Increasing Ethanol Tolerance in Industrially Important Ethanol Fermenting Organisms

Abstract

The growing population and increased urbanization have set great demand for energy. With the deprivation of fossil fuels, interest has focused on renewable sources. Among them, bioethanol is an attractive and green source being used as a promising alternative to reduce environmental pollution. In the process of ethanol production, varied ranges of feedstocks are being converted through microbial fermentation. Widespread industrial biocatalysts are exploited in bioethanol production like bacteria, yeast, fungi, and algae according to the type of raw material, environmental conditions, and resources available. However, the organisms leading the ethanol industries are Saccharomyces cerevisiae, Pichia stipites, Zymomonas mobilis, and Clostridium thermocellum. Their capacity to yield higher ethanol, ability to ferment sugars, growth in simple and inexpensive media, resistance to inhibitors, and contaminants makes them potential candidates in the fermentation process. While the maintenance of growth and metabolic efficiency of these microbes with resistance to various stresses is the desirable factor, several challenges predominate in the microbial fermentation inhibiting the overall ethanol production in which ethanol toxicity is a significant concern. The key advantages of microbial strain employed for industrial ethanol production are, ethanol tolerance limits and substrate concentration, ultimately decreasing productivity and increasing the costs of bioreactor. Ethanol toxicity is the complex and multi loci trait, affecting various cell functions and inhibits key glycolytic enzymes by denaturation mechanisms. The process renders the microbial cell to undergo reprogramming in cellular and metabolic activities with increased repair functions. However, it is crucial to understand the consequences of ethanol stress defense mechanism for ethanol tolerance improvement strategies. Various approaches have been attempted to circumvent the ethanol stress and enhance ethanol tolerance from optimizing its media to rewiring of genetic setup. Towards the establishment of ethanol tolerant phenotypes, integrative system, and the interplay of genes involving complex network is essential. With the assistance of molecular tools, the strain with improved ethanol tolerance can be achieved.