Repression of xylose‐specific enzymes by ethanol inScheffersomyces(Pichia)stipitisand utility of repitching xylose‐grown populations to eliminate diauxic lag

Abstract

During the fermentation of lignocellulosic hydrolyzates to ethanol by native pentose-fermenting yeasts such as Scheffersomyces (Pichia) stipitis NRRL Y-7124 (CBS 5773) and Pachysolen tannophilus NRRL Y-2460, the switch from glucose to xylose uptake results in a diauxic lag unless process strategies to prevent this are applied. When yeast were grown on glucose and resuspended in mixed sugars, the length of this lag was observed to be a function of the glucose concentration consumed (and consequently, the ethanol concentration accumulated) prior to the switch from glucose to xylose fermentation. At glucose concentrations of 95 g/L, the switch to xylose utilization was severely stalled such that efficient xylose fermentation could not occur. Further investigation focused on the impact of ethanol on cellular xylose transport and the induction and maintenance of xylose reductase and xylitol dehydrogenase activities when large cell populations of S. stipitis NRRL Y-7124 were pre-grown on glucose or xylose and then presented mixtures of glucose and xylose for fermentation. Ethanol concentrations around 50 g/L fully repressed enzyme induction although xylose transport into the cells was observed to be occurring. Increasing degrees of repression were documented between 15 and 45 g/L ethanol. Repitched cell populations grown on xylose resulted in faster fermentation rates, particularly on xylose but also on glucose, and eliminated diauxic lag and stalling during mixed sugar conversion by P. tannophilus or S. stipitis, despite ethanol accumulations in the 60 or 70 g/L range, respectively. The process strategy of priming cells on xylose was key to the successful utilization of high mixed sugar concentrations because specific enzymes for xylose utilization could be induced before ethanol concentration accumulated to an inhibitory level.

Zymomonas mobilis IMMOBILIZED ON LOOFA SPONGE AND SUGARCANE BAGASSE FOR LEVAN AND ETHANOL PRODUCTION USING REPEATED BATCH FERMENTATION

“…There was a reduction after the fourth cycle to values next to zero, with no statistically significant difference between them, which could be due to the death of the microorganisms immobilized on the support (Table 4 and Figure 4). This behavior could be due to exposition of microorganisms to long periods of stress, such as, high temperatures and inhibitors like acetic acid, which are potentially inhibitors of the microbiota at concentrations above 3 g.L -1 (Fan et al, 2013;Slininger et al, 2011;Jin et al, 2012).…”

Improved ethanol production by a xylose-fermenting recombinant yeast strain constructed through a modified genome shuffling method

“…It was reported that metabolic lag existed for substrate transition [11]. This indicates that yeast strain precultured on glucose prior to its use as inoculum for xylose fermentation may lead to longer metabolic lag phase.…”

“…On the contrary, it resulted in the decreased ethanol titre and a longer fermentation time for both ScF2 and P. stipitis . It was reported that ethanol plays a dramatic role as a repressor preventing the induction of specific enzymes needed for xylose utilization in P. stipitis and when ethanol concentration was greater than 30 g/L, induction of xylose reductase (XR) and xylitol dehydrogenase (XDH) was greatly decreased [11]. Ethanol concentration was topped at around 50 g/L for ScF2 in fermentation medium initially containing increased xylose concentration (100 – 250 g/L), indicating the repression of xylose utilization pathway by ethanol.…”

Fermentation of rice bran hydrolysate to ethanol using Zymomonas mobilis biofilm immobilization on DEAE-cellulose

“…Repeated batch with the cell recycle probably allowed cells to become conditioned and adapted to the fermentation process, hence the ethanol production in the repeated batch was risen up from the first batch [41]. Z. mobilis biofilm of ZM4 had shown its potential to be reused in the repeated batch after the adaptation from the previous batch and the biofilm remained intact.…”