Submitted to: Biotechnology and Bioengineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 11, 2008
Publication Date: February 15, 2009
Citation: Slininger, P.J., Gorsich, S.W., Liu, Z. 2009. Culture Nutrition and Physiology Impact the Inhibitor Tolerance of the Yeast Pichia stipitis NRRL Y-7124. Biotechnology and Bioengineering. 102(3):788-790. Interpretive Summary: To support the expansion of the biofuels industry, efficient fermentation processes are sought to produce ethanol from low-cost plant biomass (i.e. from crop and forest residues, energy crops, or municipal wastes). Stress-tolerant microorganisms are needed that are able to consume both the hexose and pentose sugars and also withstand, survive, and function in the presence of stress factors common to fermentations of biomass-derived sugars, including inhibitors such as furfural, 5-hydroxymethylfurfural (HMF), and ethanol. Pichia stipitis NRRL Y-7124 is one of the natural yeast strains best able to convert sugars from biomass because it is able to ferment not only the hexose sugars but also the pentose sugar, xylose, to ethanol. Our studies showed that mineral and nitrogen source composition supplied to fermentations had significant impact on the ability of P. stiptis to survive and detoxify furan inhibitors and to convert high xylose concentrations efficiently to ethanol. The culture age and sugar source type (whether glucose or xylose) were also found to influence inhibitor tolerance and nutritional needs. These findings will be used by others involved in biomass fermentation process technology research and development and the biofuels industry. The new knowledge on culture nutrition and management contributes to the design of a low cost fermentation process which fosters inhibitor tolerance of the yeast biocatalyst for more efficient ethanol production from renewable biomass, and it makes progress toward our national priorities of achieving energy independence, strengthening our rural economy, and preserving our environment.
Technical Abstract: Robust microorganisms are needed to consume both hexose and pentose sugars and to withstand, survive, and function in the presence of stress factors common to fermentations of lignocellulose hydrolysates, including the inhibitors furfural, 5-hydroxymethylfurfural (HMF), and ethanol. Pichia stipitis NRRL Y-7124 is one of the natural yeast strains best able to utilize biomass because it is able to ferment hexoses and the pentose, xylose, to ethanol. Previous studies have shown that mineral and nitrogen source composition supplied to fermentations significantly impact the ability of P. stipitis to efficiently convert high xylose concentrations to economically recoverable ethanol. Current studies focused on the dependency of inhibitor tolerance on culture nutrition and physiological state. Resistance of cells to ethanol, furfural, and HMF was generally greater in stationary-phase cells than log-phase cells, despite the greater exposure of stationary cells to ethanol. Consistent with this, the specific productivity of detoxification products (furfuryl alcohol or furan-2,5-dimethanol) from spikes of furfural or HMF, respectively, increased as cultures progressed into stationary phase. However, when xylose was substrate, ethanol resistance was greater for log-phase cells than stationary cells. Amino acid enrichment of the growth medium significantly enhanced ethanol tolerance if xylose was the carbon source, but had no impact if glucose was. Regardless of the carbon source, amino acid enrichment of the culture medium enhanced the ability of cells to resist furfural and HMF exposure. Mineral compositions had little impact on inhibitor resistance except stationary-phase xylose-grown cells were more susceptible to inhibitor exposure when magnesium sulfate level was too high. These findings shape process strategies to produce a tolerant yeast population, and then to foster and sustain tolerance during growth and ethanol fermentation.