Overcoming lignocellulose‐derived microbial inhibitors: advancing the Saccharomyces cerevisiae resistance toolbox

Abstract

Lignocellulose-derived biofuels present an attractive carbon-neutral alternative to fossil fuels amidst growing global energy and climate concerns. Bioethanol in particular, has been shown to be a viable additive to and / or replacement for petroleum in countries such as Brazil. The bioconversion of lignocellulose biomass to bioethanol is a developing technology with the yeast Saccharomyces cerevisiae playing a pivotal role in the fermentation-based processes. This yeast however, is challenged to ferment under harsh conditions, specifically, during exposure to lignocellulose-derived microbial inhibitors that are formed during pretreatment processes. This detrimentally affects biocatalyst performance, ultimately decreasing ethanol productivity and yield. To this end, S. cerevisiae strains are in development to increase the yeast microbial inhibitor resistance towards cost-effective cellulosic bioethanol production. This review discusses the current status of inhibitor resistance in S. cerevisiae strains and perspectives on the future of multi-tolerant phenotypes with a specific focus on existing and emerging strain development strategies designed to improve resistance phenotypes.

Nutrient-supplemented propagation of Saccharomyces cerevisiae improves its lignocellulose fermentation ability

“…Physiological parameters such as viability, biomass yield, and fermentation capacity have been reported to increase during fermentation following short-term adaptation compared to cultures propagated in a medium without hydrolysate (Alkasrawi et al 2006 ; Nielsen et al 2015 ; Zhang et al Zhang et al 2019 ; van Dijk et al 2019 ). Alternatively, evolutionary engineering, or long-term adaptation, of yeast strains has been shown to be successful in increasing the efficiency of fermentation through increased inhibitor tolerance (Marti´n and Jönsson 2003 ; Tomás-Pejó et al 2010 ; Brandt et al 2019 ). However, hydrolysate composition, and thus inhibitor abundance, varies depending on the feedstock (Klinke et al 2004 ; Almeida et al 2007 ), seasonality (Bunnell et al 2013 ; Greenhalf et al 2013 ), and method of substrate pretreatment (Chundawat et al 2010 ).…”

Rational Engineering of Saccharomyces Cerevisiae Towards Improved Tolerance to Multiple Inhibitors in Lignocellulose Fermentations

“…Physicochemical pretreatment is thus required to disrupt the compact crystalline structure and allow enzymatic access to the polysaccharides within, to release fermentable sugars [3,5,6]. The majority of such pre-treatment methods result in signi cant quantities of degradation products being formed, which have inhibitory effects of subsequent biological conversions [5,[7][8][9][10][11].…”

“…Interestingly, an unanticipated phenotype that has emerged from xylose strain development is hypersensitivity of the introduced heterologous metabolic pathways to stressful conditions [2]. Due to the interconnectivity between metabolism and stress response, strain development for lignocellulose bioconversion technologies have to simultaneously address both xylose utilization and microbial stresses [10]. Xylose engineered industrial strains are thus the ideal genetic background in which to study the impact of microbial stresses, as well as introducing stress resistance genes.…”

“…Furans are degradation products of sugars, phenolics are derived from solubilizing lignin and weak acids such as acetic and formic acids are formed during furan degradation and/or de-acetylation of hemicellulose [5,8,22,23]. Such microbial inhibitors negatively impact growth, fermentation and xylose utilisation ability of yeast, which results in suboptimal ethanol productivity and yields [9,10,13,16]. Given the toxicity of these compounds, studies have been undertaken to develop yeast strains capable of not only withstanding the harsh conditions associated with lignocellulosic biomass fermentations, but to also generate ethanol yields expected in industrial processes [5,22].…”

“…Inhibitor resistance is characterised as a complex function of multiple genes [10,[26][27][28][29], however, relatively few over-expression studies have undertaken to genetically engineer yeast towards multiple inhibitor tolerance phenotypes. Typical rational design strategies are often limited to only a few genes involved in highly speci c in situ detoxi cation mechanisms, resulting in strains with inhibitor-speci c detoxi cation phenotypes [30][31][32][33][34].…”