Elucidation of Growth Inhibition and Acetic Acid Production by Clostridium thermoaceticum

Thermophilic Moorella thermoacetica as a platform microorganism for C1 gas utilization: physiology, engineering, and applications

In the context of the rapid development of low-carbon economy, there has been increasing interest in utilizing naturally abundant and cost-effective one-carbon (C1) substrates for sustainable production of chemicals and fuels. Moorella thermoacetica, a model acetogenic bacterium, has attracted significant attention due to its ability to utilize carbon dioxide (CO2) and carbon monoxide (CO) via the Wood-Ljungdahl (WL) pathway, thereby showing great potential for the utilization of C1 gases. However, natural strains of M. thermoacetica are not yet fully suitable for industrial applications due to their limitations in carbon assimilation and conversion efficiency as well as limited product range. Over the past decade, progresses have been made in the development of genetic tools for M. thermoacetica, accelerating the understanding and modification of this acetogen. Here, we summarize the physiological and metabolic characteristics of M. thermoacetica and review the recent advances in engineering this bacterium. Finally, we propose the future directions for exploring the real potential of M. thermoacetica in industrial applications.

Removal of Acetic Acid from Bacterial Culture Media by Adsorption onto a Two-Component Composite Polymer Gel

Organic acids, including acetic acid, are the metabolic products of many microorganisms. Acetic acid is a target product useful in the fermentation process. However, acetic acid has an inhibitory effect on microorganisms and limits fermentation. Thus, it would be beneficial to recover the acid from the culture medium. However, conventional recovery processes are expensive and environmentally unfriendly. Here, we report the use of a two-component hydrogel to adsorb dissociated and undissociated acetic acid from the culture medium. The Langmuir model revealed the maximum adsorption amount to be 44.8 mg acetic acid/g of dry gel at neutral pH value. The adsorption capacity was similar to that of an ion-exchange resin. In addition, the hydrogel maintained its adsorption capability in a culture medium comprising complex components, whereas the ion-exchange did not adsorb in this medium. The adsorbed acetic acid was readily desorbed using a solution containing a high salt concentration. Thus, the recovered acetic acid can be utilized for subsequent processes, and the gel-treated fermentation broth can be reused for the next round of fermentation. Use of this hydrogel may prove to be a more sustainable downstream process to recover biosynthesized acetic acid.

Initial pH-driven production of volatile fatty acid from hybrid Pennisetum

In this work, the impact of initial pH on the production of volatile fatty acids (VFAs) of hybrid Pennisetum was investigated. The batch experiments were conducted under six distinct beginning pH at a mesophilic temperature. Initial pH had an obvious effect on VFA yield and composition, and severe alkaline circumstances (pH_in = 11.0) could boost VFA production and acetic acid selectivity. The highest VFAs yield and acetate proportion were obtained when the initial pH was 11.0, with 518 ± 29 mg g^-1VS and 92%. Furthermore, microbial community analysis showed that alkaliphilic acetogenic anaerobe such as Amphibacillus, Tissierella, and Natronincola were the dominant species when the initial pH was 11.0. The Amphibacillus is the main hydrolysis bacterium under these conditions because of its high ability for xylan degradation at pH 9.0-10.0. Because of the increased VFA yield and superior acetic acid selectivity, the results suggest that adjusting the initial pH to 11.0 in batch mode would be possible for scaling-up purposes.

Microbial assemblages and methanogenesis pathways impact methane production and foaming in manure deep-pit storages

Foam accumulation in swine manure deep-pits has been linked to explosions and flash fires that pose devastating threats to humans and livestock. It is clear that methane accumulation within these pits is the fuel for the fire; it is not understood what microbial drivers cause the accumulation and stabilization of methane. Here, we conducted a 13-month field study to survey the physical, chemical, and biological changes of pit-manure across 46 farms in Iowa. Our results showed that an increased methane production rate was associated with less digestible feed ingredients, suggesting that diet influences the storage pit’s microbiome. Targeted sequencing of the bacterial 16S rRNA and archaeal mcrA genes was used to identify microbial communities’ role and influence. We found that microbial communities in foaming and non-foaming manure were significantly different, and that the bacterial communities of foaming manure were more stable than those of non-foaming manure. Foaming manure methanogen communities were enriched with uncharacterized methanogens whose presence strongly correlated with high methane production rates. We also observed strong correlations between feed ration, manure characteristics, and the relative abundance of specific taxa, suggesting that manure foaming is linked to microbial community assemblage driven by efficient free long-chain fatty acid degradation by hydrogenotrophic methanogenesis.

Carboxylic acids production and electrosynthetic microbial community evolution under different CO2 feeding regimens

Microbial electrosynthesis (MES) is a potential technology for CO₂ recycling, but insufficient information is available on the microbial interactions underpinning electrochemically-assisted reactions. In this study, a MES reactor was operated for 225 days alternately with bicarbonate or CO₂ as carbon source, under batch or continuous feeding regimens, to evaluate the response of the microbial communities, and their productivity, to dynamic operating conditions. A stable acetic acid production rate of 9.68 g m^-2 d^-1, and coulombic efficiency up to 40%, was achieved with continuous CO₂ sparging, higher than the rates obtained with bicarbonate (0.94 g m^-2 d^-1) and CO₂ under fed-batch conditions (2.54 g m^-2 d^-1). However, the highest butyric acid production rate (0.39 g m^-2 d^-1) was achieved with intermittent CO₂ sparging. The microbial community analyses focused on differential amplicon sequence variants (ASVs), allowing detection of ASVs significantly different across consecutive samples. This analysis, combined with co-occurence network analysis, and cyclic voltammetry, indicated that hydrogen-mediated acetogenesis was carried out by Clostridium, Eubacterium and Acetobacterium, whereas Oscillibacter and Caproiciproducens were involved in butyric acid production. The cathodic community was spatially inhomogeneous, with potential electrotrophs, such as Sulfurospirillum and Desulfovibrio, most prevalent near the current collector. The abundance of Sulfurospirillum positively correlated with that of Acetobacterium, supporting the syntrophic metabolism of both organisms.

Changes in the Substrate Source Reveal Novel Interactions in the Sediment-Derived Methanogenic Microbial Community

Methanogenesis occurs in many natural environments and is used in biotechnology for biogas production. The efficiency of methane production depends on the microbiome structure that determines interspecies electron transfer. In this research, the microbial community retrieved from mining subsidence reservoir sediment was used to establish enrichment cultures on media containing different carbon sources (tryptone, yeast extract, acetate, CO₂/H₂). The microbiome composition and methane production rate of the cultures were screened as a function of the substrate and transition stage. The relationships between the microorganisms involved in methane formation were the major focus of this study. Methanogenic consortia were identified by next generation sequencing (NGS) and functional genes connected with organic matter transformation were predicted using the PICRUSt approach and annotated in the KEGG. The methane production rate (exceeding 12.8 mg CH₄ L⁻¹ d⁻¹) was highest in the culture grown with tryptone, yeast extract, and CO₂/H_2. The analysis of communities that developed on various carbon sources casts new light on the ecophysiology of the recently described bacterial phylum Caldiserica and methanogenic Archaea representing the genera Methanomassiliicoccus and Methanothrix. Furthermore, it is hypothesized that representatives of Caldiserica may support hydrogenotrophic methanogenesis.

Efficient acetogenesis of anaerobic co-digestion of food waste and maize straw in a HSAD reactor

In this study, food waste and maize straw were used as feedstock, and the two-phase high-solid anaerobic digestion (TP-HSAD) technology was used to optimize the process parameters of leachate reflux in acid-production stage. Results indicated that compared with other waste activated sludge, pig manure digestate (PM) as leachate can achieve better hydrolysis and acidification effect. The increase of leachate reflux ratio can shorten the fermentation time of the acid-producing stage and increase the fermentation efficiency. When the reflux ratio was 32:1, peak concentration of volatile fatty acids (VFAs) was 45.4 g/L and the volatile solids (VS) removal rate was 61.7%. Reflux frequency has minimal effect on the concentration of VFAs and the degree of degradation of VS, but a higher reflux frequency will prolong the reaction time of acid-production stage. When PM is used as reflux leachate, the HSAD reactor can improve the hydrolysis and acidification of the anaerobic fermentation.

Real-time recovery strategies for volatile fatty acid-inhibited anaerobic digestion of food waste for methane production

This study investigated effects of real-time recovery strategies on VFA (volatile fatty acid)-inhibited anaerobic system of FW (food waste) and identified key driver of process recovery. The long-term anaerobic system of FW encountered serious VFA (mainly propionate) inhibition. The pH adjustment (PA) strategy could not reverse process imbalance but only delayed the process failure. The short-term effect of reinoculation (RI) strategy was greatly effective, but its long-term effect was non-sustainable. Trace elements were key drivers of process recovery owing to their indispensable roles in activating methanogenesis and therefore stimulating propionate conversion. From the viewpoint of economic feasibility, the single strategy of trace elements supplementation (TE) and the combinational strategy of PA + TE were respectively recommended in the initial and medium VFA-inhibition stages. The three-in-one strategy of PA + TE + RI was always effective but was costly. This study provided practical guidance on real-time recovery of VFA-inhibited anaerobic system of FW.

Determination of volumetric gas-liquid mass transfer coefficient of carbon monoxide in a batch cultivation system using kinetic simulations

A mathematical model of microbial kinetics was introduced to predict the overall volumetric gas-liquid mass transfer coefficient (k_La) of carbon monoxide (CO) in a batch cultivation system. The cell concentration (X), acetate concentration (C_ace), headspace gas (N_co and [Formula: see text] ), dissolved CO concentration in the fermentation medium (C_co), and mass transfer rate (R) were simulated using a variety of k_La values. The simulated results showed excellent agreement with the experimental data for a k_La of 13/hr. The C_co values decreased with increase in cultivation times, whereas the maximum mass transfer rate was achieved at the mid-log phase due to vigorous microbial CO consumption rate higher than R. The model suggested in this study may be applied to a variety of microbial systems involving gaseous substrates.

Effects of gas condition on acetic acid fermentation by Clostridium thermocellum and Moorella thermoacetica (C. thermoaceticum)

Fermentation with acetogens can be affected by cultivation gas phase, but to date, there is not enough evidence on that matter for Clostridium thermocellum and Moorella thermoacetica. In this work, the effects of sparged CO₂ as well as sparged and non-sparged N₂ on these microorganisms were studied using glucose and cellobiose as substrates. It was revealed that sparged CO₂ and non-sparged N₂ supported growth and acetic acid production by C. thermocellum and M. thermoacetica, while sparged N₂ inhibited both of the microorganisms. Notably, part of the sparged CO₂ was fermented by the co-culture system and contributed to an overestimation of the products from the actual substrate as well as an erring material balance. The best condition for the co-culture was concluded to be N₂ without sparging. These results demonstrate the importance of cultivation conditions for efficient fermentation by anaerobic clostridia species.

Homolactic Acid Fermentation by the Genetically Engineered Thermophilic Homoacetogen Moorella thermoacetica ATCC 39073

For the efficient production of target metabolites from carbohydrates, syngas, or H₂-CO₂ by genetically engineered Moorella thermoacetica, the control of acetate production (a main metabolite of M. thermoacetica) is desired. Although propanediol utilization protein (PduL) was predicted to be a phosphotransacetylase (PTA) involved in acetate production in M. thermoacetica, this has not been confirmed. Our findings described herein directly demonstrate that two putative PduL proteins, encoded by Moth_0864 (pduL1) and Moth_1181 (pduL2), are involved in acetate formation as PTAs. To disrupt these genes, we replaced each gene with a lactate dehydrogenase gene from Thermoanaerobacter pseudethanolicus ATCC 33223 (T-ldh). The acetate production from fructose as the sole carbon source by the pduL1 deletion mutant was not deficient, whereas the disruption of pduL2 significantly decreased the acetate yield to approximately one-third that of the wild-type strain. The double-deletion (both pduL genes) mutant did not produce acetate but produced only lactate as the end product from fructose. These results suggest that both pduL genes are associated with acetate formation via acetyl-coenzyme A (acetyl-CoA) and that their disruption enables a shift in the homoacetic pathway to the genetically synthesized homolactic pathway via pyruvate.IMPORTANCE This is the first report, to our knowledge, on the experimental identification of PTA genes in M. thermoacetica and the shift of the native homoacetic pathway to the genetically synthesized homolactic pathway by their disruption on a sugar platform.

Fermentation of lignocellulosic sugars to acetic acid by Moorella thermoacetica

A systematic study of bioconversion of lignocellulosic sugars to acetic acid by Moorella thermoacetica (strain ATCC 39073) was conducted. Four different water-soluble fractions (hydrolysates) obtained after steam pretreatment of lignocellulosic biomass were selected and fermented to acetic acid in batch fermentations. M. thermoacetica can effectively ferment xylose and glucose in hydrolysates from wheat straw, forest residues, switchgrass, and sugarcane straw to acetic acid. Xylose and glucose were completely utilized, with xylose being consumed first. M. thermoacetica consumed up to 62 % of arabinose, 49 % galactose and 66 % of mannose within 72 h of fermentation in the mixture of lignocellulosic sugars. The highest acetic acid yield was obtained from sugarcane straw hydrolysate, with 71 % of theoretical yield based on total sugars (17 g/L acetic acid from 24 g/L total sugars). The lowest acetic acid yield was observed in forest residues hydrolysate, with 39 % of theoretical yield based on total sugars (18 g/L acetic acid from 49 g/L total sugars). Process derived compounds from steam explosion pretreatment, including 5-hydroxymethylfurfural (0.4 g/L), furfural (0.1 g/L) and total phenolics (3 g/L), did not inhibit microbial growth and acetic acid production yield. This research identified two major factors that adversely affected acetic acid yield in all hydrolysates, especially in forest residues: (i) glucose to xylose ratio and (ii) incomplete consumption of arabinose, galactose and mannose. For efficient bioconversion of lignocellulosic sugars to acetic acid, it is imperative to have an appropriate balance of sugars in a hydrolysate. Hence, the choice of lignocellulosic biomass and steam pretreatment design are fundamental steps for the industrial application of this process.

Anaerobic Mesophilic Codigestion of Rice Straw and Chicken Manure: Effects of Organic Loading Rate on Process Stability and Performance

To investigate the effects of organic loading rate (OLR) on performance and stability of mesophilic co-digestion of rice straw (RS) and chicken manure (CM), benchtop experiments (40 L) were carried out at OLRs of 3.0, 3.6, 4.2, 4.8, 6.0, 8.0, and 12.0 kg volatile solid (VS)/(m(3)·day) with volatile solid (VS) ratio of 1:1 (RS/CM) which was based on batch tests. Anaerobic co-digestion was slightly and severely inhibited by the accumulation of ammonia when the digester was overloaded at an OLR of 6 and 12 kg VS/(m(3)·day), respectively. The recommended OLR for co-digestion is 4.8 kg VS/(m(3)·day), which corresponds to average specific biogas production (SBP) of 380 L/kg VS and volumetric biogas production rate (VBPR) of 1.8 m(3)/(m(3)·day). An OLR of 6-8 kg VS/(m(3)·d) with SBP of 360-440 L/kg VS and VBPR of 2.1-3.5 m(3)/(m(3)·day) could be considered, if an Anaerobic digestion (AD) system assisted by in situ removal of ammonia was adopted.

Ethanol and acetic acid production from carbon monoxide in a Clostridium strain in batch and continuous gas-fed bioreactors

The effect of different sources of nitrogen as well as their concentrations on the bioconversion of carbon monoxide to metabolic products such as acetic acid and ethanol by Clostridium autoethanogenum was studied. In a first set of assays, under batch conditions, either NH4Cl, trypticase soy broth or yeast extract (YE) were used as sources of nitrogen. The use of YE was found statistically significant (p < 0.05) on the product spectrum in such batch assays. In another set of experiments, three bioreactors were operated with continuous CO supply, in order to estimate the effect of running conditions on products and biomass formation. The bioreactors were operated under different conditions, i.e., EXP1 (pH = 5.75, YE 1g/L), EXP2 (pH = 4.75, YE 1 g/L) and EXP3 (pH = 5.75, YE 0.2 g/L). When compared to EXP2 and EXP3, it was found that EXP1 yielded the maximum biomass accumulation (302.4 mg/L) and products concentrations, i.e., acetic acid (2147.1 mg/L) and ethanol (352.6 mg/L). This can be attributed to the fact that the higher pH and higher YE concentration used in EXP1 stimulated cell growth and did, consequently, also enhance metabolite production. However, when ethanol is the desired end-product, as a biofuel, the lower pH used in EXP2 was more favourable for solventogenesis and yielded the highest ethanol/acetic acid ratio, reaching a value of 0.54.

Effects of feedstock ratio and organic loading rate on the anaerobic mesophilic co-digestion of rice straw and cow manure

In order to investigate the effects of feedstock ratio and organic loading rate (OLR) on the anaerobic mesophilic co-digestion of rice straw (RS) and cow manure (CM), batch tests (2.5L) were carried out at volatile solid (VS) ratios of 0:1, 1:2, 1:1, 2:1, and 1:0 (RS/CM), and continuous bench experiments (40 L) were carried out at OLRs of 3.0, 3.6, 4.2, 4.8, 6.0, 8.0, and 12.0 kg VS/(m(3) d) with optimal VS ratio. The optimal VS ratio was found to be 1:1. Stable and efficient co-digestion with average specific biogas production of 383.5L/kg VS and volumetric biogas production rate of 2.30 m(3)/(m(3) d) was obtained at an OLR of 6 kg VS/(m(3) d). Anaerobic co-digestion was severely inhibited by the accumulation of volatile fatty acids instead of ammonia when the OLR was 12 kg VS/(m(3) d). Further, significant foaming was observed at OLR ⩾ 8 kg VS/(m(3) d).

Cell growth kinetics of Chlorella sorokiniana and nutritional values of its biomass

The present study investigates the effects of different physico-chemical parameters for the growth of Chlorella sorokiniana and subsequently determination of nutritional values of its biomass. Most suitable temperature, light intensity, pH, and acetic acid concentration were 30°C, 100 μmol m(-2)s(-1), pH 7.5, and 34.8mM, respectively for the growth of this microorganism. Arrhenius growth activation energy, Ea was calculated as 7.08 kJ mol(-1). Monod kinetics constants: maximum specific growth rate (μ max) and substrate (acetic acid) affinity coefficient (Ks) were determined as 0.1 ± 0.01 h(-1) and 76 ± 8 mg L(-1), respectively. Stoichiometric analysis revealed the capture of 1.83 g CO2 and release of 1.9 g O2 for 1g algal biomass synthesis. Algal biomass of C. sorokiniana was found rich in protein and several important minerals such as Mg, Ca, and Fe. Astaxanthin and β-carotene were extracted and quantified using high performance liquid chromatography.

Continuous anaerobic digestion of food waste and design of digester with lipid removal

Separation of municipal solid waste has been implemented in many cities in China. As a major component of municipal solid waste, food waste can be treated by anaerobic digestion (AD) for energy production. To provide reference data for disposing of food waste through engineering applications, continuous AD was carried out under various organic loading rates (OLRs) at 27 +/- 2 degrees C in the laboratory. The anaerobic reactor was stable with pH 7.0-7.1, total volatile fatty acid (VFA) concentrations of 206-746 mg/L, and NH4+ -N concentrations of 525-1293 mg/L when the OLR was 1.118-5.588 kg volatile solids (VS)/m(3) x d. The maximum volumetric biogas production rate was 4.41 L/L x d when the OLR was increased to 5.588 kg VS/m(3) x d with a hydraulic retention time of 30 d. When the OLR was increased to 6.706 and 8.382 kg VS/m(3) x d, biogas production was seriously inhibited by VFAs, with maximum total VFA and propionate concentrations of 8738 mg/L and 2864 mg/L, respectively. Due to the incomplete degradation of lipids, the specific methane production rate of 353-488 L/kg VS accounted for 55.2-76.3% of the theoretical methane potential calculated based on the component composition. A retrofitted anaerobic digester with lipid removal was designed to improve the efficiency.

Augmentation of mass transfer through electrical means for hydrogel-entrapped Escherichia coli cultivation

Nutrient depletion and inhibitory end-product accumulation are the major problems for hydrogel-entrapment cell cultures. An electrokinetic technique was developed to enhance intrahydrogel mass transfer to overcome these problems. Escherichia coli cells (ATCC 15224) were entrapped in 3.2-mm-thick potassium-K-carrageenan and agarose hydrogel slabs. With a electric current density of 180A/m(2) the cell densities were increased by 140%(from 3.9 to 9.6 dry cell weight [DCW] g/L) in potassium-K-carrageenan and by 80% (from 3.9 to 7.0 DCW g/L) in agarose. A mathematical model taking into account nutrient depletion, inhibitory end-product formation, and cell growth kinetics under facultatively anaerobic conditions was developed to rationalize the overall transport and biological behaviors in the hydrogel. The cell growth in hydrogel was successfully simulated. It is concluded that the augmented transports for glucose and inhibitory end-products accounted for these increases in cell growth. The increase in cell density in potassium-K-carrageenan was due to the enhanced removal of inhibitory end-products (through electroosmosis and electro-phoresis: 80%) and due to the augmented glucose transport (through electroosmosis: 20%). (c) 1995 John Wiley & Sons, Inc.

Re-fermentation of washed spent solids from batch hydrogenogenic fermentation for additional production of biohydrogen from the organic fraction of municipal solid waste

In the first batch solid substrate anaerobic hydrogenogenic fermentation with intermittent venting (SSAHF-IV) of the organic fraction of municipal solid waste (OFMSW), a cumulative production of 16.6 mmol H(2)/reactor was obtained. Releases of hydrogen partial pressure first by intermittent venting and afterward by flushing headspace of reactors with inert gas N(2) allowed for further hydrogen production in a second to fourth incubation cycle, with no new inoculum nor substrate nor inhibitor added. After the fourth cycle, no more H(2) could be harvested. Interestingly, accumulated hydrogen in 4 cycles was 100% higher than that produced in the first cycle alone. At the end of incubation, partial pressure of H(2) was near zero whereas high concentrations of organic acids and solvents remained in the spent solids. So, since approximate mass balances indicated that there was still a moderate amount of biodegradable matter in the spent solids we hypothesized that the organic metabolites imposed some kind of inhibition on further fermentation of digestates. Spent solids were washed to eliminate organic metabolites and they were used in a second SSAHF-IV. Two more cycles of H(2) production were obtained, with a cumulative production of ca. 2.4 mmol H(2)/mini-reactor. As a conclusion, washing of spent solids of a previous SSAHF-IV allowed for an increase of hydrogen production by 15% in a second run of SSAHF-IV, leading to the validation of our hypothesis.

Dissimilation of Carbon Monoxide to Acetic Acid by Glucose-Limited Cultures of Clostridium thermoaceticum

Clostridium thermoaceticum was cultivated in glucose-limited media, and the dissimilation of CO to acetic acid was evaluated. We found that cultures catalyzed the rapid dissimilation of CO to acetic acid and CO(2), with the stoichiometry obtained for conversion approximating that predicted from the following reaction: 4CO + 2H(2)O --> CH(3)CO(2)H + 2CO(2). Growing cultures formed approximately 50 mmol (3 g) of CO-derived acetic acid per liter of culture, with the rate of maximal consumption approximating 9.1 mmol of CO consumed/h per liter of culture. In contrast, resting cells were found not to dissimilate CO to acetic acid. CO was incorporated, with equal distribution between the carboxyl and methyl carbons of acetic acid when the initial cultivation gas phase was 100% CO, whereas CO(2) preferentially entered the carboxyl carbon when the initial gas phase was 100% CO(2). Significantly, in the presence of saturating levels of CO, CO(2) preferentially entered the methyl carbon, whereas saturating levels of CO(2) yielded CO-derived labeling predominantly in the carboxyl carbon. These findings are discussed in relation to the path of carbon flow to acetic acid.

Continuous Production of Thermostable beta-Amylase with Clostridium thermosulfurogenes: Effect of Culture Conditions and Metabolite Levels on Enzyme Synthesis and Activity

A beta-amylase-overproducing mutant of Clostridium thermosulfurogenes was grown in continuous culture on soluble starch to produce thermostable beta-amylase. Enzyme productivity was reasonably stable over periods of weeks to months. The pH and temperature optima for beta-amylase production were pH 6.0 and 60 degrees C, respectively. Enzyme concentration was maximized by increasing biomass concentration by using high substrate concentrations and by maintaining a low growth rate. beta-Amylase concentration reached 90 U ml at a dilution rate of 0.07 h in a 3% starch medium. A further increase in enzyme activity levels was limited by acetic acid inhibition of growth and low beta-amylase productivity at low growth rates.

Effect of hydrogen sulfide on growth of sulfate reducing bacteria

A culture of sulfate reducing bacteria (SRB) growing on lactate and sulfate was incubated at different pH values in the range of 5.8-7.0. The effect of pH on growth rate was determined in this pH range; the highest growth rate was observed at pH 6.7. Hydrogen sulfide produced from sulfate reduction was found to have a direct and reversible toxicity effect on the SRB. A hydrogen sulfide Concentration of 547 mg/L (16.1 mM) completely inhibited the culture growth. Comparison between acetic acid and hydrogen sulfide inhibition is presented and the concomitant inhibition kinetics are mathematically described.

Effects of pH and acetic acid on homoacetic fermentation of lactate by Clostridium formicoaceticum

Clostridium formicoaceticum homofermentatively converts lactate to acetate at 37 degrees C and pH 6.6-9.6. However, this fermentation is strongly inhibited by acetic acid at acidic pH. The specific growth rate of this organism decreased from a maximum at pH 7.6 to zero at pH 6.6. This inhibition effect was found to be attributed to both H(+) and undissociated acetic acid. At pH values below 7.6, the H(+) inhibited the fermentation following non-competitive inhibition kinetics. The acetic acid inhibition was found to be stronger at a lower medium pH. At pH 6.45-6.8, cell growth was found to be primarily limited by a maximum undissociated acetic acid concentration of 0.358 g/L (6mM). This indicates that the undissociated acid, not the dissociated acid, is the major acid inhibitor. At pH 7.6 or higher, this organism could tolerate acetate concentrations of higher than 0.8M, but salt (Na(+)) became a strong inhibitor at concentrations of higher than 0.4M. Acetic acid inhibition also can be represented by noncompetitive inhibition kinetics. A mathematical model for this homoacetic fermentation was also developed. This model can be used to simulate batch fermentation at any pH between 6.9 and 7.6.

Analysis of mass transfer for immobilized cells in an extractive lactic acid fermentation

The use of immobilization in extractive lactic acid fermentation by Lactobacillus delbrueckii is preferred. In this article, the mathematical simulations to examine the influences of substrate and product transport were performed to assess the overall performance. The simulations showed that transport of the substrate in k-carrageenan beads was not a rate limiting factor. However, the model observed significant buildup of inhibitory product in large beads. The model was validated through comparisons with the experimental results. Finally, the model was used to predict the performance of the extractive fermentation under different operating strategies.

Effect of salts addition on hydrogen production by C. acetobutylicum

The objective of this study is to investigate the effect of salts addition to fermentation medium on hydrogen production, under anaerobic batch culture system. In this study, batch experiments were conducted to investigate the inhibitory effect of both NaCl and sodium acetate on hydrogen production. The optimum pH and temperature for hydrogen production were at initial pH of 7.0 and 30 degrees C. Enhanced production of hydrogen, using glucose as substrate was achieved. In the absence of Sodium Chloride and Sodium Acetate enhanced hydrogen yield (Y(P/S)) from 350 mL g(-1) glucose utilized to 391 mL g(-1) glucose utilized with maximum hydrogen productivity of 77.5 ml/L/h. Results also show that sodium chloride and sodium acetate in the medium adversely affect growth. Hydrogen yield per biomass (Y(P/X)) of 254 ml/L/g, biomass per substrate utilized (Y(X/S)) of 0.268 and (Y(H2/S) of 0.0349. The results suggested that Sodium at any concentration resulted to inhibit the bacterial productivity of hydrogen.

Combination effect of pH and acetate on enzymatic cellulose hydrolysis

The productivity and efficiency of cellulase are significant in cellulose hydrolysis. With the accumulation of volatile fatty acids (VFAs), the pH value in anaerobic digestion system is reduced. Therefore, this study will find out how the pH and the amount of acetate influence the enzymatic hydrolysis of cellulose. The effects of pH and acetate on cellulase produced from Bacillus coagulans were studied at various pH 5-8, and acetate concentrations (0-60 mmol/L). A batch kinetic model for enzymatic cellulose hydrolysis was constructed from experimental data and performed. The base hypothesis was as follows: the rates of enzymatic cellulose hydrolysis rely on pH and acetate concentration. The results showed that the suitable pH range for cellulase production and cellulose hydrolysis (represents efficiency of cellulase) was 2.6-7.5, and 5.3-8.3, respectively. Moreover, acetate in the culture medium had an effect on cellulase production (K(I) = 49.50 mmol/L, n = 1.7) less than cellulose hydrolysis (K(I) = 37.85 mmol/L, n = 2.0). The results indicated that both the pH of suspension and acidogenic products influence the enzymatic hydrolysis of cellulose in an anaerobic environment. To enhance the cellulose hydrolysis rate, the accumulated acetate concentration should be lower than 25 mmol/L, and pH should be maintained at 7.

A rotational drum fermentation system with water flushing for enhancing hydrolysis and acidification of solid organic wastes

In this work, fresh soybean meal was used as the substrate for both batch and continuous experiments in a rotational drum fermentation (RDF) system to characterize the acidogenic process of solid organic waste degradation at high unionized volatile acid (U-VA) level and evaluate the effect of water flushing on the acidogenic performance. The experiments were conducted under mesophilic condition with a reaction time of 20 days. The results of the batch experiment showed that U-VA had a growing adverse effect on the volatile acid (VA) production and hydrolysis of the substrate as the initially added U-VA concentration increased (0, 5, 15, 25 g/L). VA formation deteriorated drastically when the initial U-VA concentration exceeded 5 g/L. VS degradation ratios decreased from 43.8% to 7.3%, and the hydrolysis rate constants varied between 28.8 and 3.8 x 10(-3)/d in response to the initial U-VA concentration. In the continuous experiment, two cascade process configurations (CP1 and CP2) without and with VA removal by water flushing, respectively, were developed. The results showed that the hydrolysis rate constants and VS degradation ratios were 13.1 x 10(-3)/d and 23%, respectively, in CP2, while only 9.1 x 10(-3)/d and 16.7% in CP2. Compared to CP1, the VA spectrum varied little in CP2 with water flushing. It suggested that the higher U-VA level had a significant inhibition on the acidogenic process of solid organic waste degradation, and the VA removal by water flushing improved the acidogenic performance.

Bioprocess monitoring and computer control: key roots of the current PAT initiative

This review article has been written for the journal, Biotechnology and Bioengineering, to commemorate the 70th birthday of Daniel I.C. Wang, who served as doctoral thesis advisor to each of the co-authors, but a decade apart. Key roots of the current PAT initiative in bioprocess monitoring and control are described, focusing on the impact of Danny Wang's research as a professor at MIT. The history of computer control and monitoring in biochemical processing has been used to identify the areas that have already benefited and those that are most likely to benefit in the future from PAT applications. Past applications have included the use of indirect estimation methods for cell density, expansion of on-line/at-line and on-line/in situ measurement techniques, and development of models and expert systems for control and optimization. Future applications are likely to encompass additional novel measurement technologies, measurements for multi-scale and disposable bioreactors, real time batch release, and more efficient data utilization to achieve process validation and continuous improvement goals. Dan Wang's substantial contributions in this arena have been one key factor in steering the PAT initiative towards realistic and attainable industrial applications.

Inhibitory effect of acetic acid on growth of hyperthermophilic archaeon Pyrococcus furiosus

The growth inhibition of Pyrococcus furiosus by acetic acid was stronger than that by hydrogen and could be described by a non-competitive inhibition model in which the inhibition constants of undissociated acetic acid, K(p) and n, were estimated to be 0.69 mM (25 mM total acetic acid at pH 6.5; pKa=4.96; 98 degrees C) and 1.0, respectively. In order to reduce the acetic acid inhibition, repeated-batch culturing was performed using a filtration module. This yielded 0.49 g of dry cells l(-1) after growth for 12 h after inoculation. It became impossible, however, to continue repeated-batch culturing manually because the time intervals for medium replacement became too short. In order to automatically maintain a low concentration of acetic acid, a perfusion culture was carried out in which medium feeding coupled to a pH-auxostat was performed. In this perfusion culture, it was possible to maintain the acetic acid concentration below 7.6 mM during exponential growth of P. furiosus, resulting in 1.8 g of dry cells l(-1) at 15 h after inoculation.

Acetate and ethanol production from H2 and CO2by Moorella sp. using a repeated batch culture

The growth inhibition of Moorella sp. HUC22-1 by undissociated acetic acid was analyzed using a non-competitive inhibition model coupled with a pH inhibition model. In the cells grown on H2 and CO2, the inhibition constant, K(p) of the undissociated acetic acid was 6.2 mM (164 mM as the total acetate at pH 6.2, pKa = 4.795, 55 degrees C), which was 1.5-fold higher than that obtained in cells grown on fructose. When a pH-controlled batch culture was performed using a fermentor at pH 6.2 with H2 and CO2, a maximum of 0.92 g/l of dry cell weight and 339 mM of acetate were produced after 220 h, which were 4.4- and 6.8-fold higher than those produced in the pH-uncontrolled batch culture, respectively. In order to reduce acetate inhibition in the culture medium, a repeated batch culture with cell recycling was performed at a constant pH with H2 and CO2. At a pH of 6.2, the total acetate production reached 840 mmol/l-reactor with 4.7 mmol/l-reactor of total ethanol production after 420 h. When the culture pH was maintained at 5.8, which was the optimum for ethanol production, the total ethanol production reached 15.4 mmol/l-reactor after 430 h, although the total acetate production was decreased to 675 mmol/l-reactor.

Effect of acid speciation on solid waste liquefaction in an anaerobic acid digester

Laboratory scale experiments were carried out to evaluate and determine the suitable working conditions for the enhancement of volatile fatty acids (VFAs) production with the stabilization of solid pineapple waste (peel) simultaneously when operating two-phase anaerobic digestion. It was found that acid production, enhanced when the digester was operated at neutral pH. VFAs, could be obtained at about 50 g/kg-waste within 14 days when operating at pH 6.5-7.5 compared to 33 g/kg-waste for digester operated at pH 5.0-6.0 for 32 days. The total volatile solid reduction of 42-48% was obtained with a 51-57% volume reduction at neutral pH. The effect of undissociated acids on the activity of acetogenic bacteria was also studied. The inhibitory level of undissociated acids for acidogenesis was found to be about 2300, 650, and 120 mg/L at pH 5, 6, and 7, respectively. The results revealed that at low pH, undissociated acids are inhibitory. It was found that for efficient operation of two-phase anaerobic digestion, both VFAs concentration and pH should be controlled.

Substrate and product inhibition of hydrogen production by the extreme thermophile, Caldicellulosiruptor saccharolyticus

Substrate and product inhibition of hydrogen production during sucrose fermentation by the extremely thermophilic bacterium Caldicellulosiruptor saccharolyticus was studied. The inhibition kinetics were analyzed with a noncompetitive, nonlinear inhibition model. Hydrogen was the most severe inhibitor when allowed to accumulate in the culture. Concentrations of 5-10 mM H(2) in the gas phase (identical with partial hydrogen pressure (pH(2)) of (1-2) x 10(4) Pa) initiated a metabolic shift to lactate formation. The extent of inhibition by hydrogen was dependent on the density of the culture. The highest tolerance for hydrogen was found at low volumetric hydrogen production rates, as occurred in cultures with low cell densities. Under those conditions the critical hydrogen concentration in the gas phase was 27.7 mM H(2) (identical with pH(2) of 5.6 x 10(4) Pa); above this value hydrogen production ceased completely. With an efficient removal of hydrogen sucrose fermentation was mainly inhibited by sodium acetate. The critical concentrations of sucrose and acetate, at which growth and hydrogen production was completely inhibited (at neutral pH and 70 degrees C), were 292 and 365 mM, respectively. Inorganic salts, such as sodium chloride, mimicked the effect of sodium acetate, implying that ionic strength was responsible for inhibition. Undissociated acetate did not contribute to inhibition of cultures at neutral or slightly acidic pH. Exposure of exponentially growing cultures to concentrations of sodium acetate or sodium chloride higher than ca. 175 mM caused cell lysis, probably due to activation of autolysins.

Potassium acetate by fermentation with Clostridium thermoaceticum

Potassium acetate is currently made by reacting petroleum-based acetic acid with potassium hydroxide. An alternate process, anaerobic fermentation of dextrose with Clostridium thermoaceticum, could be used and could possibly be cheaper. Growth characteristics and productivity of the fermentation were optimized to maximize acetate concentration in the broth. The effects of pH, type, and concentrations of nutrients and reducing agents were also evaluated. Corn steep liquor and stillage from an ethanol plant were effective and much cheaper substitutes for yeast extract. Preconcentrating the cells by ultrafiltration improved productivity, resulting in an acetic acid concentration of 53.6 g/L in 50 h at pH 6.5 using corn steep liquor. Sodium sulfide could be substituted for cysteine as the reducing agent with yields greater than 0.9 g acetic acid/g glucose.

Differential toxic effects of lactate and acetate on the metabolism of Streptococcus mutans and Streptococcus sanguis

Experiments were conducted with Streptococcus mutans NCTC 10449 and Streptococcus sanguis ATCC 10556 to determine whether the acid end-products, lactate and acetate, were involved in the regulation of cellular growth and metabolism. The growth rate and culture biomass of both organisms was inhibited by the addition of lactate and acetate at concentrations as high as 200 mM to the cultures, although the final pH values of the lactate and acetate cultures were similar. In addition, the metabolic conversion of glucose to lactate was decreased by external lactate but stimulated by acetate. In spite of this, calculation of the yield of cell biomass per mole of ATP (YATP) showed that the yield of both organisms actually increased in the presence of added lactate, but decreased with acetate. This indicates that the two acids interacted with the cells of the organisms by different mechanisms. For both organisms, the final external undissociated lactic acid was relatively constant at concentrations between 0 and 200 mM added lactate, 24.9-32.5 mM for S. mutans and 8.0-11.5 mM for S. sanguis. On the other hand, the final concentration of undissociated acetic acid in the S. mutans cultures increased from 2.9 to 83.7 mM as the medium acetate concentration increased, and from 1.0 to 36.0 mM with the S. sanguis cultures. Counterflow experiments provided evidence for a lactate carrier in both S. mutans and S. sanguis, but an acetate carrier in these organisms could not be demonstrated. [14C]-lactate and [14C]-acetate were taken up into de-energized, chemostatgrown cells of S. mutans and S. sanguis in response to an artificially generated pH gradient but not by an imposed electrical gradient. Thus, under these conditions lactate uptake occurred via a symport process with only one proton. Growth of both organisms in the presence of increasing concentrations of acetate resulted in a small reduction (27%) in the transmembrane pH gradient (delta pH) as measured by the permeant acid, [14C]-salicylate. However, the uptake of [14C]-acetate for the estimation of delta pH revealed significant inhibition of the acetate concentration gradient in the presence external acetate, indicating that the cells expelled the acetate anion. The results indicate that, unlike acetate uptake, lactate transport by S. mutans and S. sanguis was strictly regulated via the lactate carrier in order to prevent excessive dissipation of the pH gradient. Clearly, the formation of acetate by oral streptococci is more problematic for cellular homeostasis than the formation of lactate.