Fermentative production of butanol—the industrial perspective

Benzenoid Aromatics from Renewable Resources

In this Review, all known chemical methods for the conversion of renewable resources into benzenoid aromatics are summarized. The raw materials that were taken into consideration are CO2; lignocellulose and its constituents cellulose, hemicellulose, and lignin; carbohydrates, mostly glucose, fructose, and xylose; chitin; fats and oils; terpenes; and materials that are easily obtained via fermentation, such as biogas, bioethanol, acetone, and many more. There are roughly two directions. One much used method is catalytic fast pyrolysis carried out at high temperatures (between 300 and 700 °C depending on the raw material), which leads to the formation of biochar; gases, such as CO, CO2, H2, and CH4; and an oil which is a mixture of hydrocarbons, mostly aromatics. The carbon selectivities of this method can be reasonably high when defined small molecules such as methanol or hexane are used but are rather low when highly oxygenated compounds such as lignocellulose are used. The other direction is largely based on the multistep conversion of platform chemicals obtained from lignocellulose, cellulose, or sugars and a limited number of fats and terpenes. Much research has focused on furan compounds such as furfural, 5-hydroxymethylfurfural, and 5-chloromethylfurfural. The conversion of lignocellulose to xylene via 5-chloromethylfurfural and dimethylfuran has led to the construction of two large-scale plants, one of which has been operational since 2023.

The potential of native and engineered Clostridia for biomass biorefining

Since their first industrial application in the acetone-butanol-ethanol (ABE) fermentation in the early 1900s, Clostridia have found large application in biomass biorefining. Overall, their fermentation products include organic acids (e.g., acetate, butyrate, lactate), short chain alcohols (e.g., ethanol, n-butanol, isobutanol), diols (e.g., 1,2-propanediol, 1,3-propanediol) and H2 which have several applications such as fuels, building block chemicals, solvents, food and cosmetic additives. Advantageously, several clostridial strains are able to use cheap feedstocks such as lignocellulosic biomass, food waste, glycerol or C1-gases (CO2, CO) which confer them additional potential as key players for the development of processes less dependent from fossil fuels and with reduced greenhouse gas emissions. The present review aims to provide a survey of research progress aimed at developing Clostridium-mediated biomass fermentation processes, especially as regards strain improvement by metabolic engineering.

Study on the Effect of Secondary Injection on the Combustion and Emission of n-Butanol Engines

n-Butanol, as a biological alternative fuel containing oxygen, has similar physical and chemical properties to gasoline and has a wide range of sources, which has attracted more and more attention and research. Direct injection technology has been widely used in the field of internal combustion engine due to its advantages of flexibility and control ability. In this paper, the secondary injection of n-butanol engine under the mode of in-cylinder direct injection is discussed to organize stratified combustion of the mixture, optimize combustion to improve the thermal efficiency, and reduce emission. A four-cylinder four-stroke spark ignition (SI) engine was selected to carry out the secondary injection experiment of n-butanol under the excess air ratio (λ) of 1, an engine speed of 1500 r/min, and a low load, and the variables were the second injection ratio and timing. The results show that the secondary injection of n-butanol can achieve stratified combustion of the mixture, but only at a specific second injection timing such as 100°CA before compression top dead center (BTDC) or 125°CA BTDC, the combustion effect is the best. A small second injection ratio can optimize combustion, improve brake thermal efficiency, and reduce hydrocarbon and carbon monoxide emissions. When the second injection ratio is greater than 60%, it results in incomplete fuel combustion, a 3 to 4% reduction in thermal efficiency, and an increase in emissions. Coefficient of variation (COV) was increased by secondary injection, but the effect was insignificant in the small injection ratio, and it will increase with the increase of the second injection ratio. The change of particle number is mainly affected by the nuclear particle number, and with the increase of the second injection ratio, the total particulate number is more affected by the second injection timing. The second injection ratio of 40% can reduce the total particle number under the mixed-gas stratification condition.

Solvent-Producing Clostridia Revisited

The review provides an overview of the current status of the solvent-producing clostridia. The origin and development of industrial clostridial species, as well as the history of the industrial Acetone Butanol Ethanol fermentation process, is reexamined, and the recent resurgence of interest in the production of biobutanol is reviewed. Over 300 fully sequenced genomes for solvent-producing and closely related clostridial species are currently available in public databases. These include 270 genomes sourced from the David Jones culture collection. These genomes were allocated arbitrary DJ codes, and a conversion table to identify the species and strains has now been provided. The expanded genomic database facilitated new comparative genomic and phylogenetic analysis. A synopsis of the common features, molecular taxonomy, and phylogeny of solvent-producing clostridia and the application of comparative phylogenomics are evaluated. A survey and analysis of resident prophages in solvent-producing clostridia are discussed, and the discovery, occurrence, and role of novel R-type tailocins are reported. Prophage genomes with R-type tailocin-like features were detected in all 12 species investigated. The widespread occurrence of tailocins in Gram-negative species is well documented; this survey has indicated that they may also be widespread in clostridia.

Avoiding acid crash: From apple pomace hydrolysate to butanol through acetone-butanol-ethanol fermentation in a zero-waste approach

Apple pomace (AP) is a lignocellulosic residue from the juice and cider industries that can be valorized in a multi-product biorefinery to generate multiple value-added compounds, including biofuels such as butanol. Butanol is produced biologically by acetone-butanol-ethanol (ABE) fermentation using bacteria of the genus Clostridium from sugar-based feedstocks. In this study, AP hydrolysate was used as a substrate for producing butanol by ABE fermentation. Various environmental factors influence the amount of butanol produced, but only under certain conditions the so-called 'acid crash', an undesirable phenomenon characterized by a total arrest of cell growth and solvent production, can be avoided. Operational parameters that may influence the prevention of acid crash occurrence, such as pH, CaCO₃ concentration and culture temperature, were optimized in C. beijerinckii CECT 508 cultures applying a Box-Behnken experimental design. The mathematical model of the fermentation found the optimal conditions of pH 7, 6.8 g/L of CaCO₃ and 30 °C, and this was validated in an independent experiment carried out at the optimal conditions, reaching 10.75 g/L of butanol. Also, the comparison of butanol production between the supernatant of the AP hydrolysate (10.57 g/L) and the full hydrolysate with solids (11.69 g/L) indicated that it is possible to eliminate the centrifugation step after hydrolysis, which may allow to reduce process costs and the full utilization of apple pomace, aiming a zero-waste approach.

Using oils and fats to replace sugars as feedstocks for biomanufacturing: Challenges and opportunities for the yeast Yarrowia lipolytica

More than 200 million tons of plant oils and animal fats are produced annually worldwide from oil, crops, and the rendered animal fat industry. Triacylglycerol, an abundant energy-dense compound, is the major form of lipid in oils and fats. While oils or fats are very important raw materials and functional ingredients for food or related products, a significant portion is currently diverted to or recovered as waste. To significantly increase the value of waste oils or fats and expand their applications with a minimal environmental footprint, microbial biomanufacturing is presented as an effective strategy for adding value. Though both bacteria and yeast can be engineered to use oils or fats as the biomanufacturing feedstocks, the yeast Yarrowia lipolytica is presented as one of the most attractive platforms. Y. lipolytica is oleaginous, generally regarded as safe, demonstrated as a promising industrial producer, and has unique capabilities for efficient catabolism and bioconversion of lipid substrates. This review summarizes the major challenges and opportunities for Y. lipolytica as a new biomanufacturing platform for the production of value-added products from oils and fats. This review also discusses relevant cellular and metabolic engineering strategies such as fatty acid transport, fatty acid catabolism and bioconversion, redox balances and energy yield, cell morphology and stress response, and bioreaction engineering. Finally, this review highlights specific product classes including long-chain diacids, wax esters, terpenes, and carotenoids with unique synthesis opportunities from oils and fats in Y. lipolytica.

Recovery of Aconitic Acid from Sweet Sorghum Plant Extract Using a Solvent Mixture, and Its Potential Use as a Nematicide

Trans-aconitic acid (TAA) is naturally present in sweet sorghum juice and syrup, and it has been promoted as a potential biocontrol agent for nematodes. Therefore, we developed a process for the extraction of aconitic acid from sweet sorghum syrup. The process economics were evaluated, and the extract was tested for its capability to suppress the motility of the nematodes Caenorhabditis elegans and Meloidogyne incognita. Aconitic acid could be efficiently extracted from sweet sorghum syrup using acetone:butanol:ethanol mixtures, and it could be recovered from this solvent with a sodium carbonate solution, with an overall extraction and recovery efficiency of 86%. The estimated production cost was USD 16.64/kg of extract and this was highly dependent on the solvent cost, as the solvent was not recycled but was resold for recovery at a fraction of the cost. The extract was effective in reducing the motility of the parasitic M. incognita and causing over 78% mortality of the nematode when 2 mg/mL of TAA extract was added. However, this positive result could not conclusively be linked solely to TAA. An unidentified component (or components) in the acetone:butanol:ethanol sweet sorghum extract appears to be an effective nematode inhibitor, and it may merit further investigation. The impact of aconitic acid on C. elegans appeared to be entirely controlled by pH.

Structural and Biochemical Analyses of the Butanol Dehydrogenase from Fusobacterium nucleatum

Butanol dehydrogenase (BDH) plays a significant role in the biosynthesis of butanol in bacteria by catalyzing butanal conversion to butanol at the expense of the NAD(P)H cofactor. BDH is an attractive enzyme for industrial application in butanol production; however, its molecular function remains largely uncharacterized. In this study, we found that Fusobacterium nucleatum YqdH (FnYqdH) converts aldehyde into alcohol by utilizing NAD(P)H, with broad substrate specificity toward aldehydes but not alcohols. An in vitro metal ion substitution experiment showed that FnYqdH has higher enzyme activity in the presence of Co²⁺. Crystal structures of FnYqdH, in its apo and complexed forms (with NAD and Co²⁺), were determined at 1.98 and 2.72 Å resolution, respectively. The crystal structure of apo- and cofactor-binding states of FnYqdH showed an open conformation between the nucleotide binding and catalytic domain. Key residues involved in the catalytic and cofactor-binding sites of FnYqdH were identified by mutagenesis and microscale thermophoresis assays. The structural conformation and preferred optimal metal ion of FnYqdH differed from that of TmBDH (homolog protein of FnYqdH). Overall, we proposed an alternative model for putative proton relay in FnYqdH, thereby providing better insight into the molecular function of BDH.

Insights into the susceptibility of Pseudomonas putida to industrially relevant aromatic hydrocarbons that it can synthesize from sugars

Pseudomonas putida DOT-T1E is a highly solvent tolerant strain for which many genetic tools have been developed. The strain represents a promising candidate host for the synthesis of aromatic compounds-opening a path towards a green alternative to petrol-derived chemicals. We have engineered this strain to produce phenylalanine, which can then be used as a raw material for the synthesis of styrene via trans-cinnamic acid. To understand the response of this strain to the bioproducts of interest, we have analyzed the in-depth physiological and genetic response of the strain to these compounds. We found that in response to the exposure to the toxic compounds that the strain can produce, the cell launches a multifactorial response to enhance membrane impermeabilization. This process occurs via the activation of a cis to trans isomerase that converts cis unsaturated fatty acids to their corresponding trans isomers. In addition, the bacterial cells initiate a stress response program that involves the synthesis of a number of chaperones and ROS removing enzymes, such as peroxidases and superoxide dismutases. The strain also responds by enhancing the metabolism of glucose through the specific induction of the glucose phosphorylative pathway, Entner-Doudoroff enzymes, Krebs cycle enzymes and Nuo. In step with these changes, the cells induce two efflux pumps to extrude the toxic chemicals. Through analyzing a wide collection of efflux pump mutants, we found that the most relevant pump is TtgGHI, which is controlled by the TtgV regulator.

Effective Production of Selected Dioxolanes by Sequential Bio- and Chemocatalysis Enabled by Adapted Solvent Switching

Most combinations of chemo- and biocatalysis take place in aqueous media or require a solvent change with complex intermediate processing. Using enzymes in the same organic solvent as the chemocatalyst eliminates this need. Here, it was shown that a complete chemoenzymatic cascade to form dioxolanes could be carried out in a purely organic environment. The result, including downstream processing, was compared with a classical mode, shifting solvent. First, a two-step enzyme cascade starting from aliphatic aldehydes to chiral diols (3,4-hexanediol and 4,5-octanediol) was run either in an aqueous buffer or in the potentially biobased solvent cyclopentyl methyl ether. Subsequently, a ruthenium molecular catalyst enabled the conversion to dioxolanes [e. g., (4S,5S)-dipropyl-1,3-dioxolane]. Importantly, the total synthesis of this product was not only highly stereoselective but also based on the combination of biomass, CO₂ , and hydrogen, thus providing an important example of a bio-hybrid chemical.

Guerbet upgrading of ethanol to n-butanol using Ru(iii) catalysts under air

A series of in situ prepared Ru(iii) complexes supported by easily accessible N-donor organic pincer ligands were used as catalysts in the Guerbet upgrading reaction of ethanol under aerobic conditions.

PEBA/PDMS Composite Multilayer Hollow Fiber Membranes for the Selective Separation of Butanol by Pervaporation

The growing interest in the production of biofuels has motivated numerous studies on separation techniques that allow the separation/concentration of organics produced by fermentation, improving productivity and performance. In this work, the preparation and characterization of new butanol-selective membranes was reported. The prepared membranes had a hollow fiber configuration and consisted of two dense selective layers: a first layer of PEBA and a second (outer) layer of PDMS. The membranes were tested to evaluate their separation performance in the selective removal of organics from a synthetic ABE solution. Membranes with various thicknesses were prepared in order to evaluate the effect of the PDMS protective layer on permeant fluxes and membrane selectivity. The mass transport phenomena in the pervaporation process were characterized using a resistances-in-series model. The experimental results showed that PEBA as the material of the dense separating layer is the most favorable in terms of selectivity towards butanol with respect to the other components of the feed stream. The addition of a protective layer of PDMS allows the sealing of possible pinholes; however, its thickness should be kept as small as possible since permeation fluxes decrease with increasing thickness of PDMS and this material also has greater selectivity towards acetone compared to other feed components.

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.

Addressing the energy crisis: using microbes to make biofuels

Much of the energy being used to power our lives comes from fossil fuels such as coal, natural gas and petroleum. These energy sources are non-renewable, are being exhausted and also pollute the air, water and soil with toxic chemicals. Their mining, transportation, refining and use are associated with a large carbon footprint that contributes significantly to global warming. In addition, the geopolitical complexities surrounding the main fossil fuel producers create risks and uncertainties around the world. Replacing fossil fuels with clean, renewable forms of energy is paramount to creating a sustainable and healthy future, and for laying the foundations for global political stability and prosperity. Using biomass from plants, microbes can produce biofuels that are identical to or perform as well as fossil fuels. In addition of creating sustainable energy, advancing the biofuel industry will create new, high-quality rural jobs whilst improving energy security.

A paradigm shift towards production of sustainable bioenergy and advanced products from Cannabis/hemp biomass in Canada

The global cannabis (Cannabis sativa) market was 17.7 billion in 2019 and is expected to reach up to 40.6 billion by 2024. Canada is the 2nd nation to legalize cannabis with a massive sale of $246.9 million in the year 2021. Waste cannabis biomass is managed using disposal strategies (i.e., incineration, aerobic/anaerobic digestion, composting, and shredding) that are not good enough for long-term environmental sustainability. On the other hand, greenhouse gas emissions and the rising demand for petroleum-based fuels pose a severe threat to the environment and the circular economy. Cannabis biomass can be used as a feedstock to produce various biofuels and biochemicals. Various research groups have reported production of ethanol 9.2-20.2 g/L, hydrogen 13.5 mmol/L, lipids 53.3%, biogas 12%, and biochar 34.6% from cannabis biomass. This review summarizes its legal and market status (production and consumption), the recent advancements in the lignocellulosic biomass (LCB) pre-treatment (deep eutectic solvents (DES), and ionic liquids (ILs) known as "green solvents") followed by enzymatic hydrolysis using glycosyl hydrolases (GHs) for the efficient conversion efficiency of pre-treated biomass. Recent advances in the bioconversion of hemp into oleochemicals, their challenges, and future perspectives are outlined. A comprehensive insight is provided on the trends and developments of metabolic engineering strategies to improve product yield. The thermochemical processing of disposed-off hemp lignin into bio-oil, bio-char, synthesis gas, and phenol is also discussed. Despite some progress, barricades still need to be met to commercialize advanced biofuels and compete with traditional fuels.

Potentials of bio-butanol conversion to valuable products

In the last decade, there was observed a growing demand for both n-butanol as a potential fuel or fuel additive, and propylene as the only raw material for production of alcohol and other more bulky propylene chemical derivatives with faster growing outputs (polymers, propylene oxide, and acrylic acid). The predictable oilfield depletion and the European Green Deal adoption stimulated interest in alternative processes for n-butanol production, especially those involving bio-based materials. Their commercialization will promote additional market penetration of n-butanol for its application as a basic chemical. We analyze briefly the current status of two most advanced bio-based processes, i.e. ethanol–to-n-butanol and acetone–butanol–ethanol (ABE) fermentation. In the second part of the review, studies of n-butanol and ABE conversion to valuable products are considered with an emphasis on the most perspective catalytic systems and variants of the future processes realization.

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.

State of art of valorising of diverse potential feedstocks for the production of alcohols and ethers: Current changes and perspectives

Alcohols could be the biggest factor for the improvement of world biofuel economy in the present century due to their excellent properties compared to petroleum products. The primary concerns of sustainable alcohol production for meeting the growing energy demand owing to the selection of viable feedstock and this might enhance the opportunities for developing numerous advanced techniques. In this review, the valorization of alcohol production from several production routes has been exposed by covering the traditional routes to the present state of the art technologies. Even though the fossil fuel conversion could be dominant method for methanol production, many recent innovations like photo electrochemical synthesis and electrolysis methods might play vital role in production of renewable methanol in future. There have been several production routes for production of ethanol and among which the fermentation of lignocellulose biomass would be the ultimate choice for large scale shoot up. The greenhouse gas recovery in the form of alcohols through electrochemistry technique and hydrogenation method are the important methods for commercialization of alcohols in future. It is also observed that algae based renewable bio-alcohols is highly influenced by carbohydrate content and sustainable approaches in algae conversion to bio-alcohols would bring greater demand in future market. There is a lack of innovation in higher alcohols production in single process and this could be bounded by combining dehydrogenation and decarboxylation techniques. Finally, this review enlists the opportunities and challenges of existing alcohols production and recommended the possible routes for making significant enhancement in production.

Deletion of glyceraldehyde-3-phosphate dehydrogenase (gapN) in Clostridium saccharoperbutylacetonicum N1-4(HMT) using CLEAVE™ increases the ATP pool and accelerates solvent production

The development and advent of mutagenesis tools for solventogenic clostridial species in recent years has allowed for the increased refinement of industrially relevant strains. In this study we have utilised CLEAVE™, a CRISPR/Cas genome editing system developed by Green Biologics Ltd., to engineer a strain of Clostridium saccharoperbutylacetonicum N1-4(HMT) with potentially useful solvents titres and energy metabolism. As one of two enzymes responsible for the conversion of glyceraldehyde-3-phosphate (GAP) to 3-phosphoglyceric acid in glycolysis, it was hypothesised that deletion of gapN would increase ATP and NADH production that could in turn improve solvent production. Herein, whole genome sequencing has been used to evaluate CLEAVE™ and the successful knockout of gapN, demonstrating a clean knockout with no other detectable variations from the wild type sequence. Elevated solvent levels were detected during the first 24 h of batch fermentation, indicating an earlier shift to solventogenesis. A 2.4-fold increase in ATP concentration was observed, and quantitation of NAD(P)H derivatives revealed a more reducing cytoplasm for the gapN strain. These findings expand our understanding of clostridium carbon metabolism and report a new approach to optimising biofuel production.

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.

Recent progress on n-butanol production by lactic acid bacteria

n-Butanol is an essential chemical intermediate produced through microbial fermentation. However, its toxicity to microbial cells has limited its production to a great extent. The anaerobe lactic acid bacteria (LAB) are the most resistant to n-butanol, so it should be the first choice for improving n-butanol production. The present article aims to review the following aspects of n-butanol production by LAB: (1) the tolerance of LAB to n-butanol, including its tolerance level and potential tolerance mechanisms; (2) genome editing tools in the n-butanol-resistant LAB; (3) methods of LAB modification for n-butanol production and the production levels after modification. This review will provide a theoretical basis for further research on n-butanol production by LAB.

Enhanced sucrose fermentation by introduction of heterologous sucrose transporter and invertase into Clostridium beijerinckii for acetone-butanol-ethanol production

A heterologous pathway for sucrose transport and metabolism was introduced into Clostridium beijerinckii to improve sucrose use for n-butanol production. The combined expression of StSUT1, encoding a sucrose transporter from potato (Solanum tuberosum), and SUC2, encoding a sucrose invertase from Saccharomyces cerevisiae, remarkably enhanced n-butanol production. With sucrose, sugarcane molasses and sugarcane juice as substrates, the C. beijerinckii strain harbouring StSUT1 and SUC2 increased acetone-butanol-ethanol production by 38.7%, 22.3% and 52.8%, respectively, compared with the wild-type strain. This is the first report to demonstrate enhanced sucrose fermentation due to the heterologous expression of a sucrose transporter and invertase in Clostridium. The metabolic engineering strategy used in this study can be widely applied in other microorganisms to enhance the production of high-value compounds from sucrose-based biomass.

Rhenium Complexes Bearing Tridentate and Bidentate Phosphinoamine Ligands in the Production of Biofuel Alcohols via the Guerbet Reaction

We report a variety of rhenium complexes supported by bidentate and tridentate phosphinoamine ligands and their use in the formation of the advanced biofuel isobutanol from methanol and ethanol. Rhenium pincer complexes 1-3 are effective catalysts for this process, with 2 giving isobutanol in 35% yields, with 97% selectivity in the liquid fraction, over 16 h with catalyst loadings as low as 0.07 mol %. However, these catalysts show poorer overall selectivity, with the formation of a significant amount of carboxylate salt solid byproduct also being observed. Production of the active catalyst 1d has been followed by ³¹P NMR spectroscopy, and the importance of the presence of base and elevated temperatures to catalyst activation has been established. Complexes supported by diphosphine ligands are inactive for Guerbet chemistry; however, complexes supported by bidentate phosphinoamine ligands show greater selectivity for isobutanol formation over carboxylate salts. The novel complex 7 was able to produce isobutanol in 28% yield over 17 h. The importance of the N-H moiety to the catalytic performance has also been established, giving further weight to the hypothesis that these catalysts operate via a cooperative mechanism.

Interfacing non-enzymatic catalysis with living microorganisms

Interfacing non-enzymatic catalysis with cellular metabolism is emerging as a powerful approach to produce a range of high value small molecules and polymers. In this review, we highlight recent examples from this promising young field. Specifically, we discuss demonstrations of living cells mediating redox processes for biopolymer production, interfacing solar-light driven chemistry with microbial metabolism, and intra- and extracellular non-enzymatic catalysis to generate high value molecules. This review highlights the vast potential of this nascent field to bridge the two disciplines of synthetic chemistry and synthetic biology for a sustainable chemical industry.

Ionic liquid-based in situ product removal design exemplified for an acetone-butanol-ethanol fermentation

Selecting an appropriate separation technique is essential for the application of in situ product removal (ISPR) technology in biological processes. In this work, a three-stage systematic design method is proposed as a guide to integrate ionic liquid (IL)-based separation techniques into ISPR. This design method combines the selection of a suitable ISPR processing scheme, the optimal design of an IL-based liquid-liquid extraction (LLE) system followed by process simulation and evaluation. As a proof of concept, results for a conventional acetone-butanol-ethanol fermentation are presented (40,000 ton/year butanol production). In this application, ILs tetradecyl(trihexyl)phosphonium tetracyanoborate ([TDPh][TCB]) and tetraoctylammonium 2-methyl-1-naphthoate ([TOA] [MNaph]) are identified as the optimal solvents from computer-aided IL design (CAILD) method and reported experimental data, respectively. The dynamic simulation results for the fermentation process show that, the productivity of IL-based in situ (fed-batch) process and in situ (batch) process is around 2.7 and 1.8fold that of base case. Additionally, the IL-based in situ (fed-batch) process and in situ (batch) process also have significant energy savings (79.6% and 77.6%) when compared to the base case.

Sporulation in solventogenic and acetogenic clostridia

The Clostridium genus harbors compelling organisms for biotechnological production processes; while acetogenic clostridia can fix C1-compounds to produce acetate and ethanol, solventogenic clostridia can utilize a wide range of carbon sources to produce commercially valuable carboxylic acids, alcohols, and ketones by fermentation. Despite their potential, the conversion by these bacteria of carbohydrates or C1 compounds to alcohols is not cost-effective enough to result in economically viable processes. Engineering solventogenic clostridia by impairing sporulation is one of the investigated approaches to improve solvent productivity. Sporulation is a cell differentiation process triggered in bacteria in response to exposure to environmental stressors. The generated spores are metabolically inactive but resistant to harsh conditions (UV, chemicals, heat, oxygen). In Firmicutes, sporulation has been mainly studied in bacilli and pathogenic clostridia, and our knowledge of sporulation in solvent-producing or acetogenic clostridia is limited. Still, sporulation is an integral part of the cellular physiology of clostridia; thus, understanding the regulation of sporulation and its connection to solvent production may give clues to improve the performance of solventogenic clostridia. This review aims to provide an overview of the triggers, characteristics, and regulatory mechanism of sporulation in solventogenic clostridia. Those are further compared to the current knowledge on sporulation in the industrially relevant acetogenic clostridia. Finally, the potential applications of spores for process improvement are discussed.Key Points• The regulatory network governing sporulation initiation varies in solventogenic clostridia.• Media composition and cell density are the main triggers of sporulation.• Spores can be used to improve the fermentation process.

Modeling Growth Kinetics, Interspecies Cell Fusion, and Metabolism of a Clostridium acetobutylicum/Clostridium ljungdahlii Syntrophic Coculture

Clostridium acetobutylicum and Clostridium ljungdahlii grown in a syntrophic culture were recently shown to fuse membranes and exchange cytosolic contents, yielding hybrid cells with significant shifts in gene expression and growth phenotypes. Here, we introduce a dynamic genome-scale metabolic modeling framework to explore how cell fusion alters the growth phenotype and panel of metabolites produced by this binary community. Computational results indicate C. ljungdahlii persists in the coculture through proteome exchange during fusing events, which endow C. ljungdahlii cells with expanded substrate utilization, and access to additional reducing equivalents from C. acetobutylicum-evolved H₂ and through acquisition of C. acetobutylicum-native cofactor-reducing enzymes. Simulations predict maximum theoretical ethanol and isopropanol yields that are increased by 0.64 mmol and 0.39 mmol per mmol hexose sugar consumed, respectively, during exponential growth when cell fusion is active. This modeling effort provides a mechanistic explanation for the metabolic outcome of cellular fusion and altered homeostasis achieved in this syntrophic clostridial community.IMPORTANCE Widespread cell fusion and protein exchange between microbial organisms as observed in synthetic C. acetobutylicum/C. ljungdahlii culture is a novel observation that has not been explored in silico The mechanisms responsible for the observed cell fusion events in this culture are still unknown. In this work, we develop a modeling framework that captures the observed culture composition and metabolic phenotype, use it to offer a mechanistic explanation for how the culture achieves homeostasis, and identify C. ljungdahlii as primary beneficiary of fusion events. The implications for the events described in this study are far reaching, with potential to reshape our understanding of microbial community behavior synthetically and in nature.

Challenges Associated With the Formation of Recombinant Protein Inclusion Bodies in Escherichia coli and Strategies to Address Them for Industrial Applications

Recombinant proteins are becoming increasingly important for industrial applications, where Escherichia coli is the most widely used bacterial host for their production. However, the formation of inclusion bodies is a frequently encountered challenge for producing soluble and functional recombinant proteins. To overcome this hurdle, different strategies have been developed through adjusting growth conditions, engineering host strains of E. coli, altering expression vectors, and modifying the proteins of interest. These approaches will be comprehensively highlighted with some of the new developments in this review. Additionally, the unique features of protein inclusion bodies, the mechanism and influencing factors of their formation, and their potential advantages will also be discussed.

Butanol production from lignocellulosic sugars by Clostridium beijerinckii in microbioreactors

BACKGROUND

Butanol (n-butanol) has been gaining attention as a renewable energy carrier and an alternative biofuel with superior properties to the most widely used ethanol. We performed 48 anaerobic fermentations simultaneously with glucose and xylose as representative lignocellulosic sugars by Clostridium beijerinckii NCIMB 8052 in BioLector® microbioreactors to understand the effect of different sugar mixtures on fermentation and to demonstrate the applicability of the micro-cultivation system for high-throughput anaerobic cultivation studies. We then compared the results to those of similar cultures in serum flasks to provide insight into different setups and measurement methods.

RESULTS

ANOVA results showed that the glucose-to-xylose ratio affects both growth and production due to Carbon Catabolite Repression. The study demonstrated successful use of BioLector® system for the first time for screening several media and sugar compositions under anaerobic conditions by using online monitoring of cell mass and pH in real-time and at unprecedented time-resolution. Fermentation products possibly interfered with dissolved oxygen (DO) measurements, which require a careful interpretation of DO monitoring results.

CONCLUSIONS

The statistical approach to evaluate the microbioreactor setup, and information obtained in this study will support further research in bioreactor and bioprocess design, which are very important aspects of industrial fermentations of lignocellulosic biomass.

Structural-Genetic Characterization Of Novel Butaryl co-A Dehydrogenase and Proposition of Butanol Biosynthesis Pathway in Pusillimonas ginsengisoli SBSA

Despite extensive use in the biofuel industry, only butyryl co-A dehydrogenase enzymes from the Clostridia group have undergone extensive structural and genetic characterization. The present study, portrays the characterization of structural, functional and phylogenetic properties of butyryl co-A dehydrogenase identified within the genome of Pusillimonas ginsengisoli SBSA. In silico characterization, homology modelling and docking data indicates that this protein is a homo-tetramer and 388 amino acid residue long, rich in alanine and leucine residue; having molecular weight of 42347.69 dalton. Its isoelectric point value is 5.78; indicate its neutral nature while 38.38 instability index value indicate its stable nature. Its thermostable nature evidenced by its high aliphatic index (93.14); makes its suitable for industry-based use. The secondary structure prediction analysis of butyryl co-A dehydrogenase unveiled that the proteins has secondary arrangements of 54% α-helix, 13% β-stand and 5% disordered conformation. However, phylogenetic analysis clearly indicates that probably horizontal gene transfer is the primary mechanism of spreading of this gene in this organism. Notably, multiple sequence alignment study of phylogenetically diverse butyryl co-A dehydrogenase sequence highlighted the presence of conserved amino acid residues i.e. YXV/LGXKXWXS/T. Physicochemical characterization of other relevant proteins involved in butanol metabolism of SBSA also has been carried out. However, metabolic construction of functional butanol biosynthesis pathway in SBSA, enlightened its cost-effective potential use in biofuel industry as an alternate to Clostridia system.

Energy-Saving and Sustainable Separation of Bioalcohols by Adsorption on Bone Char

The separation of ethanol, propanol, and butanol from aqueous solutions was studied using adsorption on bone char. Adsorption kinetics and thermodynamic parameters of this separation method were studied at different conditions of pH and temperature. Results showed that the maximum adsorption capacities of these bioalcohols were obtained at pH 6 and 20°C. An exothermic separation was identified, which can be mainly associated to hydrophobic interactions between bone char surface and bioalcohols. Binary adsorption studies were also performed using mixtures of these bioalcohols. An antagonistic adsorption was observed for all bioalcohols where the ethanol and propanol separation was significantly affected by butanol. A model based on an artificial neural network was proposed to correlate both single and binary adsorption isotherms of these bioalcohols with bone char. It was concluded that the bone char could be an interesting adsorbent for the sustainable separation and recovery of bioalcohols from fermentation broths, which are actually considered emerging liquid biofuels and relevant industrial chemicals.

Electrochemical tuning of alcohol oxidase and dehydrogenase catalysis via biosensing towards butanol-1 detection in fermentation media

A novel approach for electrochemical tuning of alcohol oxidase (AOx) and alcohol dehydrogenase (ADH) biocatalysis towards butanol-1 oxidation by incorporating enzymes in various designs of amperometric biosensors is presented. The biosensors were developed by using commercial graphene oxide-based screen-printed electrodes and varying enzyme producing strains, encapsulation approaches (layer-by-layer (LbL) or one-step electrodeposition (EcD)), layers composition and structure, operating conditions (applied potential values) and introducing mediators (Meldola Blue and Prussian Blue) or Pd-nanoparticles (Pd-NPs). Simultaneous analysis/screening of multiple crucial system parameters during the enzyme engineering process allowed to identify within a period of one month that four out of twelve proposed designs demonstrated a good signal reproducibility and linear response (up to 14.6 mM of butanol) under very low applied potentials (from -0.02 to -0.32 V). Their mechanical stability was thoroughly investigated by multi-analytical techniques prior to butanol determination in cell-free samples from an anaerobic butanol fermentation. The EcD-based biosensor that incorporates ADH, NAD⁺, Pd-NPs and Nafion showed no loss of enzyme activity after preparation and demonstrated capabilities towards low potential (-0.12 V) detection of butanol-1 in fermentation medium (4 mM) containing multiple electroactive species with almost 15 times enhanced sensitivity (0.2282 μA/mM ± 0.05) when compared to the LbL design. Furthermore, the ADH-Nafion bonding for the S. cerevisiae strain was confirmed to be 3 times higher than for E. coli.

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.

Manganese Diphosphine and Phosphinoamine Complexes Are Effective Catalysts for the Production of Biofuel Alcohols via the Guerbet Reaction

We report a variety of manganese-based catalysts containing both chelating diphosphine (bis(diphenylphosphino)methane (dppm: 1, 2, and 7) or 1,2-bis(diphenylphosphino)ethane (dppe: 3)), and mixed-donor phosphinoamine (2-(diphenylphosphino)ethylamine (dppea: 4–6)) ligands for the upgrading of ethanol and methanol to the advanced biofuel isobutanol. These catalysts show moderate selectivity up to 74% along with turnover numbers greater than 100 over 90 h, with catalyst 2 supported by dppm demonstrating superior performance. The positive effect of substituting the ligand backbone was also displayed with a catalyst supported by C-phenyl-substituted dppm (8) having markedly improved performance compared to the parent dppm catalysts. Catalysts supported by the phosphinoamine ligand dppea are also active for the upgrading of ethanol to n-butanol. These results show that so-called PNP-pincer ligands are not a prerequisite for the use of manganese catalysts in Guerbet chemistry and that simple chelates can be used effectively.