Estudio de la respuesta de levaduras transformadoras de xilosa a condiciones de estrés en hidrolizados de bagazo de caña de azúcar

Abstract

The use of microorganisms as biocatalysts for the transformation of residual biomass has been studied in the last decades. However, within the parameters for the selection of these microorganisms, the ability to tolerate and adapt to stressful conditions is one of the greatest limitations for its use on an industrial scale. The aim of this work is to describe the diversity of the community of xylose transforming yeasts associated with oak forests in the Colombian Andes and to study the phenotypic response to stress conditions of non-conventional yeasts, isolated in natural and fermentative environments in Colombia capable of D-xylose metabolism, for its implementation in the fermentation of sugarcane bagasse hemicellulosic hydrolysates. For this, the isolation and taxonomic identification of 175 D-xylose-assimilating yeasts from oak forests of the central region of the eastern Andes Mountain range in Colombia was carried out belonging to 35 species previously known and 15 possible new species. It was observed that 75 isolates presented the metabolic capacity for the transformation of D-xylose to ethanol or xylitol. We also evaluated 50 yeasts from the biological collection Bank of Strain and Genes of IBUN-UNAL, isolated in the municipality of Puerto López (Meta, Colombia) and Paipa (Boyacá), of which 12 have the capacity to ferment xylose and produce xylitol as the main product. The implementation of thermal stress conditions was evaluated as a strategy for the preparation of the inoculum in two yeast species associated with fermentative environments, Meyerozyma caribbica and Candida tropicalis, for the fermentation of D-xylose in media supplemented with acetic acid. M. caribbica showed the acquisition of cross-stress protection in yeasts exposed to 10 °C with a 29 % increase in xylitol production compared to cells grown at 30 °C in the fermentation of D-xylose in the presence of the acetic acid (6 g/L). Exposure to acid, oxidative, and thermal stress prior to fermentation of hemicellulosic hydrolysate from sugarcane bagasse was applied to two yeasts isolated in natural environments, Debaryomyeces nepalensis and Scheffersomyces cryptocercus/Sc. virginianus, to produce xylitol and ethanol, respectively. The phenotypic response to the different stress conditions was species-specific and related to metabolic wiring for pentose transformation. D. nepalensis showed a 32% increase in xylitol production compared to untreated cells in fermentation of diluted sugarcane bagasse hemicellulosic hydrolysate (75% v/v). The inoculum preparation strategy with prior stress exposure coupled with serial fermentation improves fitness in Scheffersomyces cryptocercus/Sc. virginianus, achieving fermenting the D-xylose present in the sugarcane bagasse hemicellulosic hydrolysate undiluted. This is the first work carried out on the bioprospecting of the yeast diversity from oak forests in the eastern Andes Mountain range (Colombia) and the results demonstrate the potential for the exploration of non-timber ecosystem services present in these forest habitats.