Ethanol stress responses in Kluyveromyces marxianus: current knowledge and perspectives.

The rising concern with the emission of greenhouse gases has boosted new incentives for biofuels production, which are less polluting than fossil fuels. Special attention has been given to the second-generation ethanol, as it is produced from abundant feedstocks which do not compete with food production, such as lignocellulosic biomass and whey. Kluyveromyces marxianus stands out in second-generation ethanol production due to its capacity of assimilating lactose, the sugar found in whey, and tolerating high temperatures used in simultaneous saccharification processes. Nonetheless, contrary to Saccharomyces cerevisiae, K. marxianus does not tolerate high ethanol concentrations. Ethanol causes a broad range of toxic effects on yeasts, acting on cell membrane and proteins, as well as inducing the generation of reactive oxygen species (ROS). The ethanol stress responses are not fully understood, mainly in non-conventional yeasts such as K. marxianus. Indeed, many molecular responses to ethanol stress are still inferred from S. cerevisiae. As such, a better understanding of the ethanol stress responses in K. marxianus may provide the basis for improving its use in the biofuel industry. Additionally, the selection of ethanol-tolerant strains by metabolic engineering is useful to provide strains with improved capacity to withstand stressful conditions, as well as to obtain new insights about the ethanol stress responses. Key points • It is still not totally clear why K. marxianus is less tolerant to ethanol than S. cerevisiae. • Understanding the ethanol stress response in K. marxianus is pivotal for improving its application in the biofuel industry. • The Metabolic engineering is expected to improve the ethanol tolerance in K. marxianus.