Sulfolobus acidocaldarius utilizes glucose and xylose as sole carbon sources, but its ability to metabolize these sugars simultaneously is not known. We report the absence of diauxie during growth of S. acidocaldarius on glucose and xylose as co-carbon sources. The presence of glucose did not repress xylose utilization. The organism utilized a mixture of 1 g/liter of each sugar simultaneously with a specific growth rate of 0.079 h ؊1 and showed no preference for the order in which it utilized each sugar. The organism grew faster on 2 g/liter xylose (0.074 h ؊1 ) as the sole carbon source than on an equal amount of glucose (0.022 h ؊1 ). When grown on a mixture of the two carbon sources, the growth rate of the organism increased from 0.052 h ؊1 to 0.085 h ؊1 as the ratio of xylose to glucose increased from 0.25 to 4. S. acidocaldarius appeared to utilize a mixture of glucose and xylose at a rate roughly proportional to their concentrations in the medium, resulting in complete utilization of both sugars at about the same time. Gene expression in cells grown on xylose alone was very similar to that in cells grown on a mixture of xylose and glucose and substantially different from that in cells grown on glucose alone. The mechanism by which the organism utilized a mixture of sugars has yet to be elucidated.Sulfolobus acidocaldarius is a hyperthermophilic archaeon that grows optimally at 75°C and pH 2.0 to 3.0 (10, 20, 24) and has been shown to utilize a broad range of sugars (20,24). Numerous studies have shown that bacteria as well as eukaryotes sequentially utilize individual sugars when grown on a mixture of sugars. These organisms preferentially utilize the sugar that best supports their growth (mostly glucose) by repressing the utilization of other sugars in the growth medium until the preferential sugar is completely consumed. This phenomenon, termed "carbon catabolite repression" (CCR), or "diauxie," is characterized by a diphasic growth pattern when an organism is grown on a mixture of glucose and other sugars. Most studies of CCR have focused mainly on bacterial or eukaryotic systems. However, a few studies have reported that the presence of glucose in a growth medium represses the metabolism of other sugars in species closely related to Sulfolobus solfataricus via a mechanism that is similar to CCR (22,23,25,34).S. acidocaldarius metabolizes the smallest number of sugars of all known Sulfolobus species, but the sugars metabolized by the organism include glucose and xylose, the key constituents of ligno-cellulose (cellulosic) biomass (20,24,37). The lack of diauxie on 5-and 6-carbon sugars and the ability to grow at high temperature and low pH are excellent characteristics for a host that produces biofuel from cellulosic biomass deconstructed by acidic and/or high-temperature pretreatment methods. The production of biofuels from cellulosic biomass as an alternative to fossil fuels has received increased attention in the past few years. However, the development of microbial system(s) that can efficiently and...

BackgroundCurrent biomass pretreatment by hydrothermal treatment (including acid hydrolysis, steam explosion, and high-temperature steaming) and ionic liquids generally generate inhibitors to the following fermentation process. Furfural is one of the typical inhibitors generated in hydrothermal treatment of biomass. Furfural could inhibit cell growth rate and decrease biofuel productivity of microbes. Candida tropicalis is a promising microbe for the production of biofuels and value-added chemicals using hemicellulose hydrolysate as carbon source. In this study, C. tropicalis showed a comparable ability of furfural tolerance during fermentation. We investigated the mechanism of C. tropicalis’s robust tolerance to furfural and relevant metabolic responses to obtain more information for metabolic engineering of microbes for efficient lignocellulose fermentation.Results Candida tropicalis showed comparable intrinsic tolerance to furfural and a fast rate of furfural detoxification. C. tropicalis’s half maximal inhibitory concentration for furfural with xylose as the sole carbon source was 3.69 g/L, which was higher than that of most wild-type microbes reported in the literature to our knowledge. Even though furfural prolonged the lag phase of C. tropicalis, the final biomass in the groups treated with 1 g/L furfural was slightly greater than that in the control groups. By real-time PCR analysis, we found that the expression of ADH1 in C. tropicalis (ctADH1) was induced by furfural and repressed by ethanol after furfural depletion. The expression of ctADH1 could be regulated by both furfural and ethanol. After the disruption of gene ctADH1, we found that C. tropicalis’s furfural tolerance was weakened. To further confirm the function of ctADH1 and enhance Escherichia coli’s furfural tolerance, ctADH1 was overexpressed in E. coli BL21 (DE3). The rate of furfural degradation in E. coli BL21 (DE3) with pET-ADH1 (high-copy plasmid) and pCS-ADH1 (medium-copy plasmid) was increased by 1.59-fold and 1.28-fold, respectively.Conclusions Candida tropicalis was a robust strain with intrinsic tolerance to inhibitor furfural. The mechanism of furfural detoxification and metabolic responses were identified by multiple analyses. Alcohol dehydrogenase 1 was confirmed to be responsible for furfural detoxification. C. tropicalis showed a complex regulation system during furfural detoxification to minimize adverse effects caused by furfural. Furthermore, the mechanism we uncovered in this work was successfully applied to enhance E. coli’s furfural tolerance by heterologous expression of ctADH1. The study provides deeper insights into strain modification for biofuel production by efficient lignocellulose fermentation.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0668-x) contains supplementary material, which is available to authorized users.

e Microbial communities that deconstruct plant biomass have broad relevance in biofuel production and global carbon cycling. Biomass pretreatments reduce plant biomass recalcitrance for increased efficiency of enzymatic hydrolysis. We exploited these chemical pretreatments to study how thermophilic bacterial consortia adapt to deconstruct switchgrass (SG) biomass of various compositions. Microbial communities were adapted to untreated, ammonium fiber expansion (AFEX)-pretreated, and ionicliquid (IL)-pretreated SG under aerobic, thermophilic conditions using green waste compost as the inoculum to study biomass deconstruction by microbial consortia. After microbial cultivation, gravimetric analysis of the residual biomass demonstrated that both AFEX and IL pretreatment enhanced the deconstruction of the SG biomass approximately 2-fold. Two-dimensional nuclear magnetic resonance (2D-NMR) experiments and acetyl bromide-reactive-lignin analysis indicated that polysaccharide hydrolysis was the dominant process occurring during microbial biomass deconstruction, and lignin remaining in the residual biomass was largely unmodified. Small-subunit (SSU) rRNA gene amplicon libraries revealed that although the dominant taxa across these chemical pretreatments were consistently represented by members of the Firmicutes, the Bacteroidetes, and Deinococcus-Thermus, the abundance of selected operational taxonomic units (OTUs) varied, suggesting adaptations to the different substrates. Combining the observations of differences in the community structure and the chemical and physical structure of the biomass, we hypothesize specific roles for individual community members in biomass deconstruction.