(191b) Enhanced Ethanol Tolerance in Yeast by Micronutrients

Ethanol will continue to be the dominant mass-market biofuel in the near to intermediate term.  The yeast Saccharomyces cerevisiae is the catalyst of choice in the conversion of plant sugars to ethanol.  One of the primary bottlenecks to higher ethanol production is the toxicity of ethanol itself as well as compounds present in the fermentation medium.  Cell death occurs at ethanol concentrations greater than 15-20% (v/v), and hydrolysates of cellulosic biomass (widely considered to be the next generation feedstock for renewable fuels) contain a variety of organic molecules inhibitory to growth.  Conditions that increase ethanol tolerance could improve both product yield and the viability of yeast for reuse in subsequent fermentations.  However, the biological basis for ethanol tolerance remains elusive: whole genome studies have shown that tolerance is influenced by many genes, and thus far, none have elicited significant improvement when manipulated alone.  An alternative approach is to alter extracellular conditions such that cells become less sensitized to high ethanol concentrations.  To this end, we have screened and identified several micronutrients which, when added to a fermentation culture, can increase ethanol production in S. cerevisiae by ~50%.  These modifications to the medium do not require any genetic manipulation of strains and, in fact, lead to increased ethanol output in both laboratory and industrial strains.  Furthermore, we have found that the boost in ethanol production is attributable directly to an enhancement of population viability (i.e., tolerance), and occurs in both glucose- and xylose-fermenting strains alike.  Our findings simultaneously hint at a novel, univariate determinant of ethanol tolerance and are a step forward in unlocking the economic potential of cellulosic biomass.