Comparative fermentation of Saccharomyces cerevisiae and Zymomonas mobilis to study microbial responses during the conversion of lignocellulosic biomass to biofuels

For sustainable biofuel production, large-scale commercial biofuel synthesis will need to accommodate variability in feedstocks resulting from different soils, annual weather patterns, and climate change in addition to differences in the plant species and genotypes. Using AFEX-pretreated feedstocks, we produced hydrolysates from various feedstocks and used a three-tiered strategy to study how the feedstock diversity impacts microbial responses. The strategy includes hydrolysate compositional analysis, chemical genomics for fingerprinting of hydrolysates and comparative fermentation.  We have evaluated the different methods for hydrolysate production and studied the impact of climate interannual variability using corn stover and switchgrass harvested in three different years with different interannual climate conditions. We also compared the hydrolysates that were produced with high-solid loading from pretreated corn stover.  During fermentation of different biomass hydrolysates, the growth, glucose and xylose utilization, and ethanol production were monitored and quantitated, and RNA samples were collected for RNAseq analysis. We found that Saccharomyces cerevisiae and Zymomonas mobilis showed different responses to these hydrolysates. Specifically Z. mobilis showed very similar growth and glucose/xylose utilization in corn stover hydrolysate (ACSH) and switchgrass hydrolysate (ASGH), while S. cerevisiae grew significantly better in ACSH than ASGH.  The most interesting, result was that S. cerevisiae was unable to grow in ASGH that generated from 2012 switchgrass. Both Z. mobilis and S. cerevisiae showed slow growth and poor xylose utilization in highly concentrated corn stover hydrolysate (ACSH). The chemical genomics and RNAseq data analysis revealed specific microbial stress responses in these different feedstock hydrolysates.