4.1. Xylose fermenting yeast in natural environments of Ecuador

Lignocellulosic biomass is composed by mainly three different fractions of molecules: cellulose, hemicellulose and lignin. We have already talked about some applications for cellulose and lignin, mainly as ethanol and fuel biomass. Nevertheless, hemicellulose which is mainly composed of xylose, a five carbon sugar, is a very important and abundant source that accounts for 23% to 32% of the dry lignocellulosic biomass weight. This sugar can also be used for the production of ethanol as well as other valuable products.

As part of a survey in biodiversity in Ecuador, the CLQCA or Catholic University Yeast Collection has collected some isolates of yeast that exhibit fermentation skills when xylose and a Nitrogen base are mixed up in culture broths.

Xylose fermenting yeasts have been collected from different provinces of Ecuador, including the Galapagos Islands and the Amazonia. Nevertheless, none of these yeast isolates present high ethanol tolerance nor quick fermentation rates, which make these organisms not suitable for industrial processes. It can probably discourage someone to study natural occurring xylose fermenting yeasts, nonetheless, the genes involved in this physiological processes are still useful for metabolic engineering approaches.

As seen on table 4, only two strains of Schffersomyces (Pichia) stipitis are positive to ferment xylose. This yeast species has been reported to ferment xylose as will be seen further in this chapter.

In terms of ethanol production, there have been a lot of different approaches; in the last times the metabolic engineering of Saccharomyces cerevisiae was regarded as a suitable solution to ferment xylose, arabinose and other non-conventional sugars.

One example of this line of research is to be shown in the following scheme, where there are three different genes in charge of the transport, isomerization and phosphorylation in the process to ferment xylose. This construct has been designed at the Neotropical Center for the Biomass Research as part of the RESETA project. This genetic tool is very versatile and has been thought to be used as a genetic platform where to assay a wide variety of genes. The design of this construct was made by Carvajal et al. in 2011.

This genetic construction is being tested in laboratory conditions, integrated in industrial Saccharomyces cerevisiae. This first step forward to the metabolic engineering is expected to give new perspectives to the residual biomass transformation into valuable products in the context of biorefineries in Ecuador and worldwide.