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Caleb Walker.

Caleb Walker Earns Presentation Award


Caleb Walker, PhD student in chemical engineering, made a strong impression at his first American Institute of Chemical Engineers (AIChE) conference. He earned the AIChE Division 15 (Food, Pharmaceutical and Bioengineering Division) oral presentation award at the recent 2019 AIChE Annual Meeting.

Walker spent nearly four years researching and experimenting to produce the results in his presentation, “Engineering Exceptional Solvent Tolerance in Yarrowia Lipolytica for Biocatalysis.”

“This award acknowledges my commitment to producing industrially relevant research and encourages further development of my academic performance as a PhD student,” he said. “In my research, the majority of ideas and experiments either fail or take months to years to successfully validate, so any form of appreciation or validation of my work is humbly appreciated.”

Walker is from Kingsport, Tennessee, earned his undergraduate degree at UT Chattanooga, and previously worked as a process and development engineer at Steward Advanced Materials. Walker’s winning doctoral research seeks to improve bioprocessing techniques by increasing the robustness of microorganisms so that they better survive and perform in the presence of chemical inhibitors.

We have many exciting projects and ideas that have developed from this research. Including new collaborations and resources to further understand and engineer truly novel, robust microorganisms for innovative biocatalysis.”

—Caleb Walker

Walker’s Abstract

“Engineering Exceptional Solvent Tolerance in Yarrowia Lipolytica for Biocatalysis”
Microbial biocatalysis in organic solvents such as ionic liquids (ILs) is attractive for making fuels and chemicals from complex substrates including lignocellulosic biomass. However, low IL concentrations of 0.5−1.0 % (v/v) can drastically inhibit microbial activity. In this study, we engineered an exceptionally robust oleaginous yeast Yarrowia lipolytica, YlCW001, by adaptive laboratory evolution (ALE). The mutant YlWC001 shows robust growth in up to 18% (v/v) 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), which makes it more tolerant than any engineered microorganisms and naturally screened isolates. Remarkably, YlCW001 exhibits broad tolerance in most commonly used hydrophilic ILs beyond [EMIM][OAc]. Scanning electron microscopy revealed that ILs significantly damage cell wall and/or membrane of wildtype Y. lipolytica with observed cavities, dents, and wrinkles while YlCW001 maintains healthy morphology even in high concentrations of ILs up to 18% (v/v). By performing comprehensive metabolomics, lipidomics, and transcriptomics to elucidate this unique phenotype, we discovered that both wildtype Y. lipolytica and YlCW001 reconfigured membrane composition (e.g., glycerophospholipids and sterols) and cell wall structure (e.g., chitin) under IL-stressful environments. By probing the steroid pathway at transcriptomic, enzymatic, and metabolic levels, we validated that sterols (i.e., ergosterol) are a key component of the cell membrane that enables Y. lipolytica to resist IL-responsive membrane damage and hence tolerate high IL concentrations. This study provides a better understanding of exceptional robustness of Y. lipolytica that can be potentially harnessed as a microbial manufacturing platform for production of fuels and chemicals in organic solvents.

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