Characterization of sulfur-compound metabolism underlying wax-ester fermentation in Euglena gracilis

Koji Yamada,Chieko Higuchi,Satoshi Oshiro,Keiichi Mochida,Kengo Suzuki,Kohei Atsuji,Nobuko Nitta,Maeka Shiroyama,Komaki Inoue,Osamu Iwata,Iwao Ohtsu,Tomoaki Nitta

doi:10.1038/s41598-018-36600-z

Abstract

Euglena gracilis is a microalga, which has been used as a model organism for decades. Recent technological advances have enabled mass cultivation of this species for industrial applications such as feedstock in nutritional foods and cosmetics. E. gracilis degrades its storage polysaccharide (paramylon) under hypoxic conditions for energy acquisition by an oxygen-independent process and accumulates high amount of wax-ester as a by-product. Using this sequence of reactions referred to as wax-ester fermentation, E. gracilis is studied for its application in biofuel production. Although the wax-ester production pathway is well characterized, little is known regarding the biochemical reactions underlying the main metabolic route, especially, the existence of an unknown sulfur-compound metabolism implied by the nasty odor generation accompanying the wax-ester fermentation. In this study, we show sulfur-metabolomics of E. gracilis in aerobic and hypoxic conditions, to reveal the biochemical reactions that occur during wax-ester synthesis. Our results helped us in identifying hydrogen sulfide (H2S) as the nasty odor-producing component in wax-ester fermentation. In addition, the results indicate that glutathione and protein degrades during hypoxia, whereas cysteine, methionine, and their metabolites increase in the cells. This indicates that this shift of abundance in sulfur compounds is the cause of H2S synthesis.

Highlights

Biofuel produced from algae is categorized as a third generation biofuel and is expected to attain high yields in the future[1]
No report exists so far, wax-ester fermentation is empirically known to accompany the generation of a nasty odor, which is predicted to be of sulfur compounds, indicating that sulfur metabolism occurs at the background of wax-ester fermentation
To achieve deeper understanding of wax-ester fermentation mechanism in E. gracilis, we studied the basis for nasty odor production during wax-ester fermentation by performing sulfur metabolomics

Summary

Introduction

Biofuel produced from algae is categorized as a third generation biofuel and is expected to attain high yields in the future[1]. An important role of sulfur in biological systems is that it controls redox state in the cells; for example, glutathione (GSH), which is a tripeptide composed of glutamate, cysteine, and glycine, reduces the reactive oxygen species to protect cells from free radical damages. In this reaction, GSH is altered to an oxidized form of glutathione (GSSG), which is formed by a covalent disulfide bond between two GSHs and can be restored to separate GSHs by the activity of NADPH-glutathione reductase[17,20]. The results indicate that the metabolism of sulfur-containing compounds is altered in hypoxic condition, suggesting a process of H2S synthesis from intracellular sulfur-containing compounds

Methods

Results

Conclusion