Production of n-butanol from cassava bagasse hydrolysate by engineered Clostridium tyrobutyricum overexpressing adhE2: Kinetics and cost analysis

Metabolic Engineering of Clostridium tyrobutyricum for High-Yield n-Butanol Production by Increasing Intracellular Reducing Equivalent with NADPH-Dependent 3-Hydroxybutyryl-CoA Dehydrogenase

Clostridium tyrobutyricum was engineered to overexpress adhE2 encoding the aldehyde/alcohol dehydrogenase and an exogenous NADPH-dependent 3-hydroxybutyryl-CoA dehydrogenase from Clostridium kluyveri (Ckhbd) for n-butanol production. In general, large amounts of butyrate, acetate, and ethanol are also produced from glucose when butanol biosynthesis is hindered by limited intracellular NADH pools. In silico flux balance analysis showed that coupling NADP+/NADPH turnover with butanol production increased the reducing equivalent supply and butanol selectivity over ethanol and acids, thus increasing butanol production from glucose. This was verified with the coexpression of Ckhbd and adhE2 in C. tyrobutyricum wild type (WT), Ack, ΔhydA, and Δcat1 strains. Except for the Δcat1 strains, strains coexpressing Ckhbd showed significant (>5%) increase in reducing equivalents, 50-60% increase in butanol production (butanol yield: 0.24-0.28 vs. 0.15-0.18 g/g), and 2.5- to 4.5-fold increases in butanol/ethanol and alcohols/acids ratios due to increased flux from acetyl-CoA to butyryl-CoA and reducing equivalents compared to the strains expressing only adhE2. In the presence of methyl viologen, the strain Ack-adhE2-Ckhbd produced the highest butanol yield of 0.36 g/g, ∼88% of the theoretical yield from glucose, which was among the highest yields reported for known solventogenic clostridia.

Development of inducible promoters for regulating gene expression in Clostridium tyrobutyricum for biobutanol production

Clostridium tyrobutyricum is an anaerobe known for its ability to produce short-chain fatty acids, alcohols, and esters. We aimed to develop inducible promoters for fine-tuning gene expression in C. tyrobutyricum. Synthetic inducible promoters were created by employing an Escherichia coli lac operator to regulate the thiolase promoter (PCathl) from Clostridium acetobutylicum, with the best one (LacI-Pto4s) showing a 5.86-fold dynamic range with isopropyl β- d-thiogalactoside (IPTG) induction. A LT-Pt7 system with a dynamic range of 11.6-fold was then created by combining LacI-Pto4s with a T7 expression system composing of RNA polymerase (T7RNAP) and Pt7lac promoter. Furthermore, two inducible expression systems BgaR-PbgaLA and BgaR-PbgaLB with a dynamic range of ~40-fold were developed by optimizing a lactose-inducible expression system from Clostridium perfringens with modified 5' untranslated region (5' UTR) and ribosome-binding site (RBS). BgaR-PbgaLB was then used to regulate the expressions of a bifunctional aldehyde/alcohol dehydrogenase encoded by adhE2 and butyryl-CoA/acetate Co-A transferase encoded by cat1 in C. tyrobutyricum wild type and Δcat1::adhE2, respectively, demonstrating its efficient inducible gene regulation. The regulated cat1 expression also confirmed that the Cat1-catalyzed reaction was responsible for acetate assimilation in C. tyrobutyricum. The inducible promoters offer new tools for tuning gene expression in C. tyrobutyricum for industrial applications.

Harnessing lignocellulosic biomass for butanol production through clostridia for sustainable waste management: recent advances and perspectives

The escalating waste generation rates, driven by population growth, urbanization, and consumption patterns, have made waste management a critical global concern with significant environmental, social, and economic repercussions. Among the various waste sources, lignocellulosic biomass represents a significant proportion of agricultural, agro-industrial, and municipal wastes. Biofuels are gaining attention as a promising substitute to fossil fuels, and butanol is one such biofuel that has been identified as a potential candidate due to its compatibility with existing fuel infrastructure, lower volatility, and higher energy density. Sustainable management of lignocellulosic biomass waste and its utilization in fermentation are viable alternatives to produce butanol via the promising microbial catalyst clostridia. This review provides an overview of lignocellulosic biomass waste management, focusing on recent advances in strain development for butanol production from renewable biomass with an emphasis on future perspectives.

Multi-omics analysis unravels positive effect of rotenone on the cordycepin biosynthesis in submerged fermentation of Cordyceps militaris

Cordycepin is the key pharmacologically active compound of Cordyceps militaris, and various fermentation strategies have been developed to increase cordycepin production. This study aimed to investigate the effect of rotenone on cordycepin biosynthesis in submerged fermentation of C. militaris, and also to explore its possible induction mechanisms via multi-omics analysis. Adding 5 mg/L rotenone significantly increased the cordycepin production by 316.09 %, along with mycelial growth inhibition and cell wall destruction. Moreover, transcriptomic analysis and metabolomic analysis revealed the accumulation of cordycepin was promoted by alterations in energy metabolism and amino acid metabolism pathways. Finally, the integration analysis of the two omics confirmed rotenone altered the nucleotide metabolism pathway toward adenosine and up-regulated the cordycepin synthesis genes (cns1-3) to convert adenosine to cordycepin. This work reports, for the first time, rotenone could act as an effective inducer of cordycepin synthesis.

Integrated Biorefinery and Life Cycle Assessment of Cassava Processing Residue-From Production to Sustainable Evaluation

Commonly known as a subsistence culture, cassava came to be considered a commodity and key to adding value. However, this tuber's processing for starch and flour production is responsible for generating a large amount of waste that causes serious environmental problems. This biomass of varied biochemical composition has excellent potential for producing fuels (biogas, bioethanol, butanol, biohydrogen) and non-energetic products (succinic acid, glucose syrup, lactic acid) via biorefinery. However, there are environmental challenges, leading to uncertainties related to the sustainability of biorefineries. Thus, the provision of information generated in life cycle assessment (LCA) can help reduce bottlenecks found in the productive stages, making production more competitive. Within that, this review concentrates information on the production of value-added products, the environmental impact generated, and the sustainability of biorefineries.

A review on the conversion of cassava wastes into value-added products towards a sustainable environment

The solid and liquid wastes generated from cassava-based industries are organic and acidic in nature, which leads to various global concerns-primarily global warming and biodiversity loss. But the conversion of these wastes into value-added products associated with environmental pollution control contributes to sustainable development. Generally, the thermochemical process such as pyrolysis and gasification and biochemical processes such as anaerobic digestion have been applied for the conversion of cassava waste into value-added products. This review addresses the valorization of cassava wastes, which fulfill almost all needs of the hour, such as energy (biofuel), wastewater treatment (adsorbents), bioplastics, starch nanoparticles, organic acid production, and antimicrobial agents. The major aim of this paper is to analyze and provide the disclosure of the efficiency of cassava-based industrial waste as a source to minimize the problem associated with conventional fossil fuels and through which mitigate the impact of global warming and climate change. Furthermore, recent research and achievements in the valorization of cassava waste have been highlighted.

Current progress on engineering microbial strains and consortia for production of cellulosic butanol through consolidated bioprocessing

In the last decades, fermentative production of n-butanol has regained substantial interest mainly owing to its use as drop-in-fuel. The use of lignocellulose as an alternative to traditional acetone-butanol-ethanol fermentation feedstocks (starchy biomass and molasses) can significantly increase the economic competitiveness of biobutanol over production from non-renewable sources (petroleum). However, the low cost of lignocellulose is offset by its high recalcitrance to biodegradation which generally requires chemical-physical pre-treatment and multiple bioreactor-based processes. The development of consolidated processing (i.e., single-pot fermentation) can dramatically reduce lignocellulose fermentation costs and promote its industrial application. Here, strategies for developing microbial strains and consortia that feature both efficient (hemi)cellulose depolymerization and butanol production will be depicted, that is, rational metabolic engineering of native (hemi)cellulolytic or native butanol-producing or other suitable microorganisms; protoplast fusion of (hemi)cellulolytic and butanol-producing strains; and co-culture of (hemi)cellulolytic and butanol-producing microbes. Irrespective of the fermentation feedstock, biobutanol production is inherently limited by the severe toxicity of this solvent that challenges process economic viability. Hence, an overview of strategies for developing butanol hypertolerant strains will be provided.

High-rate continuous n-butanol production by Clostridium acetobutylicum from glucose and butyric acid in a single-pass fibrous bed bioreactor

Biobutanol produced in acetone-butanol-ethanol (ABE) fermentation at batch mode cannot compete with chemically derived butanol because of the low reactor productivity. Continuous fermentation can dramatically enhance productivity and lower capital and operating costs but are rarely used in industrial fermentation because of increased risks in culture degeneration, cell washout, and contamination. In this study, cells of the asporogenous Clostridium acetobutylicum ATCC55025 were immobilized in a single-pass fibrous-bed bioreactor (FBB) for continuous production of butanol from glucose and butyrate at various dilution rates. Butyric acid in the feed medium helped maintaining cells in the solventogenic phase for stable continuous butanol production. At the dilution rate of 1.88 h^-1 , butanol was produced at 9.55 g/L with a yield of 0.24 g/g and productivity of 16.8 g/L/h, which was the highest productivity ever achieved for biobutanol fermentation and an 80-fold improvement over the conventional ABE fermentation. The extremely high productivity was attributed to the high density of viable cells (~100 g/L at >70% viability) immobilized in the fibrous matrix, which also enabled the cells to better tolerate butanol and butyric acid. The FBB was stable for continuous operation for an extended period of over one month. This article is protected by copyright. All rights reserved.

Post-industrial context of cassava bagasse and trend of studies towards a sustainable industry: A scoping review - Part I

Background: The cassava starch industry is recognized as a source of negative externalities caused by the agroindustrial waste 'cassava bagasse'. Even though options for bioconversion of cassava bagasse have been introduced, it is also true that hundreds of tons of this waste are produced annually with the consequent negative environmental impact. This agroindustrial context highlights the need for further research in technological proposals aimed at lowering the water contained in cassava bagasse. Methods: We report a scoping review of studies from 2010-2021 that mention the uses of cassava bagasse, as well as the technological options that have become effective for drying fruits and vegetables. The method used for selecting articles was based on the Preferred Reporting Items for Systematic Review and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) method. Articles selected were taken from the databases of ScienceDirect, Google Scholar, Scopus and Springer. Results : This review highlights fruit and vegetable osmotic dehydration and drying studies assisted by the combination of emerging technologies of osmotic pressure, ultrasound, and electrical pulses. Studies that take advantage of cassava bagasse have focused on biotechnological products, animal and human food industry, and development of biofilms and biomaterials. Conclusions: In this review, we found 60 studies out of 124 that show the advantages of the residual components of cassava bagasse for the development of new products. These studies do not mention any potential use of bagasse fiber for post-industrial purposes, leaving this end products' final use/disposal unaddressed. A viable solution is osmotic dehydration and drying assisted with electrical pulse and ultrasound that have been shown to improve the drying efficiency of fruits, vegetables and tubers. This greatly improves the drying efficiency of agro-industrial residues such as husks and bagasse, which in turn, directly impacts its post-industrial use.

ACETONE-BUTYL FERMENTATION PECULIARITIES OF THE BUTANOL STRAINS -PRODUCER

The aim of this review was to generalize and analyze the features of acetone-butyl fermentation as a type of butyric acid fermentation in the process of obtaining butanol as an alternative biofuel. Methods. The methods of analysis and generalization of analytical information and literature sources were used in the review. The results were obtained using the following methods such as microbiological (morphological properties of strains), chromatographic (determination of solvent concentration), spectrophotometric (determination of bacterial concentration), and molecular genetic (phylogenetic analysis of strains). Results. The process of acetone-butyl fermentation was analyzed, the main producer strains were considered, the features of the relationship between alcohol formation and sporulation were described, the possibility of butanol obtaining from synthesis gas was shown, and the features of the industrial production of butanol were considered. Conclusions. The features of the mechanism of acetone-butyl fermentation (the relationships between alcohol formation and sporulation, the duration of the acid-forming and alcohol-forming stages during batch fermentation depending on the change in the concentration of H2, CO, partial pressure, organic acids and mineral additives) and obtaining an enrichment culture during the production of butanol as an alternative fuel were shown. The possibility of using synthesis gas as a substrate for reducing atmospheric emissions during the fermentation process was shown. The direction of increasing the productivity of butanol-producing strains to create a competitive industrial biofuel technology was proposed.

Butanol production from Saccharina japonica hydrolysate by engineered Clostridium tyrobutyricum: The effects of pretreatment method and heat shock protein overexpression

Macroalgal biomass is currently considered as a potential candidate for biofuel production. In this study, the effects of pretreatment method and heat shock protein overexpression were investigated for efficient butanol production from Saccharina japonica using engineered Clostridium tyrobutyricum. First, various pretreatment methods including acid hydrolysis, acid hydrolysis and enzymatic saccharification, and ultrasonic-assisted acid hydrolysis were employed to obtain the fermentable sugars, and the resulted hydrolysates were evaluated for butanol fermentation. The results showed that ultrasonic-assisted acid hydrolysate obtained the highest butanol yield (0.26 g/g) and productivity (0.19 g/L⋅h). Then, the effects of homologous or heterologous heat shock protein overexpression on butanol production and tolerance were examined. Among all the engineered strains, Ct-pMA12G exhibited improved butanol tolerance and enhanced butanol production (12.15 g/L butanol with a yield of 0.34 g/g and productivity of 0.15 g/L⋅h) from 1.8-fold concentrated S. japonica hydrolysate, which was the highest level ever reported for macroalgal biomass.

Engineering Clostridium cellulovorans for highly selective n-butanol production from cellulose in consolidated bioprocessing

Cellulosic n-butanol from renewable lignocellulosic biomass has gained increased interest. Previously, we have engineered Clostridium cellulovorans, a cellulolytic acidogen, to overexpress the bifunctional butyraldehyde/butanol dehydrogenase gene adhE2 from C. acetobutylicum for n-butanol production from crystalline cellulose. However, butanol production by this engineered strain had a relatively low yield of approximately 0.22 g/g cellulose due to the coproduction of ethanol and acids. We hypothesized that strengthening the carbon flux through the central butyryl-CoA biosynthesis pathway and increasing intracellular NADH availability in C. cellulovorans adhE2 would enhance n-butanol production. In this study, thiolase (thlA^CA ) from C. acetobutylicum and 3-hydroxybutyryl-CoA dehydrogenase (hbd^CT ) from C. tyrobutyricum were overexpressed in C. cellulovorans adhE2 to increase the flux from acetyl-CoA to butyryl-CoA. In addition, ferredoxin-NAD(P)⁺ oxidoreductase (fnr), which can regenerate the intracellular NAD(P)H and thus increase butanol biosynthesis, was also overexpressed. Metabolic flux analyses showed that mutants overexpressing these genes had a significantly increased carbon flux toward butyryl-CoA, which resulted in increased production of butyrate and butanol. The addition of methyl viologen as an electron carrier in batch fermentation further directed more carbon flux towards n-butanol biosynthesis due to increased reducing equivalent or NADH. The engineered strain C. cellulovorans adhE2-fnr^CA -thlA^CA -hbd^CT produced n-butanol from cellulose at a 50% higher yield (0.34 g/g), the highest ever obtained in batch fermentation by any known bacterial strain. The engineered C. cellulovorans is thus a promising host for n-butanol production from cellulosic biomass in consolidated bioprocessing.

Recent Progress on Chemical Production From Non-food Renewable Feedstocks Using Corynebacterium glutamicum

Due to the non-renewable nature of fossil fuels, microbial fermentation is considered a sustainable approach for chemical production using glucose, xylose, menthol, and other complex carbon sources represented by lignocellulosic biomass. Among these, xylose, methanol, arabinose, glycerol, and other alternative feedstocks have been identified as superior non-food sustainable carbon substrates that can be effectively developed for microbe-based bioproduction. Corynebacterium glutamicum is a model gram-positive bacterium that has been extensively engineered to produce amino acids and other chemicals. Recently, in order to reduce production costs and avoid competition for human food, C. glutamicum has also been engineered to broaden its substrate spectrum. Strengthening endogenous metabolic pathways or assembling heterologous ones enables C. glutamicum to rapidly catabolize a multitude of carbon sources. This review summarizes recent progress in metabolic engineering of C. glutamicum toward a broad substrate spectrum and diverse chemical production. In particularly, utilization of lignocellulosic biomass-derived complex hybrid carbon source represents the futural direction for non-food renewable feedstocks was discussed.

Effects of benzyl viologen on increasing NADH availability, acetate assimilation, and butyric acid production by Clostridium tyrobutyricum

Clostridium tyrobutyricum produces butyric and acetic acids from glucose. The butyric acid yield and selectivity in the fermentation depend on NADH available for acetate reassimilation to butyric acid. In this study, benzyl viologen (BV), an artificial electron carrier that inhibits hydrogen production, was used to increase NADH availability and butyric acid production while eliminating acetic acid accumulation by facilitating its reassimilation. To better understand the mechanism of and find the optimum condition for BV effect on enhancing acetate assimilation and butyric acid production, BV at various concentrations and addition times during the fermentation were studied. Compared with the control without BV, the addition of 1 μM BV increased butyric acid production from glucose by ∼50% in yield and ∼29% in productivity while acetate production was completely inhibited. Furthermore, BV also increased the coutilization of glucose and exogenous acetate for butyric acid production. At a concentration ratio of acetate (g/L) to BV (mM) of 4, both acetate assimilation and butyrate biosynthesis increased with increasing the concentrations of BV (0-6.25 μM) and exogenous acetate (0-25 g/L). In a fed-batch fermentation with glucose and ∼15 g/L acetate and 3.75 μM BV, butyrate production reached 55.9 g/L with productivity 0.93 g/L/h, yield 0.48 g/g, and 97.4% purity, which would facilitate product purification and reduce production cost. Manipulating metabolic flux and redox balance via BV and acetate addition provided a simple to implement metabolic process engineering approach for butyric acid production from sugars and biomass hydrolysates.

Recent advances in n-butanol and butyrate production using engineered Clostridium tyrobutyricum

Acidogenic clostridia naturally producing acetic and butyric acids has attracted high interest as a novel host for butyrate and n-butanol production. Among them, Clostridium tyrobutyricum is a hyper butyrate-producing bacterium, which re-assimilates acetate for butyrate biosynthesis by butyryl-CoA/acetate CoA transferase (CoAT), rather than the phosphotransbutyrylase-butyrate kinase (PTB-BK) pathway widely found in clostridia and other microbial species. To date, C. tyrobutyricum has been engineered to overexpress a heterologous alcohol/aldehyde dehydrogenase, which converts butyryl-CoA to n-butanol. Compared to conventional solventogenic clostridia, which produce acetone, ethanol, and butanol in a biphasic fermentation process, the engineered C. tyrobutyricum with a high metabolic flux toward butyryl-CoA produced n-butanol at a high yield of > 0.30 g/g and titer of > 20 g/L in glucose fermentation. With no acetone production and a high C4/C2 ratio, butanol was the only major fermentation product by the recombinant C. tyrobutyricum, allowing simplified downstream processing for product purification. In this review, novel metabolic engineering strategies to improve n-butanol and butyrate production by C. tyrobutyricum from various substrates, including glucose, xylose, galactose, sucrose, and cellulosic hydrolysates containing the mixture of glucose and xylose, are discussed. Compared to other recombinant hosts such as Clostridium acetobutylicum and Escherichia coli, the engineered C. tyrobutyricum strains with higher butyrate and butanol titers, yields and productivities are the most promising hosts for potential industrial applications.

Butanol production by Saccharomyces cerevisiae: perspectives, strategies and challenges

The search for gasoline substitutes has grown in recent decades, leading to the increased production of ethanol as viable alternative. However, research in recent years has shown that butanol exhibits various advantages over ethanol as a biofuel. Furthermore, butanol can also be used as a chemical platform, serving as an intermediate product and as a solvent in industrial reactions. This alcohol is naturally produced by some Clostridium species; however, Clostridial fermentation processes still have inherent problems, which focuses the interest on Saccharomyces cerevisiae for butanol production, as an alternative organism for the production of this alcohol. S. cerevisiae exhibits great adaptability to industrial conditions and can be modified with a wide range of genetic tools. Although S. cerevisiae is known to naturally produce isobutanol, the n-butanol synthesis pathway has not been well established in wild S. cerevisiae strains. Two strategies are most commonly used for of S. cerevisiae butanol production: the heterologous expression of the Clostridium pathway or the amino acid uptake pathways. However, butanol yields produced from S. cerevisiae are lower than ethanol yield. Thus, there are still many challenges needed to be overcome, which can be minimized through genetic and evolutive engineering, for butanol production by yeast to become a reality.

Pathway dissection, regulation, engineering and application: lessons learned from biobutanol production by solventogenic clostridia

The global energy crisis and limited supply of petroleum fuels have rekindled the interest in utilizing a sustainable biomass to produce biofuel. Butanol, an advanced biofuel, is a superior renewable resource as it has a high energy content and is less hygroscopic than other candidates. At present, the biobutanol route, employing acetone-butanol-ethanol (ABE) fermentation in Clostridium species, is not economically competitive due to the high cost of feedstocks, low butanol titer, and product inhibition. Based on an analysis of the physiological characteristics of solventogenic clostridia, current advances that enhance ABE fermentation from strain improvement to product separation were systematically reviewed, focusing on: (1) elucidating the metabolic pathway and regulation mechanism of butanol synthesis; (2) enhancing cellular performance and robustness through metabolic engineering, and (3) optimizing the process of ABE fermentation. Finally, perspectives on engineering and exploiting clostridia as cell factories to efficiently produce various chemicals and materials are also discussed.

Co-valorization of paper mill sludge and corn steep liquor for enhanced n-butanol production with Clostridium tyrobutyricum Δcat1::adhE2

In this study, hyper-butanol producing Clostridium tyrobutyricum Δcat1::adhE2 was used for butanol production from paper mill sludge (PMS) and corn steep liquor (CSL). Our results demonstrated that CSL can not only serve as a cheap nitrogen source, but also provide lactic acid that can be assimilated by C. tyrobutyricum for enhanced butanol production. Through a separate hydrolysis and fermentation, 16.5 g/L butanol with a yield of 0.26 g/g was obtained from PMS hydrolysates supplemented with 5% CSL. Further, a separate repeated hydrolysis was conducted to improve PMS hydrolysis rate and enhance sugar yield. Fermentation using hydrolysates from such process also generated high-level butanol with high yield. Our results suggested an innovative bioprocess for efficient biobutanol production from low-value waste streams.