Adaptive Laboratory Evolution for Multistress Tolerance, including Fermentability at High Glucose Concentrations in Thermotolerant Candida tropicalis

Koudkeo Phommachan,Mamoru Yamada,Vanhnavong Savanhnaly,Savitree Limtong,Minenosuke Matsutani,Mochamad Nurcholis,Nookhao Vongvilaisak,Chansom Keo-Oudone,Tomoyuki Kosaka,Somchanh Bounphanmy,Mingkhuan Chanhming

doi:10.3390/en15020561

Abstract

Candida tropicalis, a xylose-fermenting yeast, has the potential for converting cellulosic biomass to ethanol. Thermotolerant C. tropicalis X-17, which was isolated in Laos, was subjected to repetitive long-term cultivation with a gradual increase in temperature (RLCGT) in the presence of a high concentration of glucose, which exposed cells to various stresses in addition to the high concentration of glucose and high temperatures. The resultant adapted strain demonstrated increased tolerance to ethanol, furfural and hydroxymethylfurfural at high temperatures and displayed improvement in fermentation ability at high glucose concentrations and xylose-fermenting ability. Transcriptome analysis revealed the up-regulation of a gene for a glucose transporter of the major facilitator superfamily and genes for stress response and cell wall proteins. Additionally, hydropathy analysis revealed that three genes for putative membrane proteins with multiple membrane-spanning segments were also up-regulated. From these findings, it can be inferred that the up-regulation of genes, including the gene for a glucose transporter, is responsible for the phenotype of the adaptive strain. This study revealed part of the mechanisms of fermentability at high glucose concentrations in C. tropicalis and the results of this study suggest that RLCGT is an effective procedure for improving multistress tolerance.

Highlights

Solutions for the problems of global warming and the unsustainability of future energy supplies are crucial for continued economic development [1,2]
Candida tropicalis X-17, which was isolated in Laos, is a thermotolerant xylose-fermenting yeast with similar growth rates in glucose and xylose media
C. tropicalis is superior to K. marxianus in the ability to convert xylose to ethanol but inferior in fermentation ability at high glucose concentrations

Summary

Introduction

Solutions for the problems of global warming and the unsustainability of future energy supplies are crucial for continued economic development [1,2]. Much interest has been shown in biomass as carbon sources for biofuel production due to the environmentally friendly low carbon emissions of biofuels [3,4,5,6]. Biomass conversion has been performed by fermentation or other biomass utilization technologies, fast pyrolysis. In anaerobic digested wastewater and converted it by fast pyrolysis to biofuel. In recent years, increasing interest has been shown in lignocellulose biomass as renewable and sustainable energy resources. Bioethanol, a biofuel from lignocellulosic biomass, is gaining increasing attention as an alternative fuel due to fluctuations in oil prices, reduced oil reserves, and important environmental issues associated with greenhouse gas emissions [8,9,10,11,12,13]. Hemicellulose hydrolysates contain xylose as an abundant sugar [14,15]

Methods

Results

Conclusion