Prolonged conversion of n-butyrate to n-butanol with Clostridium saccharoperbutylacetonicum in a two-stage continuous culture with in-situ product removal

Effect of changes in continuous carboxylate feeding on the specific production rate of butanol using Clostridium saccharoperbutylacetonicum

Substrate variability in multi-feedstock biorefineries has implications for the stability of downstream bioprocesses. Here, we studied potential effects of fluctuating feed rates and pH on substrate uptake and butanol production by Clostridium saccharoperbutylacetonicum during continuous co-feeding with butyric and acetic acid. Monitoring the fermentation extensively and at high frequency, enabled us to perform irregular fraction experimental designs. The total acid feed rate and the ratio of butyric acid to acetic acid in the feed were found to be significant factors in their uptake by the culture. Furthermore, to maximize the specific butanol production rate, glucose may not be limited and butyric acid should be supplied at a rate of 7.5 mmol L^-1 h^-1. Surprisingly, pH played a role only indirectly, in its effect on process stability. Obtained results facilitate the control of feed rates based on physiological descriptors, which will be a critical factor in the establishment of multi-feedstock biorefineries.

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.

Effective continuous acetone-butanol-ethanol production with full utilization of cassava by immobilized symbiotic TSH06

BACKGROUND

Butanol production by fermentation has recently attracted increasingly more attention because of its mild reaction conditions and environmentally friendly properties. However, traditional feedstocks, such as corn, are food supplies for human beings and are expensive and not suitable for butanol production at a large scale. In this study, acetone, butanol, and ethanol (ABE) fermentation with non-pretreated cassava using a symbiotic TSH06 was investigated.

RESULTS

In batch fermentation, the butanol concentration of 11.6 g/L was obtained with a productivity of 0.16 g/L/h, which was similar to that obtained from glucose system. A full utilization system of cassava was constructed to improve the fermentation performance, cassava flour was used as the substrate and cassava peel residue was used as the immobilization carrier. ABE fermentation with immobilized cells resulted in total ABE and butanol concentrations of 20 g/L and 13.3 g/L, which were 13.6% and 14.7% higher, respectively, than those of free cells. To further improve the solvent productivity, continuous fermentation was conducted with immobilized cells. In single-stage continuous fermentation, the concentrations of total ABE and butanol reached 9.3 g/L and 6.3 g/L with ABE and butanol productivities of 1.86 g/L/h and 1.26 g/L/h, respectively. In addition, both of the high product concentration and high solvent productivity were achieved in a three-stage continuous fermentation. The ABE productivity and concentration was 1.12 g/L/h and 16.8 g/L, respectively.

CONCLUSIONS

The results indicate that TSH06 could produce solvents from cassava effectively. This study shows that ABE fermentation with cassava as a substrate could be an efficient and economical method of butanol production.

Recent advances and strategies in process and strain engineering for the production of butyric acid by microbial fermentation

Butyric acid is an important platform chemical, which is widely used in the fields of food, pharmaceutical, energy, etc. Microbial fermentation as an alternative approach for butyric acid production is attracting great attention as it is an environmentally friendly bioprocessing. However, traditional fermentative butyric acid production is still not economically competitive compared to chemical synthesis route, due to the low titer, low productivity, and high production cost. Therefore, reduction of butyric acid production cost by utilization of alternative inexpensive feedstock, and improvement of butyric acid production and productivity has become an important target. Recently, several advanced strategies have been developed for enhanced butyric acid production, including bioprocess techniques and metabolic engineering methods. This review provides an overview of advances and strategies in process and strain engineering for butyric acid production by microbial fermentation. Additionally, future perspectives on improvement of butyric acid production are also proposed.

High-efficient n-butanol production by co-culturing Clostridium acetobutylicum and Saccharomyces cerevisiae integrated with butyrate fermentative supernatant addition

Butanol is not only an important chemical intermediate and solvent in pharmaceutical and cosmetics industries, but also considered as an advanced biofuel. Although species of the natural host Clostridium have been engineered, butanol titers in the anaerobe seem to be limited by its intolerance to butanol less than 13 g/L. Here we aimed to develop a technology for enhancing butanol production by a co-culture system with butyrate fermentative supernatant addition. First, when adding 4.0 g/L butyrate into the acetone-butanol-ethanol (ABE) fermentation broth with single-shot at 24 h, the "acid crash" phenomenon occurred and the ABE fermentation performance deteriorated. Subsequently, we found that adding certain amino acids could effectively enhance butyrate re-assimilation, butanol tolerance and titer (from 11.1 to 14.8 g/L). Additionally, in order to decrease the raw material cost, butyrate fermentative supernatant produced by Clostridium tyrobutyricum was applied to butanol production in the Clostridium acetobutylicum/Saccharomyces cerevisiae co-culture system, instead of adding synthetic butyrate. Final butanol and total ABE concentrations reached higher levels of 16.3 and 24.8 g/L with increments of 46.8 and 37.8%, respectively. These results show that the proposed fermentation strategy has great potential for efficiently butanol production with an economic approach.

Upgrading syngas fermentation effluent using Clostridium kluyveri in a continuous fermentation

BACKGROUND

The product of current syngas fermentation systems is an ethanol/acetic acid mixture and the goal is to maximize ethanol recovery. However, ethanol currently has a relatively low market value and its separation from the fermentation broth is energy intensive. We can circumvent these disadvantages of ethanol production by converting the dilute ethanol/acetic acid mixture into products with longer carbon backbones, which are of higher value and are more easily extracted than ethanol. Chain elongation, which is the bioprocess in which ethanol is used to elongate short-chain carboxylic acids to medium-chain carboxylic acids (MCCAs), has been studied with pure cultures and open cultures of microbial consortia (microbiomes) with several different substrates. While upgrading syngas fermentation effluent has been studied with open cultures, to our knowledge, no study exists that has performed this with pure cultures.

RESULTS

Here, pure cultures of Clostridium kluyveri were used in continuous bioreactors to convert ethanol/acetic acid mixtures into MCCAs. Besides changing the operating conditions in regards to substrate loading rates and composition, the effect of in-line product extraction, pH, and the use of real syngas fermentation effluent on production rates were tested. Increasing the organic loading rates resulted in proportionally higher production rates of n-caproic acid, which were up to 40 mM day^-1 (4.64 g L^-1 day^-1) at carbon conversion efficiencies of 90% or higher. The production rates were similar for bioreactors with and without in-line product extraction. Furthermore, a lower ethanol/acetic acid ratio (3:1 instead of 10:1) enabled faster and more efficient n-caproic acid production. In addition, n-caprylic acid production was observed for the first time with C. kluyveri (up to 2.19 ± 0.34 mM in batch). Finally, the use of real effluent from syngas fermentation, without added yeast extract, but with added defined growth factors, did maintain similar production rates. Throughout the operating period, we observed that the metabolism of C. kluyveri was inhibited at a mildly acidic pH value of 5.5 compared to a pH value of 7.0, while reactor microbiomes perform successfully at mildly acidic conditions.

CONCLUSIONS

Clostridium kluyveri can be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5.

Microbiome-based carboxylic acids production: from serum bottles to bioreactors

We propose a cheap, time and resource efficient screening and upscaling strategy for microbiome-based production of bio-based fuels and chemicals.

Development of a High-Efficiency Transformation Method and Implementation of Rational Metabolic Engineering for the Industrial Butanol Hyperproducer Clostridium saccharoperbutylacetonicum Strain N1-4

While a majority of academic studies concerning acetone, butanol, and ethanol (ABE) production by Clostridium have focused on Clostridium acetobutylicum, other members of this genus have proven to be effective industrial workhorses despite the inability to perform genetic manipulations on many of these strains. To further improve the industrial performance of these strains in areas such as substrate usage, solvent production, and end product versatility, transformation methods and genetic tools are needed to overcome the genetic intractability displayed by these species. In this study, we present the development of a high-efficiency transformation method for the industrial butanol hyperproducer Clostridium saccharoperbutylacetonicum strain N1-4 (HMT) ATCC 27021. Following initial failures, we found that the key to creating a successful transformation method was the identification of three distinct colony morphologies (types S, R, and I), which displayed significant differences in transformability. Working with the readily transformable type I cells (transformation efficiency, 1.1 × 10⁶ CFU/μg DNA), we performed targeted gene deletions in C. saccharoperbutylacetonicum N1-4 using a homologous recombination-mediated allelic exchange method. Using plasmid-based gene overexpression and targeted knockouts of key genes in the native acetone-butanol-ethanol (ABE) metabolic pathway, we successfully implemented rational metabolic engineering strategies, yielding in the best case an engineered strain (Clostridium saccharoperbutylacetonicum strain N1-4/pWIS13) displaying an 18% increase in butanol titers and 30% increase in total ABE titer (0.35 g ABE/g sucrose) in batch fermentations. Additionally, two engineered strains overexpressing aldehyde/alcohol dehydrogenases (encoded by adh11 and adh5) displayed 8.5- and 11.8-fold increases (respectively) in batch ethanol production.

IMPORTANCE

This paper presents the first steps toward advanced genetic engineering of the industrial butanol producer Clostridium saccharoperbutylacetonicum strain N1-4 (HMT). In addition to providing an efficient method for introducing foreign DNA into this species, we demonstrate successful rational engineering for increasing solvent production. Examples of future applications of this work include metabolic engineering for improving desirable industrial traits of this species and heterologous gene expression for expanding the end product profile to include high-value fuels and chemicals.

Consolidated Bioprocessing for Butyric Acid Production from Rice Straw with Undefined Mixed Culture

Lignocellulosic biomass is a renewable source with great potential for biofuels and bioproducts. However, the cost of cellulolytic enzymes limits the utilization of the low-cost bioresource. This study aimed to develop a consolidated bioprocessing without the need of supplementary cellulase for butyric acid production from lignocellulosic biomass. A stirred-tank reactor with a working volume of 21 L was constructed and operated in batch and semi-continuous fermentation modes with a cellulolytic butyrate-producing microbial community. The semi-continuous fermentation with intermittent discharging of the culture broth and replenishment with fresh medium achieved the highest butyric acid productivity of 2.69 g/(L· d). In semi-continuous operation mode, the butyric acid and total carboxylic acid concentrations of 16.2 and 28.9 g/L, respectively, were achieved. Over the 21-day fermentation period, their cumulative yields reached 1189 and 2048 g, respectively, corresponding to 41 and 74% of the maximum theoretical yields based on the amount of NaOH pretreated rice straw fed in. This study demonstrated that an undefined mixed culture-based consolidated bioprocessing for butyric acid production can completely eliminate the cost of supplementary cellulolytic enzymes.

Kinetic Study of Acetone-Butanol-Ethanol Fermentation in Continuous Culture

Acetone-butanol-ethanol (ABE) fermentation by clostridia has shown promise for industrial-scale production of biobutanol. However, the continuous ABE fermentation suffers from low product yield, titer, and productivity. Systems analysis of the continuous ABE fermentation will offer insights into its metabolic pathway as well as into optimal fermentation design and operation. For the ABE fermentation in continuous Clostridium acetobutylicum culture, this paper presents a kinetic model that includes the effects of key metabolic intermediates and enzymes as well as culture pH, product inhibition, and glucose inhibition. The kinetic model is used for elucidating the behavior of the ABE fermentation under the conditions that are most relevant to continuous cultures. To this end, dynamic sensitivity analysis is performed to systematically investigate the effects of culture conditions, reaction kinetics, and enzymes on the dynamics of the ABE production pathway. The analysis provides guidance for future metabolic engineering and fermentation optimization studies.

Glutamate and histidine improve both solvent yields and the acid tolerance response of Clostridium beijerinckii NCP 260

AIMS

This study aims to examine the effect of amino acid supplementation on solvent production by Clostridium beijerinckii during the acetone-butanol fermentation and to determine whether amino acids are involved in the acid tolerance response (ATR), which results in increased solvents.

METHODS AND RESULTS

Fermentation studies with Cl. beijerinckii NCP 260 in limited-nitrogen media supplemented with glutamate, glutamine, lysine, proline, histidine or asparagine revealed that only glutamate, glutamine or histidine increased butanol titres comparable to control media. Acid survival tests at pH 5 showed that glutamate and histidine were effective in protecting Cl. beijerinckii cells against acid shock, and may be involved in the ATR. Using quantitative PCR, the transcription of the glutamine synthetase, nitrogen regulator and glutamate synthase operon (glnA-nitR-gltAB) was monitored during acid shock conditions, and expression of both the nitR and gltA genes was shown to be increased twofold.

CONCLUSIONS

Glutamate and histidine specifically enhance the ATR in Cl. beijerinckii NCP 260, and the genes encoding glutamate synthase and the NitR regulator are both upregulated, predicted to lead to increased endogenous glutamate pools during acidogenesis. This may enhance the ATR and allow more viable cells to enter solventogenesis, thereby increasing butanol titres. Glutamine, glutamate and histidine may also afford protection from butanol stress directly.

SIGNIFICANCE AND IMPACT OF THE STUDY

Using substrates naturally rich in glutamine, glutamate and histidine in industrial fermentations is a promising means to increase acid survival and solvent yields in solventogenic Clostridium.

Effective multiple stages continuous acetone-butanol-ethanol fermentation by immobilized bioreactors: Making full use of fresh corn stalk

In order to make full use of the fresh corn stalk, the sugar containing juice was used as the sole substrate for acetone-butanol-ethanol production without any nutrients supplement, and the bagasse after squeezing the juice was used as the immobilized carrier. A total 21.34g/L of ABE was produced in batch cells immobilization system with ABE yield of 0.35g/g. A continuous fermentation containing three stages with immobilized cells was conducted and the effect of dilution rate on fermentation was investigated. As a result, the productivity and ABE solvents concentration reached 0.80g/Lh and 19.93g/L, respectively, when the dilution rate in each stage was 0.12/h (corresponding to a dilution rate of 0.04/h in the whole system). And the long-term operation indicated the continuous multiple stages ABE fermentation process had good stability and showed the great potential in future industrial applications.

Production of biobutanol from cellulose hydrolysate by the Escherichia coli co-culture system

The commercialization of the n-butanol bioprocess is largely dependent on the price of feedstocks. Renewable cellulose appears to be an appealing feedstock. The microbial production of n-butanol still remains challenging because of the limited availability of intracellular NADH. To address this issue, an Escherichia coli strain carrying the clostridial CoA-dependent pathway was supplied with heterologous formate dehydrogenase. With the cellulose hydrolysate of rice straw, this single strain produced cellulosic biobutanol with a production yield at 35% of the theoretical and a productivity of 0.093 g L(-1) h(-1). In an alternative method, the production involved a co-culture system consisting of two separate strains provided with the full CoA-dependent pathway. This system achieved a production yield and productivity reaching 62.8% of the theoretical and 0.163 g L(-1) h(-1), respectively. The result indicates that the E. coli co-culture system is technically promising for the production of cellulosic biobutanol.

Metabolic analysis of butanol production from acetate in Clostridium saccharoperbutylacetonicum N1-4 using13C tracer experiments

This study directly verified butanol production from exogenously added acetate byClostridium saccharoperbutylacetonicumN1-4, and illustrated its metabolismvia¹³C-tracer experiments.

Production of Fatty Acid-derived valuable chemicals in synthetic microbes

Fatty acid derivatives, such as hydroxy fatty acids, fatty alcohols, fatty acid methyl/ethyl esters, and fatty alka(e)nes, have a wide range of industrial applications including plastics, lubricants, and fuels. Currently, these chemicals are obtained mainly through chemical synthesis, which is complex and costly, and their availability from natural biological sources is extremely limited. Metabolic engineering of microorganisms has provided a platform for effective production of these valuable biochemicals. Notably, synthetic biology-based metabolic engineering strategies have been extensively applied to refactor microorganisms for improved biochemical production. Here, we reviewed: (i) the current status of metabolic engineering of microbes that produce fatty acid-derived valuable chemicals, and (ii) the recent progress of synthetic biology approaches that assist metabolic engineering, such as mRNA secondary structure engineering, sensor-regulator system, regulatable expression system, ultrasensitive input/output control system, and computer science-based design of complex gene circuits. Furthermore, key challenges and strategies were discussed. Finally, we concluded that synthetic biology provides useful metabolic engineering strategies for economically viable production of fatty acid-derived valuable chemicals in engineered microbes.

Potential production platform of n-butanol in Escherichia coli

We proposed a potential production platform of n-butanol in Escherichia coli. First, a butyrate-conversion strain was developed by removal of undesired genes and recruiting endogenous atoDA and Clostridium adhE2. Consequently, this E. coli strain grown on the M9 mineral salt with yeast extract (M9Y) was shown to produce 6.2g/L n-butanol from supplemented butyrate at 36h. The molar conversion yield of n-butanol on butyrate reaches 92%. Moreover, the production platform was advanced by additional inclusion of a butyrate-producing strain. This strain was equipped with a pathway comprising atoDA and heterologous genes for the synthesis of butyrate. Without butyrate, the butyrate-conversion and the butyrate-producing strains were co-cultured in M9Y medium and produced 5.5g/L n-butanol from glucose at 24h. The production yield on glucose accounts for 69% of the theoretical yield. Overall, it indicates a promise of the developed platform for n-butanol production in E. coli.

Biocatalytic reduction of carboxylic acids

An increasing demand for non-petroleum-based products is envisaged in the near future. Carboxylic acids such as citric acid, succinic acid, fatty acids, and many others are available in abundance from renewable resources and they could serve as economic precursors for bio-based products such as polymers, aldehyde building blocks, and alcohols. However, we are confronted with the problem that carboxylic acid reduction requires a high level of energy for activation due to the carboxylate's thermodynamic stability. Catalytic processes are scarce and often their chemoselectivity is insufficient. This review points at bio-alternatives: currently known enzyme classes and organisms that catalyze the reduction of carboxylic acids are summarized. Two totally distinct biocatalyst lines have evolved to catalyze the same reaction: aldehyde oxidoreductases from anaerobic bacteria and archea, and carboxylate reductases from aerobic sources such as bacteria, fungi, and plants. The majority of these enzymes remain to be identified and isolated from their natural background in order to evaluate their potential as industrial biocatalysts.

Effects of supplementary butyrate on butanol production and the metabolic switch in Clostridium beijerinckii NCIMB 8052: genome-wide transcriptional analysis with RNA-Seq

BACKGROUND

Butanol (n-butanol) has high values as a promising fuel source and chemical feedstock. Biobutanol is usually produced by the solventogenic clostridia through a typical biphasic (acidogenesis and solventogenesis phases) acetone-butanol-ethanol (ABE) fermentation process. It is well known that the acids produced in the acidogenic phase are significant and play important roles in the switch to solventogenesis. However, the mechanism that triggers the metabolic switch is still not clear.

RESULTS

Sodium butyrate (40 mM) was supplemented into the medium for the ABE fermentation with Clostridium beijerinckii NCIMB 8052. With butyrate addition (reactor R1), solvent production was triggered early in the mid-exponential phase and completed quickly in < 50 h, while in the control (reactor R2), solventogenesis was initiated during the late exponential phase and took > 90 h to complete. Butyrate supplementation led to 31% improvement in final butanol titer, 58% improvement in sugar-based yield, and 133% improvement in butanol productivity, respectively. The butanol/acetone ratio was 2.4 versus 1.8 in the control, indicating a metabolic shift towards butanol production due to butyrate addition. Genome-wide transcriptional dynamics was investigated with RNA-Seq analysis. In reactor R1, gene expression related to solventogenesis was induced about 10 hours earlier when compared to that in reactor R2. Although the early sporulation genes were induced after the onset of solventogenesis in reactor R1 (mid-exponential phase), the sporulation events were delayed and uncoupled from the solventogenesis. In contrast, in reactor R2, sporulation genes were induced at the onset of solventogenesis, and highly expressed through the solventogenesis phase. The motility genes were generally down-regulated to lower levels prior to stationary phase in both reactors. However, in reactor R2 this took much longer and gene expression was maintained at comparatively higher levels after entering stationary phase.

CONCLUSIONS

Supplemented butyrate provided feedback inhibition to butyrate formation and may be re-assimilated through the reversed butyrate formation pathway, thus resulting in an elevated level of intracellular butyryl phosphate, which may act as a phosphate donor to Spo0A and then trigger solventogenesis and sporulation events. High-resolution genome-wide transcriptional analysis with RNA-Seq revealed detailed insights into the biochemical effects of butyrate on solventogenesis related-events at the gene regulation level.

Engineering E. coli strain for conversion of short chain fatty acids to bioalcohols

BACKGROUND

Recent progress in production of various biofuel precursors and molecules, such as fatty acids, alcohols and alka(e)nes, is a significant step forward for replacing the fossil fuels with renewable fuels. A two-step process, where fatty acids from sugars are produced in the first step and then converted to corresponding biofuel molecules in the second step, seems more viable and attractive at this stage. We have engineered an Escherichia coli strain to take care of the second step for converting short chain fatty acids into corresponding alcohols by using butyrate kinase (Buk), phosphotransbutyrylase (Ptb) and aldehyde/alcohol dehydrogenase (AdhE2) from Clostridium acetobutylicum.

RESULTS

The engineered E. coli was able to convert butyric acid and other short chain fatty acids of chain length C3 to C7 into corresponding alcohols and the efficiency of conversion varied with different E. coli strain type. Glycerol proved to be a better donor of ATP and electron as compared to glucose for converting butyric acid to butanol. The engineered E. coli was able to tolerate up to 100 mM butyric acid and produced butanol with the conversion rate close to 100% under anaerobic condition. Deletion of native genes, such as fumarate reductase (frdA) and alcohol dehydrogenase (adhE), responsible for side products succinate and ethanol, which act as electron sink and could compete with butyric acid uptake, did not improve the butanol production efficiency. Indigenous acyl-CoA synthetase (fadD) was found to play no role in the conversion of butyric acid to butanol. Engineered E. coli was cultivated in a bioreactor under controlled condition where 60 mM butanol was produced within 24 h of cultivation. A continuous bioreactor with the provision of cell recycling allowed the continuous production of butanol at the average productivity of 7.6 mmol/l/h until 240 h.

CONCLUSIONS

E. coli engineered with the pathway from C. acetobutylicum could efficiently convert butyric acid to butanol. Other short chain fatty acids with the chain length of C3 to C7 were also converted to the corresponding alcohols. The ability of engineered strain to convert butyric acid to butanol continuously demonstrates commercial significance of the system.

Direct conversion of sugars and organic acids to biobutanol by non-growing cells of Clostridium spp. incubated in a nitrogen-free medium

Several Clostridium spp. were incubated in a nitrogen-free medium (non-growth medium) containing only butyric acid as a sole precursor for performing butanol production by non-growing cells. Non-growing cells of Clostridium spp., especially Clostridium beijerinckii TISTR 1461, could convert butyric acid to butanol via their sole solventogenic activity. This activity was further enhanced in the presence of glucose as a co-substrate. In addition to glucose, other monosaccharides (i.e., galactose and xylose) and disaccharides (i.e., maltose, sucrose, and lactose) could also be used as a co-substrate with butyric acid. Among the organic acids tested (i.e., formic, acetic, propionic, and butyric acids), only butyric and acetic acids were converted to butanol. This study has shown that it is possible to use the non-growing cells of Clostridium spp. for direct conversion of sugars and organic acids to biobutanol. With this strategy, C. beijerinckii TISTR 1461 produced 12 g/L butanol from 15 g/L glucose and 10 g/L butyric acid with a high butanol yield of 0.68 C-mol/C-mol and a high butanol ratio of 88 %.

Integrating syngas fermentation with the carboxylate platform and yeast fermentation to reduce medium cost and improve biofuel productivity

To ensure economic implementation of syngas fermentation as a fuel-producing platform, engineers and scientists must both lower operating costs and increase product value. A considerable part of the operating costs is spent to procure chemicals for fermentation medium that can sustain sufficient growth of carboxydotrophic bacteria to convert synthesis gas (syngas: carbon monoxide, hydrogen, and carbon dioxide) into products such as ethanol. Recently, we have observed that wildtype carboxydotrophic bacteria (including Clostridium ljungdahlii) can produce alcohols with a longer carbon chain than ethanol via syngas fermentation when supplied with the corresponding carboxylic acid precursors, resulting in possibilities of increasing product value. Here, we evaluated a proof-of-concept system to couple syngas fermentation with the carboxylate platform to both lower medium costs and increase product value. Our carboxylate platform concept consists of an open culture, anaerobic fermentor that is fed with corn beer from conventional yeast fermentation in the corn kernel-to-ethanol industry. The mixed-culture anaerobic fermentor produces a mixture ofcarboxylic acids at dilute concentrations within the carboxylate platform effluent (CPE). Besides providing carboxylic acid precursors, this effluent may represent an inexpensive growth medium. An elemental analysis demonstrated that the CPE lacked certain essential trace metals, but contained ammonium, phosphate, sodium, chloride, potassium, magnesium, calcium, and sulphate at required concentrations. CPE medium with the addition of a trace metal solution supported growth and alcohol production of C. ljungdahlii at similar or better levels compared with an optimized synthetic medium (modified ATCC 1754 medium). Other expensive supplements, such as yeast extract or macro minerals (ammonium, phosphate), were not required. Finally, n-butyric acid and n-caproic acid within the CPE were converted into their corresponding medium-chain alcohols n-butanol and n-hexanol.

The two stage immobilized column reactor with an integrated solvent recovery module for enhanced ABE production

The production of acetone, butanol, and ethanol (ABE) by fermentation is a process that had been used by industries for decades. Two stage immobilized column reactor system integrated with liquid-liquid extraction was used with immobilized Clostridium acetobutylicum DSM 792, to enhance the ABE productivity and yield. The sugar mixture (glucose, mannose, galactose, arabinose, and xylose) representative to the lignocellulose hydrolysates was used as a substrate for continuous ABE production. Maximum total ABE solvent concentration of 20.30 g L(-1) was achieved at a dilution rate (D) of 0.2h(-1), with the sugar mixture as a substrate. The maximum solvent productivity (10.85 g L(-1)h(-1)) and the solvent yield (0.38 g g(-1)) were obtained at a dilution rate of 1.0 h(-1). The maximum sugar mixture utilization rate was achieved with the present set up which is difficult to reach in a single stage chemostat. The system was operated for 48 days without any technical problems.

"In situ" removal of isopropanol, butanol and ethanol from fermentation broth by gas stripping

In this study, the removal of IBE from aqueous solutions by gas stripping has been characterized. The effect of one or more components in the solution on the kinetics of the separation has been studied, both at 37°C and at 70°C. Gas stripping has been applied to batch, repeated batch and continuous cultures of Clostridium beijerinckii grown on a glucose/xylose mixed sugar substrate mimicking lignocellulosic hydrolysates, with the aim of finding optimal conditions for a stable IBE-producing culture with high productivity. An innovative repeated-batch process has been demonstrated in which the gas-stripping is performed at 70°C, resulting in a prolonged stable IBE culture.

Enhancing butanol production with Clostridium pasteurianum CH4 using sequential glucose-glycerol addition and simultaneous dual-substrate cultivation strategies

Adding butyrate significantly enhanced butanol production from glycerol with Clostridium pasteurianum CH4, which predominantly produces butyrate (instead of butanol) when grown on glucose. Hence, the butyrate produced from assimilating glucose can be used to stimulate butanol production from glycerol under dual-substrate cultivation with glucose and glycerol. This proposed butanol production process was conducted by employing sequential or simultaneous addition of the two substrates. The latter approach exhibited better carbon source utilization and butanol production efficiencies. Under the optimal glucose to glycerol ratio (20 g L(-1) to 60 g L(-1)), the simultaneous dual-substrate strategy obtained maximum butanol titer, productivity and yield of 13.3 g L(-1), 0.28 g L(-1) h(-1), and 0.38 mol butanol/mol glycerol, respectively. Moreover, bagasse and crude glycerol as dual-substrates were also converted into butanol efficiently with a maximum butanol concentration, productivity and yield of 11.8 g L(-1), 0.14 g L(-1) h(-1), and 0.33 mol butanol/mol glycerol, respectively.

Investigation of gas stripping and pervaporation for improved feasibility of two-stage butanol production process

Gas stripping and pervaporation are investigated for butanol recovery in a two-stage acetone-butanol-ethanol (ABE) fermentation process. The first stage is operated in a continuous mode and the second stage as a fed-batch. Gas stripping coupled to the second stage and operated intermittently enabled additional glucose feeding in the second stage and up to 59 g/L butanol and 73 g/L total ABE solvents in the condensate. Concentration of 167 g/L butanol and 269 g/L ABE in the permeate was measured in ex situ pervaporation experiments using a PDMS membrane at temperature of 37 °C and pressure of 10mbars. The "operating window" tool is introduced to evaluate the feasibility of the existing ABE fermentations operated as continuous with cell recycle, as two-stages, with biomass immobilization or with integrated product removal. This tool enables the identification of the most favorable process configuration, which is the combination of cell immobilization and integrated product removal.

Shaping reactor microbiomes to produce the fuel precursor n-butyrate from pretreated cellulosic hydrolysates

To maximize the production of carboxylic acids with open cultures of microbial consortia (reactor microbiomes), we performed experiments to understand which factors affect the community dynamics and performance parameters. We operated six thermophilic (55 °C) bioreactors to test how the factors: (i) biomass pretreatment; (ii) bioreactor operating conditions; and (iii) bioreactor history (after perturbations during the operating period) affected total fermentation product and n-butyrate performance parameters with corn fiber as the cellulosic biomass waste. We observed a maximum total fermentation product yield of 39%, a n-butyrate yield of 23% (both on a COD basis), a maximum total fermentation production rate of 0.74 g COD l(-1) d(-1) and n-butyrate production rate of 0.47 g COD l(-1) d(-1) in bioreactors that were fed with dilute-acid pretreated corn fiber at a pH of 5.5. Pyrosequencing of 16S rRNA genes with constrained ordination and other statistical methods showed that changes in operating conditions to enable dilution of toxic carboxylic acid products, which lead to these maximum performance parameters, also altered the composition of the microbiome, and that the microbiome, in turn, affected the performance. Operating conditions are an important factor (tool for operators) to shape reactor microbiomes, but other factors, such as substrate composition after biomass pretreatment and bioreactor history are also important. Further optimization of operating conditions must relieve the toxicity of carboxylic acids at acidic bioreactor pH levels even more, and this can, for example, be accomplished by extracting the product from the bioreactor solutions.

Integrated bioprocess for long-term continuous cultivation of Clostridium acetobutylicum coupled to pervaporation with PDMS composite membranes

A continuous cultivation of Clostridium acetobutylicum ATCC 824 is described using a two-stage design to mimic the two phases of batch culture growth of the organism. A hydrophobic pervaporation unit was coupled to the second fermentor containing the highest solvent titers. This in situ product recovery technology efficiently decreased butanol toxicity in the fermentor while the permeate was enriched to 57-195 g L(-1) total solvents depending on the solvent concentrations in the fermentor. By the alleviation of product inhibition, the glucose concentration could be increased from 60 to 126 g L(-1) while the productivity increased concomitantly from 0.13 to 0.30 g L(-1)h(-1). The continuous fermentation was conducted for 1172 h during which the pervaporation was coupled to the second fermentor for 475 h with an average flux of 367 g m(-2)h(-1). The energy consumption was calculated for a 2 wt.% n-butanol fermentation broth and compared with the conventional process.