatment, 49 g L(-1) xylose was aerobically converted into 15.8 g L(-1) of xylitol. In addition, 25.4 g L(-1) glucose released after the enzymatic hydrolysis of biomass was fermented by the same yeast to obtain an ethanol titer of 7.3 g L(-1).

Conclusions: During the present study, a new strain of C. tropicalis was isolated and found to have potential for consolidated bioprocessing (CBP) applications. The strain could grow in a wide range of process conditions (temperature, pH) and in the presence of lignocellulosic inhibitors such as furfural, HMF and acetic acid. The new yeast produced cellulolytic enzymes over a wide temperature range and in the presence of various cellulosic substrates. The cellulolytic enzymes produced by the yeast were effectively used for the hydrolysis of pretreated biomass. The released sugars, xylose and glucose were, respectively, converted into xylitol and ethanol. The potential shown by the new inhibitor tolerant cellulolytic C. tropicalis to produce ethanol or xylitol is of great industrial significance.

Keywords: Candida tropicalis; Cellulase; Ethanol; Wheat straw; Xylanase; Xylitol.

Fig. 1

Effect of a different inhibitors (furfural, HMF, acetate) concentration, b temperature and c initial pH on growth of Candida tropicalis. The error bar range represents standard deviation from mean, t test significance level p < 0.005

Fig. 2

Effect of different substrates (CMC and Wheat Straw-WS) concentrations (0.5, 1 and 2 %) on a endoglucanase activity, b exoglucanase activity and c xylanase activity at 32 and 42 °C. The error bar range represents standard deviation from mean, t-test significance level p < 0.005

Fig. 3

Fermentation of glucose under aerobic a, anaerobic (b) and of xylose under aerobic c conditions by Candida tropicalis at 32 °C in synthetic media. The error bar range represents standard deviation from mean, t-test significance level p < 0.005

Fig. 4

Fermentation of synthetic media containing glucose and xylose under a anaerobic and b aerobic conditions to produce ethanol and xylitol by Candida tropicalis at 32 °C. The error bar range represents standard deviation from mean, t-test significance level p < 0.005

Fig. 5

Fermentation of pretreated hydrolysate under anaerobic a and aerobic b conditions to produce ethanol and xylitol by Candida tropicalis at 32 °C. The error bar range represents standard deviation from mean, t-test significance level p < 0.005