Native xylose and arabinose yeasts-assimilating: study of the assimilation and metabolization of the pentoses and use of these sugars from lignocellulosic biomass for obtainment bio-products

Abstract

In the period from 2011 to 2015 the research group developed a thematic project (Proc. FAPESP 2010 / 12624-0), under the coordination of the proponent of the present project, whose main objective was the enzymatic saccharification of sugarcane bagasse for the production of second generation ethanol. In this work thermophilic filamentous fungi producing fibrolytic enzymes and yeasts capable of assimilating the pentoses from hemicellulose hydrolysis were isolated. Among the various specimens of yeast capable of growing in xylose or arabinose as single carbon sources, some of them produced ethanol while others did not produce ethanol but reached high biomass rates when growing at both pentoses. However, other yeasts, that consumed from 80 to 100% of the xylose or arabinose present in the medium, produced little biomass and did not produce ethanol or xylitol. An exploratory analysis of the culture medium of these strains revealed numerous chromatographic peaks referring to low molar mass compounds, including various organic acids. In these previous studies, 36 strains were studied for the assimilation of L-arabinose, of which 17 showed excellent growth from this sugar as the only source of carbon, 3 strains produced ethanol and 5, citric acid and lactic acid. The results showed that most of the 36 strains consumed 100% of the xylose in the medium, presented similar growth to that in the presence of glucose and classified organic acids identified as oxalic, citric, maleic, tartaric, malic, pyruvic acids , Lactic, fumaric, acetic, propionic, isobutyric and butyric. Several of the studied strains were able to tolerate phenolic derivatives at various concentrations and were grown in acidic sugar cane bagasse hydrolysates. Data on pH effects of culture media, incubation temperature and on the influence of mineral nutrients on the assimilation of pentoses have also been obtained. At the end of the thematic project, it was clear the need for the continuity of the investigations of these yeasts and their potential for the assimilation of pentoses to the use of these sugars resulting from the hydrolysis of lignocellulosic biomass for the production of byproducts alternative to ethanol. Another question that needs to be answered is related to the ability of these assimilating yeasts of xylose and arabinose to produce enzymes that hydrolyze hemicellulose, especially the ²-xylosidase enzyme which is a rare enzyme in its extracellular form. Thus, it will be sought to know the production profiles of the xylanases by these yeasts. Considering that the physiology of the yeasts of pentose assimilation is not yet well known, a study will be necessary to know the efficiency of the assimilation of pentoses, as well as the bio-products formed from the consumption of pentoses, such as organic acids. The researchers who will be part of this current project are the same ones who participated in the thematic and now intend to investigate these yeasts more deeply considering, in addition to the bio-products obtained, the knowledge of the pentose transport system acting on them. It is understood that the yields in bio-products obtained when using native (not genetically modified) microorganisms are not always economically interesting, but what is sought in this project is the knowledge and understanding of this peculiar group of yeasts that efficiently assimilate the pentoses and thus to provide subsidies to those surveyed involving pentose transporters for future metabolic engineering projects of these and other yeasts. (AU)