Novel and neglected issues of acetone-butanol-ethanol (ABE) fermentation by clostridia: Clostridium metabolic diversity, tools for process mapping and continuous fermentation systems

Steering the product spectrum in high-pressure anaerobic processes: CO2 partial pressure as a novel tool in biorefinery concepts

BACKGROUND

Elevated CO₂ partial pressure (pCO₂) has been proposed as a potential steering parameter for selective carboxylate production in mixed culture fermentation. It is anticipated that intermediate product spectrum and production rates, as well as changes in the microbial community, are (in)directly influenced by elevated pCO₂. However, it remains unclear how pCO₂ interacts with other operational conditions, namely substrate specificity, substrate-to-biomass (S/X) ratio and the presence of an additional electron donor, and what effect pCO₂ has on the exact composition of fermentation products. Here, we investigated possible steering effects of elevated pCO₂ combined with (1) mixed substrate (glycerol/glucose) provision; (2) subsequent increments in substrate concentration to increase the S/X ratio; and (3) formate as an additional electron donor.

RESULTS

Metabolite predominance, e.g., propionate vs. butyrate/acetate, and cell density, depended on interaction effects between pCO₂-S/X ratio and pCO₂-formate. Individual substrate consumption rates were negatively impacted by the interaction effect between pCO₂-S/X ratio and were not re-established after lowering the S/X ratio and adding formate. The product spectrum was influenced by the microbial community composition, which in turn, was modified by substrate type and the interaction effect between pCO₂-formate. High propionate and butyrate levels strongly correlated with Negativicutes and Clostridia predominance, respectively. After subsequent pressurized fermentation phases, the interaction effect between pCO₂-formate enabled a shift from propionate towards succinate production when mixed substrate was provided.

CONCLUSIONS

Overall, interaction effects between elevated pCO₂, substrate specificity, high S/X ratio and availability of reducing equivalents from formate, rather than an isolated pCO₂ effect, modified the proportionality of propionate, butyrate and acetate in pressurized mixed substrate fermentations at the expense of reduced consumption rates and increased lag-phases. The interaction effect between elevated pCO₂ and formate was beneficial for succinate production and biomass growth with a glycerol/glucose mixture as the substrate. The positive effect may be attributed to the availability of extra reducing equivalents, likely enhanced carbon fixating activity and hindered propionate conversion due to increased concentration of undissociated carboxylic acids.

Construction of a Microbial Consortium for the De Novo Synthesis of Butyl Butyrate from Renewable Resources

Butyl butyrate has shown wide applications in food, cosmetic, and biofuel sectors. Currently, biosynthesis of butyl butyrate still requires exogenous addition of precursors and lipase, which increases the production cost and limits further large-scale development. In this study, a microbial consortium was first designed to realize direct butyl butyrate production from lignocellulose. The highest butyl butyrate concentration of 34.42 g/L was detected in the solvent phase from 60 g/L glucose using a microbial coculture system composed of Clostridium acetobutylicum NJ4 and Clostridium tyrobutyricum LD with the elimination of butyric acid supplementation. Meanwhile, 13.52 g/L butyl butyrate was synthesized from 60 g/L glucose using a microbial consortium composed of three strains including strain NJ4, strain LD, and Escherichia coli BL21- pET-29a(+)-LE without the addition of any exogenous precursors and lipase. In addition, 2.94 g/L butyl butyrate could be directly produced from 60 g/L microcrystalline cellulose when Trichoderma asperellum was added to the above-mentioned three-strain microbial consortium. This four-strain microbial consortium represents the first study regarding the direct butyl butyrate production from lignocellulose without the supplementation of exogenous precursors and lipase, which may be extended to the biosynthesis of other short-chain esters, such as ethyl acetate and butyl lactate.

Adding glucose delays the conversion of ethanol and acetic acid to caproic acid in Lacrimispora celerecrescens JSJ-1

Caproic acid is an important fatty acid with diverse applications. In this study, the biomass growth and metabolites of Lacrimispora celerecrescens JSJ-1 were investigated under different carbon sources (ethanol, starch, sucrose, and glucose), with a focus on the effect of the coexistence of glucose and ethanol on the synthesis of caproic acid. The results showed that starch, glucose, and sucrose all contributed to the biomass of L. celerecrescens JSJ-1. Under the three carbon sources, L. celerecrescens JSJ-1 produced acetic acid, butyric acid, lactic acid, ethanol, and butanol, but caproic acid was not produced. Ethanol was the optimal substrate for the production of caproic acid. When glucose and ethanol coexisted, the generation time of caproic acid was delayed compared with that in ethanol sodium acetate medium (ES medium). This was because glucose was preferentially consumed over ethanol. Lactic acid was generated as a result of glucose consumption, which led to a significant decrease in pH from 6.45 to 4.68. The low pH (< 5) inhibited the synthesis of caproic acid. Then, the strain's usage of lactic acid and the reaction between CaCO₃ and lactic acid caused the pH to increase. L. celerecrescens JSJ-1 did not start producing caproic acid using ethanol and acetic acid until the pH increased to 5.8. This research enriches the knowledge regarding the metabolism of L. celerecrescens JSJ-1 and provides guidelines for the industrial production of caproic acid by using L. celerecrescences JSJ-1. KEY POINTS: • Ethanol is the optimal substrate for the synthesis of caproic acid by Lacrimispora celerecrescens JSJ-1. • Lacrimispora celerecrescens JSJ-1 produced lactic acid rapidly when it used glucose, causing a sharp drop in pH. • pH is a crucial factor affecting the synthesis of caproic acid from ethanol by Lacrimispora celerecrescens JSJ-1.

Recent Advances In Microbe-Photocatalyst Hybrid Systems for Production of Bulk Chemicals: A Review

Solar-driven biocatalysis technologies can combine inorganic photocatalytic materials with biological catalysts to convert CO₂, light, and water into chemicals, offering the promise of high energy efficiency and a broader product scope than that of natural photosynthesis. Solar energy is the most abundant renewable energy source on earth, but it cannot be directly utilized by current industrial microorganisms. Therefore, the establishment of a solar-driven bio-catalysis platform, a bridge between solar energy and heterotrophic microorganisms, can dramatically increase carbon flux in biomanufacturing systems and consequently may revolutionize the biorefinery. This review first discusses the main applications of microbe-photocatalyst hybrid (MPH) systems in biorefinery processes. Then, various strategies to improve the electron transfer by microorganisms at the inorganic photocatalytic material interface are discussed, especially biohybrid systems based on autotrophic or heterotrophic bacteria and photocatalytic materials. Finally, we discuss the current challenges and offer potential solutions for the development of MPH systems.

An Alternative Approach to Improve the Butanol Production Efficiency from Sweet Sorghum Stem Juice Using Immobilized Cells Combined with an In Situ Gas Stripping System

The effects of the nitrogen source and buffers used in butanol production with Clostridium beijerinckii TISTR 1461 from sweet sorghum stem juice (SSJ) containing 60 g/L of total sugar were first studied in this paper. Among the various nitrogen sources (dried spent yeast, urea, ammonium acetate, ammonium sulfate), urea was found to be the most suitable for butanol production. SSJ supplemented with urea (0.64 g/L) and cocktail buffers (KH2PO4, 0.5 g/L; K2HPO4, 0.5 g/L; ammonium acetate, 2.2 g/L) gave the highest butanol concentration (PB, 10.13 g/L). Then, the capability of immobilized C. beijerinckii TISTR 1461 cells for butanol fermentation was investigated. Two residual waste materials were examined as immobilized cell carriers. Bamboo chopstick pieces were more appropriate as carriers for cell immobilization than cigarette filter tips. The PB value of the immobilized cells on the bamboo chopstick pieces was ~13% higher than that on the cigarette filter tips. Using the response surface methodology (RSM), 1.9 cm bamboo chopstick pieces with a carrier loading of 1:32 (w/v) were the optimum conditions for cell immobilization for butanol production. Under these conditions, the PB value was 11.62 g/L. To improve the butanol production efficiency, a gas stripping system (GS) was connected to the fermenter. It was found that the PB (14.02 g/L) and butanol productivity (QB, 0.29 g/L·h) values improved by ~21% compared to butanol fermentation using no gas stripping.

An updated review on advancement in fermentative production strategies for biobutanol using Clostridium spp

A significant concern of our fuel-dependent era is the unceasing exhaustion of petroleum fuel supplies. In parallel to this, environmental issues such as the greenhouse effect, change in global climate, and increasing global temperature must be addressed on a priority basis. Biobutanol, which has fuel characteristics comparable to gasoline, has attracted global attention as a viable green fuel alternative among the many biofuel alternatives. Renewable biomass could be used for the sustainable production of biobutanol by the acetone-butanol-ethanol (ABE) pathway. Non-extinguishable resources, such as algal and lignocellulosic biomass, and starch are some of the most commonly used feedstock for fermentative production of biobutanol, and each has its particular set of advantages. Clostridium, a gram-positive endospore-forming bacterium that can produce a range of compounds, along with n-butanol is traditionally known for its biobutanol production capabilities. Clostridium fermentation produces biobased n-butanol through ABE fermentation. However, low butanol titer, a lack of suitable feedstock, and product inhibition are the primary difficulties in biobutanol synthesis. Critical issues that are essential for sustainable production of biobutanol include (i) developing high butanol titer producing strains utilizing genetic and metabolic engineering approaches, (ii) renewable biomass that could be used for biobutanol production at a larger scale, and (iii) addressing the limits of traditional batch fermentation by integrated bioprocessing technologies with effective product recovery procedures that have increased the efficiency of biobutanol synthesis. Our paper reviews the current progress in all three aspects of butanol production and presents recent data on current practices in fermentative biobutanol production technology.

Effectiveness of Low-Cost Bioreactors Integrated with a Gas Stripping System for Butanol Fermentation from Sugarcane Molasses by Clostridium beijerinckii

The effectiveness of column bioreactors for butanol fermentation from sugarcane molasses by Clostridium beijerinckii TISTR 1461 was investigated. This fermentation was operated at an initial pH of 6.5 and temperature of 37 °C under anaerobic conditions. A 1-L bubble column bioreactor was used with various gas circulation rates ranging from 0.2 to 1.0 L/min. The highest butanol concentration (PB, 8.72 g/L), productivity (QB, 0.24 g/L∙h) and yield (YB/S, 0.21 g/g) were obtained with a gas circulation of 0.2 L/min. To improve butanol production efficiency, gas-lift column bioreactors with internal and external loops at 0.2 L/min of circulating gas were used. Higher PB (10.50–10.58 g/L), QB (0.29 g/L∙h) and YB/S (0.22–0.23 g/g) values were obtained in gas-lift column bioreactors. These values were similar to those using a more complex 2-L stirred-tank bioreactor (PB, 10.10 g/L; QB, 0.28 g/L h and YB/S, 0.22 g/g). Hence, gas-lift column bioreactors have potential for use as low-cost fermenters instead of stirred-tank bioreactors for butanol fermentation. When the gas-lift column bioreactor with an internal loop was coupled with a gas stripping system, it yielded an enhanced PB and sugar consumption of approximately 9% and 7%, respectively, compared to a system with no gas stripping.

Unraveling the Genotypic and Phenotypic Diversity of the Psychrophilic Clostridium estertheticum Complex, a Meat Spoilage Agent

The spoilage of vacuum-packed meat by Clostridium estertheticum complex (CEC), which is accompanied by or without production of copious amounts of gas, has been linked to the acetone–butyrate–ethanol fermentation, but the mechanism behind the variable gas production has not been fully elucidated. The reconstruction and comparison of intra- and interspecies metabolic pathways linked to meat spoilage at the genomic level can unravel the genetic basis for the variable phenotype. However, this is hindered by unavailability of CEC genomes, which in addition, has hampered the determination of genetic diversity and its drivers within CEC. Therefore, the current study aimed at determining the diversity of CEC through comprehensive comparative genomics. Fifty CEC genomes from 11 CEC species were compared. Recombination and gene gain/loss events were identified as important sources of natural variation within CEC, with the latter being pronounced in genomospecies2 that has lost genes related to flagellar assembly and signaling. Pan-genome analysis revealed variations in carbohydrate metabolic and hydrogenases genes within the complex. Variable inter- and intraspecies gas production in meat by C. estertheticum and Clostridium tagluense were associated with the distribution of the [NiFe]-hydrogenase hyp gene cluster whose absence or presence was associated with occurrence or lack of pack distention, respectively. Through comparative genomics, we have shown CEC species exhibit high genetic diversity that can be partly attributed to recombination and gene gain/loss events. We have also shown genetic basis for variable gas production in meat can be attributed to the presence/absence of the hyp gene cluster.

Transcriptomic studies of solventogenic clostridia, Clostridium acetobutylicum and Clostridium beijerinckii

Solventogenic clostridia are not a strictly defined group within the genus Clostridium but its representatives share some common features, i.e. they are anaerobic, non-pathogenic, non-toxinogenic and endospore forming bacteria. Their main metabolite is typically 1-butanol but depending on species and culture conditions, they can form other metabolites such as acetone, isopropanol, ethanol, butyric, lactic and acetic acids, and hydrogen. Although these organisms were previously used for the industrial production of solvents, they later fell into disuse, being replaced by more efficient chemical production. A return to a more biological production of solvents therefore requires a thorough understanding of clostridial metabolism. Transcriptome analysis, which reflects the involvement of individual genes in all cellular processes within a population, at any given (sampling) moment, is a valuable tool for gaining a deeper insight into clostridial life. In this review, we describe techniques to study transcription, summarize the evolution of these techniques and compare methods for data processing and visualization of solventogenic clostridia, particularly the species Clostridium acetobutylicum and Clostridium beijerinckii. Individual approaches for evaluating transcriptomic data are compared and their contributions to advancements in the field are assessed. Moreover, utilization of transcriptomic data for reconstruction of computational clostridial metabolic models is considered and particular models are described. Transcriptional changes in glucose transport, central carbon metabolism, the sporulation cycle, butanol and butyrate stress responses, the influence of lignocellulose-derived inhibitors on growth and solvent production, and other respective topics, are addressed and common trends are highlighted.

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.

Phenotypic and genomic analysis of isopropanol and 1,3-propanediol producer Clostridium diolis DSM 15410

Clostridium diolis DSM 15410 is a type strain of solventogenic clostridium capable of conducting isopropanol-butanol-ethanol fermentation. By studying its growth on different carbohydrates, we verified its ability to utilize glycerol and produce 1,3-propanediol and discovered its ability to produced isopropanol. Complete genome sequencing showed that its genome is a single circular chromosome and belongs to the cluster I (sensu scricto) of the genus Clostridium. By cultivation analysis we highlighted its specific behavior in comparison to two selected closely related strains. Despite the fact that several CRISPR loci were found, 16 putative prophages showed the ability to receive foreign DNA. Thus, the strain has the necessary features for future engineering of its 1,3-propanediol biosynthetic pathway and for the possible industrial utilization in the production of biofuels.

Towards continuous industrial bioprocessing with solventogenic and acetogenic clostridia: challenges, progress and perspectives

The sustainable production of solvents from above ground carbon is highly desired. Several clostridia naturally produce solvents and use a variety of renewable and waste-derived substrates such as lignocellulosic biomass and gas mixtures containing H2/CO2 or CO. To enable economically viable production of solvents and biofuels such as ethanol and butanol, the high productivity of continuous bioprocesses is needed. While the first industrial-scale gas fermentation facility operates continuously, the acetone-butanol-ethanol (ABE) fermentation is traditionally operated in batch mode. This review highlights the benefits of continuous bioprocessing for solvent production and underlines the progress made towards its establishment. Based on metabolic capabilities of solvent producing clostridia, we discuss recent advances in systems-level understanding and genome engineering. On the process side, we focus on innovative fermentation methods and integrated product recovery to overcome the limitations of the classical one-stage chemostat and give an overview of the current industrial bioproduction of solvents.

Transcriptomic and Phenotypic Analysis of a spoIIE Mutant in Clostridium beijerinckii

SpoIIE is a phosphatase involved in the activation of the first sigma factor of the forespore, σ F , during sporulation. A ΔspoIIE mutant of Clostridium beijerinckii NCIMB 8052, previously generated by CRISPR-Cas9, did not sporulate but still produced granulose and solvents. Microscopy analysis also showed that the cells of the ΔspoIIE mutant are elongated with the presence of multiple septa. This observation suggests that in C. beijerinckii, SpoIIE is necessary for the completion of the sporulation process, as seen in Bacillus and Clostridium acetobutylicum. Moreover, when grown in reactors, the spoIIE mutant produced higher levels of solvents than the wild type strain. The impact of the spoIIE inactivation on gene transcription was assessed by comparative transcriptome analysis at three time points (4 h, 11 h and 23 h). Approximately 5% of the genes were differentially expressed in the mutant compared to the wild type strain at all time points. Out of those only 12% were known sporulation genes. As expected, the genes belonging to the regulon of the sporulation specific transcription factors (σ F , σ E , σ G , σ K ) were strongly down-regulated in the mutant. Inactivation of spoIIE also caused differential expression of genes involved in various cell processes at each time point. Moreover, at 23 h, genes involved in butanol formation and tolerance, as well as in cell motility, were up-regulated in the mutant. In contrast, several genes involved in cell wall composition, oxidative stress and amino acid transport were down-regulated. These results indicate an intricate interdependence of sporulation and stationary phase cellular events in C. beijerinckii.

Metabolic engineering strategies for butanol production in Escherichia coli

The global market of butanol is increasing due to its growing applications as solvent, flavoring agent, and chemical precursor of several other compounds. Recently, the superior properties of n-butanol as a biofuel over ethanol have stimulated even more interest. (Bio)butanol is natively produced together with ethanol and acetone by Clostridium species through acetone-butanol-ethanol fermentation, at noncompetitive, low titers compared to petrochemical production. Different butanol production pathways have been expressed in Escherichia coli, a more accessible host compared to Clostridium species, to improve butanol titers and rates. The bioproduction of butanol is here reviewed from a historical and theoretical perspective. All tested rational metabolic engineering strategies in E. coli to increase butanol titers are reviewed: manipulation of central carbon metabolism, elimination of competing pathways, cofactor balancing, development of new pathways, expression of homologous enzymes, consumption of different substrates, and molecular biology strategies. The progress in the field of metabolic modeling and pathway generation algorithms and their potential application to butanol production are also summarized here. The main goals are to gather all the strategies, evaluate the respective progress obtained, identify, and exploit the outstanding challenges.

Alterations of the Predominant Fecal Microbiota and Disruption of the Gut Mucosal Barrier in Patients with Early-Stage Colorectal Cancer

Growing evidence indicated that the gut microbiota was the intrinsic and essential component of the cancer microenvironment, which played vital roles in the development and progression of colorectal cancer (CRC). In our present study, we investigated the alterations of fecal abundant microbiota with real-time quantitative PCR and the changes of indicators of gut mucosal barrier from 53 early-stage CRC patients and 45 matched healthy controls. We found that the traditional beneficial bacteria such as Lactobacillus and Bifidobacterium decreased significantly and the carcinogenic bacteria such as Enterobacteriaceae and Fusobacterium nucleatum were significantly increased in CRC patients. We also found gut mucosal barrier dysfunction in CRC patients with increased levels of endotoxin (LPS), D-lactate, and diamine oxidase (DAO). With Pearson's correlation analysis, D-lactate, LPS, and DAO were correlated negatively with Lactobacillus and Bifidobacterium and positively with Enterobacteriaceae and F. nucleatum. Our present study found dysbiosis of the fecal microbiota and dysfunction of the gut mucosal barrier in patients with early-stage CRC, which implicated that fecal abundant bacteria and gut mucosal barrier indicators could be used as targets to monitor the development and progression of CRC in a noninvasive and dynamic manner.

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.

Investigation of secondary metabolism in the industrial butanol hyper-producer Clostridium saccharoperbutylacetonicum N1-4

Clostridium saccharoperbutylacetonicum N1-4 (Csa) is a historically significant anaerobic bacterium which can perform saccharolytic fermentations to produce acetone, butanol, and ethanol (ABE). Recent genomic analyses have highlighted this organism's potential to produce polyketide and nonribosomal peptide secondary metabolites, but little is known regarding the identity and function of these metabolites. This study provides a detailed bioinformatic analysis of seven biosynthetic gene clusters (BGCs) present in the Csa genome that are predicted to produce polyketides/nonribosomal peptides. An RNA-seq-based untargeted transcriptomic approach revealed that five of seven BGCs were expressed during ABE fermentation. Additional characterization of a highly expressed nonribosomal peptide synthetase gene led to the discovery of its associated metabolite and its biosynthetic pathway. Transcriptomic analysis suggested an association of this nonribosomal peptide synthetase gene with butanol tolerance, which was supported by butanol challenge assays.

Smart fermentation engineering for butanol production: designed biomass and consolidated bioprocessing systems

There is a renewed interest in acetone-butanol-ethanol (ABE) fermentation from renewable substrates for the sustainable and environment-friendly production of biofuel and platform chemicals. However, the ABE fermentation is associated with several challenges due to the presence of heterogeneous components in the renewable substrates and the intrinsic characteristics of ABE fermentation process. Hence, there is a need to select optimal substrates and modify their characteristics suitable for the ABE fermentation process or microbial strain. This "designed biomass" can be used to establish the consolidated bioprocessing systems. As there are very few reports on designed biomass, the main objectives of this review are to summarize the main challenges associated with ABE fermentation from renewable substrates and to introduce feasible strategies for designing the substrates through pretreatment and hydrolysis technologies as well as through the establishment of consolidated bioprocessing systems. This review offers new insights on improving the efficiency of ABE fermentation from designed renewable substrates.

Butanol production from lignocellulosic biomass: revisiting fermentation performance indicators with exploratory data analysis

After just more than 100 years of history of industrial acetone-butanol-ethanol (ABE) fermentation, patented by Weizmann in the UK in 1915, butanol is again today considered a promising biofuel alternative based on several advantages compared to the more established biofuels ethanol and methanol. Large-scale fermentative production of butanol, however, still suffers from high substrate cost and low product titers and selectivity. There have been great advances the last decades to tackle these problems. However, understanding the fermentation process variables and their interconnectedness with a holistic view of the current scientific state-of-the-art is lacking to a great extent. To illustrate the benefits of such a comprehensive approach, we have developed a dataset by collecting data from 175 fermentations of lignocellulosic biomass and mixed sugars to produce butanol that reported during the past three decades of scientific literature and performed an exploratory data analysis to map current trends and bottlenecks. This review presents the results of this exploratory data analysis as well as main features of fermentative butanol production from lignocellulosic biomass with a focus on performance indicators as a useful tool to guide further research and development in the field towards more profitable butanol manufacturing for biofuel applications in the future.

The significance of proline on lignocellulose-derived inhibitors tolerance in Clostridium acetobutylicum ATCC 824

When lignocellulosic biomass was used for acetone-butanol-ethanol (ABE) fermentation, several lignocellulose-derived inhibitors, which are toxic to Clostridium acetobutylicum, were generated during acid hydrolysis process and seriously hindered the industrialization of lignocellulosic butanol. In this study, an engineered strain 824(proABC) with significantly improved tolerance to multiple lignocellulose-derived inhibitors (formic acid and phenolic compounds) was constructed by strengthening the proline biosynthesis. The engineered strain exhibited more effective synthesis ability of proline and scavenging ability of reactive oxygen species (ROS). Consequently, the butanol produced by 824(proABC) was 1-, 2.4- or 3.4-fold higher than that of the wild type strain when using the undetoxified hydrolysate of soybean straw, rice straw or corn straw as the substrate, respectively. Therefore, enhancing the proline biosynthesis can be used as an effective strategy to improve the tolerance of C. acetobutylicum to multiple lignocellulose-derived inhibitors, and 824(proABC) has great potential to produce butanol from undetoxified lignocellulosic hydrolysates.

n-Butanol production in S. cerevisiae: co-ordinate use of endogenous and exogenous pathways

n-Butanol represents a key commodity chemical and holds significant potential as a biofuel. It can be produced naturally by Clostridia species via the ABE pathway. However, butanol production via such systems can be associated with significant drawbacks. Therefore, substantial efforts have been made toward engineering a suitable industrial host for butanol production. For instance, we previously generated a metabolically engineered Saccharomyces cerevisiae strain that produces ~300 mg/L butanol from combined endogenous and exogenous pathways. In this current study, the endogenous and exogenous pathways of butanol production were further characterised, and their relative contribution to the overall butanol titre was assessed. Deletion of any single component of the exogenous ABE pathway was sufficient to significantly reduce butanol production. Further evidence for a major contribution from the ABE pathway came with the discovery that specific yeast deletion mutants only affected butanol production from this pathway and had a significant impact on butanol levels. In previous studies, the threonine-based ketoacid (TBK) pathway has been proposed to explain endogenous butanol synthesis in ADH1 mutants. However, we find that key mutants in this pathway have little impact on endogenous butanol production; hence, this pathway does not explain endogenous butanol production in our strains. Instead, endogenous butanol production appears to rely on glycine metabolism via an α-ketovalerate intermediate. Indeed, yeast cells can utilise α-ketovalerate as a supplement to generate high butanol titres (> 2 g/L). The future characterisation and optimisation of the enzymatic activities required for this pathway provides an exciting area in the generation of robust butanol production strategies.

Enhancement of microalgal growth and biocomponent-based transformations for improved biofuel recovery: A review

Microalgal biomass has received much attention as feedstock for biofuel production due to its capacity to accumulate a substantial amount of biocomponents (including lipid, carbohydrate, and protein), high growth rate, and environmental benefit. However, commercial realization of microalgal biofuel is a challenge due to its low biomass production and insufficient technology for complete utilization of biomass. Recently, advanced strategies have been explored to overcome the challenges of conventional approaches and to achieve maximum possible outcomes in terms of growth. These strategies include a combination of stress factors; co-culturing with other microorganisms; and addition of salts, flue gases, and phytohormones. This review summarizes the recent progress in the application of single and combined abiotic stress conditions to stimulate microalgal growth and its biocomponents. An innovative schematic model is presented of the biomass-energy conversion pathway that proposes the transformation of all potential biocomponents of microalgae into biofuels.

Transcription profiling of butanol producer Clostridium beijerinckii NRRL B-598 using RNA-Seq

BACKGROUND

Thinning supplies of natural resources increase attention to sustainable microbial production of bio-based fuels. The strain Clostridium beijerinckii NRRL B-598 is a relatively well-described butanol producer regarding its genotype and phenotype under various conditions. However, a link between these two levels, lying in the description of the gene regulation mechanisms, is missing for this strain, due to the lack of transcriptomic data.

RESULTS

In this paper, we present a transcription profile of the strain over the whole fermentation using an RNA-Seq dataset covering six time-points with the current highest dynamic range among solventogenic clostridia. We investigated the accuracy of the genome sequence and particular genome elements, including pseudogenes and prophages. While some pseudogenes were highly expressed, all three identified prophages remained silent. Furthermore, we identified major changes in the transcriptional activity of genes using differential expression analysis between adjacent time-points. We identified functional groups of these significantly regulated genes and together with fermentation and cultivation kinetics captured using liquid chromatography and flow cytometry, we identified basic changes in the metabolism of the strain during fermentation. Interestingly, C. beijerinckii NRRL B-598 demonstrated different behavior in comparison with the closely related strain C. beijerinckii NCIMB 8052 in the latter phases of cultivation.

CONCLUSIONS

We provided a complex analysis of the C. beijerinckii NRRL B-598 fermentation profile using several technologies, including RNA-Seq. We described the changes in the global metabolism of the strain and confirmed the uniqueness of its behavior. The whole experiment demonstrated a good reproducibility. Therefore, we will be able to repeat the experiment under selected conditions in order to investigate particular metabolic changes and signaling pathways suitable for following targeted engineering.

Diauxic growth of Clostridium acetobutylicum ATCC 824 when grown on mixtures of glucose and cellobiose

Clostridium acetobutylicum, a promising organism for biomass transformation, has the capacity to utilize a wide variety of carbon sources. During pre-treatments of (ligno) cellulose through thermic and/or enzymatic processes, complex mixtures of oligo saccharides with beta 1,4-glycosidic bonds can be produced. In this paper, the capability of C. acetobutylicum to ferment glucose and cellobiose, alone and in mixtures was studied. Kinetic studies indicated that a diauxic growth occurs when both glucose and cellobiose are present in the medium. In mixtures, D-glucose is the preferred substrate even if cells were pre grown with cellobiose as the substrate. After the complete consumption of glucose, the growth kinetics exhibits an adaptation time, of few hours, before to be able to use cellobiose. Because of this diauxic phenomenon, the nature of the carbon source deriving from a cellulose hydrolysis pre-treatment could strongly influence the kinetic performances of a fermentation process with C. acetobutylicum.

In situ biobutanol recovery from clostridial fermentations: a critical review

Butanol is a precursor of many industrial chemicals, and a fuel that is more energetic, safer and easier to handle than ethanol. Fermentative biobutanol can be produced using renewable carbon sources such as agro-industrial residues and lignocellulosic biomass. Solventogenic clostridia are known as the most preeminent biobutanol producers. However, until now, solvent production through the fermentative routes is still not economically competitive compared to the petrochemical approaches, because the butanol is toxic to their own producer bacteria, and thus, the production capability is limited by the butanol tolerance of producing cells. In order to relieve butanol toxicity to the cells and improve the butanol production, many recovery strategies (either in situ or downstream of the fermentation) have been attempted by many researchers and varied success has been achieved. In this article, we summarize in situ recovery techniques that have been applied to butanol production through Clostridium fermentation, including liquid-liquid extraction, perstraction, reactive extraction, adsorption, pervaporation, vacuum fermentation, flash fermentation and gas stripping. We offer a prospective and an opinion about the past, present and the future of these techniques, such as the application of advanced membrane technology and use of recent extractants, including polymer solutions and ionic liquids, as well as the application of these techniques to assist the in situ synthesis of butanol derivatives.

The Physiological Functions and Structural Determinants of Catalytic Bias in the [FeFe]-Hydrogenases CpI and CpII of Clostridium pasteurianum Strain W5

The first generation of biochemical studies of complex, iron-sulfur-cluster-containing [FeFe]-hydrogenases and Mo-nitrogenase were carried out on enzymes purified from Clostridium pasteurianum (strain W5). Previous studies suggested that two distinct [FeFe]-hydrogenases are expressed differentially under nitrogen-fixing and non-nitrogen-fixing conditions. As a result, the first characterized [FeFe]-hydrogenase (CpI) is presumed to have a primary role in central metabolism, recycling reduced electron carriers that accumulate during fermentation via proton reduction. A role for capturing reducing equivalents released as hydrogen during nitrogen fixation has been proposed for the second hydrogenase, CpII. Biochemical characterization of CpI and CpII indicated CpI has extremely high hydrogen production activity in comparison to CpII, while CpII has elevated hydrogen oxidation activity in comparison to CpI when assayed under the same conditions. This suggests that these enzymes have evolved a catalytic bias to support their respective physiological functions. Using the published genome of C. pasteurianum (strain W5) hydrogenase sequences were identified, including the already known [NiFe]-hydrogenase, CpI, and CpII sequences, and a third hydrogenase, CpIII was identified in the genome as well. Quantitative real-time PCR experiments were performed in order to analyze transcript abundance of the hydrogenases under diazotrophic and non-diazotrophic growth conditions. There is a markedly reduced level of CpI gene expression together with concomitant increases in CpII gene expression under nitrogen-fixing conditions. Structure-based analyses of the CpI and CpII sequences reveal variations in their catalytic sites that may contribute to their alternative physiological roles. This work demonstrates that the physiological roles of CpI and CpII are to evolve and to consume hydrogen, respectively, in concurrence with their catalytic activities in vitro, with CpII capturing excess reducing equivalents under nitrogen fixation conditions. Comparison of the primary sequences of CpI and CpII and their homologs provides an initial basis for identifying key structural determinants that modulate hydrogen production and hydrogen oxidation activities.

Utilization of inulin-containing waste in industrial fermentations to produce biofuels and bio-based chemicals

Inulins are polysaccharides that belong to an important class of carbohydrates known as fructans and are used by many plants as a means of storing energy. Inulins contain 20 to several thousand fructose units joined by β-2,1 glycosidic bonds, typically with a terminal glucose unit. Plants with high concentrations of inulin include: agave, asparagus, coffee, chicory, dahlia, dandelion, garlic, globe artichoke, Jerusalem artichoke, jicama, onion, wild yam, and yacón. To utilize inulin as its carbon and energy source directly, a microorganism requires an extracellular inulinase to hydrolyze the glycosidic bonds to release fermentable monosaccharides. Inulinase is produced by many microorganisms, including species of Aspergillus, Kluyveromyces, Penicillium, and Pseudomonas. We review various inulinase-producing microorganisms and inulin feedstocks with potential for industrial application as well as biotechnological efforts underway to develop sustainable practices for the disposal of residues from processing inulin-containing crops. A multi-stage biorefinery concept is proposed to convert cellulosic and inulin-containing waste produced at crop processing operations to valuable biofuels and bioproducts using Kluyveromyces marxianus, Yarrowia lipolytica, Rhodotorula glutinis, and Saccharomyces cerevisiae as well as thermochemical treatments.

Mathematical modelling of clostridial acetone-butanol-ethanol fermentation

Clostridial acetone-butanol-ethanol (ABE) fermentation features a remarkable shift in the cellular metabolic activity from acid formation, acidogenesis, to the production of industrial-relevant solvents, solventogensis. In recent decades, mathematical models have been employed to elucidate the complex interlinked regulation and conditions that determine these two distinct metabolic states and govern the transition between them. In this review, we discuss these models with a focus on the mechanisms controlling intra- and extracellular changes between acidogenesis and solventogenesis. In particular, we critically evaluate underlying model assumptions and predictions in the light of current experimental knowledge. Towards this end, we briefly introduce key ideas and assumptions applied in the discussed modelling approaches, but waive a comprehensive mathematical presentation. We distinguish between structural and dynamical models, which will be discussed in their chronological order to illustrate how new biological information facilitates the ‘evolution’ of mathematical models. Mathematical models and their analysis have significantly contributed to our knowledge of ABE fermentation and the underlying regulatory network which spans all levels of biological organization. However, the ties between the different levels of cellular regulation are not well understood. Furthermore, contradictory experimental and theoretical results challenge our current notion of ABE metabolic network structure. Thus, clostridial ABE fermentation still poses theoretical as well as experimental challenges which are best approached in close collaboration between modellers and experimentalists.

Enhanced butanol production by solvent tolerance Clostridium acetobutylicum SE25 from cassava flour in a fibrous bed bioreactor

To enhance the butanol productivity and reduce the material cost, acetone, butanol, and ethanol fermentation by Clostridium acetobutylicum SE25 was investigated using batch, repeated-batch and continuous cultures in a fibrous bed bioreactor, where cassava flour was used as the substrate. With periodical nutrient supplementation, stable butanol production was maintained for about 360h in a 6-cycle repeated-batch fermentation with an average butanol productivity of 0.28g/L/h and butanol yield of 0.32g/g-starch. In addition, the highest butanol productivity of 0.63g/L/h and butanol yield of 0.36g/g-starch were achieved when the dilution rate were investigated in continuous production of acetone, butanol, and ethanol using a fibrous bed bioreactor, which were 231.6% and 28.6% higher than those of the free-cell fermentation. On the other hand, this study also successfully comfirmed that the biofilm can provide an effective protection for the microbial cells which are growing in stressful environment.

Clostridia: a flexible microbial platform for the production of alcohols

Solventogenic clostridia are native producers of ethanol and many higher alcohols employing a broad range of cheap renewable substrates, such as lignocellulosic materials and C1 gases (CO and CO₂). These characteristics enable solventogenic clostridia to act as flexible microbial platforms for the production of liquid biofuels. With the rapid development of genetic tools in recent years, the intrinsic intractability of clostridia has been largely overcome, thus, engineering clostridia for production of chemicals and fuels has attracted increasing interests. Here, we provide an overview of recent progress in the production of alcohols based on solventogenic clostridia. Saccharolytic, cellulolytic and gas-fermenting clostridia are discussed, with a special focus on strategies for metabolic engineering to enable and to improve clostridia for the production of higher alcohols.

Evaluation of viability, metabolic activity and spore quantity in clostridial cultures during ABE fermentation

Flow cytometry, in combination with fluorescent staining, was used to evaluate population heterogeneity in acetone-butanol-ethanol fermentation that was carried out with type strain Clostridium beijerinckii NCIMB 8052 and non-type C. pasteurianum NRRL B-598. A combination of propidium iodide (PI) and carboxyfluorescein diacetate (CFDA), PI plus Syto-9 and bis-oxonol (BOX) alone were employed to distinguish between active and damaged cells together with simultaneous detection of spores. These strategies provided valuable information on the physiological state of clostridia. CFDA and PI staining gave the best separation of four distinct subpopulations of enzymatically active cells, doubly stained cells, damaged cells and spores. Proportional representation of cells in particular sub-regions correlated with growth characteristics, fermentation parameters such as substrate consumption and product formation in both species under different cultivation conditions.

Enhancement of butanol production in Clostridium acetobutylicum SE25 through accelerating phase shift by different phases pH regulation from cassava flour

A prominent delay with 12h was encountered in the phase shift from acidogenesis to solventogenesis in butanol production when the substrate-glucose was replaced by cassava flour. To solve this problem, different phase of pH regulation strategies were performed to shorten this delay time. With this effort, the phase shift occurred smoothly and the fermentation time was shortened. Under the optimal conditions, 16.24g/L butanol and 72h fermentation time were achieved, which were 25.3% higher and 14.3% shorter than those in the case of without pH regulation. Additionally, the effect of CaCO3 on "acid crash" and butanol production was also investigated. It was found that organic acids reassimilation would be of benefit to enhance butanol production. These results indicated that the simple but effective approach for acceleration of phase shift is a promising technique for shortening the fermentation time and improvement of butanol production.

Converting Sugars to Biofuels: Ethanol and Beyond

To date, the most significant sources of biofuels are starch- or sugarcane-based ethanol, which have been industrially produced in large quantities in the USA and Brazil, respectively. However, the ultimate goal of biofuel production is to produce fuels from lignocellulosic biomass-derived sugars with optimal fuel properties and compatibility with the existing fuel distribution infrastructure. To achieve this goal, metabolic pathways have been constructed to produce various fuel molecules that are categorized into fermentative alcohols (butanol and isobutanol), non-fermentative alcohols from 2-keto acid pathways, fatty acids-derived fuels and isoprenoid-derived fuels. This review will focus on current metabolic engineering efforts to improve the productivity and the yield of several key biofuel molecules. Strategies used in these metabolic engineering efforts can be summarized as follows: (1) identification of better enzymes; (2) flux control of intermediates and precursors; (3) elimination of competing pathways; (4) redox balance and cofactor regeneration; and (5) bypassing regulatory mechanisms. In addition to metabolic engineering approaches, host strains are optimized by improving sugar uptake and utilization, and increasing tolerance to toxic hydrolysates, metabolic intermediates and/or biofuel products.

Potential and Prospects of Continuous Polyhydroxyalkanoate (PHA) Production

Together with other so-called “bio-plastics”, Polyhydroxyalkanoates (PHAs) are expected to soon replace established polymers on the plastic market. As a prerequisite, optimized process design is needed to make PHAs attractive in terms of costs and quality. Nowadays, large-scale PHA production relies on discontinuous fed-batch cultivation in huge bioreactors. Such processes presuppose numerous shortcomings such as nonproductive time for reactor revamping, irregular product quality, limited possibility for supply of certain carbon substrates, and, most of all, insufficient productivity. Therefore, single- and multistage continuous PHA biosynthesis is increasingly investigated for production of different types of microbial PHAs; this goes for rather crystalline, thermoplastic PHA homopolyesters as well as for highly flexible PHA copolyesters, and even blocky-structured PHAs consisting of alternating soft and hard segments. Apart from enhanced productivity and constant product quality, chemostat processes can be used to elucidate kinetics of cell growth and PHA formation under constant process conditions. Furthermore, continuous enrichment processes constitute a tool to isolate novel powerful PHA-producing microbial strains adapted to special environmental conditions. The article discusses challenges, potential and case studies for continuous PHA production, and shows up new strategies to further enhance such processes economically by developing unsterile open continuous processes combined with the application of inexpensive carbon feedstocks.

Changes in membrane plasmalogens of Clostridium pasteurianum during butanol fermentation as determined by lipidomic analysis

Changes in membrane lipid composition of Clostridium pasteurianum NRRL B-598 were studied during butanol fermentation by lipidomic analysis, performed by high resolution electrospray ionization tandem mass spectrometry. The highest content of plasmalogen phospholipids correlated with the highest butanol productivity, which indicated a probable role of these compounds in the complex responses of cells toward butanol stress. A difference in the ratio of saturated to unsaturated fatty acids was found between the effect of butanol produced by the cells and butanol added to the medium. A decrease in the proportion of saturated fatty acids during conventional butanol production was observed while a rise in the content of these acids appeared when butanol was added to the culture. The largest change in total plasmalogen content was observed one hour after butanol addition i.e. at the 7th hour of cultivation. When butanol is produced by bacterial cells, then the cells are not subjected to severe stress and responded to it by relatively slowly changing the content of fatty acids and plasmalogens, while after a pulse addition of external butanol (to a final non-lethal concentration of 0.5 % v/v) the cells reacted relatively quickly (within a time span of tens of minutes) by increasing the total plasmalogen content.

Genome Sequence of Clostridium acetobutylicum GXAS18-1, a Novel Biobutanol Production Strain

Clostridium acetobutylicum is an organism involved in the production of acetone and butanol by traditional acetone-butanol-ethanol fermentation (ABE). We report the draft genome sequence of C. acetobutylicum strain GXAS18-1, which can produce ABE directly from cassava flour.

Biobutanol from cheese whey

At present, due to environmental and economic concerns, it is urgent to evolve efficient, clean and secure systems for the production of advanced biofuels from sustainable cheap sources. Biobutanol has proved better characteristics than the more widely used bioethanol, however the main disadvantage of biobutanol is that it is produced in low yield and titer by ABE (acetone-butanol-ethanol) fermentation, this process being not competitive from the economic point of view. In this review we summarize the natural metabolic pathways for biobutanol production by Clostridia and yeasts, together with the metabolic engineering efforts performed up to date with the aim of either enhancing the yield of the natural producer Clostridia or transferring the butanol production ability to other hosts with better attributes for industrial use and facilities for genetic manipulation. Molasses and starch-based feedstocks are main sources for biobutanol production at industrial scale hitherto. We also review herewith (and for the first time up to our knowledge) the research performed for the use of whey, the subproduct of cheese making, as another sustainable source for biobutanol production. This represents a promising alternative that still needs further research. The use of an abundant waste material like cheese whey, that would otherwise be considered an environmental pollutant, for biobutanol production, makes economy of the process more profitable.

Biobutanol production by Clostridium acetobutylicum using xylose recovered from birch Kraft black liquor

Acetone-butanol-ethanol (ABE) fermentation was studied using acid-hydrolyzed xylan recovered from hardwood Kraft black liquor by CO2 acidification as the only carbon source. Detoxification of hydrolyzate using activated carbon was conducted to evaluate the impact of inhibitor removal and fermentation. Xylose hydrolysis yields as high as 18.4% were demonstrated at the highest severity hydrolysis condition. Detoxification using active carbon was effective for removal of both phenolics (76-81%) and HMF (38-52%). Batch fermentation of the hydrolyzate and semi-defined P2 media resulted in a total solvent yield of 0.12-0.13g/g and 0.34g/g, corresponding to a butanol concentration of 1.8-2.1g/L and 7.3g/L respectively. This work is the first study of a process for the production of a biologically-derived biofuel from hemicelluloses solubilized during Kraft pulping and demonstrates the feasibility of utilizing xylan recovered directly from industrial Kraft pulping liquors as a feedstock for biological production of biofuels such as butanol.