Comparative fermentation of Saccharomyces cerevisiae and Zymomonas mobilis in lignocellulosic hydrolysates produced from corn stover and switchgrass to study microbial responses to different feedstock hydrolysates

In order to identify and overcome key barriers in the sustainable conversion of lignocellulosic biomass to biofuels, we produced hydrolysates using AFEX^TM-pretreated corn stover and switchgrass and performed chemical genomics and fermentation to study microbial responses to these different feedstock hydrolysates. To investigate the effect of interannual climate variability on hydrolysate and microbial performance, feedstocks were chosen that were harvested from three different years with significantly different levels of precipitation. We produced most hydrolysates with 6% glucan-loading for corn stover (ACSH) or 7% glucan-loading for switchgrass (ASGH), but also produced more highly concentrated ACSH (9% glucan loading) from corn stover. The chemical compositions of these lignocellulosic hydrolysates were analyzed for the concentrations of sugars, organic acids, heavy metals and minerals, amino acids, and various low-molecular weight inhibitors. Using deletion mutant libraries of both Saccharomyces cerevisiae and Zymomonas mobilis, we performed chemical genomic analysis to understand the biological response of fermentative ethanologens to the different hydrolysates. We also carried out fermentation experiments with these hydrolysates using S. cerevisiae and Z. mobilis strains. The growth, glucose and xylose utilization, and ethanol production were monitored during fermentation, and RNA samples were collected for RNAseq. We found that S. cerevisiae and Z. mobilis showed different responses to these hydrolysates.  Specifically Z. mobilis showed very similar growth and glucose/xylose utilization in 6% ACSH and 7% ASGH, while S. cerevisiae grew significantly better in 6% ACSH than 7% ASGH.  However, both Z. mobilis and S. cerevisiae had slow growth and poor xylose utilization in 9% ACSH.