A new search for thermotolerant yeasts, its characterization and optimization using response surface methodology for ethanol production

The progressive rise in energy crisis followed by green house gas (GHG) emissions is serving as the driving force for bioethanol production from renewable resources. Current bioethanol research focuses on lignocellulosic feedstocks as these are abundantly available, renewable, sustainable and exhibit no competition between the crops for food and fuel. However, the technologies in use have some drawbacks including incapability of pentose fermentation, reduced tolerance to products formed, costly processes, etc. Therefore, the present study was carried out with the objective of isolating hexose and pentose fermenting thermophilic/thermotolerant ethanologens with acceptable product yield. Two thermotolerant isolates, NIRE-K1 and NIRE-K3 were screened for fermenting both glucose and xylose and identified as Kluyveromyces marxianus NIRE-K1 and K. marxianus NIRE-K3. After optimization using Face-centered Central Composite Design (FCCD), the growth parameters like temperature and pH were found to be 45.17°C and 5.49, respectively for K. marxianus NIRE-K1 and 45.41°C and 5.24, respectively for K. marxianus NIRE-K3. Further, batch fermentations were carried out under optimized conditions, where K. marxianus NIRE-K3 was found to be superior over K. marxianus NIRE-K1. Ethanol yield (Y x∕s ), sugar to ethanol conversion rate (%), microbial biomass concentration (X) and volumetric product productivity (Q p ) obtained by K. marxianus NIRE-K3 were found to be 9.3, 9.55, 14.63, and 31.94% higher than that of K. marxianus NIRE-K1, respectively. This study revealed the promising potential of both the screened thermotolerant isolates for bioethanol production.

Candida baotianmanensis sp. nov. and Candida pseudoviswanathii sp. nov., two ascosporic yeast species isolated from the gut of beetles

Four yeast strains were isolated from the gut of beetles collected on Baotianman Mountain and People's Park of Nanyang in Henan Province, China. These strains produced unconjugated asci with one or two ellipsoidal to elongate ascospores in a persistent ascus. Phylogenetic analysis of the D1/D2 domains of the LSU rRNA gene sequences indicated that the isolates represent two novel sexual species in the Candida/Lodderomyces clade. Candida baotianmanensis sp. nov. was located in a statistically well-supported branch together with Candida maltosa. Candida pseudoviswanathii sp. nov. formed a subclade with its closest relative Candida viswanathii supported by a strong bootstrap value. The two novel species were distinguished from their most closely related described species, Candida maltosa and Candida viswanathii, in the D1/D2 LSU rRNA gene and internal transcribed spacer (ITS) sequences and in phenotypic traits. The type strain of Candida baotianmanensis sp. nov. is NYNU 14719T ( = CBS 13915T = CICC 33052T), and the type strain of Candida pseudoviswanathii sp. nov. is NYNU 14772T ( = CBS 13916T = CICC 33053T). The MycoBank numbers for Candida baotianmanensis sp. nov. and Candida pseudoviswanathii sp. nov. are MB 812621 and MB 812622.

Yamadazyma ubonensis f.a., sp. nov., a novel xylitol-producing yeast species isolated in Thailand

Three hundred and thirty-seven xylose-utilizing yeast strains were isolated from various natural samples. Among these, 68 strains produced xylitol in the range of 0.1-0.69 g xylitol/g xylose. Thirty-nine xylitol-producing strains were identified to be Candida tropicalis. Ten strains were found belonging to 14 known species in the genus Candida, Cyberlindnera, Meyerozyma, Pichia, Wickerhamomyces, Yamadazyma and Cryptococcus. Two strains were identified to be two Candida species and two strains (DMKU-XE142(T) and DMKU-XE332) were found to be a novel species. Strain DMKU-XE142(T) was isolated from tree bark and DMKU-XE332 was obtained from decaying plant leaf collected in Thailand. On the basis of morphological, biochemical, physiological and chemotaxonomic characteristics and sequence analysis of the D1/D2 region of the large subunit rRNA gene (LSU) and the internal transcribed spacer (ITS) region, the two strains were determined to represent a novel Yamadazyma species although formation of ascospores was not observed. The sequences of the D1/D2 region of the LSU rRNA gene and the ITS region of the two strains were identical but differed from Yamadazyma phyllophila, the closest species in terms of pairwise sequence similarity of the D1/D2 region, by 1.7 % nucleotide substitutions and 3.5 % nucleotide substitutions in the ITS region. The name Yamadazyma ubonensis f.a., sp. nov. is proposed (type strain is DMKU-XE142(T) = BCC 61020(T) = CBS 12859(T)).

Diversity and fermentation products of xylose-utilizing yeasts isolated from buffalo feces in Thailand

Twenty-eight xylose-utilizing yeast strains were isolated by enrichment culture from 11 samples of feces from the rectum of Murrah buffalo and Swamp buffalo in Thailand. On the basis of their morphological and biochemical characteristics, including sequence analysis of the D1/D2 region of the large-subunit ribosomal RNA gene (LSU rDNA), they were identified as Candida tropicalis (designated as Group I, 11 isolates), Candida parasilosis (Group II, 2 isolates), Candida mengyuniae (Group III, 2 isolates), Sporopachydermia lactativora (Group IV, 2 isolates), Geotrichum sp. (Group V, 5 isolates) and Trichosporon asahii (Group VI, 6 isolates). All isolates utilized xylose as the sole carbon source but 27 isolates could ferment xylose to ethanol (0.006–0.602 g L⁻¹) and 21 isolates could ferment xylose to xylitol (0.19–22.84 g L⁻¹). Candida tropicalis isolates produced the highest yield of xylitol (74.80%). Their ability to convert xylose to xylitol and ethanol ranged from 15.06 g L⁻¹ to 22.84 g L⁻¹ xylitol and 0.110 g L⁻¹ to 0.602 g L⁻¹ ethanol, respectively.

Wickerhamomyces xylosica sp. nov. and Candida phayaonensis sp. nov., two xylose-assimilating yeast species from soil

Two strains (NT29T and NT31T) of xylose-assimilating yeasts were obtained from soils collected in northern Thailand. On the basis of morphological, biochemical, physiological and chemotaxonomic characteristics, and sequence analysis of the D1/D2 domain of the large subunit rRNA gene and the internal transcribed spacer region, the two strains were found to represent two novel ascomycete yeast species. Strain NT29T was assigned to the genus Candida belonging to the Pichia clade as a representative of Candida phayaonensis sp. nov.; the type strain is NT29T ( = BCC 47634T = NBRC 108868T = CBS 12319T). Strain NT31T represented a novel Wickerhamomyces species, which was named Wickerhamomyces xylosica sp. nov.; the type strain is NT31T ( = BCC 47635T = NBRC 108869T = CBS 12320T).