Enhanced butanol production by optimization of medium parameters using Clostridium acetobutylicum YM1

New insights into the influence of pre-culture on robust solvent production of C. acetobutylicum

Clostridia are known for their solvent production, especially the production of butanol. Concerning the projected depletion of fossil fuels, this is of great interest. The cultivation of clostridia is known to be challenging, and it is difficult to achieve reproducible results and robust processes. However, existing publications usually concentrate on the cultivation conditions of the main culture. In this paper, the influence of cryo-conservation and pre-culture on growth and solvent production in the resulting main cultivation are examined. A protocol was developed that leads to reproducible cultivations of Clostridium acetobutylicum. Detailed investigation of the cell conservation in cryo-cultures ensured reliable cell growth in the pre-culture. Moreover, a reason for the acid crash in the main culture was found, based on the cultivation conditions of the pre-culture. The critical parameter to avoid the acid crash and accomplish the shift to the solventogenesis of clostridia is the metabolic phase in which the cells of the pre-culture were at the time of inoculation of the main culture; this depends on the cultivation time of the pre-culture. Using cells from the exponential growth phase to inoculate the main culture leads to an acid crash. To achieve the solventogenic phase with butanol production, the inoculum should consist of older cells which are in the stationary growth phase. Considering these parameters, which affect the entire cultivation process, reproducible results and reliable solvent production are ensured. KEY POINTS: • Both cryo- and pre-culture strongly impact the cultivation of C. acetobutylicum • Cultivation conditions of the pre-culture are a reason for the acid crash • Inoculum from cells in stationary growth phase ensures shift to solventogenesis.

Butanol as a major product during ethanol and acetate chain elongation

Chain elongation is a relevant bioprocess in support of a circular economy as it can use a variety of organic feedstocks for production of valuable short and medium chain carboxylates, such as butyrate (C4), caproate (C6), and caprylate (C8). Alcohols, including the biofuel, butanol (C4), can also be generated in chain elongation but the bioreactor conditions that favor butanol production are mainly unknown. In this study we investigated production of butanol (and its precursor butyrate) during ethanol and acetate chain elongation. We used semi-batch bioreactors (0.16 L serum bottles) fed with a range of ethanol concentrations (100-800 mM C), a constant concentration of acetate (50 mM C), and an initial total gas pressure of ∼112 kPa. We showed that the butanol concentration was positively correlated with the ethanol concentration provided (up to 400 mM C ethanol) and to chain elongation activity, which produced H₂ and further increased the total gas pressure. In bioreactors fed with 400 mM C ethanol and 50 mM C acetate, a concentration of 114.96 ± 9.26 mM C butanol (∼2.13 g L^-1) was achieved after five semi-batch cycles at a total pressure of ∼170 kPa and H₂ partial pressure of ∼67 kPa. Bioreactors with 400 mM C ethanol and 50 mM C acetate also yielded a butanol to butyrate molar ratio of 1:1. At the beginning of cycle 8, the total gas pressure was intentionally decreased to ∼112 kPa to test the dependency of butanol production on total pressure and H₂ partial pressure. The reduction in total pressure decreased the molar ratio of butanol to butyrate to 1:2 and jolted H₂ production out of an apparent stall. Clostridium kluyveri (previously shown to produce butyrate and butanol) and Alistipes (previously linked with butyrate production) were abundant amplicon sequence variants in the bioreactors during the experimental phases, suggesting the microbiome was resilient against changes in bioreactor conditions. The results from this study clearly demonstrate the potential of ethanol and acetate-based chain elongation to yield butanol as a major product. This study also supports the dependency of butanol production on limiting acetate and on high total gas and H₂ partial pressures.

Structural and Biochemical Analyses of the Butanol Dehydrogenase from Fusobacterium nucleatum

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

Improving carbohydrate accumulation in Chlamydomonas debaryana induced by sulfur starvation using response surface methodology

Most methods that promote carbohydrate production negatively affect cell growth and microalgal biomass production. This study explores, in a two-stage cultivation strategy, in Chlamydomonas debaryana the optimization of certain culture conditions for high carbohydrate production without loss of biomass. In the first stage, the interaction between sodium bicarbonate supplementation, aeration, and different growth periods was optimized using the response surface methodology (RMS). The 3-factor Box-Behnken design (BBD) was applied, and a second-order polynomial regression analysis was used to analyze the experimental data. The results showed that 0.45 g L^-1 of sodium bicarbonate combined with a good aerated agitation (0.6 L min^-1) and a cultivation period of 18 days are optimal to produce 5.02 g L^-1 of biomass containing 43% of carbohydrates.Under these optimized growth conditions, accumulation of carbohydrates was studied using different modes of nutritional stress. The results indicated that carbohydrate content was improved and the maximum accumulation (about 60% of the dry weight) was recorded under sulfur starvation with only a 14% reduction in biomass as compared to control. This study showed promising results as to biomass production and carbohydrate yield by the microalgae C. debaryana in view of production of third-generation biofuels.

Anaerobic biobutanol production from black strap molasses using Clostridium acetobutylicum MTCC11274: Media engineering and kinetic analysis

Microbial reduction of black strap molasses (BSM) by Clostridium acetobutylicum MTCC 11,274 was performed for the production of biobutanol. The optimum fermentation conditions were predicted using one factor at a time (OFAT) method. The identification of significant parameters was performed using Plackett-Burman Design (PBD). Furthermore the fermentation conditions were optimized using central composite design (CCD). The kinetics of substrate utilization and product formation were investigated. Initial pH, yeast extract concentration (g/L) and total reducing sugar concentration (g/L) were found as significant parameters affecting butanol production using C. acetobutylicum MTCC11274. The maximum butanol production under optimal condition was 10.27 + 0.82 g/L after 24 h. The waste black strap molasses obtained from sugar industry could be used as promising substrate for the production of next generation biofuel.

Performance of Anisole and Isobutanol as Gasoline Bio-Blendstocks for Spark Ignition Engines

Several countries have set ambitious targets for the transport sector that mandate a gradual increase in advanced biofuel content in the coming years. The current work addresses this transition and indicates two promising gasoline bio-blendstocks: Anisole and isobutanol. The whole value chains of these bio-components were considered, focusing on end-use performance, but also analyzing feedstock and its conversion, well-to wheel (WTW) greenhouse gas (GHG) emissions and costs. Three alternative fuels, namely a ternary blend (15% anisole, 15% isobutanol, 70% fossil gasoline on an energy basis) and two binary blends (15% anisole with fossil gasoline and 30% isobutanol with fossil gasoline), were tested, focusing on their drop-in applicability in spark ignition (SI) engines. The formulated liquid fuels performed well and showed the potential to increase brake thermal efficiency (BTE) by 1.4% on average. Measured unburned hydrocarbons (HC) and carbon monoxide (CO) emissions were increased on average by 12–29% and 17–51%, respectively. However, HC and CO concentrations and exhaust temperatures were at acceptable levels for proper catalyst operation. The studied blends were estimated to bring 11–22% of WTW GHG emission reductions compared to base gasoline. Additionally, the fleet performance and benefits of flexi-fuel vehicles (FFV) were modeled for ternary blends.

Production of butanol from distillers' grain waste by a new aerotolerant strain of Clostridium beijerinckii LY-5

A new aerotolerant strain of Clostridium beijerinckii LY-5 was isolated from the pit mud of the Chinese Baijiu-making process for butanol production. Plackett-Burman design and artificial neural network were used to optimize the fermentation medium and a total of 13.54 ± 0.22 g/L butanol and 19.91 ± 0.52 g/L ABE were attained under aerotolerant condition. Moreover, distillers' grain waste (DGW), the main by-product in the Baijiu production process, was utilized as potential substrate for butanol production. DGW was hydrolyzed by α-amylase and glucoamylase and then fermented after a detoxifying process of overliming. Butanol and ABE concentrations were 9.02 ± 0.18 and 9.57 ± 0.19 g/L with the yield of 0.21 and 0.23 g/g sugar, respectively. The higher ratio of butanol to ABE might be caused by the inhibitors in DGW medium affecting the metabolic pathways of C. beijerinckii LY-5 and approximately 1.48 ± 0.04 g/L isopropanol was found at the end of fermentation. This work highlights the feasibility of using DGW as a promising feedstock for butanol production by a new aerotolerant strain of C. beijerinckii LY-5, with benefit to the environment.

Identification and Investigation of Autolysin Genes in Clostridium saccharoperbutylacetonicum Strain N1-4 for Enhanced Biobutanol Production

Biobutanol is a valuable biochemical and one of the most promising biofuels. Clostridium saccharoperbutylacetonicum N1-4 is a hyperbutanol-producing strain. However, its strong autolytic behavior leads to poor cell stability, especially during continuous fermentation, thus limiting the applicability of the strain for long-term and industrial-scale processes. In this study, we aimed to evaluate the role of autolysin genes within the C. saccharoperbutylacetonicum genome related to cell autolysis and further develop more stable strains for enhanced butanol production. First, putative autolysin-encoding genes were identified in the strain based on comparison of amino acid sequence with homologous genes in other strains. Then, by overexpressing all these putative autolysin genes individually and characterizing the corresponding recombinant strains, four key genes were pinpointed to be responsible for significant cell autolysis activities. Further, these key genes were deleted using CRISPR-Cas9. Fermentation characterization demonstrated enhanced performance of the resultant mutants. Results from this study reveal valuable insights concerning the role of autolysins for cell stability and solvent production, and they provide an essential reference for developing robust strains for enhanced biofuel and biochemical production.IMPORTANCE Severe autolytic behavior is a common issue in Clostridium and many other microorganisms. This study revealed the key genes responsible for the cell autolysis within Clostridium saccharoperbutylacetonicum, a prominent platform for biosolvent production from lignocellulosic materials. The knowledge generated in this study provides insights concerning cell autolysis in relevant microbial systems and gives essential references for enhancing strain stability through rational genome engineering.

Enhancing starch accumulation/production in Chlorococcum humicola through sulphur limitation and 2,4- D treatment for butanol production

Depleting fuel resources is a global concern worldwide due to the unstable and cost of fuel resources. Increased transportation has gradually depleted the fossil-based fuel resources leading to find a cost-effective, readily available, and renewable source. Considering these issues, various private and government organizations have focussed on producing bio-based fuels from natural sources. In this scenario, algae are a potential emerging source of feedstock or biomass for biobutanol production, which can effectively replace fossil fuels and their environmental drawbacks. The present study focussed on evaluating the potential of freshwater microalga Chlorococcum humicola isolated from temple pond as feedstock for biobutanol production using Clostridium acetobutylicum. The results indicated that C. humicola produced 846.33 μgmg^-1of starch under full strength Chu10 medium. While under sulphur and phosphorus limitation, the accumulation of starch was 947.33 μg mg^-1 and 766.67 μgmg^-1, respectively. Also, C. humicola was exposed to different concentrations of 2,4-Dichlorophenoxyacetic acid (2,4-D). At 10μgml^-1 of 2,4-D, the highest starch concentration of 989μgmg^-1was achieved in C. humicola. Finally, starch in C. humicola were hydrolysed and ABE fermentation was performed using C. acetobutylicum under anaerobic condition in a 5 L automated fermenter. After 72 h of fermentation, the fermented broth is analyed in Gas Chromatography showing the fermented product containing Acetone: Butanol: Ethanol. The present study is the first report on the production of biobutanol from C. humicola isolated from Temple pond. This study emphasizes the importance of local isolates of microalgae as a third-generation substrate to produce butanol to replace fossil-based fuels.

Conversion of Exhausted Sugar Beet Pulp into Fermentable Sugars from a Biorefinery Approach

In this study, the production of a hydrolysate rich in fermentable sugars, which could be used as a generic microbial culture medium, was carried out by using exhausted sugar beet pulp pellets (ESBPPs) as raw material. For this purpose, the hydrolysis was performed through the direct addition of the fermented ESBPPs obtained by fungal solid-state fermentation (SSF) as an enzyme source. By directly using this fermented solid, the stages for enzyme extraction and purification were avoided. The effects of temperature, fermented to fresh solid ratio, supplementation of fermented ESBPP with commercial cellulase, and the use of high-solid fed-batch enzymatic hydrolysis were studied to obtain the maximum reducing sugar (RS) concentration and productivity. The highest RS concentration and productivity, 127.3 g·L⁻¹ and 24.3 g·L⁻¹·h⁻¹ respectively, were obtained at 50 °C and with an initial supplementation of 2.17 U of Celluclast^® per gram of dried solid in fed-batch mode. This process was carried out with a liquid to solid ratio of 4.3 mL·g⁻¹ solid, by adding 15 g of fermented solid and 13.75 g of fresh solid at the beginning of the hydrolysis, and then the same amount of fresh solid 3 times every 2.5 h. By this procedure, ESBPP can be used to produce a generic microbial feedstock, which contains a high concentration of monosaccharides.

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.

A Feasibility Study of Cellulosic Isobutanol Production—Process Simulation and Economic Analysis

Renewable liquid biofuels for transportation have recently attracted enormous global attention due to their potential to provide a sustainable alternative to fossil fuels. In recent years, the attention has shifted from first-generation bioethanol to the production of higher molecular weight alcohols, such as biobutanol, from cellulosic feedstocks. The economic feasibility of such processes depends on several parameters such as the cost of raw materials, the fermentation performance and the energy demand for the pretreatment of biomass and downstream processing. In this work, two conceptual process scenarios for isobutanol production, one with and one without integrated product removal from the fermentor by vacuum stripping, were developed and evaluated using SuperPro Designer®. In agreement with previous publications, it was concluded that the fermentation titer is a crucial parameter for the economic competitiveness of the process as it is closely related to the energy requirements for product purification. In the first scenario where the product titer was 22 g/L, the energy demand for downstream processing was 15.8 MJ/L isobutanol and the unit production cost of isobutanol was $2.24/L. The integrated product removal by vacuum stripping implemented in the second scenario was assumed to improve the isobutanol titer to 50 g/L. In this case, the energy demand for the product removal (electricity) and downstream processing were 1.8 MJ/L isobutanol and 10 MJ/L isobutanol, respectively, and the unit production cost was reduced to $1.42/L. The uncertainty associated with the choice of modeling and economic parameters was investigated by Monte Carlo simulation sensitivity analysis.