Inhibition of microbial biofuel production in drought-stressed switchgrass hydrolysate.
BACKGROUND
Interannual variability in precipitation, particularly drought, can affect lignocellulosic crop biomass yields and composition, and is expected to increase biofuel yield variability. However, the effect of precipitation on downstream fermentation processes has never been directly characterized. In order to investigate the impact of interannual climate variability on biofuel production, corn stover and switchgrass were collected during 3 years with significantly different precipitation profiles, representing a major drought year (2012) and 2 years with average precipitation for the entire season (2010 and 2013). All feedstocks were AFEX (ammonia fiber expansion)-pretreated, enzymatically hydrolyzed, and the hydrolysates separately fermented using xylose-utilizing strains ofSaccharomyces cerevisiaeandZymomonas mobilis.A chemical genomics approach was also used to evaluate the growth of yeast mutants in the hydrolysates.


RESULTS
While most corn stover and switchgrass hydrolysates were readily fermented, growth ofS. cerevisiaewas completely inhibited in hydrolysate generated from drought-stressed switchgrass. Based on chemical genomics analysis, yeast strains deficient in genes related to protein trafficking within the cell were significantly more resistant to the drought-year switchgrass hydrolysate. Detailed biomass and hydrolysate characterization revealed that switchgrass accumulated greater concentrations of soluble sugars in response to the drought and these sugars were subsequently degraded to pyrazines and imidazoles during ammonia-based pretreatment. When added ex situ to normal switchgrass hydrolysate, imidazoles and pyrazines caused anaerobic growth inhibition ofS.cerevisiae.


CONCLUSIONS
In response to the osmotic pressures experienced during drought stress, plants accumulate soluble sugars that are susceptible to degradation during chemical pretreatments. For ammonia-based pretreatment, these sugars degrade to imidazoles and pyrazines. These compounds contribute toS. cerevisiaegrowth inhibition in drought-year switchgrass hydrolysate. This work discovered that variation in environmental conditions during the growth of bioenergy crops could have significant detrimental effects on fermentation organisms during biofuel production. These findings are relevant to regions where climate change is predicted to cause an increased incidence of drought and to marginal lands with poor water-holding capacity, where fluctuations in soil moisture may trigger frequent drought stress response in lignocellulosic feedstocks.
DOE Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA ; Department of Chemical Engineering, Michigan State University, East Lansing, MI USA ; Department of Chemical Engineering, Michigan Technological University, Houghton, MI USA.
Rebecca Garlock
RG
Ong
Department of Chemistry, University of Wisconsin-Madison, Madison, WI USA.
Alan
A
Higbee
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Scott
S
Bottoms
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Quinn
Q
Dickinson
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Dan
D
Xie
RTSF Mass Spectrometry & Metabolomics Core, Michigan State University, East Lansing, MI USA.
Scott A
SA
Smith
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Jose
J
Serate
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Edward
E
Pohlmann
RTSF Mass Spectrometry & Metabolomics Core, Michigan State University, East Lansing, MI USA ; Department of Biochemistry & Molecular Biology, Michigan State University, East Lansing, MI USA ; Department of Chemistry, Michigan State University, East Lansing, MI USA.
Arthur Daniel
AD
Jones
Department of Chemistry, University of Wisconsin-Madison, Madison, WI USA ; Department of Biomolecular Chemistry, University of Wisconsin-Madison, Madison, WI USA ; Genome Center of Wisconsin, University of Wisconsin-Madison, Madison, WI USA.
Joshua J
JJ
Coon
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Trey K
TK
Sato
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA ; Department of Agronomy, University of Wisconsin-Madison, Madison, WI USA.
Gregg R
GR
Sanford
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Dustin
D
Eilert
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA ; Department of Agronomy, University of Wisconsin-Madison, Madison, WI USA.
Lawrence G
LG
Oates
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Jeff S
JS
Piotrowski
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Donna M
DM
Bates
DOE Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI USA.
David
D
Cavalier
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, Madison, WI USA.
Yaoping
Y
Zhang
Biofuel; Corn stover; Drought; Fermentation inhibition; Lignocellulose; Saccharomyces cerevisiae; Switchgrass