Interannual climate variability affects the microbial response to and fermentability of lignocellulosic biofuel crops

In order to investigate the impact of interannual climate variability on downstream biofuel production processes, corn stover and switchgrass were collected during three years with significantly different precipitation profiles (2010, 2012, and 2013). These were chosen to represent a wet, dry, and more average year, respectively. During fermentation of AFEX-treated biomass hydrolysates, Zymomonas mobilis 2032 showed no major difference in response, with the exception of slightly lower cell growth in the switchgrass hydrolysates. Saccharomyces cerevisiae Y128 likewise showed reduced growth in the switchgrass hydrolysates, but was completely unable to grow in hydrolysate generated from 2012 switchgrass (SG). Statistical analysis of the chemical composition of the hydrolysates did not implicate any compounds for the poor performance. Chemical genomic analysis of the yeast response to these hydrolysates pointed to inhibition of ergosterol biosynthesis as a reason for growth inhibition. Further investigation revealed the presence of high quantities of pyrazines and imidazoles in the pretreated biomass and hydrolysate. Of these compounds, 2-methylpyrazine was particularly inhibitory to yeast growth. Addition of these compounds to synthetic hydrolysate and media also inhibited growth of S. cerevisiae. During the drought year, soluble sugars accumulated in the switchgrass, possibly because of incomplete utilization of the sugars due to early senescence or as an osmotic stress response, and were ultimately degraded during AFEX pretreatment to chemicals that were severely inhibitory to yeast growth. These results show a complex interplay between the weather impacts on biomass crop composition, pretreatment chemistry, and the response of the microorganisms during fermentation.