Anaerobic glycerol production by Saccharomyces cerevisiae strains under hyperosmotic stress

Commercial wine yeast nitrogen requirement influences the production of secondary metabolites (aroma, hydroxytyrosol, melatonin and other bioactives) during alcoholic fermentation

During alcoholic fermentation, Saccharomyces cerevisiae synthesizes different compounds, which are crucial for product quality: volatile compounds with sensory impact, and bioactive compounds such as melatonin (MEL) and hydroxytyrosol (HT), linked to health benefits. As many of these compounds are related with yeast's nitrogen metabolism, their production have been studied in four different commercial strains with different nitrogen requirement (Red Fruit, Uvaferm VRB, Lalvin Rhone 2323 and Lalvin QA23) being, Uvaferm UVR the higher nitrogen demander strain. All strains produced the secondary metabolites, notably Uvaferm UVR produced the highest HT concentration, despite its low growth. Uvaferm UVR emerged also as a significant producer of MEL, indicating a potential role in fermentation related stress. Moreover, Uvaferm UVR shows the highest total concentrations of volatile compounds. Multivariate analysis revealed distinct clustering based on nitrogen requirements of the strains, highlighting the strain-dependent metabolic responses.

Phosphorus-solubilizing bacteria improve the growth of Nicotiana benthamiana on lunar regolith simulant by dissociating insoluble inorganic phosphorus

In-situ utilization of lunar soil resources will effectively improve the self-sufficiency of bioregenerative life support systems for future lunar bases. Therefore, we have explored the microbiological method to transform lunar soil into a substrate for plant cultivation. In this study, five species of phosphorus-solubilizing bacteria are used as test strains, and a 21-day bio-improving experiment with another 24-day Nicotiana benthamiana cultivation experiment are carried out on lunar regolith simulant. We have observed that the phosphorus-solublizing bacteria Bacillus mucilaginosus, Bacillus megaterium, and Pseudomonas fluorescens can tolerate the lunar regolith simulant conditions and dissociate the insoluble phosphorus from the regolith simulant. The phosphorus-solubilizing bacteria treatment improves the available phosphorus content of the regolith simulant, promoting the growth of Nicotiana benthamiana. Here we demonstrate that the phosphorus-solubilizing bacteria can effectively improve the fertility of lunar regolith simulant, making it a good cultivation substrate for higher plants. The results can lay a technical foundation for plant cultivation based on lunar regolith resources in future lunar bases.

Complex yeast-bacteria interactions affect the yield of industrial ethanol fermentation

Sugarcane ethanol fermentation represents a simple microbial community dominated by S. cerevisiae and co-occurring bacteria with a clearly defined functionality. In this study, we dissect the microbial interactions in sugarcane ethanol fermentation by combinatorically reconstituting every possible combination of species, comprising approximately 80% of the biodiversity in terms of relative abundance. Functional landscape analysis shows that higher-order interactions counterbalance the negative effect of pairwise interactions on ethanol yield. In addition, we find that Lactobacillus amylovorus improves the yeast growth rate and ethanol yield by cross-feeding acetaldehyde, as shown by flux balance analysis and laboratory experiments. Our results suggest that Lactobacillus amylovorus could be considered a beneficial bacterium with the potential to improve sugarcane ethanol fermentation yields by almost 3%. These data highlight the biotechnological importance of comprehensively studying microbial communities and could be extended to other microbial systems with relevance to human health and the environment.

Glycerol Is an Osmoprotectant in Two Antarctic Chlamydomonas Species From an Ice-Covered Saline Lake and Is Synthesized by an Unusual Bidomain Enzyme

Glycerol, a compatible solute, has previously been found to act as an osmoprotectant in some marine Chlamydomonas species and several species of Dunaliella from hypersaline ponds. Recently, Chlamydomonas reinhardtii and Dunaliella salina were shown to make glycerol with an unusual bidomain enzyme, which appears to be unique to algae, that contains a phosphoserine phosphatase and glycerol-3-phosphate dehydrogenase. Here we report that two psychrophilic species of Chlamydomonas (C. spp. UWO241 and ICE-MDV) from Lake Bonney, Antarctica also produce high levels of glycerol to survive in the lake's saline waters. Glycerol concentration increased linearly with salinity and at 1.3 M NaCl, exceeded 400 mM in C. sp. UWO241, the more salt-tolerant strain. We also show that both species expressed several isoforms of the bidomain enzyme. An analysis of one of the isoforms of C. sp. UWO241 showed that it was strongly upregulated by NaCl and is thus the likely source of glycerol. These results reveal another adaptation of the Lake Bonney Chlamydomonas species that allow them to survive in an extreme polar environment.

Flow-cytometry-based physiological characterisation and transcriptome analyses reveal a mechanism for reduced cell viability in yeast engineered for increased lipid content

BACKGROUND

Yeast has been the focus of development of cell biofactories for the production of lipids and interest in the field has been driven by the need for sustainably sourced lipids for use in a broad range of industrial applications. Previously, we reported a metabolic engineering strategy for enhanced lipid production in yeast which delivered high per-cell lipid but with low cell growth and compromised physiology. To investigate the relationship between lipid engineering and cellular physiological responses and to identify further metabolic engineering targets, we analysed transcriptomes and measured cell physiology parameters in engineered strains.

RESULTS

In the engineering strategy, the central carbon pathway was reprogrammed to provide more precursors for lipid production and lipid accumulation and sequestration steps were enhanced through the expression of heterologous genes. Genes coding for enzymes within the pentose phosphate, beta-oxidation pathways, ATP and NADPH biosynthesis had lower transcript levels in engineered cells. Meanwhile, flow-cytometry analysis of fluorescent-dye stained cells showed the highest reactive oxygen species (ROS) levels and mitochondrial membrane potential (Δψm) in cells with the highest lipid content, supporting the known relationship between mitochondrial activity and ROS generation. High intracellular ROS and low membrane integrity were not ameliorated by application of antioxidants.

CONCLUSIONS

The limited intracellular energy supplies and the unbalanced redox environment could be regarded as targets for further lipid engineering, similarly for native lipid accumulation genes that were upregulated. Thus, lipid pathway engineering has an important effect on the central carbon pathway, directing these towards lipid production and sacrificing the precursors, energy and cofactor supply to satisfy homeostatic metabolic requirements.

A synthetic medium to simulate sugarcane molasses

BACKGROUND

Developing novel microbial cell factories requires careful testing of candidates under industrially relevant conditions. However, this frequently occurs late during the strain development process. The availability of laboratory media that simulate industrial-like conditions might improve cell factory development, as they allow for strain construction and testing in the laboratory under more relevant conditions. While sugarcane molasses is one of the most important substrates for the production of biofuels and other bioprocess-based commodities, there are no defined media that faithfully simulate it. In this study, we tested the performance of a new synthetic medium simulating sugarcane molasses.

RESULTS

Laboratory scale simulations of the Brazilian ethanol production process, using both sugarcane molasses and our synthetic molasses (SM), demonstrated good reproducibility of the fermentation performance, using yeast strains, PE-2 and Ethanol Red™. After 4 cycles of fermentation, the final ethanol yield (g_p g_s^-1) values for the SM ranged from 0.43 ± 0.01 to 0.44 ± 0.01 and from 0.40 ± 0.01 to 0.46 ± 0.01 for the molasses-based fermentations. The other fermentation parameters (i.e., biomass production, yeast viability, and glycerol and acetic acid yield) were also within similar value ranges for all the fermentations. Sequential pairwise competition experiments, comparing industrial and laboratory yeast strains, demonstrated the impact of the media on strain fitness. After two sequential cocultivations, the relative abundance of the laboratory yeast strain was 5-fold lower in the SM compared to the yeast extract-peptone-dextrose medium, highlighting the importance of the media composition on strain fitness.

CONCLUSIONS

Simulating industrial conditions at laboratory scale is a key part of the efficient development of novel microbial cell factories. In this study, we have developed a synthetic medium that simulated industrial sugarcane molasses media. We found good agreement between the synthetic medium and the industrial media in terms of the physiological parameters of the industrial-like fermentations.

Citeromyces matritensis M37 is a salt-tolerant yeast that produces ethanol from salted algae

Algae are referred to as a third-generation biomass for ethanol production. However, salinity treatment is a problem that needs to be solved, because algal hydrolysates often contain high salt. Here, we isolated the salt-tolerant ethanol-producing yeast Citeromyces matritensis M37 from the east coast of Miura Peninsula in Japan. This yeast grew under osmotic stress conditions (20% NaCl or 60% glucose). It produced 6.55 g/L ethanol from YPD medium containing 15% NaCl after 48 h, and the ethanol accumulation was observed even at 20% NaCl. Using salted Undaria pinnatifida (wakame), we obtained 6.33 g/L glucose from approx. 150 g/L of the salted wakame powder with acidic and heat pretreatment followed by enzymatic saccharification, and the ethanol production reached 2.58 g/L for C. matritensis M37. The ethanol concentration was 1.4 times higher compared with that using the salt-tolerant ethanol-producing yeast Zygosaccharomyces rouxii S11.

Effect of acetic acid in recycling water on ethanol production for cassava in an integrated ethanol-methane fermentation process

Recently, the integrated ethanol-methane fermentation process has been studied to prevent wastewater pollution. However, when the anaerobic digestion reaction runs poorly, acetic acid will accumulate in the recycling water. In this paper, we studied the effect of low concentration of acetic acid (≤25 mM) on ethanol fermentation at different initial pH values (4.2, 5.2 or 6.2). At an initial pH of 4.2, ethanol yields increased by 3.0% and glycerol yields decreased by 33.6% as the acetic acid concentration was increased from 0 to 25 mM. Raising the concentration of acetic acid to 25 mM increased the buffering capacity of the medium without obvious effects on biomass production in the cassava medium. Acetic acid was metabolized by Saccharomyces cerevisiae for the reason that the final concentration of acetic acid was 38.17% lower than initial concentration at pH 5.2 when 25 mM acetic acid was added. These results confirmed that a low concentration of acetic acid in the process stimulated ethanol fermentation. Thus, reducing the acetic acid concentration to a controlled low level is more advantageous than completely removing it.

Identification and Characterization of a Novel Issatchenkia orientalis GPI-Anchored Protein, IoGas1, Required for Resistance to Low pH and Salt Stress

The use of yeasts tolerant to acid (low pH) and salt stress is of industrial importance for several bioproduction processes. To identify new candidate genes having potential roles in low-pH tolerance, we screened an expression genomic DNA library of a multiple-stress-tolerant yeast, Issatchenkia orientalis (Pichia kudriavzevii), for clones that allowed Saccharomyces cerevisiae cells to grow under highly acidic conditions (pH 2.0). A genomic DNA clone containing two putative open reading frames was obtained, of which the putative protein-coding gene comprising 1629 bp was retransformed into the host. This transformant grew significantly at pH 2.0, and at pH 2.5 in the presence of 7.5% Na2SO4. The predicted amino acid sequence of this new gene, named I. orientalis GAS1 (IoGAS1), was 60% identical to the S. cerevisiae Gas1 protein, a glycosylphosphatidylinositol-anchored protein essential for maintaining cell wall integrity, and 58-59% identical to Candida albicans Phr1 and Phr2, pH-responsive proteins implicated in cell wall assembly and virulence. Northern hybridization analyses indicated that, as for the C. albicans homologs, IoGAS1 expression was pH-dependent, with expression increasing with decreasing pH (from 4.0 to 2.0) of the medium. These results suggest that IoGAS1 represents a novel pH-regulated system required for the adaptation of I. orientalis to environments of diverse pH. Heterologous expression of IoGAS1 complemented the growth and morphological defects of a S. cerevisiae gas1Δ mutant, demonstrating that IoGAS1 and the corresponding S. cerevisiae gene play similar roles in cell wall biosynthesis. Site-directed mutagenesis experiments revealed that two conserved glutamate residues (E161 and E262) in the IoGas1 protein play a crucial role in yeast morphogenesis and tolerance to low pH and salt stress. Furthermore, overexpression of IoGAS1 in S. cerevisiae remarkably improved the ethanol fermentation ability at pH 2.5, and at pH 2.0 in the presence of salt (5% Na2SO4), compared to that of a reference strain. Our results strongly suggest that constitutive expression of the IoGAS1 gene in S. cerevisiae could be advantageous for several fermentation processes under these stress conditions.

Physiological and transcriptional characterization of Saccharomyces cerevisiae engineered for production of fatty acid ethyl esters

Saccharomyces cerevisiae has previously been engineered to become a cell factory for the production of fatty acid ethyl esters (FAEEs), molecules suitable for crude diesel replacement. To find new metabolic engineering targets for the improvement of FAEE cell factories, three different FAEE-producing strains of S. cerevisiae, constructed previously, were compared and characterized by quantification of key fluxes and genome-wide transcription analysis. From both the physiological and the transcriptional data, it was indicated that strain CB2I20, with high expression of a heterologous wax ester synthase gene (ws2) and strain BdJ15, containing disruptions of genes DGA1, LRO1, ARE1, ARE2 and POX1, which prevent the conversion of acyl-CoA to sterol esters, triacylglycerides and the degradation to acetyl-CoA, triggered oxidative stress that consequently influenced cellular growth. In the latter strain, stress was possibly triggered by disabling the buffering capacity of lipid droplets in encapsulating toxic fatty acids such as oleic acid. Additionally, it was indicated that there was an increased demand for NADPH required for the reduction steps in fatty acid biosynthesis. In conclusion, our analysis clearly shows that engineering of fatty acid biosynthesis results in transcriptional reprogramming and has a significant effect on overall cellular metabolism.

Anhydrobiosis in yeast: FT-IR spectroscopic studies of yeast grown under conditions of severe oxygen limitation

Anhydrobiosis is a unique state of living organisms when metabolism is temporarily and reversibly delayed in response to the extreme desiccation of cells. The production of dry active preparations of yeast grown under anaerobic conditions is not currently possible because preparations are extremely sensitive to the dehydration procedure, though they could be very helpful in different biotechnological processes, including bioethanol production. To characterize mechanisms responsible for such sensitivity to the dehydration procedure, Fourier transform infrared spectroscopy was used to study the composition of aerobically grown yeast Saccharomyces cerevisiae resistant to dehydration and grown under conditions of severe oxygen limitation and sensitive to dehydration. Results indicated that significantly lower amounts of lipids in cells, grown under conditions of severe oxygen limitation, may be related to the mechanisms of sensitivity. Dehydration of both resistant and sensitive S. cerevisiae cells was accompanied by similar changes in main cellular compounds. Amounts of nucleic acids and proteins decreased slightly, whereas that of lipids and carbohydrates increased. Artificially reduced sensitivity to dehydration in S. cerevisiae cells, grown under conditions of severe oxygen limitation, led to the increase in the lipid concentration. The chemical composition of S. cerevisiae membranes is proposed to dictate the resistance to dehydration in resistant and sensitive cells.

Anhydrobiosis in yeast: is it possible to reach anhydrobiosis for yeast grown in conditions with severe oxygen limitation?

The yeast Saccharomyces cerevisiae was shown to be extremely sensitive to dehydration-rehydration treatments when stationary phase cells were subjected to conditions of severe oxygen limitation, unlike the same cells grown in aerobic conditions. The viability of dehydrated anaerobically grown yeast cells never exceeded 2 %. It was not possible to increase this viability using gradual rehydration of dry cells in water vapour, which usually strongly reduces damage to intracellular membranes. Specific pre-dehydration treatments significantly increased the resistance of anaerobic yeast to drying. Thus, incubation of cells with trehalose (100 mM), increased the viability of dehydrated cells after slow rehydration in water vapour to 30 %. Similarly, pre-incubation of cells in 1 M xylitol or glycerol enabled up to 50-60 % of cells to successfully enter a viable state of anhydrobiosis after subsequent rehydration. We presume that trehalose and sugar alcohols function mainly according to a water replacement hypothesis, as well as initiating various protective intracellular reactions.

Effect of salts on the Co-fermentation of glucose and xylose by a genetically engineered strain of Saccharomyces cerevisiae

BACKGROUND

A challenge currently facing the cellulosic biofuel industry is the efficient fermentation of both C5 and C6 sugars in the presence of inhibitors. To overcome this challenge, microorganisms that are capable of mixed-sugar fermentation need to be further developed for increased inhibitor tolerance. However, this requires an understanding of the physiological impact of inhibitors on the microorganism. This paper investigates the effect of salts on Saccharomyces cerevisiae 424A(LNH-ST), a yeast strain capable of effectively co-fermenting glucose and xylose.

RESULTS

In this study, we show that salts can be significant inhibitors of S. cerevisiae. All 6 pairs of anions (chloride and sulfate) and cations (sodium, potassium, and ammonium) tested resulted in reduced cell growth rate, glucose consumption rate, and ethanol production rate. In addition, the data showed that the xylose consumption is more strongly affected by salts than glucose consumption at all concentrations. At a NaCl concentration of 0.5M, the xylose consumption rate was reduced by 64.5% compared to the control. A metabolomics study found a shift in metabolism to increased glycerol production during xylose fermentation when salt was present, which was confirmed by an increase in extracellular glycerol titers by 4 fold. There were significant differences between the different cations. The salts with potassium cations were the least inhibitory. Surprisingly, although salts of sulfate produced twice the concentration of cations as compared to salts of chloride, the degree of inhibition was the same with one exception. Potassium salts of sulfate were less inhibitory than potassium paired with chloride, suggesting that chloride is more inhibitory than sulfate.

CONCLUSIONS

When developing microorganisms and processes for cellulosic ethanol production, it is important to consider salt concentrations as it has a significant negative impact on yeast performance, especially with regards to xylose fermentation.

Aldehyde dehydrogenases in cellular responses to oxidative/electrophilic stress

Reactive oxygen species (ROS) are continuously generated within living systems and the inability to manage ROS load leads to elevated oxidative stress and cell damage. Oxidative stress is coupled to the oxidative degradation of lipid membranes, also known as lipid peroxidation. This process generates over 200 types of aldehydes, many of which are highly reactive and toxic. Aldehyde dehydrogenases (ALDHs) metabolize endogenous and exogenous aldehydes and thereby mitigate oxidative/electrophilic stress in prokaryotic and eukaryotic organisms. ALDHs are found throughout the evolutionary gamut, from single-celled organisms to complex multicellular species. Not surprisingly, many ALDHs in evolutionarily distant, and seemingly unrelated, species perform similar functions, including protection against a variety of environmental stressors such as dehydration and ultraviolet radiation. The ability to act as an "aldehyde scavenger" during lipid peroxidation is another ostensibly universal ALDH function found across species. Upregulation of ALDHs is a stress response in bacteria (environmental and chemical stress), plants (dehydration, salinity, and oxidative stress), yeast (ethanol exposure and oxidative stress), Caenorhabditis elegans (lipid peroxidation), and mammals (oxidative stress and lipid peroxidation). Recent studies have also identified ALDH activity as an important feature of cancer stem cells. In these cells, ALDH expression helps abrogate oxidative stress and imparts resistance against chemotherapeutic agents such as oxazaphosphorine, taxane, and platinum drugs. The ALDH superfamily represents a fundamentally important class of enzymes that contributes significantly to the management of electrophilic/oxidative stress within living systems. Mutations in various ALDHs are associated with a variety of pathological conditions in humans, highlighting the fundamental importance of these enzymes in physiological and pathological processes.

Stress-related challenges in pentose fermentation to ethanol by the yeast Saccharomyces cerevisiae

Conversion of agricultural residues, energy crops and forest residues into bioethanol requires hydrolysis of the biomass and fermentation of the released sugars. During the hydrolysis of the hemicellulose fraction, substantial amounts of pentose sugars, in particular xylose, are released. Fermentation of these pentose sugars to ethanol by engineered Saccharomyces cerevisiae under industrial process conditions is the subject of this review. First, fermentation challenges originating from the main steps of ethanol production from lignocellulosic feedstocks are discussed, followed by genetic modifications that have been implemented in S. cerevisiae to obtain xylose and arabinose fermenting capacity per se. Finally, the fermentation of a real lignocellulosic medium is discussed in terms of inhibitory effects of furaldehydes, phenolics and weak acids and the presence of contaminating microbiota.