Insoluble solids at high concentrations repress yeast’s response against stress and increase intracellular ROS levels

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.

Stress modulation as a means to improve yeasts for lignocellulose bioconversion

“…During lignocellulosic fermentation, both the substrate (for, e.g., xylose and biomass solids) and the end-product ethanol can induce stress in S. cerevisiae (Cunha et al 2019a;Moreno et al 2019;Osiro et al 2019). LCB is processed through pretreatment into a solid fraction composed of insoluble solids (IS), as well as a liquid fraction containing soluble sugars such as xylose.…”

“…LCB is processed through pretreatment into a solid fraction composed of insoluble solids (IS), as well as a liquid fraction containing soluble sugars such as xylose. During fermentation, the solids can negatively interact with yeast, decreasing fermentation performance by (i) deforming cellular morphology, (ii) disrupting the plasma membrane, (iii) increasing ROS formation, and (iv) altering gene expression and negating cellular stress responses (Moreno et al 2019).…”

Predicted yeast interactome and network‐based interpretation of transcriptionally changed genes

“…Authors of this dataset indicated that 200 genes were overexpressed and 513 genes were downregulated between insolublesolid (IS) cultures and ordinary cultures. Three biological repeats were used for both the IS cultures and ordinary cultures (Moreno, González‐Fernández, Ballesteros, & Tomás‐Pejó, 2019). The online tool GEO2R was used to reanalyze this dataset with default parameters.…”

“…Moreno et al evaluated the effects of high concentrations of IS on S. cerevisiae F12. In the presence of 40-60% IS (w/w), significantly lower glucose consumption and reduced ethanol production were observed (Moreno, González-Fernández, Ballesteros, & Tomás-Pejó, 2019). Furthermore, in the presence of IS, yeast cells exhibited a wrinkled surface, reduced cell size, and increased levels of reactive oxygen species (ROS).…”

“…Furthermore, in the presence of IS, yeast cells exhibited a wrinkled surface, reduced cell size, and increased levels of reactive oxygen species (ROS). In this study, the authors compared the transcriptomic profiles of yeast at 4 h with and without 40% IS (GEO accession: GSE115460) (Moreno, González-Fernández, Ballesteros, & Tomás-Pejó, 2019). Results showed that 200 genes were upregulated, and 513 genes were downregulated.…”