Insoluble solids at high concentrations repress yeast\u2019s response against stress and increase intracellular ROS levels

Antonio D Moreno,Elia Tomás-Pejó,Mercedes Ballesteros,Cristina González-Fernández

doi:10.1038/s41598-019-48733-w

Abstract

Lignocellulosic ethanol production requires high substrate concentrations for its cost-competitiveness. This implies the presence of high concentrations of insoluble solids (IS) at the initial stages of the process, which may limit the fermentation performance of the corresponding microorganism. The presence of 40–60% IS (w/w) resulted in lower glucose consumption rates and reduced ethanol volumetric productivities of Saccharomyces cerevisiae F12. Yeast cells exposed to IS exhibited a wrinkled cell surface and a reduced mean cell size due to cavity formation. In addition, the intracellular levels of reactive oxygen species (ROS) increased up to 40%. These ROS levels increased up to 70% when both lignocellulose-derived inhibitors and IS were simultaneously present. The general stress response mechanisms (e.g. DDR2, TPS1 or ZWF1 genes, trehalose and glycogen biosynthesis, and DNA repair mechanisms) were found repressed, and ROS formation could not be counteracted by the induction of the genes involved in repairing the oxidative damage such as glutathione, thioredoxin and methionine scavenging systems (e.g. CTA1, GRX4, MXR1, and TSA1; and the repression of cell cycle progression, CLN3). Overall, these results clearly show the role of IS as an important microbial stress factor that affect yeast cells at physical, physiological, and molecular levels.

Highlights

Bioethanol fuel has increased in many countries as an effective alternative to reduce CO2 emissions in the transport sector
Fermentation tests with 40% and 60% insoluble solids (IS) (w/w) were performed to envisage any effect exerted by IS on the ethanol production of S. cerevisiae F12
The presence of IS during the fermentation process resulted in lower glucose consumption rates and reduced ethanol volumetric productivities at early times, independently of the IS concentration (Fig. 1A). 5.2 ± 0.6 and 10.5 ± 1.2 g/L of glucose remained in the media after 6 h of fermentation in presence of 40% and 60% IS (w/w), respectively, while total glucose depletion was observed in absence of IS (w/w) (Fig. 1A, Table 1)

Summary

Introduction

Bioethanol fuel has increased in many countries as an effective alternative to reduce CO2 emissions in the transport sector. The present work evaluates the effects exerted by IS on Saccharomyces cerevisiae F12, focusing in aspects such as fermentation performance, cell viability, cell morphology, accumulation of reactive oxygen species (ROS), and the differences on the gene expression pattern. These results will boost the setting of the physiological and molecular basis towards a comprehensive understanding of the mechanisms beyond the effects exerted by IS on yeast cells, which has been often underestimated

Methods

Results

Conclusion