Economical challenges to microbial producers of butanol: Feedstock, butanol ratio and titer

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.

Membrane applications in the food industry

Current trends in the food industry for the application of membrane techniques are presented. Industrial solutions as well as laboratory research, which can contribute to the improvement of membrane efficiency and performance in this field, are widely discussed. Special attention is given to the main food industries related to dairy, sugar and biotechnology. In addition, the potential of membrane techniques to assist in the treatment of waste sources arising from food production is highlighted.

Renewable Butanol Production via Catalytic Routes

Fluctuating crude oil price and global environmental problems such as global warming and climate change lead to growing demand for the production of renewable chemicals as petrochemical substitutes. Butanol is a nonpolar alcohol that is used in a large variety of consumer products and as an important industrial intermediate. Thus, the production of butanol from renewable resources (e.g., biomass and organic waste) has gained a great deal of attention from researchers. Although typical renewable butanol is produced via a fermentative route (i.e., acetone-butanol-ethanol (ABE) fermentation of biomass-derived sugars), the fermentative butanol production has disadvantages such as a low yield of butanol and the formation of byproducts, such as acetone and ethanol. To avoid the drawbacks, the production of renewable butanol via non-fermentative catalytic routes has been recently proposed. This review is aimed at providing an overview on three different emerging and promising catalytic routes from biomass/organic waste-derived chemicals to butanol. The first route involves the conversion of ethanol into butanol over metal and oxide catalysts. Volatile fatty acid can be a raw chemical for the production of butanol using porous materials and metal catalysts. In addition, biomass-derived syngas can be transformed to butanol on non-noble metal catalysts promoted by alkali metals. The prospect of catalytic renewable butanol production is also discussed.

Systems metabolic engineering of Vibrio natriegens for the production of 1,3-propanediol

The economic viability of current bio-production systems is often limited by its low productivity due to slow cell growth and low substrate uptake rate. The fastest-growing bacterium Vibrio natriegens is a highly promising next-generation workhorse of the biotechnology industry which can utilize various industrially relevant carbon sources with high substrate uptake rates. Here, we demonstrate the first systematic engineering example of V. natriegens for the heterologous production of 1,3-propanediol (1,3-PDO) from glycerol. Systems metabolic engineering strategies have been applied in this study to develop a superior 1,3-PDO producer, including: (1) heterologous pathway construction and optimization; (2) engineering cellular transcriptional regulators and global transcriptomic analysis; (3) enhancing intracellular reducing power by cofactor engineering; (4) reducing the accumulation of toxic intermediate by pathway engineering; (5) systematic engineering of glycerol oxidation pathway to eliminate byproduct formation. A final engineered strain can efficiently produce 1,3-PDO with a titer of 56.2 g/L, a yield of 0.61 mol/mol, and an average productivity of 2.36 g/L/h. The strategies described in this study would be useful for engineering V. natriegens as a potential chassis for the production of other useful chemicals and biofuels.

Bio-purification of sugar industry wastewater and production of high-value industrial products with a zero-waste concept

In recent years, biorefinery approach with a zero-waste concept has gained a lot research impetus to boost the environment and bioeconomy in a sustainable manner. The wastewater from sugar industries contains miscellaneous compounds and need to be treated chemically or biologically before being discharged into water bodies. Efficient utilization of wastewater produced by sugar industries is a key point to improve its economy. Thus, interest in the sugar industry wastes has grown in both fundamental and applied research fields, over the years. Although, traditional methods being used to process such wastewaters are effective yet are tedious, laborious and time intensive. Considering the diverse nature of wastewaters from various sugar-manufacturing processes, the development of robust, cost-competitive, sustainable and clean technologies has become a challenging task. Under the recent scenario of cleaner production and consumption, the biorefinery and/or close-loop concept, though using different technologies and multi-step processes, namely, bio-reduction, bio-accumulation or biosorption using a variety of microbial strains, has stepped-up as the method of choice for a sustainable exploitation of a wide range of organic waste matter along with the production of high-value products of industrial interests. This review comprehensively describes the use of various microbial strains employed for eliminating the environmental pollutants from sugar industry wastewater. Moreover, the main research gaps are also critically discussed along with the prospects for the efficient purification of sugar industry wastewaters with the concomitant production of high-value products using a biorefinery approach. In this review, we emphasized that the biotransformation/biopurification of sugar industry waste into an array of value-added compounds such as succinic acid, L-arabinose, solvents, and xylitol is a need of hour and is futuristic approach toward achieving cleaner production and consumption.

Optimization of n-butanol synthesis in Lactobacillus brevis via the functional expression of thl, hbd, crt and ter

Abstract

N-butanol is an important chemical and can be naturally synthesized by Clostridium species; however, the poor n-butanol tolerance of Clostridium impedes the further improvement in titer. In this study, Lactobacillus brevis, which possesses a higher butanol tolerance, was selected as host for heterologous butanol production. The Clostridium acetobutylicum genes thl, hbd, and crt which encode thiolase, β-hydroxybutyryl-CoA dehydrogenase, and crotonase, and the Treponema denticola gene ter, which encodes trans-enoyl-CoA reductase were cloned into a single plasmid to express the butanol synthesis pathway in L. brevis. A titer of 40 mg/L n-butanol was initially achieved with plasmid pLY15-opt, in which all pathway genes are codon-optimized. A titer of 450 mg/L of n-butanol was then synthesized when ter was further overexpressed in this pathway. The role of metabolic flux was reinforced with pLY15, in which only the ter gene was codon-optimized, which greatly increased the n-butanol titer to 817 mg/L. Our strategy significantly improved n-butanol synthesis in L. brevis and the final titer is the highest achieved amongst butanol-tolerant lactic acid bacteria.

Graphic abstract

How to outwit nature: Omics insight into butanol tolerance

The energy crisis, depletion of oil reserves, and global climate changes are pressing problems of developed societies. One possibility to counteract that is microbial production of butanol, a promising new fuel and alternative to many petrochemical reagents. However, the high butanol toxicity to all known microbial species is the main obstacle to its industrial implementation. The present state of the art review aims to expound the recent advances in modern omics approaches to resolving this insurmountable to date problem of low butanol tolerance. Genomics, transcriptomics, and proteomics show that butanol tolerance is a complex phenomenon affecting multiple genes and their expression. Efflux pumps, stress and multidrug response, membrane transport, and redox-related genes are indicated as being most important during butanol challenge, in addition to fine-tuning of global regulators of transcription (Spo0A, GntR), which may further improve tolerance. Lipidomics shows that the alterations in membrane composition (saturated lipids and plasmalogen increase) are very much species-specific and butanol-related. Glycomics discloses the pleiotropic effect of CcpA, the role of alternative sugar transport, and the production of exopolysaccharides as alternative routes to overcoming butanol stress. Unfortunately, the strain that simultaneously syntheses and tolerates butanol in concentrations that allow its commercialization has not yet been discovered or produced. Omics insight will allow the purposeful increase of butanol tolerance in natural and engineered producers and the effective heterologous expression of synthetic butanol pathways in strains hereditary butanol-resistant up to 3.2 - 4.9% (w/v). Future breakthrough can be achieved by a detailed study of the membrane proteome, of which 21% are proteins with unknown functions.

A Novel Butanol Tolerance-Promoting Function of the Transcription Factor Rob in Escherichia coli

Producing high concentrations of biobutanol is challenging, primarily because of the toxicity of butanol toward cells. In our previous study, several butanol tolerance-promoting genes were identified from butanol-tolerant Escherichia coli mutants and inactivation of the transcriptional regulator factor Rob was shown to improve butanol tolerance. Here, the butanol tolerance characteristics and mechanism regulated by inactivated Rob are investigated. Comparative transcriptome analysis of strain DTrob, with a truncated rob in the genome, and the control BW25113 revealed 285 differentially expressed genes (DEGs) to be associated with butanol tolerance and categorized as having transport, localization, and oxidoreductase activities. Expression of 25 DEGs representing different functional categories was analyzed by quantitative reverse transcription PCR (qRT-PCR) to assess the reliability of the RNA-Seq data, and 92% of the genes showed the same expression trend. Based on functional complementation experiments of key DEGs, deletions of glgS and yibT increased the butanol tolerance of E. coli, whereas overexpression of fadB resulted in increased cell density and a slight increase in butanol tolerance. A metabolic network analysis of these DEGs revealed that six genes (fadA, fadB, fadD, fadL, poxB, and acs) associated with acetyl-CoA production were significantly upregulated in DTrob, suggesting that Rob inactivation might enhance butanol tolerance by increasing acetyl-CoA. Interestingly, DTrob produced more acetate in response to butanol stress than the wild-type strain, resulting in the upregulation expression of some genes involved in acetate metabolism. Altogether, the results of this study reveal the mechanism underlying increased butanol tolerance in E. coli regulated by Rob inactivation.

Milling byproducts are an economically viable substrate for butanol production using clostridial ABE fermentation

Butanol is a platform chemical that is utilized in a wide range of industrial products and is considered a suitable replacement or additive to liquid fuels. So far, it is mainly produced through petrochemical routes. Alternative production routes, for example through biorefinery, are under investigation but are currently not at a market competitive level. Possible alternatives, such as acetone-butanol-ethanol (ABE) fermentation by solventogenic clostridia are not market-ready to this day either, because of their low butanol titer and the high costs of feedstocks. Here, we analyzed wheat middlings and wheat red dog, two wheat milling byproducts available in large quantities, as substrates for clostridial ABE fermentation. We could identify ten strains that exhibited good butanol yields on wheat red dog. Two of the best ABE producing strains, Clostridium beijerinckii NCIMB 8052 and Clostridium diolis DSM 15410, were used to optimize a laboratory-scale fermentation process. In addition, enzymatic pretreatment of both milling byproducts significantly enhanced ABE production rates of the strains C. beijerinckii NCIMB 8052 and C. diolis DSM 15410. Finally, a profitability analysis was performed for small- to mid-scale ABE fermentation plants that utilize enzymatically pretreated wheat red dog as substrate. The estimations show that such a plant could be commercially successful.Key points• Wheat milling byproducts are suitable substrates for clostridial ABE fermentation.• Enzymatic pretreatment of wheat red dog and middlings increases ABE yield.• ABE fermentation plants using wheat red dog as substrate are economically viable. Graphical abstract.

Metabolic and Process Engineering of Clostridium beijerinckii for Butyl Acetate Production in One Step

n-Butyl acetate is an important food additive commonly produced via concentrated sulfuric acid catalysis or immobilized lipase catalysis of butanol and acetic acid. Compared with chemical methods, an enzymatic approach is more environmentally friendly; however, it incurs a higher cost due to lipase production. In vivo biosynthesis via metabolic engineering offers an alternative to produce n-butyl acetate. This alternative combines substrate production (butanol and acetyl-coenzyme A (acetyl-CoA)), alcohol acyltransferase expression, and esterification reaction in one reactor. The alcohol acyltransferase gene ATF1 from Saccharomyces cerevisiae was introduced into Clostridium beijerinckii NCIMB 8052, enabling it to directly produce n-butyl acetate from glucose without lipase addition. Extractants were compared and adapted to realize glucose fermentation with in situ n-butyl acetate extraction. Finally, 5.57 g/L of butyl acetate was produced from 38.2 g/L of glucose within 48 h, which is 665-fold higher than that reported previously. This demonstrated the potential of such a metabolic approach to produce n-butyl acetate from biomass.

Genetic engineering of non-native hosts for 1-butanol production and its challenges: a review

BACKGROUND

Owing to the increase in energy consumption, fossil fuel resources are gradually depleting which has led to the growing environmental concerns; therefore, scientists are being urged to produce sustainable and ecofriendly fuels. Thus, there is a growing interest in the generation of biofuels from renewable energy resources using microbial fermentation.

MAIN TEXT

Butanol is a promising biofuel that can substitute for gasoline; unfortunately, natural microorganisms pose challenges for the economical production of 1-butanol at an industrial scale. The availability of genetic and molecular tools to engineer existing native pathways or create synthetic pathways have made non-native hosts a good choice for the production of 1-butanol from renewable resources. Non-native hosts have several distinct advantages, including using of cost-efficient feedstock, solvent tolerant and reduction of contamination risk. Therefore, engineering non-native hosts to produce biofuels is a promising approach towards achieving sustainability. This paper reviews the currently employed strategies and synthetic biology approaches used to produce 1-butanol in non-native hosts over the past few years. In addition, current challenges faced in using non-native hosts and the possible solutions that can help improve 1-butanol production are also discussed.

CONCLUSION

Non-native organisms have the potential to realize commercial production of 1- butanol from renewable resources. Future research should focus on substrate utilization, cofactor imbalance, and promoter selection to boost 1-butanol production in non-native hosts. Moreover, the application of robust genetic engineering approaches is required for metabolic engineering of microorganisms to make them industrially feasible for 1-butanol production.

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.

Consolidated bioprocessing for butanol production of cellulolytic Clostridia: development and optimization

Butanol is an important bulk chemical, as well as a promising renewable gasoline substitute, that is commonly produced by solventogenic Clostridia. The main cost of cellulosic butanol fermentation is caused by cellulases that are required to saccharify lignocellulose, since solventogenic Clostridia cannot efficiently secrete cellulases. However, cellulolytic Clostridia can natively degrade lignocellulose and produce ethanol, acetate, butyrate and even butanol. Therefore, cellulolytic Clostridia offer an alternative to develop consolidated bioprocessing (CBP), which combines cellulase production, lignocellulose hydrolysis and co-fermentation of hexose/pentose into butanol in one step. This review focuses on CBP advances for butanol production of cellulolytic Clostridia and various synthetic biotechnologies that drive these advances. Moreover, the efforts to optimize the CBP-enabling cellulolytic Clostridia chassis are also discussed. These include the development of genetic tools, pentose metabolic engineering and the improvement of butanol tolerance. Designer cellulolytic Clostridia or consortium provide a promising approach and resource to accelerate future CBP for butanol production.

High butanol production from glycerol by using Clostridium sp. strain CT7 integrated with membrane assisted pervaporation

In this study, a unique butanol-ethanol fermentation process from glycerol integrated with pervaporation by using Clostridium sp. strain CT7 was investigated. 20.4 g/L of butanol and 0.3 g/L of ethanol were produced from 51.3 g/L of glycerol in the PV coupled batch fermentation process with butanol productivity of 0.15 g/L/h and yield of 0.40 g/g due to the reduced butanol inhibition by butanol removal. Subsequently, 41.9 g/L of butanol and 0.4 g/L of ethanol were obtained from 103.3 g/L of glycerol, with butanol productivity of 0.21 g/L/h and yield of 0.41 g/g in the PV coupled fed-batch fermentation process. The high butanol production could be attributed to the thin PDMS layer and negligible transportation resistance of the support. These results indicated the PV coupled fermentation process from glycerol using PDMS/ceramic composite membrane by Clostridium sp. strain CT7 might show a great potential for sustainable biobutanol production from low-cost carbon sources.

EMUlator: An Elementary Metabolite Unit (EMU) Based Isotope Simulator Enabled by Adjacency Matrix

Stable isotope based metabolic flux analysis is currently the unique methodology that allows the experimental study of the integrated responses of metabolic networks. This method primarily relies on isotope labeling and modeling, which could be a challenge in both experimental and computational biology. In particular, the algorithm implementation for isotope simulation is a critical step, limiting extensive usage of this powerful approach. Here, we introduce EMUlator a Python-based isotope simulator which is developed on Elementary Metabolite Unit (EMU) algorithm, an efficient and powerful algorithm for isotope modeling. We propose a novel adjacency matrix method to implement EMU modeling and exemplify it stepwise. This method is intuitively straightforward and can be conveniently mastered for various customized purposes. We apply this arithmetic pipeline to understand the phosphoketolase flux in the metabolic network of an industrial microbe Clostridium acetobutylicum. The resulting design enables a high-throughput and non-invasive approach for estimating phosphoketolase flux in vivo. Our computational insights allow the systematic design and prediction of isotope-based metabolic models and yield a comprehensive understanding of their limitations and potentials.

The draft genome sequence of Clostridium sp. strain LJ4 with high furan and phenolic derivates' tolerances occurring from lignocellulosic hydrolysates

The genome of a wild-type solventogenic Clostridium sp. strain LJ4 that could directly convert undetoxified lignocellulosic hydrolysate to butanol and tolerate high concentration of furan and phenolic derivates occurring in the lignocellulosic hydrolysate is described. 16S rDNA gene sequencing and analysis indicated that it is closely related to Clostridium acetobutylicum. The genome size of strain LJ4 is 3.90 Mp, which has a G + C content of 30.72% and encodes 2711 proteins. It also has one 0.19 Mp plasmid with 181 predicted encoding proteins. Alcohol dehydrogenases (ADs) and a nicotinamide adenine dinucleotide phosphate (NADPH)-dependent flavin mononucleotide (FMN) reductase were identified, which may play key roles in inhibitors' resistance in lignocellulosic hydrolysate.

Alginate Adsorbent Immobilization Technique Promotes Biobutanol Production by Clostridium acetobutylicum Under Extreme Condition of High Concentration of Organic Solvent

In Acetone-Butanol-Ethanol fermentation, bacteria should tolerate high concentrations of solvent products, which inhibit bacteria growth and limit further increase of solvents to more than 20 g/L. Moreover, this limited solvent concentration significantly increases the cost of solvent separation through traditional approaches. In this study, alginate adsorbent immobilization technique was successfully developed to assist in situ extraction using octanol which is effective in extracting butanol but presents strong toxic effect to bacteria. The adsorbent improved solvent tolerance of Clostridium acetobutylicum under extreme condition of high concentration of organic solvent. Using the developed technique, more than 42% of added bacteria can be adsorbed to the adsorbent. Surface area of the adsorbent was more than 10 times greater than sodium alginate. Scanning electron microscope image shows that an abundant amount of pore structure was successfully developed on adsorbents, promoting bacteria adsorption. In adsorbent assisted ABE fermentation, there was 21.64 g/L butanol in extracting layer compared to negligible butanol produced with only the extractant but without the adsorbent, for the reason that adsorbent can reduce damaging exposure of C. acetobutylicum to octanol. The strategy can improve total butanol production with respect to traditional culture approach by more than 2.5 fold and save energy for subsequent butanol recovery, which effects can potentially make the biobutanol production more economically practical.

The Draft Genome Sequence of a Novel High-Efficient Butanol-Producing Bacterium Clostridium Diolis Strain WST

A wild-type solventogenic strain Clostridium diolis WST, isolated from mangrove sediments, was characterized to produce high amount of butanol and acetone with negligible level of ethanol and acids from glucose via a unique acetone-butanol (AB) fermentation pathway. Through the genomic sequencing, the assembled draft genome of strain WST is calculated to be 5.85 Mb with a GC content of 29.69% and contains 5263 genes that contribute to the annotation of 5049 protein-coding sequences. Within these annotated genes, the butanol dehydrogenase gene (bdh) was determined to be in a higher amount from strain WST compared to other Clostridial strains, which is positively related to its high-efficient production of butanol. Therefore, we present a draft genome sequence analysis of strain WST in this article that should facilitate to further understand the solventogenic mechanism of this special microorganism.

Unique genetic cassettes in a Thermoanaerobacterium contribute to simultaneous conversion of cellulose and monosugars into butanol

The demand for cellulosic biofuels is on the rise because of the anticipation for sustainable energy and less greenhouse gas emissions in the future. However, production of cellulosic biofuels, especially cellulosic butanol, has been hampered by the lack of potent microbes that are capable of converting cellulosic biomass into biofuels. We report a wild-type Thermoanaerobacterium thermosaccharolyticum strain TG57, which is capable of using microcrystalline cellulose directly to produce butanol (1.93 g/liter) as the only final product (without any acetone or ethanol produced), comparable to that of engineered microbes thus far. Strain TG57 exhibits significant advances including unique genes responsible for a new butyrate synthesis pathway, no carbon catabolite repression, and the absence of genes responsible for acetone synthesis (which is observed as the main by-product in most Clostridium strains known today). Furthermore, the use of glucose analog 2-deoxyglucose posed a selection pressure to facilitate isolation of strain TG57 with deletion/silencing of carbon catabolite repressor genes-the ccr and xylR genes-and thus is able to simultaneously ferment glucose, xylose, and arabinose to produce butanol (7.33 g/liter) as the sole solvent. Combined analysis of genomic and transcriptomic data revealed unusual aspects of genome organization, numerous determinants for unique bioconversions, regulation of central metabolic pathways, and distinct transcriptomic profiles. This study provides a genome-level understanding of how cellulose is metabolized by T. thermosaccharolyticum and sheds light on the potential of competitive and sustainable biofuel production.

Exploitation of novel wild type solventogenic strains for butanol production

Butanol has been regarded as an important bulk chemical and advanced biofuel; however, large scaling butanol production by solventogenic Clostridium sp. is still not economically feasible due to the high cost of substrates, low butanol titer and yield caused by the toxicity of butanol and formation of by-products. Renewed interests in biobutanol as biofuel and rapid development in genetic tools have spurred technological advances to strain modifications. Comprehensive reviews regarding these aspects have been reported elsewhere in detail. Meanwhile, more wild type butanol producers with unique properties were also isolated and characterized. However, few reviews addressed these discoveries of novel wild type solventogenic Clostridium sp. strains. Accordingly, this review aims to comprehensively summarize the most recent advances on wild type butanol producers in terms of fermentation patterns, substrate utilization et al. Future perspectives using these native ones as chassis for genetic modification were also discussed.

Enhanced isopropanol-butanol-ethanol mixture production through manipulation of intracellular NAD(P)H level in the recombinant Clostridium acetobutylicum XY16

BACKGROUND

The formation of by-products, mainly acetone in acetone-butanol-ethanol (ABE) fermentation, significantly affects the solvent yield and downstream separation process. In this study, we genetically engineered Clostridium acetobutylicum XY16 isolated by our lab to eliminate acetone production and altered ABE to isopropanol-butanol-ethanol (IBE). Meanwhile, process optimization under pH control strategies and supplementation of calcium carbonate were adopted to investigate the interaction between the reducing force of the metabolic networks and IBE production.

RESULTS

After successful introduction of secondary alcohol dehydrogenase into C. acetobutylicum XY16, the recombinant XY16 harboring pSADH could completely eliminate acetone production and convert it into isopropanol, indicating great potential for large-scale production of IBE mixtures. Especially, pH could significantly improve final solvent titer through regulation of NADH and NADPH levels in vivo. Under the optimal pH level of 4.8, the total IBE production was significantly increased from 3.88 to 16.09 g/L with final 9.97, 4.98 and 1.14 g/L of butanol, isopropanol, and ethanol. Meanwhile, NADH and NADPH levels were maintained at optimal levels for IBE formation compared to the control one without pH adjustment. Furthermore, calcium carbonate could play dual roles as both buffering agency and activator for NAD kinase (NADK), and supplementation of 10 g/L calcium carbonate could finally improve the IBE production to 17.77 g/L with 10.51, 6.02, and 1.24 g/L of butanol, isopropanol, and ethanol.

CONCLUSION

The complete conversion of acetone into isopropanol in the recombinant C. acetobutylicum XY16 harboring pSADH could alter ABE to IBE. pH control strategies and supplementation of calcium carbonate were effective in obtaining high IBE titer with high isopropanol production. The analysis of redox cofactor perturbation indicates that the availability of NAD(P)H is the main driving force for the improvement of IBE production.

Characterization and genome analysis of a butanol-isopropanol-producing Clostridium beijerinckii strain BGS1

BACKGROUND

One of the main challenges of acetone-butanol-ethanol fermentation is to reduce acetone production with high butanol yield. Converting acetone into isopropanol is an alternative pathway to reduce fermentation by-products in the fermentation broth. Here, we aimed to cultivate a wild-type Clostridium strain with high isopropanol and butanol production and reveal its genome information.

RESULTS

Clostridium beijerinckii strain BGS1 was found to be capable of producing 10.21 g/L butanol and 3.41 g/L isopropanol, higher than previously known wild-type isopropanol-butanol-producing Clostridium species. Moreover, culture BGS1 exhibited a broad carbon spectrum utilizing diverse sugars such as arabinose, xylose, galactose, cellobiose, and sucrose, with 9.61 g/L butanol and 2.57 g/L isopropanol generated from 60 g/L sucrose and less amount from other sugars. Based on genome analysis, protein-based sequence of strain BGS1 was closer to C. beijerinckii NCIMB 8052, reaching 90.82% similarity, while compared to C. beijerinckii DSM 6423, the similarity was 89.53%. In addition, a unique secondary alcohol dehydrogenase (sAdhE) was revealed in the genome of strain BGS1, which distinguished it from other Clostridium species. Average nucleotide identity analysis identified strain BGS1 belonging to C. beijerinckii. The transcription profile and enzymatic activity of sAdhE proved its function of converting acetone into isopropanol.

CONCLUSIONS

Clostridium beijerinckii strain BGS1 is a potential candidate for industrial isopropanol and butanol production. Its genome provides unique information for genetic engineering of isopropanol-butanol-producing microorganisms.

Enhanced biobutanol production with high yield from crude glycerol by acetone uncoupled Clostridium sp. strain CT7

This study reports a unique acetone uncoupled Clostridium species strain CT7, which shows efficient capability of glycerol utilization with high butanol ratio. Medium compositions, such as substrate concentration, micronutrients and pH show significant effects on butanol production from glycerol by strain CT7. To further maximize butanol production, fermentation conditions were optimized by using response surface methodology (RSM). Final butanol production of 16.6g/L with yield of 0.43g/g consumed glycerol was obtained, representing the highest butanol production and yield from glycerol in the batch fermentation mode. Furthermore, strain CT7 could directly convert crude glycerol to 11.8g/L of butanol without any pretreatment. Hence, strain CT7 shows immense potential for biofuels production using waste glycerol as cheap substrate.

The Draft Genome Sequence of Clostridium beijerinckii NJP7, a Unique Bacterium Capable of Producing Isopropanol-Butanol from Hemicellulose Through Consolidated Bioprocessing

A wild type solventogenic Clostridium beijerinckii NJP7 capable of converting polysaccharides, such as hemicellulose, into butanol and isopropanol via a unique acetone-isopropanol-butanol (AIB) pathway was isolated and characterized. This represents the first wild type isopropanol-butanol generating bacterium which could achieve butanol production directly from lignocellulose through consolidated bioprocessing (CBP). Strain NJP7 was isolated from decomposite soil from Laoshan Nature Park, China, and its genome shows 98.6% identical to 89.5% of the Clostridium diolis submitted genome sequence. The assembled draft genome contains 5.76 Mb and 5101 predicted encoding proteins with a GC content of 29.73%. Among these annotated proteins, hemicellulase and the secondary alcohol dehydrogenase play key roles in achievement of AIB production from hemicellulose through CBP.

Recent advances on conversion and co-production of acetone-butanol-ethanol into high value-added bioproducts

Butanol is an important bulk chemical and has been regarded as an advanced biofuel. Large-scale production of butanol has been applied for more than 100 years, but its production through acetone-butanol-ethanol (ABE) fermentation process by solventogenic Clostridium species is still not economically viable due to the low butanol titer and yield caused by the toxicity of butanol and a by-product, such as acetone. Renewed interest in biobutanol as a biofuel has spurred technological advances to strain modification and fermentation process design. Especially, with the development of interdisciplinary processes, the sole product or even the mixture of ABE produced through ABE fermentation process can be further used as platform chemicals for high value added product production through enzymatic or chemical catalysis. This review aims to comprehensively summarize the most recent advances on the conversion of acetone, butanol and ABE mixture into various products, such as isopropanol, butyl-butyrate and higher-molecular mass alkanes. Additionally, co-production of other value added products with ABE was also discussed.

Strategies for improved isopropanol-butanol production by a Clostridium strain from glucose and hemicellulose through consolidated bioprocessing

BACKGROUND

High cost of traditional substrates and formation of by-products (such as acetone and ethanol) in acetone-butanol-ethanol (ABE) fermentation hindered the large-scale production of biobutanol. Here, we comprehensively characterized a newly isolated solventogenic and xylanolytic Clostridium species, which could produce butanol at a high ratio with elimination of ethanol and conversion of acetone to more value-added product, isopropanol. Ultimately, direct butanol production from hemicellulose was achieved with efficient expression of indigenous xylanase by the novel strain via consolidated bioprocessing.

RESULTS

A novel wild-type Clostridium sp. strain NJP7 was isolated and characterized in this study, which was capable of fermenting monosaccharides, e.g., glucose into butanol via a fermentative acetone-isopropanol-butanol pathway. With enhancement of buffering capacity and alcohol dehydrogenase activities, butanol and isopropanol titer by Clostridium sp. strain NJP7 was improved to 12.21 and 1.92 g/L, respectively, and solvent productivity could be enhanced to 0.44 g/L/h. Furthermore, with in situ extraction with biodiesel, the amount of butanol and isopropanol was finally improved to 25.58 and 5.25 g/L in the fed-batch mode. Meanwhile, Clostridium sp. strain NJP7 shows capability of direct isopropanol-butanol production from hemicelluloses with expression of indigenous xylanase. 2.06 g/L of butanol and 0.54 g/L of isopropanol were finally achieved through the temperature-shift simultaneous saccharification and fermentation, representing the highest butanol production directly from hemicellulose.

CONCLUSION

The co-production of isopropanol with butanol by the newly isolated Clostridium sp. strain NJP7 would add on the economical values for butanol fermentation. Furthermore, the high isopropanol-butanol production with in situ extraction would also greatly enhance the economic feasibility for fermentative production of butanol-isopropanol in large scale. Meanwhile, its direct production of butanol-isopropanol from polysaccharides, hemicellulose through secretion of indigenous thermostable xylanase, shows great potential using lignocellulosic wastes for biofuel production.

Advances in industrial microbiome based on microbial consortium for biorefinery

One of the important targets of industrial biotechnology is using cheap biomass resources. The traditional strategy is microbial fermentations with single strain. However, cheap biomass normally contains so complex compositions and impurities that it is very difficult for single microorganism to utilize availably. In order to completely utilize the substrates and produce multiple products in one process, industrial microbiome based on microbial consortium draws more and more attention. In this review, we first briefly described some examples of existing industrial bioprocesses involving microbial consortia. Comparison of 1,3-propanediol production by mixed and pure cultures were then introduced, and interaction relationships between cells in microbial consortium were summarized. Finally, the outlook on how to design and apply microbial consortium in the future was also proposed.

Simultaneous saccharification and fermentation of hemicellulose to butanol by a non-sporulating Clostridium species

Production of lignocellulosic butanol has drawn increasing attention. However, currently few microorganisms can produce biofuels, particularly butanol, from lignocellulosic biomass via simultaneous saccharification and fermentation. Here we report discovery of a wild-type, mesophilic Clostridium sp. strain MF28 that ferments xylan to produce butanol (up to 3.2g/L) without the addition of saccharolytic enzymes and without any chemical pretreatments. Application of selective pressure from 2-deoxy-d-glucose facilitated isolation of strain MF28, which exhibits inactivation of genes (gid and ccp genes) responsible for carbon catabolite repression, thus allowing strain MF28 to simultaneously ferment a combination of glucose (30g/L), xylose (15g/L), and arabinose (15g/L) to produce 11.9g/L of butanol. Strain MF28 possesses several unique features: (i) non-sporulating, (ii) no acetone/ethanol, (iii) complete hemicellulose-binding enzymatic domain, and (iv) absence of carbon catabolite repression. These unique characteristics demonstrate the industrial potential of strain MF28 for cost-effective biofuel generation from lignocellulosic biomass.

Enhanced production of butanol and acetoin by heterologous expression of an acetolactate decarboxylase in Clostridium acetobutylicum

Butanol is an important industrial chemical and an attractive transportation fuel. However, the deficiency of reducing equivalents NAD(P)H in butanol fermentation results in a large quantity of oxidation products, which is a major problem limiting the atom economy and economic viability of bio-butanol processes. Here, we integrated the butanol fermentation process with a NADH-generating, acetoin biosynthesis process to improve the butanol production. By overexpressing the α-acetolactate decarboxylase gene alsD from Bacillus subtilis in Clostridium acetobutylicum, acetoin yield was significantly increased at the cost of acetone. After optimization of fermentation conditions, butanol (12.9g/L), acetoin (6.5g/L), and ethanol (1.9g/L) were generated by the recombinant strain, with acetone no more than 1.8g/L. Thus, both mass yield and product value were greatly improved. This study demonstrates that reducing power compensation is effective to improve the atom economy of butanol fermentation, and provides a novel approach to improve the economic viability of bio-butanol production.

Synergistic effect of calcium and zinc on glucose/xylose utilization and butanol tolerance of Clostridium acetobutylicum

Biobutanol outperforms bioethanol as an advanced biofuel, but is not economically competitive in terms of its titer, yield and productivity associated with feedstocks and energy cost. In this work, the synergistic effect of calcium and zinc was investigated in the acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum using glucose, xylose and glucose/xylose mixtures as carbon source(s). Significant improvements associated with enhanced glucose/xylose utilization, cell growth, acids re-assimilation and butanol biosynthesis were achieved. Especially, the maximum butanol and ABE production of 16.1 and 25.9 g L(-1) were achieved from 69.3 g L(-1) glucose with butanol/ABE productivities of 0.40 and 0.65 g L(-1) h(-1) compared to those of 11.7 and 19.4 g/L with 0.18 and 0.30 g L(-1) h(-1) obtained in the control respectively without any supplement. More importantly, zinc was significantly involved in the butanol tolerance based on the improved xylose utilization under various butanol-shock conditions (2, 4, 6, 8 and 10 g L(-1) butanol). Under the same conditions, calcium and zinc co-supplementation led to the best xylose utilization and butanol production. These results suggested that calcium and zinc could play synergistic roles improving ABE fermentation by C. acetobutylicum.

Comprehensive investigations of biobutanol production by a non-acetone and 1,3-propanediol generating Clostridium strain from glycerol and polysaccharides

BACKGROUND

Low-cost feedstocks, a single product (butanol), and a high butanol titer are three key points for establishing a sustainable and economically viable process for biological butanol production. Here, we comprehensively investigated the butanol production from mono-substrates, mainly glycerol and polysaccharides, mainly starch and xylan by a newly identified wild-type Clostridium pasteurianum GL11.

RESULTS

Strain GL11 produced 14.7 g/L of butanol with a yield of 0.41 g/g from glycerol in the batch mode without formation of by-products of acetone and 1,3-propanediol (1,3-PDO). With in situ extraction with biodiesel, the amount of butanol was finally improved to 28.8 g/L in the fed-batch mode. Genomic and enzymatic analysis showed that the deficiency of key enzymes involved in acetone and 1,3-PDO pathway within strain GL11 led to the elimination of these by-products, which may also greatly simplify downstream separation. The elimination of acetone and 1,3-PDO and high butanol tolerance contributed to its high butanol production yield from glycerol. More importantly, strain GL11 could directly convert polysaccharides, such as xylan and starch to butanol with secretion of xylanase and amylase via consolidated bioprocessing.

CONCLUSIONS

The wild-type strain GL11 was found to be particularly advantageous due to its capability of efficient butanol production from glycerol and polysaccharides with elimination of acetone and 1,3-PDO formation. And the high butanol production with in situ extraction by using biodiesel would significantly enhance the economic feasibility of fermentative production of butanol from glycerol. These unique features of C. pasteurianum GL11 open the door to the possibility of cost-effective biofuels production in large scale.