Natural habitats of yeasts were examined for the presence of strains able to produce ethanol from D-xylose. Black knots, insect frass, and tree exudates were screened by enrichment in liquid D-xylose-yeast extract medium. These and each D-xylose-assimilating yeast in a collection from cactus fruits and Drosophila spp. were tested for alcohol production from this sugar. Among the 412 isolates examined, 36 produced more than 1 g of ethanol liter-' from 20 g of D-xylose liter-', all under aerated conditions. Closer examination of the strains indicated that their time courses of D-xylose fermentation followed different patterns. Some strains produced more biomass than ethanol, and among these, ethanol may or may not be assimilated rapidly after depletion of D-xylose. Others produced more ethanol than biomass, but all catabolized ethanol after carbohydrate exhaustion. Ethanol production appeared best at low pH values and under mild aeration. Possible correlations between the nutritional profiles of the yeasts and their ability to produce ethanol from D-xylose were explored by multivariate analysis. D-Xylose appeared slightly better utilized by yeasts which rate poorly in terms of fermentation. The fermentation of D-glucose had no bearing on D-xylose fermentation. No specific nutritional trait could discriminate well between better D-xylose fermentors and other yeasts.
Each species of the genus Hansenula has been tested for the extent to which cellular inactivation by ultraviolet radiation is mitigated by photoreactivation. The values obtained conform to a pattern consistent with the phylogenetic relationships between the species proposed by Wickerham. These findings corroborate previous indications that (i) fixation of the capacity for photoreactivation is an important element in the evolution of yeasts and that (ii) opportunity for environmental exposure of yeasts to solar ultraviolet radiation is not the determining factor in natural selection for their abilities to photo-reactivate.
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