Abatement of Inhibitors in Recycled Process Water from Biomass Fermentations Relieves Inhibition of a Saccharomyces cerevisiae Pentose Phosphate Pathway Mutant

Ronald E Hector,Sarah E Frazer,Jeffrey A Mertens,Nancy N Nichols

doi:10.3390/fermentation6040107

Abstract

Understanding the nature of fermentation inhibition in biomass hydrolysates and recycled fermentation process water is important for conversion of biomass to fuels and chemicals. This study used three mutants disrupted in genes important for tolerance to either oxidative stress, salinity, or osmolarity to ferment biomass hydrolysates in a xylose-fermenting Saccharomyces cerevisiae background. The S. cerevisiaeZWF1 mutant with heightened sensitivity to fermentation inhibitors was unable to ferment corn stover dilute-acid hydrolysate without conditioning of hydrolysate using a fungal strain, Coniochaeta ligniaria, to consume inhibitors. Growth of two other strains, a salt-sensitive HAL4 mutant and a GPD1 mutant sensitive to osmotic stress, was not negatively affected in hydrolysate compared to the parent xylose-metabolizing strain. In recycled fermentation process water, inhibition of the ZWF1 mutant could again be remediated by biological abatement, and no effect on growth was observed for any of the mutants compared to the parent strain.

Highlights

Conversion of lignocellulosic biomass to value-added products entails physical-chemical pretreatment to release sugar monomers, locked within the plant cell wall, as substrates
Volume lost to evaporation was replaced with distilled water and the hydrolysate was diluted with an equal volume of fresh hydrolysate that had been treated by bioabatement to mitigate fermentation inhibitors
Continued reuse of process water could result in accumulation of initially low-concentration components, causing concentrations to increase with each backset and eventually reach inhibitory levels if they are not adequately cleared during fermentation

Summary

Introduction

Conversion of lignocellulosic biomass to value-added products entails physical-chemical pretreatment to release sugar monomers, locked within the plant cell wall, as substrates. In addition to extracted sugars, hydrolysates contain other compounds formed under the harsh mechanical, thermal, and chemical conditions used to hydrolyze hemicellulose and make cellulose fibrils accessible to cellulase [1]. We used phenotypically sensitive mutant yeast strains to probe for common causes of inhibition: chemical, saline, and osmotic stresses. To demonstrate this sensor technology, we have applied it to hydrolysate that has been conditioned for fermentation following pretreatment using biological abatement, a method developed in this lab.

Materials and Methods

Yeast Growth Assays

Preparation and Analysis of Hydrolysate

Inhibitor Abatement

Ethanol Fermentations and Recycle of Hydrolysate

Results and Discussion