Eukarya; Fungi/Metazoa; Fungi; Dikarya; Ascomycota; Saccharomycotina; Saccharomycetes; Saccharomycetales; Debaryomycetaceae; Scheffersomyces
Description and Significance
The fungus P. stipitis is a yeast from the xylose fermenting clade of the Schefferomyces genus. As with most members of the Saccharomycetales order a single P. stipitis individual has a size of 3 to 5 µm in diameter. Budding from asexual reproduction may cause deviation from the usual spherical or ellipsoidal shape of an individual. Under study it has been noted P. stipitis exhibits a cream-colored, smooth-shaped colony . A suitable habitat for P. stipitis includes damp or wet areas rich in organic biomass . Hardwood forests or areas high in agricultural waste are common environments. Temperatures at which growth occurs are 25°C to 37°C with budding most commonly occurring at 25°C. Studies have shown growth ceases at temperatures around 40°C to 45°C .
The significance of P. stipitis to industrial-related purposes comes from the ability to ferment the sugar, xylose . Degrading biomass to ethanol at high yields in respect to other ethanol-producing microorganisms has made P. stipitis an important constituent of the alternative fuel and bioenergy industries .
The genome P. stipits is composed of eight linear chromosomes. The total genome is comprised of 15.4 Mbp with chromosomes ranging in size from 3.5 to 0.97 Mbp . Studies have shown that P. stipitis is haploid, because when compared to similar yeasts, who have determined to be haploid, they share a high frequency of recessive mutations .
Analysis of the genome sequence have revealed many sequences that code for putative xylose transporters similar to those found in related yeasts, D. hanseii and C. intermedia. In addition to containing genes required for the glycolysis, the tricarboxylic cycle, and the oxidative pentose phosphate pathway, it contains genes for xylose assimilation and ethanol production . Genome sequencing and analysis have led to many possible insights into the metabolism of P. stipits as indicated by the following assertion from T. W. Jeffries et al., "The presence of numerous genes for endoglucanases and Beta-glucosidases, along with xylanase, mannanase, and chitinase activities indicate that it could metabolize polysaccharides in the beetle gut" . The ability of P. stipitis metabolize the polysaccharides found in the beetle gut is not that surprising since the strain was originally isolated from insect larvae belonging to passalid beetles . However, the yeast has the highest known capacity to ferment xylose to ethanol , an abundant sugar in the polysaccharides that compose hardwood and other plant material . This aspect of the yeast's metabolism and other reported pathways, such as the fermentation of cellobiose to ethanol , have lead to interest in the possibility of using P. stipitis to produce ethanol for use as a biofuel.
Cell Structure, Metabolism and Life Cycle
Pichia stipitis has been shown to be haplotonic and homothallic  and goes through ascomycetous sexual reproduction .
Pichia stipitis is known for its ability to produce ethanol by fermenting xylose . Xylose enters the cell through a proton symport transporter . A xylose reductase then reduces xylose to xylitol, which is consequently oxidized to xylulose with a xylitol dehydrogenase. Xylulose is phosphorylated and introduced into the pentose phosphate pathway, which will yield three and six carbon sugars that can be utilized in glycolysis .
P. stipitis will readily use oxygen as a terminal electron acceptor during catabolysis of sugars, but does not produce ethanol when grown aerobically. When grown under oxygen-limited conditions P. stipitis will begin fermenting xylose and produce ethanol, but this method is inefficient so the cell gains less energy and grows slower . It is possibly for this reason, that the xylose transport system has evolved to be the rate limiting step of xylose fermentation under anaerobic conditions . Oxygen is believed to activate the transport system increasing the rate of xylose up-take . This might be because xylose transport is an energy consuming process so energized oxygenated cells would be able to transport more xylose than less energized anaerobic cells , and thus oxygen indirectly influences xylose up-take . This leads to a problem in using P. stipitis for industrial ethanol production since it grows well and transports xylose well under oxygenated conditions, but ferments xylose and produces ethanol well only under anaerobic conditions. This may be overcome through genetic engineering of P. stipitis and its genes.
Ecology and Pathogenesis
The fermentation capabilities of P. stipitis are stimulated in environments where oxygen becomes a limitation . Aside from typical forest or agricultural habitats P. stipitis also forms an endosymbiotic relationship with passalid beetles . In the case of a study by Blackwell and Nguyen (unpublished) 400 of the 400 tested passalid beetles were found to contain the microorganism in their guts' . The oxygen-limited digestive tract and hindgut environment of the wood-ingesting beetle permit the degradation of xylose by P. stipitis .
It has also been determined P. stipitis can be utilized to ferment glucose from manufactured lumber products. Construction waste in landfills rich in glucose content may contribute to the bioenergy feedstock via the microbes' biomass conversion capabilities. Ethanol yield efficiencies can be as high as 84.7 to 90.7% per unit substrate consumption . Due to the fermentative ability there is potential to utilize food and municipal wastes as well as agricultural and forest residues to significantly contribute to ethanol feedstocks .
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Page authored by Josh Wolter and John Wright, students of Prof. Jay Lennon at Michigan State University.
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