Biotechnological potential of Candida spp. for the bioconversion of D-xylose to xylitol

Cellulosic biomass is an abundant and underused substrate for biofuel production. The inability of many microbes to metabolize the pentose sugars abundant within hemicellulose creates specific challenges for microbial biofuel production from cellulosic material. Although engineered strains of Saccharomyces cerevisiae can use the pentose xylose, the fermentative capacity pales in comparison with glucose, limiting the economic feasibility of industrial fermentations. To better understand xylose utilization for subsequent microbial engineering, we sequenced the genomes of two xylose-fermenting, beetle-associated fungi, Spathaspora passalidarum and Candida tenuis. To identify genes involved in xylose metabolism, we applied a comparative genomic approach across 14 Ascomycete genomes, mapping phenotypes and genotypes onto the fungal phylogeny, and measured genomic expression across five Hemiascomycete species with different xylose-consumption phenotypes. This approach implicated many genes and processes involved in xylose assimilation. Several of these genes significantly improved xylose utilization when engineered into S. cerevisiae, demonstrating the power of comparative methods in rapidly identifying genes for biomass conversion while reflecting on fungal ecology.

Rising crude oil prices and environmental concerns have renewed interest in renewable energy. Cellulosic ethanol promises to deliver a renewable fuel from non-food feedstocks. One technical challenge producing cellulosic ethanol economically is a robust organism to utilize the different sugars present in cellulosic biomass. Unlike starch where glucose is the only sugar present, cellulosic biomass has other sugars such as xylose and arabinose, usually called C5 sugars. This review examines the most promising naturally occurring C5 fermenting organism, Pichia stipitis. In this work, the properties that make P. stipitis unique from other organisms, its physiology and fermentation results on lignocellulosic substrates have been reviewed. P. stipitis can produce 41 g ethanol/l with a potential to cleanup some of the most concentrated toxins. These results coupled with the less stringent nutritional requirements, great resistance to contamination and its thick cell walls makes P. stipitis a viable organism for scale-up. However, P. stipitis has a slower sugar consumption rate compared to Saccharomyces cerevisiae and requires microaerophilic condition for ethanol production. Finally, future studies to enhance fermentation capabilities of this yeast have been discussed.