Physiological mechanism of improved tolerance of Saccharomyces cerevisiae to lignin-derived phenolic acids in lignocellulosic ethanol fermentation by short-term adaptation

Lignin-derived compounds assisted with Kocuria marina H-2 and Pseudomonas putida B6-2 co-culture enhanced naphthalene biodegradation

The implementation of environmentally friendly and sustainable remediation strategies positively impacts solid waste management. In this study, the Kocuria marina H-2 and Pseudomonas putida B6-2 co-culture system demonstrated enhanced naphthalene biodegradation efficiency compared to single-strain cultures. Under optimal conditions of 35 °C, 200 rpm/min, and a 1:1 ratio of the co-culture system, the naphthalene biodegradation potential was further increased. Notably, the addition of both ethylenediamine-pretreated lignin and p-hydroxybenzoic acid significantly elevated naphthalene degradation rates to 68.5 %. In addition, the oil-liquid surface tension decreased, while cell surface hydrophobicity and colony-forming units increased with the addition of lignin-derived compounds. The modification of naphthalene bioavailability by ethylenediamine-pretreated lignin would accelerate the uptake and transport of hydrocarbons via ABC transporters and flagellar assembly. Importantly, genes related to bacterial chemotaxis and fatty acid biosynthesis were upregulated during the co-metabolism of naphthalene and p-hydroxybenzoic acid, further enhancing naphthalene bioconversion.

Engineering Yarrowia lipolytica to Produce l-Malic Acid from Glycerol

The declining availability of cheap fossil-based resources has sparked growing interest in the sustainable biosynthesis of organic acids. l-Malic acid, a crucial four-carbon dicarboxylic acid, finds extensive applications in the food, chemical, and pharmaceutical industries. Synthetic biology and metabolic engineering have enabled the efficient microbial production of l-malic acid, albeit not in Yarrowia lipolytica, an important industrial microorganism. The present study aimed to explore the potential of this fungal species for the production of l-malic acid. First, endogenous biosynthetic genes and heterologous transporter genes were overexpressed in Y. lipolytica to identify bottlenecks in the l-malic acid biosynthesis pathway grown on glycerol. Second, overexpression of isocitrate lyase, malate synthase, and malate dehydrogenase in the glyoxylate cycle pathway and introduction of a malate transporter from Schizosaccharomyces pombe significantly boosted l-malic acid production, which reached 27.0 g/L. A subsequent increase to 37.0 g/L was attained through shake flask medium optimization. Third, adaptive laboratory evolution allowed the engineered strain Po1g-CEE2+Sp to tolerate a lower pH and to accumulate a higher amount of l-malic acid (56.0 g/L). Finally, when scaling up to a 5 L bioreactor, a titer of 112.5 g/L was attained. In conclusion, this study demonstrates for the first time the successful production of l-malic acid in Y. lipolytica by combining metabolic engineering and laboratory evolution, paving the way for large-scale sustainable biosynthesis of this and other organic acids.

Unveiling superior phenol detoxification and degradation ability in Candida tropicalis SHC-03: a comparative study with Saccharomyces cerevisiae BY4742

This study examined the phenol degradation capabilities and oxidative stress responses of Candida tropicalis SHC-03, demonstrating its metabolic superiority and resilience compared to Saccharomyces cerevisiae BY4742 in a culture medium with phenol as the sole carbon source. Through comparative growth, transcriptomic, and metabolomic analyses under different phenol concentrations, this study revealed C. tropicalis SHC-03's specialized adaptations for thriving in phenol as the sole carbon source environments. These include a strategic shift from carbohydrate metabolism to enhanced phenol degradation pathways, highlighted by the significant upregulation of genes for Phenol 2-monoxygenase and Catechol 1,2-dioxygenase. Despite phenol levels reaching 1.8 g/L, C. tropicalis exhibits a robust oxidative stress response, efficiently managing ROS through antioxidative pathways and the upregulation of genes for peroxisomal proteins like PEX2, PEX13, and PMP34. Concurrently, there was significant upregulation of genes associated with membrane components and transmembrane transporters, enhancing the cell's capacity for substance exchange and signal transduction. Especially, when the phenol concentration was 1.6 g/L and 1.8 g/L, the degradation rates of C. tropicalis towards it were 99.47 and 95.91%, respectively. Conversely, S. cerevisiae BY4742 shows limited metabolic response, with pronounced growth inhibition and lack of phenol degradation. Therefore, our study not only sheds light on the molecular mechanisms underpinning phenol tolerance and degradation in C. tropicalis but also positions this yeast as a promising candidate for environmental and industrial processes aimed at mitigating phenol pollution.

Corynebacterium glutamicum cell factory design for the efficient production of cis, cis-muconic acid

Cis, cis-muconic acid (MA) is widely used as a key starting material in the synthesis of diverse polymers. The growing demand in these industries has led to an increased need for MA. Here, we constructed recombinant Corynebacterium glutamicum by systems metabolic engineering, which exhibit high efficiency in the production of MA. Firstly, the three major degradation pathways were disrupted in the MA production process. Subsequently, metabolic optimization strategies were predicted by computational design and the shikimate pathway was reconstructed, significantly enhancing its metabolic flux. Finally, through optimization and integration of key genes involved in MA production, the recombinant strain produced 88.2 g/L of MA with the yield of 0.30 mol/mol glucose in the 5 L bioreactor. This titer represents the highest reported titer achieved using glucose as the carbon source in current studies, and the yield is the highest reported for MA production from glucose in Corynebacterium glutamicum. Furthermore, to enable the utilization of more cost-effective glucose derived from corn straw hydrolysate, we subjected the strain to adaptive laboratory evolution in corn straw hydrolysate. Ultimately, we successfully achieved MA production in a high solid loading of corn straw hydrolysate (with the glucose concentration of 83.56 g/L), resulting in a titer of 19.9 g/L for MA, which is 4.1 times higher than that of the original strain. Additionally, the glucose yield was improved to 0.33 mol/mol. These provide possibilities for a greener and more sustainable production of MA.

Bioprocess optimization for enhanced xylitol synthesis by new isolate Meyerozyma caribbica CP02 using rice straw

The present work models the fermentation process parameters of the newly isolated, Meyerozyma caribbica CP02 for enhanced xylitol production and its fermentability study on rice straw hydrolysate. The study examined the impact of each of the process variables by one variable at a time optimization followed by statistical validation. Temperature of 32 °C, pH of 3.5, agitation of 200 rpm, 1.5% (v/v) inoculum, 80 gL-1 initial xylose was optimized. Subsequently, a sequential two-stage agitation approach was adopted for fermentation. At these optimized conditions, xylitol yield of 0.77 gg-1 and 0.64 gg-1 was achieved using media containing commercial and rice straw derived xylose, respectively. For scale up, in 3L batch bioreactor, the highest xylitol yield (0.63 gg-1) was attained at 72 h with rice straw hydrolysate media containing initial xylose (59.48 ± 0.82 gL-1) along with inhibitors (1.55 ± 0.10 gL-1 aliphatic acids, 0.0.048 ± 0.11 gL-1 furans, 0.64 ± 0.23 gL-1 total phenols). The results imply that even under circumstances characterized by an acidic pH and elevated initial xylose level, M. caribbica CP02, as an isolate, displays robustness and shows favorable fermentability of rice straw hydrolysate. Therefore, isolate CP02 has potential to be used in bio-refineries for high yield xylitol production with minimal hydrolysate processing requirements.

Solid-state fermentation by S. cerevisiae with high resistance to ferulic acid improves the physicochemical properties of wheat bran and quality of bran-rich Chinese steamed bread

Wheat bran has numerous health benefits, but its poor processing and sensory properties limit its application in the staple food industry. Fermentation by S. cerevisiae changes the performance of wheat bran. However, high levels of ferulic acid (FA) inhibit S. cerevisiae. The effects of solid-state fermentation of S. cerevisiae with high resistance to FA on the physicochemical properties of wheat bran and the quality of bran-rich Chinese steamed bread (CSB) were investigated. The results showed that the growth of S. cerevisiae was inhibited by FA in a dose-dependent manner. Short-term adaptation strategies efficiently improved the tolerance of S. cerevisiae to FA stress. Compared with the parental strain (PS), fermentation of the short-term adapted strains (adapted strains) significantly increased the FA, total phenol, and soluble dietary fiber content in wheat bran. Wheat bran fermented by the adapted strains had a higher antioxidant capacity than wheat bran fermented by PS. In addition, compared with the PS, the wheat bran fermented by the adapted strains can decrease the hardness, improve the specific volume, and the quality of CSB. Thus, solid-state fermentation of the adapted strain is a potentially effective method to improve the nutritional and physicochemical properties of wheat bran as a cereal food ingredient.

Metabolomic analysis of hydroxycinnamic acid inhibition on Saccharomyces cerevisiae

Ferulic acid (FA) and p-coumaric acid (p-CA) are hydroxycinnamic acid inhibitors that are mainly produced during the pretreatment of lignocellulose. To date, the inhibitory mechanism of hydroxycinnamic acid compounds on Saccharomyces cerevisiae has not been fully elucidated. In this study, liquid chromatography-mass spectrometry (LC-MS) and scanning electron microscopy (SEM) were used to investigate the changes in S. cerevisiae cells treated with FA and p-CA. In this experiment, the control group was denoted as group CK, the FA-treated group was denoted as group F, and the p-CA-treated group was denoted as group P. One hundred different metabolites in group F and group CK and 92 different metabolites in group P and group CK were selected and introduced to metaboanalyst, respectively. A total of 38 metabolic pathways were enriched in S. cerevisiae under FA stress, and 27 metabolic pathways were enriched in S. cerevisiae under p-CA stress as identified through Kyoto Encyclopaedia of Genes and Genomes (KEGG) analysis. The differential metabolites involved included S-adenosine methionine, L-arginine, and cysteine, which were significantly downregulated, and acetyl-CoA, L-glutamic acid, and L-threonine, which were significantly upregulated. Analysis of differential metabolic pathways showed that the differentially expressed metabolites were mainly related to amino acid metabolism, nucleotide metabolism, fatty acid degradation, and the tricarboxylic acid cycle (TCA). Under the stress of FA and p-CA, the metabolism of some amino acids was blocked, which disturbed the redox balance in the cells and destroyed the synthesis of most proteins, which was the main reason for the inhibition of yeast cell growth. This study provided a strong scientific reference to improve the durability of S. cerevisiae against hydroxycinnamic acid inhibitors. KEY POINTS: • Morphological changes of S. cerevisiae cells under inhibitors stress were observed. • Changes of the metabolites in S. cerevisiae cells were explored by metabolomics. • One of the inhibitory effects on yeast is due to changes in the metabolic network.

General mechanisms of weak acid-tolerance and current strategies for the development of tolerant yeasts

Yeast cells are often subjected to various types of weak acid stress in the process of industrial production, food processing, and preservation, resulting in growth inhibition and reduced fermentation performance. Under acidic conditions, weak acids enter the near-neutral yeast cytoplasm and dissociate into protons and anions, leading to cytoplasmic acidification and cell damage. Although some yeast strains have developed the ability to survive weak acids, the complexity and diversity of stresses during industrial production still require the application of appropriate strategies for phenotypes improvement. In this review, we summarized current knowledge concerning weak acid stress response and resistance, which may suggest important targets for further construction of more robust strains. We also highlight current feasible strategies for improving the weak acid resistance of yeasts, such as adaptive laboratory evolution, transcription factors engineering, and cell membrane/wall engineering. Moreover, the challenges and perspectives associated with improving the competitiveness of industrial strains are also discussed. This review provides effective strategies for improving the industrial phenotypes of yeast from multiple dimensions in future studies.

An Evolved Strain of the Oleaginous Yeast Rhodotorula toruloides, Multi-Tolerant to the Major Inhibitors Present in Lignocellulosic Hydrolysates, Exhibits an Altered Cell Envelope

The presence of toxic compounds in lignocellulosic hydrolysates (LCH) is among the main barriers affecting the efficiency of lignocellulose-based fermentation processes, in particular, to produce biofuels, hindering the production of intracellular lipids by oleaginous yeasts. These microbial oils are promising sustainable alternatives to vegetable oils for biodiesel production. In this study, we explored adaptive laboratory evolution (ALE), under methanol- and high glycerol concentration-induced selective pressures, to improve the robustness of a Rhodotorula toruloides strain, previously selected to produce lipids from sugar beet hydrolysates by completely using the major C (carbon) sources present. An evolved strain, multi-tolerant not only to methanol but to four major inhibitors present in LCH (acetic acid, formic acid, hydroxymethylfurfural, and furfural) was isolated and the mechanisms underlying such multi-tolerance were examined, at the cellular envelope level. Results indicate that the evolved multi-tolerant strain has a cell wall that is less susceptible to zymolyase and a decreased permeability, based on the propidium iodide fluorescent probe, in the absence or presence of those inhibitors. The improved performance of this multi-tolerant strain for lipid production from a synthetic lignocellulosic hydrolysate medium, supplemented with those inhibitors, was confirmed.

Spectrophotometric and Fluorimetric High-Throughput Assays for Phenolic Acid Decarboxylase

Biocatalytic decarboxylation of hydroxycinnamic acids yields phenolic styrenes, which are important precursors for antioxidants, epoxy coatings, adhesives and other polymeric materials. Bacillus subtilis decarboxylase (BsPAD) is a cofactor-independent enzyme that catalyzes the cleavage of carbon dioxide from p-coumaric-, caffeic-, and ferulic acid with high catalytic efficiency. Real-time spectroscopic assays for decarboxylase reactions remove the necessity of extensive sample workup, which is required for HPLC, mass spectrometry, gas chromatography, or NMR methods. This work presents two robust and sensitive assays based on photometry and fluorimetry that allow decarboxylation reactions to be followed with high sensitivity while avoiding product extraction and long analysis times. Optimized assay procedures were used to measure BsPAD activity in cell lysates and to determine the kinetic constants (KM and Vmax ) of the purified enzyme for p-coumaric-, caffeic- and ferulic acid. Substrate inhibition was shown for caffeic acid.

Relation of xylitol formation and lignocellulose degradation in yeast

One of the critical steps of the biotechnological production of xylitol from lignocellulosic biomass is the deconstruction of the plant cell wall. This step is crucial to the bioprocess once the solubilization of xylose from hemicellulose is allowed, which can be easily converted to xylitol by pentose-assimilating yeasts in a microaerobic environment. However, lignocellulosic toxic compounds formed/released during plant cell wall pretreatment, such as aliphatic acids, furans, and phenolic compounds, inhibit xylitol production during fermentation, reducing the fermentative performance of yeasts and impairing the bioprocess productivity. Although the toxicity of lignocellulosic inhibitors is one of the biggest bottlenecks of the biotechnological production of xylitol, most of the studies focus on how much xylitol production is inhibited but not how and where cells are affected. Understanding this mechanism is important in order to develop strategies to overcome lignocellulosic inhibitor toxicity. In this mini-review, we addressed how these inhibitors affect both yeast physiology and metabolism and consequently xylose-to-xylitol bioconversion. In addition, this work also addresses about cellular adaptation, one of the most relevant strategies to overcome lignocellulosic inhibitors toxicity, once it allows the development of robust and tolerant strains, contributing to the improvement of the microbial performance against hemicellulosic hydrolysates toxicity. KEY POINTS: • Impact of lignocellulosic inhibitors on the xylitol production by yeasts • Physiological and metabolic alterations provoked by lignocellulosic inhibitors • Cell adaptation as an efficient strategy to improve yeast's robustness.

Surface display of enzyme complex on Corynebacterium glutamicum as a whole cell biocatalyst and its consolidated bioprocessing using fungal-pretreated lignocellulosic biomass

A novel whole cell biocatalyst using fungal-pretreated lignocellulosic biomass was developed by displaying the enzyme complex consisting of N-acetylglucosaminidase (cNAG) and endoglucanse E (cCelE) on Corynebacterium glutamicum, hereafter called mNC. mNC showed a maximum 4.43-fold cNAG and 2.40-fold cCelE activity compared to single enzyme-secreting C. glutamicum. mNC also showed the highest efficiency of sugar production in various types of cellulose and fungal-pretreated biomass. The growth of mNC was 5.06-fold higher than that of the control. Then, the ability of mNC to produce a valuable chemical was confirmed. mNC overexpressing isopropanol biosynthesis genes showed a maximum titer of 218.9 ± 11.73 mg/L isopropanol and maintained high efficiency for isopropanol production in the recycling test, which was 90.07 ± 4.12 % during 4 cycles. This strategy can be applied to the direct saccharification of fungal-pretreated lignocellulosic biomass efficiently leading to the production of valuable products in various industrial fields.

Utilization of Sugarcane Bagasse as a Substrate for Lipid Production by Aurantiochytrium sp

Thraustochytrid, Aurantiochytrium sp., produces various lipids such as polyunsaturated and saturated fatty acids, carotenoids, and other hydrocarbons, which are useful in the fields of health foods, cosmetics, fine chemicals, and biofuels. Lignocellulosic biomass, which is abundant and cheap, is a promising feedstock for producing cheaper bulk and high-value-added products using Aurantiochytrium sp. However, the steam explosion of lignocellulosic biomass for efficient enzymatic saccharification generates substances that inhibit the growth of microorganisms. In this study, the inhibitory activities of these by-products on the growth and lipid production of Aurantiochytrium sp. were investigated. Aurantiochytrium sp. was found to be highly sensitive to furfural and vanillin and moderately sensitive to 5-hydroxymethylfurfural and syringaldehyde. Washing steam-exploded bagasse with water, followed by activated charcoal treatment, significantly reduced furfural, which was a major inhibitory component in the saccharified solution.

Effects of Inhibitors Generated by Dilute Phosphoric Acid Plus Steam-Exploded Poplar on Saccharomyces cerevisiae Growth

The pretreatment of lignocellulosic biomass is important for efficient bioethanol conversion, but causes undesirable by-products that inhibit microbial growth, conversely affecting the bioconversion efficiency. In this study, the main inhibitors derived from dilute phosphoric acid plus steam-exploded poplar wood were identified as 0.22 g/L furfural, 3.63 g/L acetic acid, 0.08 g/L syringaldehyde, etc., indicating the green nature and low toxicity of the pretreatment process. The effects of the three typical inhibitors (furfural, acetic acid, and syringaldehyde) on Saccharomyces cerevisiae 1517RM growth were analyzed and shown to prolong the lag phase of microbial growth to different degrees. In all the inhibitor groups, the ergosterol secretion was boosted, indicating low cell membrane fluidity and robustness of the strain to an adverse environment. The cell electronegativity and morphology of S. cerevisiae 1517RM also changed under different growth conditions, which was helpful for monitoring the physicochemical properties of cells. Furfural, acetic acid, and syringaldehyde had a synergistic effect on each other, providing an important reference to improving the subsequent ethanol fermentation process.

Production of butanol from lignocellulosic biomass: recent advances, challenges, and prospects

Due to energy and environmental concerns, biobutanol is gaining increasing attention as an alternative renewable fuel owing to its desirable fuel properties. Biobutanol production from lignocellulosic biomass through acetone-butanol-ethanol (ABE) fermentation has gained much interest globally due to its sustainable supply and non-competitiveness with food, but large-scale fermentative production suffers from low product titres and poor selectivity. This review presents recent developments in lignocellulosic butanol production, including pretreatment and hydrolysis of hemicellulose and cellulose during ABE fermentation. Challenges are discussed, including low concentrations of fermentation sugars, inhibitors, detoxification, and carbon catabolite repression. Some key process improvements are also summarised to guide further research and development towards more profitable and commercially viable butanol fermentation.

Antibacterial Effect of Phenolic Acids Derived from Rice Straw and in Combination with Antibiotics Against Escherichia coli

Many studies have demonstrated that natural plant extracts have inhibitory effects on microorganisms. The purpose of this study was to investigate the inhibitory effect of phenolic acids from rice straw (PAs) on Escherichia coli and their synergistic effect in combination with antibiotics. PAs can inhibit the growth of E. coli effectively by inducing the formation of H₂O₂; PA-treated cells had a tenfold greater intracellular H₂O₂ concentration than the control group. The synergistic effect caused by the interaction of PAs and antibiotics on inhibiting the growth of E. coli was significant. This effect may be caused by a PA-induced change in the permeability of E. coli cell membrane. The treatment with PAs made the extracellular K⁺ concentration reached 15 mg/L within 30 min, while the K⁺ concentration in the control group was very low and did not change significantly over time. Similarly to the extracellular K⁺, the extracellular protein concentration exceeded 150 mg/L in the PA treatment group, while it remained very low in the control group. Due to the increased cell permeability, more antibiotics can enter the cell. Hence, this study may provide a novel method of improving the safe use of antibiotics.

Response mechanisms of Saccharomyces cerevisiae to the stress factors present in lignocellulose hydrolysate and strategies for constructing robust strains

Bioconversion of lignocellulosic biomass to biofuels such as bioethanol and high value-added products has attracted great interest in recent decades due to the carbon neutral nature of biomass feedstock. However, there are still many key technical difficulties for the industrial application of biomass bioconversion processes. One of the challenges associated with the microorganism Saccharomyces cerevisiae that is usually used for bioethanol production refers to the inhibition of the yeast by various stress factors. These inhibitive effects seriously restrict the growth and fermentation performance of the strains, resulting in reduced bioethanol production efficiency. Therefore, improving the stress response ability of the strains is of great significance for industrial production of bioethanol. In this article, the response mechanisms of S. cerevisiae to various hydrolysate-derived stress factors including organic acids, furan aldehydes, and phenolic compounds have been reviewed. Organic acids mainly stimulate cells to induce intracellular acidification, furan aldehydes mainly break the intracellular redox balance, and phenolic compounds have a greater effect on membrane homeostasis. These damages lead to inadequate intracellular energy supply and dysregulation of transcription and translation processes, and then activate a series of stress responses. The regulation mechanisms of S. cerevisiae in response to these stress factors are discussed with regard to the cell wall/membrane, energy, amino acids, transcriptional and translational, and redox regulation. The reported key target genes and transcription factors that contribute to the improvement of the strain performance are summarized. Furthermore, the genetic engineering strategies of constructing multilevel defense and eliminating stress effects are discussed in order to provide technical strategies for robust strain construction. It is recommended that robust S. cerevisiae can be constructed with the intervention of metabolic regulation based on the specific stress responses. Rational design with multilevel gene control and intensification of key enzymes can provide good strategies for construction of robust strains.

Low-Temperature Biodegradation of Lignin-Derived Aromatic Model Monomers by the Cold-Adapted Yeast Rhodosporidiobolus colostri Isolated from Alpine Forest Soil

The contribution of cold-adapted yeasts to the emerging field of lignin biovalorization has not yet been studied. The red-pigmented basidiomycetous yeast strain Rhodosporidiobolus colostri DBVPG 10655 was examined for its potential to degrade five selected lignin-derived aromatic monomers (syringic acid, p-coumaric acid, 4-hydroxybenzoic acid, ferulic acid, and vanillic acid). The strain utilized p-coumaric acid, 4-hydroxybenzoic acid, and ferulic acid not only as the sole carbon source; full biodegradation occurred also in mixtures of multiple monomers. Vanillic acid was not utilized as the sole carbon source, but was degraded in the presence of p-coumaric acid, 4-hydroxybenzoic acid, and ferulic acid. Syringic acid was utilized neither as the sole carbon source nor in mixtures of compounds. Biodegradation of lignin-derived aromatic monomers was detected over a broad temperature range (1–25 °C), which is of ecological significance and of biotechnological relevance.

An effective strategy for improving the specific activity and saccharification efficiency of cellulase by pre-incubation with phenolic acids

This short communication analyzed the effects of lignin-derived phenolic acid compounds on cellulase. Vanillic acid, syringic acid, ferulic acid, and isovanillic acid improved cellulase specific activity and saccharification efficiency. In the enzymatic hydrolysis process, the promotion effect of phenolic acid was concentration-dependent. The effect of low concentration of phenolic acids (less than 5 mM) was negligible. After pre-incubating 1 g cellulase with 5 mmol phenolic acid, FPase-specific activity, CMCase-specific activity, and pNPGase-specific activity increased by 57.06%, 136.79%, and 110.61%, respectively. After digestion with pre-incubated cellulase, the saccharification efficiency of phosphoric acid-swollen cellulose increased by 45.13%. Pre-incubation with phenolic acid improved the saccharification efficiency of cellulase. It might be helpful to enhance the comprehensive utilization capacity of lignin-derived compounds.

Prediction of phenolic compounds and glucose content from dilute inorganic acid pretreatment of lignocellulosic biomass using artificial neural network modeling

Dilute inorganic acids hydrolysis is one of the most promising pretreatment strategies with high recovery of fermentable sugars and low cost for sustainable production of biofuels and chemicals from lignocellulosic biomass. The diverse phenolics derived from lignin degradation during pretreatment are the main inhibitors for enzymatic hydrolysis and fermentation. However, the content features of derived phenolics and produced glucose under different conditions are still unclear due to the highly non-linear characteristic of biomass pretreatment. Here, an artificial neural network (ANN) model was developed for simultaneous prediction of the derived phenolic contents (CPhe) and glucose yield (CGlc) in corn stover hydrolysate before microbial fermentation by integrating dilute acid pretreatment and enzymatic hydrolysis. Six processing parameters including inorganic acid concentration (CIA), pretreatment temperature (T), residence time (t), solid-to-liquid ratio (RSL), kinds of inorganic acids (kIA), and enzyme loading dosage (E) were used as input variables. The CPhe and CGlc were set as the two output variables. An optimized topology structure of 6-12-2 in the ANN model was determined by comparing root means square errors, which has a better prediction efficiency for CPhe (R2 = 0.904) and CGlc (R2 = 0.906). Additionally, the relative importance of six input variables on CPhe and CGlc was firstly calculated by the Garson equation with net weight matrixes. The results indicated that CIA had strong effects (22%-23%) on CPhe or CGlc, then followed by E and T. In conclusion, the findings provide new insights into the sustainable development and inverse optimization of biorefinery process from ANN modeling perspectives.

Biorefinery potential of chemically enhanced primary treatment sewage sludge to representative value-added chemicals - A de novo angle for wastewater treatment

Chemically enhanced primary treatment (CEPT) is an emerging sewage treatment strategy due to its high efficiency and small land requirement. CEPT sludge can be easily dewatered and used for energy recovery through incineration. However, with large amount of reusable nutrients (40% organic carbon, 23% lipids, and 17% protein), the value of CEPT sludge may have been underestimated. In this study, the biorefinery potential of CEPT sludge has been proven via production of 28.9 g/L ethanol or 50.3 g/L lactic acid (LA) or 1.43 filter paper unit (FPU)/mL cellulase from 10 g of CEPT sludge experiment. Inhibition on cell growth and potential inhibitors from plasticizers, pharmaceuticals, and surfactants were determined. Nevertheless, production titer was not affected or performed even better than the non-inhibitors controls. CEPT sludge showed significant potential in biochemical conversion, and the related products may offer an opportunity to support wastewater treatment toward sustainability and carbon neutrality.

Overexpressing CCW12 in Saccharomyces cerevisiae enables highly efficient ethanol production from lignocellulose hydrolysates

A Saccharomyces cerevisiae strain CCW12^OE was constructed by overexpressing CCW12 in a previously reported strain WXY70 harboring six xylose utilization genes. CCW12^OE produced an optimal ethanol yield of 98.8% theoretical value within 48 h in a simulated corn stover hydrolysate. CCW12^OEwas comprehensively evaluated for ethanol production in Miscanthus, maize and corncob hydrolysates, among which a 96.1% theoretical value was achieved within 12 h in corncob hydrolysates. Under normal growth conditions, CCW12^OE did not display altered cell morphology; however, in the presence of acetate, CCW12^OE maintained relatively intact cell structure and increased cell wall thickness by nearly 50%, while WXY70 had abnormal cell morphology and reduced cell wall thickness by nearly 50%. Besides, CCW12^OE had higher fermentation capacity than that of WXY70 in undetoxified and detoxified hydrolysates with both aerobic and anaerobic conditions, demonstrating that CCW12 overexpression alone exhibits improved stress resistance and better fermentation performance.

Simultaneous Improvement of Limonene Production and Tolerance in Yarrowia lipolytica through Tolerance Engineering and Evolutionary Engineering

Limonene is an important plant natural product widely used in food and cosmetics production as well as in the pharmaceutical and chemical industries. However, low efficiency of plant extraction and high energy consumption in chemical synthesis limit the sustainability of industrial limonene production. Recently, the advancement of metabolic engineering and synthetic biology has facilitated the engineering of microbes into microbial cell factories for producing limonene. However, the deleterious effects on cellular activity by the toxicity of limonene is the major obstacle in achieving high-titer production of limonene in engineered microbes. In this study, by using transcriptomics, we identified 82 genes from the nonconventional yeast Yarrowia lipolytica that were up-regulated when exposed to limonene. When overexpressed, 8 of the gene candidates improved tolerance of this yeast to exogenously added limonene. To determine whether overexpression of these genes could also improve limonene production, we individually coexpressed the tolerance-enhancing genes with a limonene synthase gene. Indeed, expression of 5 of the 8 candidate genes enhanced limonene production in Y. lipolytica. Particularly, overexpressing YALI0F19492p led to an 8-fold improvement in product titer. Furthermore, through short-term adaptive laboratory evolution strategy, in combination with morphological and cytoplasmic membrane integrity analysis, we shed light on the underlying mechanism of limonene cytotoxicity to Y. lipolytica. This study demonstrated an effective strategy for improving limonene tolerance of Y. lipolytica and limonene titer in the host strain through the combinatorial use of tolerance engineering and evolutionary engineering.

Transporter proteins in Zymomonas mobilis contribute to the tolerance of lignocellulose-derived phenolic aldehyde inhibitors

Transporter proteins are of great importance for improving the tolerance of fermentation strains to lignocellulose-derived furans and phenolic inhibitors. Different from the documented transporter proteins responsible for the tolerance of furfural and 5-hydroxymethyl-furfural (HMF), transporters responsible for that of varied phenolic aldehyde inhibitors were less investigated and elucidated. Here, an interesting phenomenon was found that no phenolic alcohols were accumulated from phenolic aldehydes degradation in Zymomonas mobilis. A transcriptional profiling of transporter genes was established in Z. mobilis ZM4 under phenolic aldehydes stress using DNA microarray. Six transporter proteins were identified as the potential candidates responsible for the tolerance of phenolic aldehydes including ABC transporter (ZMO0799 and ZMO0800), MFS transporter (ZMO1288 and ZMO1856), and RND transporter (ZMO0282 and ZMO0798). Furthermore, the analysis showed that the key transporters were significantly correlated with oxidoreductases and transcriptional regulators. This work would provide several important transporter genes serving as synthetic biology tools for improving the robustness of biorefinery strains.

Very High Gravity Bioethanol Revisited: Main Challenges and Advances

Over the last decades, the constant growth of the world-wide industry has been leading to more and more concerns with its direct impact on greenhouse gas (GHG) emissions. Resulting from that, rising efforts have been dedicated to a global transition from an oil-based industry to cleaner biotechnological processes. A specific example refers to the production of bioethanol to substitute the traditional transportation fuels. Bioethanol has been produced for decades now, mainly from energy crops, but more recently, also from lignocellulosic materials. Aiming to improve process economics, the fermentation of very high gravity (VHG) mediums has for long received considerable attention. Nowadays, with the growth of multi-waste valorization frameworks, VHG fermentation could be crucial for bioeconomy development. However, numerous obstacles remain. This work initially presents the main aspects of a VHG process, giving then special emphasis to some of the most important factors that traditionally affect the fermentation organism, such as nutrients depletion, osmotic stress, and ethanol toxicity. Afterwards, some factors that could possibly enable critical improvements in the future on VHG technologies are discussed. Special attention was given to the potential of the development of new fermentation organisms, nutritionally complete culture media, but also on alternative process conditions and configurations.

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.

Recent advances in the biological valorization of citrus peel waste into fuels and chemicals

In the quest to reduce global food loss and waste, fruit processing wastes, particularly citrus peel waste (CPW), have emerged as a promising and sustainable option for biorefinery without competing with human foods and animal feeds. CPW is largely produced and, as recent studies suggest, has the industrial potential of biological valorization into fuels and chemicals. In this review, the promising aspects of CPW as an alternative biomass were highlighted, focusing on its low lignin content. In addition, specific technical difficulties in fermenting CPW are described, highlighting that citrus peel is high in pectin that consist of non-fermentable sugars, mainly galacturonic acid. Last, recent advances in the metabolic engineering of yeast and other microbial strains that ferment CPW-derived sugars to produce value-added products, such as ethanol and mucic acid, are summarized. For industrially viable CPW-based biorefinery, more studies are needed to improve fermentation efficiency and to diversify product profiles.

Efficient bio-butanol production from lignocellulosic waste by elucidating the mechanisms of Clostridium acetobutylicum response to phenolic inhibitors

Lignocellulosic biomass is considered abundant renewable feedstock to constitute a green and environmentally friendly approach for biofuels (bio-butanol) production as an effective substitute for fossil resources. However, a variety of fermentable inhibitors can be generated in hydrolysates during the biomass pretreatment process. Among them, phenolics including phenolic acids and phenolic aldehydes are the most toxic inhibitors to solventogenic clostridia for bio-butanol production. This study elucidates the physiological mechanism of Clostridium acetobutylicum ATCC 824 response to phenolic inhibitors by the integration of kinetics and transcriptional analysis. Butanol fermentations were stressed by 0.4 g/L phenolic acids or 0.4 g/L phenolic aldehydes at 12 h at the beginning of solventogenesis. With post-stress for 12 h, butanol titer was 7.01 g/L in fermentation with phenolic acid stress, while only 5.82 g/L butanol was produced in the case of phenolic aldehydes stress. Reductions in the two fermentations were 27.6% and 40.0% in comparison with the control (without stress), indicated that phenolic aldehydes had a stronger inhibitory effect on solvents synthesis in C. acetobutylicum than phenolic acids. Additionally, the transcriptional analysis revealed that phenolics altered the gene expression profiles related to membrane transporters such as ATP-binding cassette (ABC)-transporter and phosphotransferase system (PTS), glycolysis, and heat shock proteins. The lower expression levels of PTS-related genes might result in reduced glucose consumption and finally inhibited solvents synthesis under phenolic aldehydes stress. Some genes encoding histidine kinase (CA_C0323, CA_C0903, and CA_C3319) were also affected by phenolics, which might inhibit sporulation. In conclusion, our results provide valuable guidance for the construction of robust strain to efficiently produce bio-butanol from lignocellulosic biomass.