Bioproduction of butanol from biomass: from genes to bioreactors

Role of Different Feedstocks on the Butanol Production Through Microbial and Catalytic Routes

Among the renewable fuels, butanol has become an attractive, economic and sustainable choice because of cost elevation in petroleum fuel, diminishing the oil reserves and an increase of green house effect. Butanol can be derived from renewable sources by using the natural bio-resources and agro-wastes such as orchard wastes, peanut wastes, wheat straw, barley straw and grasses via Acetone Butanol Ethanol (ABE) process. On the other hand, butanol can be directly formed from chemical route involving catalysts also such as from ethanol through aldol condensation. This review presents extensive evaluation for the production of butanol deploying microbial and catalytic routes.

A systematic optimization of styrene biosynthesis in Escherichia coli BL21(DE3)

BACKGROUND

Styrene is a versatile commodity petrochemical used as a monomer building-block for the synthesis of many useful polymers. Although achievements have been made on styrene biosynthesis in microorganisms, several bottleneck problems limit factors for further improvement in styrene production.

RESULTS

A two-step styrene biosynthesis pathway was developed and introduced into Escherichia coli BL21(DE3). Systematic optimization of styrene biosynthesis, such as enzyme screening, codon and plasmid optimization, metabolic flow balance, and in situ fermentation was performed. Candidate isoenzymes of the rate-limiting enzyme phenylalanine ammonia lyase (PAL) were screened from Arabidopsis thaliana (AtPAL2), Fagopyrum tataricum (FtPAL), Petroselinum crispum (PcPAL), and Artemisia annua (AaPAL). After codon optimization, AtPAL2 was found to be the most effective one, and the engineered strain was able to produce 55 mg/L styrene. Subsequently, plasmid optimization was performed, which improved styrene production to 103 mg/L. In addition, two upstream shikimate pathway genes, aroF and pheA, were overexpressed in the engineered strain, which resulted in styrene production of 210 mg/L. Subsequently, combined overexpression of tktA and ppsA increased styrene production to 275 mg/L. Finally, in situ product removal was used to ease the burden of end-product toxicity. By using isopropyl myristate as a solvent, styrene production reached a final titer of 350 mg/L after 48 h of shake-flask fermentation, representing a 636% improvement, which compared with that achieved in the original strain.

CONCLUSIONS

This present study achieved the highest titer of de novo production of styrene in E. coli at shake-flask fermentation level. These results obtained provided new insights for the development of microbial production of styrene in a sustainable and environment friendly manner.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Calculation of anharmonic effect on the dissociation of ethylene glycol

The unimolecular dissociation rate constants of ethylene glycol were examined using the MP2/6-311[Formula: see text]G(d,p) method based on the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The effect of anharmonicity on the dissociation rate constants was evaluated at 500–4000[Formula: see text]K temperatures of the canonical system and 25,182–50,235[Formula: see text]cm[Formula: see text] total energies of the microcanonical system. The comparison of the results showed that the H2O elimination reaction played a critical role in the decomposition processes of ethylene glycol. The results of the rate constant calculations indicated that the H2O elimination reaction dominated at low temperatures, whereas the direct C–C bond dissociation reaction (CH2OHCH2OH [Formula: see text] CH2OH[Formula: see text][Formula: see text][Formula: see text]CH2OH) dominated at high temperatures. For channel 1, CH2OH[Formula: see text][Formula: see text][Formula: see text]CH2OH, the anharmonic effect of the canonical system was not observed, while it became more obvious with the increasing total energies in the microcanonical system. For channels 2–5, CH3CHO[Formula: see text][Formula: see text][Formula: see text]H2O, CH2CHOH[Formula: see text][Formula: see text][Formula: see text]H2O, CH3OH[Formula: see text][Formula: see text][Formula: see text]CHOH, and CH2OHCHO[Formula: see text][Formula: see text][Formula: see text]H2, the anharmonic effect of canonical and microcanonical systems became more obvious with increasing temperatures and total energies. The comparison showed that, for channels 1 and 4, C–C bond dissociation and the anharmonic effect of the microcanonical system were more evident, whereas the anharmonic effect of the canonical system was more predominant for channels 2 (CH3CHO[Formula: see text][Formula: see text][Formula: see text]H2O), 3 (CH2CHOH[Formula: see text][Formula: see text][Formula: see text]H2O), and 5 (CH2OHCHO[Formula: see text][Formula: see text][Formula: see text]H2).

Enhanced robustness in acetone-butanol-ethanol fermentation with engineered Clostridium beijerinckii overexpressing adhE2 and ctfAB

Clostridium beijerinckii CC101 was engineered to overexpress aldehyde/alcohol dehydrogenase (adhE2) and CoA-transferase (ctfAB). Solvent production and acid assimilation were compared between the parental and engineered strains expressing only adhE2 (CC101-SV4) and expressing adhE2, ald and ctfAB (CC101-SV6). CC101-SV4 showed an early butanol production from glucose but stopped pre-maturely at a low butanol concentration of ∼6g/L. Compared to CC101, CC101-SV6 produced more butanol (∼12g/L) from glucose and was able to re-assimilate more acids, which prevented "acid crash" and increased butanol production, under all conditions studied. CC101-SV6 also showed better ability in using glucose and xylose present in sugarcane bagasse hydrolysate, and produced 9.4g/L solvents (acetone, butanol and ethanol) compared to only 2.6g/L by CC101, confirming its robustness and better tolerance to hydrolysate inhibitors. The engineered strain of C. beijerinckii overexpressing adhE2 and ctfAB should have good potential for producing butanol from lignocellulosic biomass hydrolysates.

High-efficiency acetone-butanol-ethanol production and recovery in non-strict anaerobic gas-stripping fed-batch fermentation

Conventional acetone-butanol-ethanol (ABE) fermentation coupled with gas stripping is conducted under strict anaerobic conditions. In this work, a fed-batch ABE fermentation integrated with gas stripping (FAFIGS) system using a non-strict anaerobic butanol-producing symbiotic system, TSH06, was investigated for the efficient production of butanol. To save energy and keep a high gas-stripping efficiency, the integrated fermentation was conducted by adjusting the butanol recovery rate. The gas-stripping efficiency increased when the butanol concentration increased from 6 to 12 g/L. However, in consideration of the butanol toxicity to TSH06, 8 g/L butanol was the optimal concentration for this FAFIGS process. A model for describing the relationship between the butanol recovery rate and the gas flow rate was developed, and the model was subsequently applied to adjust the butanol recovery rate during the FAFIGS process. In the integrated system under non-strict anaerobic condition, relatively stable butanol concentrations of 7 to 9 g/L were achieved by controlling the gas flow rate which varied between 1.6 and 3.5 vvm based on the changing butanol productivity. 185.65 g/L of butanol (267.15 g/L of ABE) was produced in 288 h with a butanol recovery ratio of 97.36%. The overall yield and productivity of butanol were 0.23 g/g and 0.64 g/L/h, respectively. This study demonstrated the feasibility of using FAFIGS under non-strict anaerobic conditions with TSH06. This work is helpful in characterizing the butanol anabolism performance of TSH06 and provides a simple and efficient scheme for butanol production.

Engineering coenzyme A-dependent pathway from Clostridium saccharobutylicum in Escherichia coli for butanol production

Clostridium saccharobutylicum has been proved to be efficient in butanol fermentation from various feedstocks. Whereas, lack of genetic manipulation system has severely hindered the engineering of C. saccharobutylicum for more extensive applications. In this study, recombinant Escherichia coli harboring heterologous coenzyme A-dependent pathway from C. saccharobutylicum DSM 13864 was constructed, which consisted of solventogenic pathway genes: acetoacetyl-CoA thiolase (thlA), aldehyde/alcohol dehydrogenase (adhE2) and bcs-operon (crt-bcd1-etfB2-fixB2-hbd). Then, a butanol titer of 67mg/L was attained. After replacing thlA with acetyl-CoA acetyltransferase (atoB) from E. coli and deleting the competitive branch genes lactate dehydrogenase (ldhA), aldehyde/alcohol dehydrogenase (adhE1) and fumarate reductase (frdBC), the butanol titer was successfully improved for 3.8-fold (254mg/L). Under the optimum fermentation conditions, the final butanol titer reached 584mg/L after 120h. This result demonstrates the feasibility of adapting CoA-dependent solventogenic pathway from C. saccharobutylicum in E. coli for butanol synthesis.

Improved n-butanol production via co-expression of membrane-targeted tilapia metallothionein and the clostridial metabolic pathway in Escherichia coli

Background

N-Butanol has favorable characteristics for use as either an alternative fuel or platform chemical. Bio-based n-butanol production using microbes is an emerging technology that requires further development. Although bio-industrial microbes such as Escherichia coli have been engineered to produce n-butanol, reactive oxygen species (ROS)-mediated toxicity may limit productivity. Previously, we show that outer-membrane-targeted tilapia metallothionein (OmpC-TMT) is more effective as an ROS scavenger than human and mouse metallothioneins to reduce oxidative stress in the host cell.

Results

The host strain (BUT1-DE) containing the clostridial n-butanol pathway displayed a decreased growth rate and limited n-butanol productivity, likely due to ROS accumulation. The clostridial n-butanol pathway was co-engineered with inducible OmpC-TMT in E. coli (BUT3-DE) for simultaneous ROS removal, and its effect on n-butanol productivity was examined. The ROS scavenging ability of cells overexpressing OmpC-TMT was examined and showed an approximately twofold increase in capacity. The modified strain improved n-butanol productivity to 320 mg/L, whereas the control strain produced only 95.1 mg/L. Transcriptomic analysis revealed three major KEGG pathways that were significantly differentially expressed in the BUT3-DE strain compared with their expression in the BUT1-DE strain, including genes involved in oxidative phosphorylation, fructose and mannose metabolism and glycolysis/gluconeogenesis.

Conclusions

These results indicate that OmpC-TMT can increase n-butanol production by scavenging ROS. The transcriptomic analysis suggested that n-butanol causes quinone malfunction, resulting in oxidative-phosphorylation-related nuo operon downregulation, which would diminish the ability to convert NADH to NAD⁺ and generate proton motive force. However, fructose and mannose metabolism-related genes (fucA, srlE and srlA) were upregulated, and glycolysis/gluconeogenesis-related genes (pfkB, pgm) were downregulated, which further assisted in regulating NADH/NAD⁺ redox and preventing additional ATP depletion. These results indicated that more NADH and ATP were required in the n-butanol synthetic pathway. Our study demonstrates a potential approach to increase the robustness of microorganisms and the production of toxic chemicals through the ability to reduce oxidative stress.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-017-0356-3) contains supplementary material, which is available to authorized users.

Understanding butanol tolerance and assimilation in Pseudomonas putida BIRD-1: an integrated omics approach

Pseudomonas putida  BIRD‐1 has the potential to be used for the industrial production of butanol due to its solvent tolerance and ability to metabolize low‐cost compounds. However, the strain has two major limitations: it assimilates butanol as sole carbon source and butanol concentrations above 1% (v/v) are toxic. With the aim of facilitating BIRD‐1 strain design for industrial use, a genome‐wide mini‐Tn5 transposon mutant library was screened for clones exhibiting increased butanol sensitivity or deficiency in butanol assimilation. Twenty‐one mutants were selected that were affected in one or both of the processes. These mutants exhibited insertions in various genes, including those involved in the TCA cycle, fatty acid metabolism, transcription, cofactor synthesis and membrane integrity. An omics‐based analysis revealed key genes involved in the butanol response. Transcriptomic and proteomic studies were carried out to compare short and long‐term tolerance and assimilation traits. Pseudomonas putida initiates various butanol assimilation pathways via alcohol and aldehyde dehydrogenases that channel the compound to central metabolism through the glyoxylate shunt pathway. Accordingly, isocitrate lyase – a key enzyme of the pathway – was the most abundant protein when butanol was used as the sole carbon source. Upregulation of two genes encoding proteins PPUBIRD1_2240 and PPUBIRD1_2241 (acyl‐CoA dehydrogenase and acyl‐CoA synthetase respectively) linked butanol assimilation with acyl‐CoA metabolism. Butanol tolerance was found to be primarily linked to classic solvent defense mechanisms, such as efflux pumps, membrane modifications and control of redox state. Our results also highlight the intensive energy requirements for butanol production and tolerance; thus, enhancing TCA cycle operation may represent a promising strategy for enhanced butanol production.

Biomass, strain engineering, and fermentation processes for butanol production by solventogenic clostridia

Butanol is considered an attractive biofuel and a commercially important bulk chemical. However, economical production of butanol by solventogenic clostridia, e.g., via fermentative production of acetone-butanol-ethanol (ABE), is hampered by low fermentation performance, mainly as a result of toxicity of butanol to microorganisms and high substrate costs. Recently, sugars from marine macroalgae and syngas were recognized as potent carbon sources in biomass feedstocks that are abundant and do not compete for arable land with edible crops. With the aid of systems metabolic engineering, many researchers have developed clostridial strains with improved performance on fermentation of these substrates. Alternatively, fermentation strategies integrated with butanol recovery processes such as adsorption, gas stripping, liquid-liquid extraction, and pervaporation have been designed to increase the overall titer of butanol and volumetric productivity. Nevertheless, for economically feasible production of butanol, innovative strategies based on recent research should be implemented. This review describes and discusses recent advances in the development of biomass feedstocks, microbial strains, and fermentation processes for butanol production.

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.

The acquisition of Clostridium tyrobutyricum mutants with improved bioproduction under acidic conditions after two rounds of heavy-ion beam irradiation

End-product inhibition is a key factor limiting the production of organic acid during fermentation. Two rounds of heavy-ion beam irradiation may be an inexpensive, indispensable and reliable approach to increase the production of butyric acid during industrial fermentation processes. However, studies of the application of heavy ion radiation for butyric acid fermentation engineering are lacking. In this study, a second ¹²C⁶⁺ heavy-ion irradiation-response curve is used to describe the effect of exposure to a given dose of heavy ions on mutant strains of Clostridium tyrobutyricum. Versatile statistical elements are introduced to characterize the mechanism and factors contributing to improved butyric acid production and enhanced acid tolerance in adapted mutant strains harvested from the fermentations. We characterized the physiological properties of the strains over a large pH value gradient, which revealed that the mutant strains obtained after a second round of radiation exposure were most resistant to harsh external pH values and were better able to tolerate external pH values between 4.5 and 5.0. A customized second round of heavy-ion beam irradiation may be invaluable in process engineering.

Proteomic analysis to elucidate degeneration of Clostridium beijerinckii NCIMB 8052 and role of Ca(2+) in strain recovery from degeneration

Degeneration of solventogenic Clostridium strains is one of the major barriers in bio-butanol production. A degenerated Clostridium beijerinckii NCIMB 8052 strain (DG-8052) was obtained without any genetic manipulation. Supplementation of CaCO3 to fermentation medium could partially recover metabolism of DG-8052 by more than 50 % increase of cell growth and solvent production. This study investigated the protein expression profile of DG-8052 and its response to CaCO3 treatment. Compared with WT-8052, the lower expressed proteins were responsible for disruption of RNA secondary structures and DNA repair, sporulation, signal transduction, transcription regulation, and membrane transport in DG-8052. Interestingly, accompanied with the decreased glucose utilization and lower solvent production, there was a decreased level of sigma-54 modulation protein which may indicate that the level of sigma-54 activity may be associated with the observed strain degeneration. For the addition of CaCO3, proteomic and biochemical study results revealed that besides buffer capacity, Ca(2+) could stabilize heat shock proteins, increase DNA synthesis and replication, and enhance expression of solventogenic enzymes in DG-8052, which has a similar contribution in WT-8052.

Enhanced butanol production by increasing NADH and ATP levels in Clostridium beijerinckii NCIMB 8052 by insertional inactivation of Cbei_4110

Clostridium beijerinckii is identified as a promising Clostridium strain for industrialization of acetone and butanol (AB) fermentation. It has been reported that high reducing power levels are associated with high butanol yield. In this study, we regulated reducing power by blocking NAD(P)H consumption in C. beijerinckii NCIMB 8052. Gene Cbei_4110, encoding NADH-quinone oxidoreductase (nuoG), is a subunit of the electron transport chain complex I. After inactivation of gene Cbei_4110, the generated mutant strain exhibited a remarkable increase in glucose utilization ratio and enhanced butanol production to 9.5 g/L in P2 medium containing 30 g/L of glucose. NAD(P)H and ATP levels were also increased by one to two times and three to five times, respectively. Furthermore, a comparative transcriptome analysis was carried out in order to determine the mechanism involved in the enhanced activity of the Cbei_4110-inactivated mutant strain. This strategy may be extended for making industrial bio-butanol more economically attractive.

Frontiers in microbial 1-butanol and isobutanol production

The heavy dependence on petroleum-derived fuel has raised concerns about energy sustainability and climate change, which have prompted researchers to explore fuel production from renewable sources. 1-Butanol and isobutanol are promising biofuels that have favorable properties and can also serve as solvents or chemical feedstocks. Microbial production of these alcohols provides great opportunities to access a wide spectrum of renewable resources. In recent years, research has improved the native 1-butanol production and has engineered isobutanol production in various organisms to explore metabolic diversity and a broad range of substrates. This review focuses on progress in metabolic engineering for the production of these two compounds using various resources.

Combining microbial production with chemical upgrading

This review presents developments in the chemical processing of fermentation-derived compounds, focusing on ethanol, lactic acid, 2,3-butanediol and the acetone-butanol-ethanol mixture. We examine pathways from these products to biologically-derived drop-in fuels, polymers, as well as commodity chemicals, highlighting the role of homogeneous and heterogeneous catalysts in the development of green processes for the production of fuels and high-value-added compounds from biomass.

Upgraded biofuel from alcohol–acetone feedstocks over a two-stage flow-through catalytic system

Over a two-stage flow-through catalytic system an advantageous mixture of various straight and branched alkanes can be obtained. In the second reactor the commercial NiMo/Al₂O₃ hydrodeoxygenating catalyst shows similar good properties as alkylating Pd-catalysts in the first stage.

Effective isopropanol-butanol (IB) fermentation with high butanol content using a newly isolated Clostridium sp. A1424

BACKGROUND

Acetone-butanol-ethanol fermentation has been studied for butanol production. Alternatively, to achieve acetone-free butanol production, use of clostridium strains producing butanol and 1,3-propanediol (1,3-PDO) from glycerol, natural and engineered isopropanol-butanol-ethanol (IBE) producers has been attempted; however, residual 1,3-PDO and acetone, low IBE production by natural IBE producers, and complicated gene modification are limitations.

RESULTS

Here, we report an effective isopropanol and butanol (IB) fermentation using a newly isolated Clostridium sp. A1424 capable of producing IB from various substrates with a small residual acetone. Notably, this strain also utilized glycerol and produced butanol and 1,3-PDO. After 46.35 g/L of glucose consumption at pH 5.5-controlled batch fermentation, Clostridium sp. A1424 produced 9.43 g/L of butanol and 13.92 g/L of IB at the productivity of 0.29 and 0.44 g/L/h, respectively, which are the highest values in glucose-based batch fermentations using natural IB producers. More interestingly, using glucose-glycerol mixtures at ratios ranging from 20:2 to 14:8 led to not only acetone-free and 1,3-PDO-free IB fermentation but also enhanced IB production along with a much higher butanol content (butanol/isopropanol ratio of 1.81 with glucose vs. 2.07-6.14 with glucose-glycerol mixture). Furthermore, when the mixture of glucose and crude glycerol at the ratio of 14:8 (total concentration of 35.68 g/L) was used, high butanol/isopropanol ratio (3.44) and butanol titer (9.86 g/L) were achieved with 1.4-fold enhanced butanol yield (0.28 g/g) and productivity (0.41 g/L/h) compared to those with glucose only at pH 5.5.

CONCLUSIONS

A newly isolated Clostridium sp. A1424 was able to produce butanol and isopropanol from various carbon sources. The productivity and titer of butanol and total alcohol obtained in this study were higher than the previously reported results obtained using other natural IB producers. Use of the mixture of glucose and glycerol was successful to achieve acetone-free, 1,3-PDO-free, and enhanced IB production with higher yield, productivity, and selectivity of butanol compared to those with glucose only, providing great advantages from the perspective of carbon recovery to alcohols. This notable result could be accomplished by isolating an effective IB producer Clostridium sp. A1424 as well as by utilizing glucose-glycerol mixtures.

Efficient Eucalypt Cell Wall Deconstruction and Conversion for Sustainable Lignocellulosic Biofuels

In order to meet the world's growing energy demand and reduce the impact of greenhouse gas emissions resulting from fossil fuel combustion, renewable plant-based feedstocks for biofuel production must be considered. The first-generation biofuels, derived from starches of edible feedstocks, such as corn, create competition between food and fuel resources, both for the crop itself and the land on which it is grown. As such, biofuel synthesized from non-edible plant biomass (lignocellulose) generated on marginal agricultural land will help to alleviate this competition. Eucalypts, the broadly defined taxa encompassing over 900 species of Eucalyptus, Corymbia, and Angophora are the most widely planted hardwood tree in the world, harvested mainly for timber, pulp and paper, and biomaterial products. More recently, due to their exceptional growth rate and amenability to grow under a wide range of environmental conditions, eucalypts are a leading option for the development of a sustainable lignocellulosic biofuels. However, efficient conversion of woody biomass into fermentable monomeric sugars is largely dependent on pretreatment of the cell wall, whose formation and complexity lend itself toward natural recalcitrance against its efficient deconstruction. A greater understanding of this complexity within the context of various pretreatments will allow the design of new and effective deconstruction processes for bioenergy production. In this review, we present the various pretreatment options for eucalypts, including research into understanding structure and formation of the eucalypt cell wall.

Impact of butyric acid on butanol formation by Clostridium pasteurianum

The butanol yield of the classic fermentative acetone-butanol-ethanol (ABE) process has been enhanced in the past decades through the development of better strains and advanced process design. Nevertheless, by-product formation and the incomplete conversion of intermediates still decrease the butanol yield. This study demonstrates the potential of increasing the butanol yield from glycerol though the addition of small amounts of butyric acid. The impact of butyric acid was investigated in a 7L stirred tank reactor. The results of this study show the positive impact of butyric acid on butanol yield under pH controlled conditions and the metabolic stages were monitored via online measurement of carbon dioxide formation, pH value and redox potential. Butyric acid could significantly increase the butanol yield at low pH values if sufficient quantities of primary carbon source (glycerol) were present.