Abstract
BackgroundA challenge currently facing the cellulosic biofuel industry is the efficient fermentation of both C5 and C6 sugars in the presence of inhibitors. To overcome this challenge, microorganisms that are capable of mixed-sugar fermentation need to be further developed for increased inhibitor tolerance. However, this requires an understanding of the physiological impact of inhibitors on the microorganism. This paper investigates the effect of salts on Saccharomyces cerevisiae 424A(LNH-ST), a yeast strain capable of effectively co-fermenting glucose and xylose.ResultsIn this study, we show that salts can be significant inhibitors of S. cerevisiae. All 6 pairs of anions (chloride and sulfate) and cations (sodium, potassium, and ammonium) tested resulted in reduced cell growth rate, glucose consumption rate, and ethanol production rate. In addition, the data showed that the xylose consumption is more strongly affected by salts than glucose consumption at all concentrations. At a NaCl concentration of 0.5M, the xylose consumption rate was reduced by 64.5% compared to the control. A metabolomics study found a shift in metabolism to increased glycerol production during xylose fermentation when salt was present, which was confirmed by an increase in extracellular glycerol titers by 4 fold. There were significant differences between the different cations. The salts with potassium cations were the least inhibitory. Surprisingly, although salts of sulfate produced twice the concentration of cations as compared to salts of chloride, the degree of inhibition was the same with one exception. Potassium salts of sulfate were less inhibitory than potassium paired with chloride, suggesting that chloride is more inhibitory than sulfate.ConclusionsWhen developing microorganisms and processes for cellulosic ethanol production, it is important to consider salt concentrations as it has a significant negative impact on yeast performance, especially with regards to xylose fermentation.
Highlights
A challenge currently facing the cellulosic biofuel industry is the efficient fermentation of both C5 and C6 sugars in the presence of inhibitors
Impact on glucose and xylose consumption rates To explore the effect of salts on glucose and xylose consumption, the specific consumption rates were calculated for each fermentation condition using the model described in Methods
Salts might have greater inhibitory effects on other recombinant Saccharomyces yeast developed for glucose /xylose co-fermentation than the 424A (LNH-ST) strain, especially if the parent strain used for development was a laboratory strain derived from CEN
Summary
A challenge currently facing the cellulosic biofuel industry is the efficient fermentation of both C5 and C6 sugars in the presence of inhibitors To overcome this challenge, microorganisms that are capable of mixed-sugar fermentation need to be further developed for increased inhibitor tolerance. A number of these ions have been shown to have a significant inhibitory effect on microorganisms that are considered for biofuel production They were found to reduce cell growth, sugar utilization rates, and ethanol productivity rates, while increasing ethanol yields and fermentation byproducts such as glycerol [16,17,18]. When exposed to high salt concentrations, organisms can experience both osmotic stress and ion toxicity [19] These previous studies have one major limitation: they were conducted with microorganisms capable of fermenting glucose, but not xylose. Salt ions may have a similar, enhanced inhibitory effect on xylose fermentation and contribute to a significantly slower fermentation rate of lignocelullosic hydrolysates when compared to simple sugar cane or corn starch hydrolysates
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