The utilization of hemicellulose, the second most abundant polysaccharide, is must for the cost-efficient production of ethanol from second generation feedstocks. Xylan, the major hemicellulose in plant biomass yields mainly xylose as pentose sugars on hydrolysis. The progress in fermentation of pentose sugars has gone on slow pace as there are few microorganisms known, which are capable of pentose metabolism. The future perhaps lies in finding organisms that would ferment high density hydrolysates without purification. This obviously has to use the genetic and metabolic engineering routes. Either a direct or a sequential fermentation system needs to be worked out. This review provides an overview of the current pentose bioconversion processes and future prospects for bioethanol production.

Bioethanol production from pentose sugars: Current status and future prospects

SUMMARY Forced interspecific hybridization has been used in yeasts for many years to study speciation or to construct artificial strains with novel fermentative and metabolic properties. Recent genome analyses indicate that natural hybrids are also generated spontaneously between yeasts belonging to distinct species, creating lineages with novel phenotypes, varied genetic stability, or altered virulence in the case of pathogens. Large segmental introgressions from evolutionarily distant species are also visible in some yeast genomes, suggesting that interspecific genetic exchanges occur during evolution. The origin of this phenomenon remains unclear, but it is likely based on weak prezygotic barriers, limited Dobzhansky-Muller (DM) incompatibilities, and rapid clonal expansions. Newly formed interspecies hybrids suffer rapid changes in the genetic contribution of each parent, including chromosome loss or aneuploidy, translocations, and loss of heterozygosity, that, except in a few recently studied cases, remain to be characterized more precisely at the genomic level by use of modern technologies. We review here known cases of natural or artificially formed interspecies hybrids between yeasts and discuss their potential importance in terms of genome evolution. Problems of meiotic fertility, ploidy constraint, gene and gene product compatibility, and nucleomitochondrial interactions are discussed and placed in the context of other known mechanisms of yeast genome evolution as a model for eukaryotes.

Evolutionary Role of Interspecies Hybridization and Genetic Exchanges in Yeasts

Arabitol belongs to the pentitol family and is used in the food industry as a sweetener and in the production of human therapeutics as an anticariogenic agent and an adipose tissue reducer. It can also be utilized as a substrate for chemical products such as arabinoic and xylonic acids, propylene, ethylene glycol, xylitol and others. It is included on the list of 12 building block C3-C6 compounds, designated for further biotechnological research. This polyol can be produced by yeasts in the processes of bioconversion or biotransformation of waste materials from agriculture, the forest industry (l-arabinose, glucose) and the biodiesel industry (glycerol). The present review discusses research on native yeasts from the genera Candida, Pichia, Debaryomyces and Zygosaccharomyces as well as genetically modified strains of Saccharomyces cerevisiae which are able to utilize biomass hydrolysates to effectively produce L- or D-arabitol. The metabolic pathways of these yeasts leading from sugars and glycerol to arabitol are presented. Although the number of reports concerning microbial production of arabitol is rather limited, the research on this topic has been growing for the last several years, with researchers looking for new micro-organisms, substrates and technologies.

Production of arabitol by yeasts: current status and future prospects

The sugar alcohol d-arabitol is one of the Department of Energy’s top twelve bio-based building block chemicals. In this study, we found that the oleaginous yeast Rhodosporidium toruloides IFO0880 produces d-arabitol during growth on xylose in nitrogen-rich medium. Efficient xylose utilization was a prerequisite for extracellular d-arabitol production. During growth in complex media, R. toruloides produced 22 ± 2, 32 ± 2, and 49 ± 2 g/L d-arabitol from 70, 105, and 150 g/L xylose, respectively. In addition, we found that R. toruloides could potentially be used for the co-production of lipids and d-arabitol from xylose. These results demonstrate that R. toruloides can be used to produce multiple value-added chemicals from xylose.

Production of D-arabitol from D-xylose by the oleaginous yeast Rhodosporidium toruloides IFO0880.

A process for industrial production of fructooligosaccharides (FOS’s) based on the conversion of sucrose by immobilized fructosyltransferase (FTase) from the cells of Aureobasidium pullulans CCY 27-1-94 was developed. Particular process operations and conditions were designed employing results of laboratory and semi-pilot scale experiments. The process flowsheet comprised three sections: FTase production, which included fermentation, isolation and purification of the enzyme, FTase immobilization and FOS’s production where a product with a high content of FOS’s was prepared by the removal of glucose, fructose and unreacted sucrose from the reaction mixture using simulated moving-bed chromatography. Two alternative process flowsheets were proposed for the annual production of 10 000 t of FOS’s: one for a powdery product and the second one for syrup. The economic analysis provided the costs for the production of immobilized FTase and FOS’s using two different price estimates for sucrose.

Jeng-Leun Mau

Papers

Construction of an intergeneric fusion from Schizosaccharomyces pombe and Lentinula edodes for xylan degradation and polyol production