Evaluation of divergent yeast genera for fermentation-associated stresses and identification of a robust sugarcane distillery waste isolate Saccharomyces cerevisiae NGY10 for lignocellulosic ethanol production in SHF and SSF

Improved xylitol production by the novel inhibitor-tolerant yeast Candida tropicalis K2

Production of potential value-added products from different lignocellulosic biomass is becoming more common due to the availability of the feedstocks in abundance and the environment- friendly nature of the microbial production process. Due to the large array of its applications in the pharmaceutical and food sectors, xylitol is considered as potential value-added compound for production. In this study, organic waste samples were collected from various habitats and screened for potential yeast isolates for xylitol production. Among 124 tested isolates, Candida tropicalis K2 showed the highest potential for xylitol production as well as inhibitors tolerance (Furfural, 5-hydroxymethyl furfural and acetic acid) phenotypes. C. tropicalis K2 produced 90 g/L of xylitol in batch fermentation (100 g/L xylose supplemented with 20 g/L of glycerol as co-substrate) with the yield and productivity of 0.90 g/g and 1.5 g/L.h, respectively, at pH 5.5 and 30°C temperature. Together, >10% higher xylitol yield was achieved when glycerol was used as a co-substrate with pure xylose. Moreover, with non-detoxified corncob and Albizia pod hydrolysates, C. tropicalis K2 isolate produced 0.62 and 0.69 g/g of xylitol yields and 1.04 and 0.75 g/L.h xylitol productivities, respectively. Thus, C. tropicalis K2 isolate could be considered as promising candidate for xylitol production from different lignocellulosic biomass.HIGHLIGHTS Candia tropicalis K2 isolate was screened from natural sites of biomass degradation and characterized for xylitol production.Non-detoxified Albizia pod and corncob hydrolysates were explored for xylitol production using selected C. tropicalis K2 isolate.A maximum of 0.90 g/g yield and 1.07 g/L.h xylitol productivity was achieved with pure xylose.A >10% increase in xylitol yield was achieved using glycerol as a co-substrate.

Sustainable production and degradation of plastics using microbes

Plastics are indispensable in everyday life and industry, but the environmental impact of plastic waste on ecosystems and human health is a huge concern. Microbial biotechnology offers sustainable routes to plastic production and waste management. Bacteria and fungi can produce plastics, as well as their constituent monomers, from renewable biomass, such as crops, agricultural residues, wood and organic waste. Bacteria and fungi can also degrade plastics. We review state-of-the-art microbial technologies for sustainable production and degradation of bio-based plastics and highlight the potential contributions of microorganisms to a circular economy for plastics.

Development of engineered Candida tropicalis strain for efficient corncob-based xylitol-ethanol biorefinery

BACKGROUND

Xylitol has a wide range of applications in the pharmaceuticals, cosmetic, food and beverage industry. Microbial xylitol production reduces the risk of contamination and is considered as environment friendly and sustainable compared to the chemical method. In this study, random mutagenesis and genetic engineering approaches were employed to develop Candida tropicalis strains with reduced xylitol dehydrogenase (XDH) activity to eliminate co-substrate requirement for corn cob-based xylitol-ethanol biorefinery.

RESULTS

The results suggest that when pure xylose (10% w/v) was fermented in bioreactor, the Ethyl methane sulfonate (EMS) mutated strain (C. tropicalis K2M) showed 9.2% and XYL2 heterozygous (XYL2/xyl2Δ::FRT) strain (C. tropicalis K21D) showed 16% improvement in xylitol production compared to parental strain (C. tropicalis K2). Furthermore, 1.5-fold improvement (88.62 g/L to 132 g/L) in xylitol production was achieved by C. tropicalis K21D after Response Surface Methodology (RSM) and one factor at a time (OFAT) applied for media component optimization. Finally, corncob hydrolysate was tested for xylitol production in biorefinery mode, which leads to the production of 32.6 g/L xylitol from hemicellulosic fraction, 32.0 g/L ethanol from cellulosic fraction and 13.0 g/L animal feed.

CONCLUSIONS

This work, for the first time, illustrates the potential of C. tropicalis K21D as a microbial cell factory for efficient production of xylitol and ethanol via an integrated biorefinery framework by utilising lignocellulosic biomass with minimum waste generation.

Technological characterisation and probiotic traits of yeasts isolated from Sha'a, a Cameroonian maize-based traditional fermented beverage

The current trend in starter selection is to combine both technological and probiotic properties to standardise and make functional artisanal fermented beverages such as Sha'a whose properties are very variable due to the lack of a known starter. The objective of this work was to study technological and probiotic properties of yeasts isolated from Sha'a sold in Bamenda, Bafoussam, Bonabérie, Dschang, Foumbot, Mbouda and Njombé (Cameroon). The isolated yeasts were studied for their ability to produce CO₂ from glucose, to grow in the presence of 8% ethanol, 20% glucose and pH 3, to assimilate maltose and to produce ethanol. Then, the survival of the pre-selected isolates was assessed in simulated gastric (pH 2 and 3) and intestinal juices, followed by self-aggregation, co-aggregation, hydrophobicity, haemolysin, gelatinase, biogenic amine production, antibiotic and antifungal susceptibility, bile salt hydrolase and antiradical activity. The selected isolates were identified by sequencing the 5.8S/28S rRNA gene. From the 98 isolates obtained, 66 produced CO₂ from glucose and 16 were then selected for their ability to grow in the presence of 8% ethanol, 20% glucose, pH 3 and maltose. The overall survival of isolates ranged from 4.12 ± 1.63 to 104.25 ± 0.19% (LT16) and from 0.56 ± 0.20 to 96.74 ± 1.60% (LT66) at pH 3 and pH 2 respectively. All of them have remarkable surface hydrophobicity properties. Based on principal component analysis, 5 isolates were selected as the best. However, only 3 of them, LT16 (the most promising), LT25 identified as Saccharomyces cerevisiae and LT80 as Nakaseomyces delphensis, do not produce a virulence factor. The latter can deconjugate bile salts with a maximum percentage of 60.54 ± 0.12% (LT16) and the highest inhibition of DPPH° radicals was 55.94 ± 1.14% (LT25). In summary, the yeast flora of Sha'a contains yeasts capable of fermenting and producing ethanol while producing bioactive compounds that would benefit the consumer.

Current Ethanol Production Requirements for the Yeast Saccharomyces cerevisiae

An increase in global energy demand has caused oil prices to reach record levels in recent times. High oil prices together with concerns over CO₂ emissions have resulted in renewed interest in renewable energy. Nowadays, ethanol is the principal renewable biofuel. However, the industrial need for increased productivity, wider substrate range utilization, and the production of novel compounds leads to renewed interest in further extending the use of current industrial strains by exploiting the immense, and still unknown, potential of natural yeast strains. This review seeks to answer the following questions: (a) which characteristics should S. cerevisiae have for the current production of first- and second-generation ethanol? (b) Why are alcohol-tolerance and thermo-tolerance characteristics required? (c) Which genes are related to these characteristics? (d) What are the advances that can be achieved with the isolation of new organisms from the environment?

Enhanced biodiesel and β-carotene production in Rhodotorula pacifica INDKK using sugarcane bagasse and molasses by an integrated biorefinery framework

Dependency on fossil fuels raises an economic and ecological concern that has urged to look for alternative sources of energy. Bio-refinery concept is one of the alternate frameworks for the biomass conversion into biofuel and other value-added by-products. The present work illustrates importance of an oleaginous yeast Rhodotorula pacifica INDKK in an integrated bio-refinery field by utilizing renewable sugars generated from lignocellulosic biomass. The maximum 11.8 g/L lipid titer, 210.4 mg/L β-carotene and 7.1 g animal feed were produced by R. pacifica INDKK in bioreactor containing 5% (v/v) molasses supplemented with enzymatically hydrolyzed and alkali-pretreated sugarcane bagasse hydrolysate (35% v/v). Furthermore, xylooligosaccharides (20.6 g/L), a beneficial prebiotics were also produced from the hemicellulosic fraction separated after alkali pretreatment of bagasse. This novel concept of integrated yeast bio-refinery for concomitant production of biodiesel and multiple value-added products with minimum waste generation is proposed as a sustainable and profitable process.

The Dynamics of Single-Cell Nanomotion Behaviour of Saccharomyces cerevisiae in a Microfluidic Chip for Rapid Antifungal Susceptibility Testing

The fast emergence of multi-resistant pathogenic yeasts is caused by the extensive—and sometimes unnecessary—use of broad-spectrum antimicrobial drugs. To rationalise the use of broad-spectrum antifungals, it is essential to have a rapid and sensitive system to identify the most appropriate drug. Here, we developed a microfluidic chip to apply the recently developed optical nanomotion detection (ONMD) method as a rapid antifungal susceptibility test. The microfluidic chip contains no-flow yeast imaging chambers in which the growth medium can be replaced by an antifungal solution without disturbing the nanomotion of the cells in the imaging chamber. This allows for recording the cellular nanomotion of the same cells at regular time intervals of a few minutes before and throughout the treatment with an antifungal. Hence, the real-time response of individual cells to a killing compound can be quantified. In this way, this killing rate provides a new measure to rapidly assess the susceptibility of a specific antifungal. It also permits the determination of the ratio of antifungal resistant versus sensitive cells in a population.

Metabolomic Profiling Revealed Diversion of Cytidinediphosphate-Diacylglycerol and Glycerol Pathway towards Denovo Triacylglycerol Synthesis in Rhodosporidium toruloides

Oleaginous yeast Rhodosporidium toruloides has great biotechnological potential and scientific interest, yet the molecular rationale of its cellular behavior to carbon and nitrogen ratios with concurrent lipid agglomeration remains elusive. Here, metabolomics adaptations of the R. toruloides in response to varying glucose and nitrogen concentrations have been investigated. In preliminary screening we found that 5% glucose (w/v) was optimal for further analysis in Rhodosporidium toruloides 3641. Hereafter, the effect of complementation to increase lipid agglomeration was evaluated with different nitrogen sources and their concentration. The results obtained illustrated that the biomass (13 g/L) and lipid (9.1 g/L) production were maximum on 5% (w/v) glucose and 0.12% (NH₄)₂SO₄. Furthermore, to shed lights on lipid accumulation induced by nitrogen-limitation, we performed metabolomic analysis of the oleaginous yeast R. toruloides 3641. Significant changes were observed in metabolite concentrations by qualitative metabolomics through gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS), which were mapped onto the governing metabolic pathways. Notable finding in this strain concerns glycerol and CDP-DAG metabolism wherein reduced production of glycerol and phospholipids induced a bypass leading to enhanced de-novo triacylglyceride synthesis. Collectively, our findings help in understanding the central carbon metabolism of R. toruloides which may assist in developing rationale metabolic models and engineering efforts in this organism.

Compost as an untapped niche for thermotolerant yeasts capable of high-temperature ethanol production

Efficient bioethanol production from lignocellulosic biomass requires thermotolerant yeasts capable of utilizing multiple sugars, tolerating inhibitors and fermenting at high temperatures. In this study, 98 thermotolerant yeasts were isolated from nine compost samples. We selected 37 yeasts that belonged to 11 species; 31 grew at 45°C; 6 strains grew at 47°C, while 9 yeasts could utilize multiple sugars. Many yeast isolates showed high ethanol production in the range of 12-24 g l^-1 , with fermentation efficiencies of 47-94% at 40°C using 5% glucose. Kluyveromyces marxianus CSV3.1 and CSC4.1 (47°C), Pichia kudriavzevii CSUA9.3 (45°C) produced 21, 22 and 23 g l^-1 of ethanol with efficiencies of 83, 87 and 90%, respectively, using 5% glucose. Among these yeasts, K. marxianus CSC4.1 and P. kudriavzevii CSUA9.3 exhibited high tolerance against furfural, 5-HMF, acetic acid and ethanol. These two strains produced high amounts of ethanol from alkali-treated RS, with 84 and 87% efficiency via separate hydrolysis and fermentation; 76 and 74% via simultaneous saccharification and fermentation at 47 and 45°C, respectively. Therefore, this study demonstrates compost as a potential anthropogenic niche for multiple sugar-utilizing, inhibitor-tolerant ethanologenic yeasts suitable for high-temperature ethanol production via SHF of rice straw.

Performance of Yeast Microbial Fuel Cell Integrated with Sugarcane Bagasse Fermentation for COD Reduction and Electricity Generation

The purpose of this analysis is to evaluate the efficiency of the Microbial Fuel Cell (MFC) system incorporated with the fermentation process, with the aim of reducing COD and generating electricity, using sugarcane bagasse extract as a substrate, in the presence and absence of sugarcane fibers. There is a possibility of turning bagasse extract into renewable bioenergy to promote the sustainability of the environment and energy. As a result, the integration of liquid fermentation (LF) with MFC has improved efficiency compared to semi-solid state fermentation (S-SSF). The maximum power generated was 14.88 mW/m2, with an average COD removal of 39.68% per cycle. The variation margin of the liquid fermentation pH readings remained slightly decrease, with a slight deflection of +0.14 occurring from 4.33. With the absence of bagasse fibers, biofilm can grow freely on the anode surface so that the transfer of electrons is fast and produces a relatively high current. Experimental data showed a positive potential after an effective integration of the LF and MFC systems in the handling of waste. The product is then simultaneously converted into electrical energy. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Salt-stress adaptation of yeast as a simple method to improve high-gravity fermentation in an industrial medium

While Saccharomyces cerevisiae is a popular organism to produce ethanol, its fermentation performance is affected at high sugar concentrations due to osmotic stress. We hypothesized that adaptation under ionic stress conditions will improve the fermentation performance at high sugar concentrations due to cross-stress adaptation. We, therefore, adapted a high-performance yeast strain, S. cerevisiae CEN.PK 122, to increasing salt concentrations in an industrial medium. Control cells were adapted in the medium without added salt. The cells adapted to 3.5% (w/v) salt concentration demonstrated a superior performance when fermenting 10-30% (w/v) glucose. When fermenting 30% (w/v) glucose, the ethanol yields of the adapted cells (0.49 ± 0.01 g g^-1) were about 30% higher than the control cells (0.37 ± 0.01 g g^-1) and are comparable with the best reported to date for any medium employed. Similar improvements were also observed when fermenting 10% (w/v) sucrose. However, little improvement in fermentation was observed at the higher temperature tested (40 °C), even though the growth of the adapted cells was greater when tested in YPD medium. The improvements in fermentation at 30 °C were primarily related to the faster growth of the adapted cells and not to an increase in specific intake rates. Additionally, a significantly reduced lag phase was also observed when fermenting 30% (w/v) glucose. Thus, our work shows the application of a simple strategy to significantly improve high-gravity fermentation (HGF) performance through adaptation. KEY POINTS: • Cell adapted on 3.5% NaCl made 28% more ethanol when fermenting 30% glucose. • The adapted cells had reduced lag phase, grew faster, and produced less glycerol. • The improvements were not related to increased specific rates of production.

Bioprospecting thermotolerant yeasts from distillery effluent and molasses for high-temperature ethanol production

AIMS

Isolation, characterization and assessment of inhibitor tolerance of thermotolerant yeasts associated with distillery effluent and molasses, and their use in high-temperature ethanol production from alkali-treated rice straw.

METHODS AND RESULTS

A total of 92 thermotolerant yeasts were isolated from seven different distillery effluent and molasses samples. Based on MSP-PCR, 34 yeasts were selected and identified by sequencing the D1/D2 domain of LSU rDNA. These yeasts belonged to eight genera and nine different species. We assessed the inhibitor tolerance of these 34 well-characterized yeasts against various pre-treatment-generated inhibitors (furfural, 5-hydroxymethyl furfural and acetic acid) and also evaluated their ethanol yields at 40, 45 and 50℃. Among selected strains, Pichia kudriavzevii DSA3.2 exhibited the highest ethanol production (24.5 g l^-1 ) with an efficiency of 95.7% at 40℃ using 5% glucose. At 45℃, P. kudriavzevii DSA3.2 and Kluyveromyces marxianus MSS6.3 yielded maximum ethanol titres; 22.3 and 23 g l^-1 with 87.4% and 90% efficiency, respectively. While using alkali-treated RS at 45℃, K. marxianus MSS6.3 produced 10.5 g l^-1 of ethanol with 84.5% fermentation efficiency via separate hydrolysis and fermentation, and 10.9 g l^-1 of ethanol with 85% efficiency via simultaneous saccharification and fermentation. Pichia kudriavzevii DSA3.2, DSA3.1 and K. marxianus MSS6.3 also exhibited significant tolerance against multiple inhibitors.

CONCLUSIONS

Yeast isolates P. kudriavzevii DSA3.2 and K. marxianus MSS6.3 exhibited significant inhibitor tolerance and proved to be suitable for high-temperature ethanol fermentation. After additional optimization and scale-up experiments, these isolates can be exemplary candidates for industrial-scale ethanol production from lignocellulosic biomass.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study recognizes distillery effluents and molasses as specialized niches for yeasts with a broad substrate range, capable of tolerating multiple inhibitors and yielding high levels of ethanol at elevated temperatures. These yeasts can further be exploited for bioethanol production through SSF/SHF at a larger scale.

Deletion of the MBP1 Gene, Involved in the Cell Cycle, Affects Respiration and Pseudohyphal Differentiation in Saccharomyces cerevisiae

Mbp1p is a component of MBF (MluI cell cycle box binding factor, Mbp1p-Swi6p) and is well known to regulate the G1-S transition of the cell cycle. However, few studies have provided clues regarding its role in fermentation. This work aimed to recognize the function of the MBP1 gene in ethanol fermentation in a wild-type industrial Saccharomyces cerevisiae strain. MBP1 deletion caused an obvious decrease in the final ethanol concentration under oxygen-limited (without agitation), but not under aerobic, conditions (130 rpm). Furthermore, the mbp1Δ strain showed 84% and 35% decreases in respiration intensity under aerobic and oxygen-limited conditions, respectively. These findings indicate that MBP1 plays an important role in responding to variations in oxygen content and is involved in the regulation of respiration and fermentation. Unexpectedly, mbp1Δ also showed pseudohyphal growth, in which cells elongated and remained connected in a multicellular arrangement on yeast extract-peptone-dextrose (YPD) plates. In addition, mbp1Δ showed an increase in cell volume, associated with a decrease in the fraction of budded cells. These results provide more detailed information about the function of MBP1 and suggest some clues to efficiently improve ethanol production by industrially engineered yeast strains. IMPORTANCE Saccharomyces cerevisiae is an especially favorable organism used for ethanol production. However, inhibitors and high osmolarity conferred by fermentation broth, and high concentrations of ethanol as fermentation runs to completion, affect cell growth and ethanol production. Therefore, yeast strains with high performance, such as rapid growth, high tolerance, and high ethanol productivity, are highly desirable. Great efforts have been made to improve their performance by evolutionary engineering, and industrial strains may be a better start than laboratory ones for industrial-scale ethanol production. The significance of our research is uncovering the function of MBP1 in ethanol fermentation in a wild-type industrial S. cerevisiae strain, which may provide clues to engineer better-performance yeast in producing ethanol. Furthermore, the results that lacking MBP1 caused pseudohyphal growth on YPD plates could shed light on the development of xylose-fermenting S. cerevisiae, as using xylose as the sole carbon source also caused pseudohyphal growth.

Rational engineering of Saccharomyces cerevisiae towards improved tolerance to multiple inhibitors in lignocellulose fermentations

BACKGROUND

The fermentation of lignocellulose hydrolysates to ethanol requires robust xylose-capable Saccharomyces cerevisiae strains able to operate in the presence of microbial inhibitory stresses. This study aimed at developing industrial S. cerevisiae strains with enhanced tolerance towards pretreatment-derived microbial inhibitors, by identifying novel gene combinations that confer resistance to multiple inhibitors (thus cumulative inhibitor resistance phenotype) with minimum impact on the xylose fermentation ability. The strategy consisted of multiple sequential delta-integrations of double-gene cassettes containing one gene conferring broad inhibitor tolerance (ARI1, PAD1 or TAL1) coupled with an inhibitor-specific gene (ADH6, FDH1 or ICT1). The performances of the transformants were compared with the parental strain in terms of biomass growth, ethanol yields and productivity, as well as detoxification capacities in a synthetic inhibitor cocktail, sugarcane bagasse hydrolysate as well as hardwood spent sulphite liquor.

RESULTS

The first and second round of delta-integrated transformants exhibited a trade-off between biomass and ethanol yield. Transformants showed increased inhibitor resistance phenotypes relative to parental controls specifically in fermentations with concentrated spent sulphite liquors at 40% and 80% v/v concentrations in 2% SC media. Unexpectedly, the xylose fermentation capacity of the transformants was reduced compared to the parental control, but certain combinations of genes had a minor impact (e.g. TAL1 + FDH1). The TAL1 + ICT1 combination negatively impacted on both biomass growth and ethanol yield, which could be linked to the ICT1 protein increasing transformant susceptibility to weak acids and temperature due to cell membrane changes.

CONCLUSIONS

The integration of the selected genes was proven to increase tolerance to pretreatment inhibitors in synthetic or industrial hydrolysates, but they were limited to the fermentation of glucose. However, some gene combination sequences had a reduced impact on xylose conversion.

Interdependence between lignocellulosic biomasses, enzymatic hydrolysis and yeast cell factories in biorefineries

Biorefineries have a pivotal role in the bioeconomy scenario for the transition from fossil‐based processes towards more sustainable ones relying on renewable resources. Lignocellulose is a prominent feedstock since its abundance and relatively low cost. Microorganisms are often protagonists of biorefineries, as they contribute both to the enzymatic degradation of lignocellulose complex polymers and to the fermentative conversion of the hydrolyzed biomasses into fine and bulk chemicals. Enzymes have therefore become crucial for the development of sustainable biorefineries, being able to provide nutrients to cells from lignocellulose. Enzymatic hydrolysis can be performed by a portfolio of natural enzymes that degrade lignocellulose, often combined into cocktails. As enzymes can be deployed in different operative settings, such as separate hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF), their characteristics need to be combined with microbial ones to maximize the process. We therefore reviewed how the optimization of lignocellulose enzymatic hydrolysis can ameliorate bioethanol production when Saccharomyces cerevisiae is used as cell factory. Expanding beyond biofuels, enzymatic cocktail optimization can also be pivotal to unlock the potential of non‐Saccharomyces yeasts, which, thanks to broader substrate utilization, inhibitor resistance and peculiar metabolism, can widen the array of feedstocks and products of biorefineries.

Variable and dose-dependent response of Saccharomyces and non-Saccharomyces yeasts toward lignocellulosic hydrolysate inhibitors

Lignocellulosic hydrolysates will also contain compounds that inhibit microbial metabolism, such as organic acids, furaldehydes, and phenolic compounds. Understanding the response of yeasts toward such inhibitors is important to the development of different bioprocesses. In this work, the growth capacity of 7 industrial Saccharomyces cerevisiae and 7 non-Saccharomyces yeasts was compared in the presence of 3 different concentrations of furaldehydes (furfural and 5-hydroxymetil-furfural), organic acids (acetic and formic acids), and phenolic compounds (vanillin, syringaldehyde, ferulic, and coumaric acids). Then, Candida tropicalis JA2, Meyerozyma caribbica JA9, Wickerhamomyces anomalus 740, S. cerevisiae JP1, B1.1, and G06 were selected for fermentation in presence of acetic acid, HMF, and vanillin because they proved to be most tolerant to the tested compounds, while Spathaspora sp. JA1 because its xylose consumption rate. The results obtained showed a dose-dependent response of the yeasts toward the eight different inhibitors. Among the compared yeasts, S. cerevisiae strains presented higher tolerance than non-Saccharomyces, 3 of them with the highest tolerance among all. Regarding the non-Saccharomyces yeasts, C. tropicalis JA2 and W. anomalus 740 appeared as the most tolerant, whereas Spathaspora strains appeared very sensitive to the different compounds.

Improving the productivity of Candida glycerinogenes in the fermentation of ethanol from non-detoxified sugarcane bagasse hydrolysate by a hexose transporter mutant

AIMS

In this study, we attempted to increase the productivity of Candida glycerinogenes yeast for ethanol production from non-detoxified sugarcane bagasse hydrolysates (NDSBH) by identifying the hexose transporter in this yeast that makes a high contribution to glucose consumption, and by adding additional copies of this transporter and enhancing its membrane localisation stability (MLS).

METHODS AND RESULTS

Based on the knockout and overexpression of key hexose transporter genes and the characterisation of their promoter properties, we found that Cghxt4 and Cghxt6 play major roles in the early and late stages of fermentation, respectively, with Cghxt4 contributing most to glucose consumption. Next, subcellular localisation analysis revealed that a common mutation of two ubiquitination sites (K9 and K538) in Cghxt4 improved its MLS. Finally, we overexpressed this Cghxt4 mutant (Cghxt4.2A) using a strong promoter, P_CgGAP , which resulted in a significant increase in the ethanol productivity of C. glycerinogenes in the NDSBH medium. Specifically, the recombinant strain showed 18 and 25% higher ethanol productivity than the control in two kinds of YP-NDSBH medium (YP-NDSBH1_G160 and YP-NDSBH2_G160 ), respectively.

CONCLUSIONS

The hexose transporter mutant Cghxt4.2A (Cghxt4^K9A,K538A ) with multiple copies and high MLS was able to significantly increase the ethanol productivity of C. glycerinogenes in NDSBH.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results provide a promising strategy for constructing efficient strains for ethanol production.

Multiple roles of ABC transporters in yeast

The ATP binding cassette (ABC) transporters, first discovered as high-affinity nutrient importers in bacteria, rose to prominence when their ability to confer multidrug resistance (MDR) to cancer cells was realized. The most characterized human permeability glycoprotein (P-gp) is a dominant exporter of anti-cancer drugs and its overexpression is directly linked to MDR. The overexpression of drug efflux pumps belonging to the ABC superfamily is also a frequent cause of resistance to antifungals. Fungi has a battery of ABC proteins, but in variable numbers and at different subcellular locations. These proteins perform many critical functions, from serving as gatekeepers for xenobiotic cleansing to translocating various structurally unrelated cargoes, including lipids, fatty acids, ions, peptides, sterols, metabolites and toxins. Their emerging additional roles in cellular physiology and virulence call for attention to analyze and re-examine their divergent functions in yeast. In brief, this review traces the history of ABC transporters in yeast and discusses their typical physiological functions that go beyond their well-known role as antifungal drug efflux pumps.

The Model System Saccharomyces cerevisiae Versus Emerging Non-Model Yeasts for the Production of Biofuels

Microorganisms are effective platforms for the production of a variety of chemicals including biofuels, commodity chemicals, polymers and other natural products. However, deep cellular understanding is required for improvement of current biofuel cell factories to truly transform the Bioeconomy. Modifications in microbial metabolic pathways and increased resistance to various types of stress caused by the production of these chemicals are crucial in the generation of robust and efficient production hosts. Recent advances in systems and synthetic biology provide new tools for metabolic engineering to design strategies and construct optimal biocatalysts for the sustainable production of desired chemicals, especially in the case of ethanol and fatty acid production. Yeast is an efficient producer of bioethanol and most of the available synthetic biology tools have been developed for the industrial yeast Saccharomyces cerevisiae. Non-conventional yeast systems have several advantageous characteristics that are not easily engineered such as ethanol tolerance, low pH tolerance, thermotolerance, inhibitor tolerance, genetic diversity and so forth. Currently, synthetic biology is still in its initial steps for studies in non-conventional yeasts such as Yarrowia lipolytica, Kluyveromyces marxianus, Issatchenkia orientalis and Pichia pastoris. Therefore, the development and application of advanced synthetic engineering tools must also focus on these underexploited, non-conventional yeast species. Herein, we review the basic synthetic biology tools that can be applied to the standard S. cerevisiae model strain, as well as those that have been developed for non-conventional yeasts. In addition, we will discuss the recent advances employed to develop non-conventional yeast strains that are efficient for the production of a variety of chemicals through the use of metabolic engineering and synthetic biology.

Xylanolytic and Ethanologenic Potential of Gut Associated Yeasts from Different Species of Termites from India

Xylophagous termites are capable of degrading lignocellulose by symbiotic gut microorganisms along with the host's indigenous enzymes. Therefore, the termite gut might be a potential niche to obtain natural yeasts with celluloytic, xylanolytic and ethanologenic traits required for bioethanol production from lignocellulosic biomass. In this study, we cultured 79 yeasts from three different termites viz. Coptotermes heimi, Odontotermes javanicus and Odontotermes obesus. After suitable screening methods, we identified 53 yeasts, which belonged to 10 genera and 16 different species of both ascomycetous and basidiomycetous yeasts. Most yeasts in the present study represent their first-ever isolation from the termite gut. Representative strains of identified yeasts were evaluated for their cellulolytic, xylanolytic, and ethanologenic abilities. None of the isolates showed cellulase activity; 22 showed xylanolytic activity, while six produced substantial quantities of ethanol. Among xylanolytic cultures, Pseudozyma hubeiensis STAG 1.7 and Hannaella pagnoccae STAG 1.14 produced 1.31 and 1.17 IU of xylanase. Among ethanologenic yeasts, the strains belonging to genera Candida and Kodamaea produced high amount of ethanol. Overall, highest ethanol level of 4.42 g/L was produced by Candida tropicalis TS32 using 1% glucose, which increased up to 22.92 g/L at 35 °C, pH 4.5 with 5% glucose. Fermentation of rice straw hydrolysate gave 8.95 g/l of ethanol with a yield of 0.42 g/g using the strain TS32. Our study highlights the gut of wood-feeding termites as a potential source of diverse yeasts that would be useful in the production of xylanase and bioethanol.

Harnessing pongamia shell hydrolysate for triacylglycerol agglomeration by novel oleaginous yeast Rhodotorula pacifica INDKK

BACKGROUND

To meet the present transportation demands and solve food versus fuel issue, microbial lipid-derived biofuels are gaining attention worldwide. This study is focussed on high-throughput screening of oleaginous yeast by microwave-aided Nile red spectrofluorimetry and exploring pongamia shell hydrolysate (PSH) as a feedstock for lipid production using novel oleaginous yeast Rhodotorula pacifica INDKK.

RESULTS

A new oleaginous yeast R. pacifica INDKK was identified and selected for microbial lipid production. R. pacifica INDKK produced maximum 12.8 ± 0.66 g/L of dry cell weight and 6.78 ± 0.4 g/L of lipid titre after 120 h of growth, showed high tolerance to pre-treatment-derived inhibitors such as 5-hydroxymethyl furfural (5-HMF), (2 g/L), furfural (0.5 g/L) and acetic acid (0.5 g/L), and ability to assimilate C3, C5 and C6 sugars. Interestingly, R. pacifica INDKK showed higher lipid accumulation when grown in alkali-treated saccharified PSH (AS-PSH) (0.058 ± 0.006 g/L/h) as compared to acid-treated detoxified PSH (AD-PSH) (0.037 ± 0.006 g/L/h) and YNB medium (0.055 ± 0.003 g/L/h). The major fatty acid constituents are oleic, palmitic, linoleic and linolenic acids with an estimated cetane number (CN) of about 56.7, indicating the good quality of fuel.

CONCLUSION

These results suggested that PSH and R. pacifica INDKK could be considered as potential feedstock for sustainable biodiesel production.

Current state-of-the-art in ethanol production from lignocellulosic feedstocks

The renewable lignocellulosic biomass is a sustainable feedstock for the production of bioethanol, which shows the potential to replace fossil fuels. Due to the recalcitrant structure of plant cell wall made of cellulose, hemicellulose, and lignin, the biomass conversion process requires the use of efficient pretreatment process before enzymatic hydrolysis and fermentation to degrade the crystallinity of cellulose fibres and to remove lignin from biomass. Proper pretreatment techniques, economical production of cellulolytic enzymes, and effective fermentation of glucose and xylose in the presence of inhibitors are key challenges for the viable production of bioethanol. Although new strains capable of fermenting xylose are being designed, they are often not resistant to toxic compounds in hydrolysates. This paper provides an in-depth review of lignocellulosic bioethanol production via biochemical route, focusing on the most widely used pretreatment technologies and key operational conditions of enzymatic hydrolysis and fermentation considering sugar/ethanol yields. In addition, this review examines the relevant detoxification strategies for the removal of toxic substances and the importance of immobilization. The review also indicates potential usage of engineered microorganisms to improve glucose and xylose fermentation, cellulolytic enzymes production, and response to stress conditions.

Single yeast cell nanomotions correlate with cellular activity

Living single yeast cells show a specific cellular motion at the nanometer scale with a magnitude that is proportional to the cellular activity of the cell. We characterized this cellular nanomotion pattern of nonattached single yeast cells using classical optical microscopy. The distribution of the cellular displacements over a short time period is distinct from random motion. The range and shape of such nanomotion displacement distributions change substantially according to the metabolic state of the cell. The analysis of the nanomotion frequency pattern demonstrated that single living yeast cells oscillate at relatively low frequencies of around 2 hertz. The simplicity of the technique should open the way to numerous applications among which antifungal susceptibility tests seem the most straightforward.