Biohydrogen production from xylose at extreme thermophilic temperatures (70 degrees C) by mixed culture fermentation

Comprehensive insights into the effects of acidogenic off-gas utilization on successive biogas production, microbial community structure and metabolite distribution during two-stage anaerobic digestion

Although two-stage anaerobic digestion (TSAD) technology has been investigated, the mechanisms regarding the impact of acidogenic off-gas (AOG) on successive methane production have not been well addressed. In this study, a novel TSAD system was designed. Food waste, as the main substrate, was co-digested with chicken manure and corn straw. The acidogenic gas beyond atmospheric pressure was introduced into the bottom of the methanogenesis reactor through a stainless steel diffuser. Results showed the addition of AOG increased the methane yield from 435.2 to 597.1 mL/g VSin in successive methanogenesis stage, improved by 37.2 %, and increased the energy yield from 9.0 to 11.3 kJ/g VSsubstrate. However, the theoretical contribution of hydrogenotrophic methanogenesis using H2 contained in AOG was only 15.2 % of the increased methane yield. After the addition of AOG, the decreased levels of ammonia nitrogen and butyrate indicate that the stability of the AD system was improved. The electron transfer system and co-enzyme F420 activity were enhanced; however, the decrease in acetate kinase activity indicates aceticlastic methanogenesis may have been weakened. The microbial diversity and species richness were improved by the added AOG. Methanosarcina was more competitive than Methanothermobacter, enhancing the syntrophic effect. The relative abundance of protein degradation bacteria norank_f_Anaerolineaceae and lipid degradation bacteria Syntrophomonas was increased. Metabolite analysis confirmed that the addition of AOG promoted amino acid metabolism, the biosynthesis of other secondary metabolism and lipid metabolism. The improved degradation of recalcitrant organic components (lipids and proteins) in food waste was responsible for the increased methane yield. This study provides an in-depth understanding of the impact of AOG utilization on successive methane production and has practical implications for the treatment of food waste.

Mixed culture biotechnology and its versatility in dark fermentative hydrogen production

Over the years, extensive research has gone into fermentative hydrogen production using pure and mixed cultures from waste biomass with promising results. However, for up-scaling of hydrogen production mixed cultures are more appropriate to overcome the operational difficulties such as a metabolic shift in response to environmental stress, and the need for a sterile environment. Mixed culture biotechnology (MCB) is a robust and stable alternative with efficient waste and wastewater treatment capacity along with co-generation of biohydrogen and platform chemicals. Mixed culture being a diverse group of bacteria with complex metabolic functions would offer a better response to the environmental variations encountered during biohydrogen production. The development of defined mixed cultures with desired functions would help to understand the microbial community dynamics and the keystone species for improved hydrogen production. This review aims to offer an overview of the application of MCB for biohydrogen production.

Adaptive Network Fuzzy Inference System and Particle Swarm Optimization of Biohydrogen Production Process

Green hydrogen is considered to be one of the best candidates for fossil fuels in the near future. Bio-hydrogen production from the dark fermentation of organic materials, including organic wastes, is one of the most cost-effective and promising methods for hydrogen production. One of the main challenges posed by this method is the low production rate. Therefore, optimizing the operating parameters, such as the initial pH value, operating temperature, N/C ratio, and organic concentration (xylose), plays a significant role in determining the hydrogen production rate. The experimental optimization of such parameters is complex, expensive, and lengthy. The present research used an experimental data asset, adaptive network fuzzy inference system (ANFIS) modeling, and particle swarm optimization to model and optimize hydrogen production. The coupling between ANFIS and PSO demonstrated a robust effect, which was evident through the improvement in the hydrogen production based on the four input parameters. The results were compared with the experimental and RSM optimization models. The proposed method demonstrated an increase in the biohydrogen production of 100 mL/L compared to the experimental results and a 200 mL/L increase compared to the results obtained using ANOVA.

Two-phase anaerobic digestion of food waste: Effect of semi-continuous feeding on acidogenesis and methane production

In present investigation, effect of diverting acidogenic off-gas from leached bed reactor (LBR) to up-flow anaerobic sludge blanket (UASB) reactor during semi-continuous food waste (FW) anaerobic digestion was evaluated. In test LBR headspace pressure (3.3 psi) was maintained with intermittent headspace gas transfer into UASB. In control, same headspace pressure was maintained without gas transfer. The semi-continuous FW addition affected the characteristics and production of leachate in control and test LBR. The cumulative COD, total soluble products and methane yields were 1.26, 1.37 and 3 times higher in the test LBR than the control. The acetate and methane yields from test LBR were 697.8 g·kgVS_added^-1 and 167.55 mL·gCOD^-1feeding. Acidogenic gas transfer maintained low partial pressure of hydrogen and the hydrogen to carbon-di-oxide ratio in the headspace of LBR, which were thermodynamically favorable for microbial metabolism and concomitant high-rate production of acetate-rich volatile fatty acid and methane-rich biogas from FW.

Reactor Designs and Configurations for Biological and Bioelectrochemical C1 Gas Conversion: A Review

Microbial C1 gas conversion technologies have developed into a potentially promising technology for converting waste gases (CO₂, CO) into chemicals, fuels, and other materials. However, the mass transfer constraint of these poorly soluble substrates to microorganisms is an important challenge to maximize the efficiencies of the processes. These technologies have attracted significant scientific interest in recent years, and many reactor designs have been explored. Syngas fermentation and hydrogenotrophic methanation use molecular hydrogen as an electron donor. Furthermore, the sequestration of CO₂ and the generation of valuable chemicals through the application of a biocathode in bioelectrochemical cells have been evaluated for their great potential to contribute to sustainability. Through a process termed microbial chain elongation, the product portfolio from C1 gas conversion may be expanded further by carefully driving microorganisms to perform acetogenesis, solventogenesis, and reverse β-oxidation. The purpose of this review is to provide an overview of the various kinds of bioreactors that are employed in these microbial C1 conversion processes.

Woody biomass as a potential feedstock for fermentative gaseous biofuel production

Biogas and biohydrogen are compatible gaseous biofuels that can be blended with natural gas for reticulated fuel supply to reduce greenhouse gas emissions. Sustainably grown woody biomass is emerging as a potential feedstock in the production of biofuels. Woody biomass is widely available, uses non-arable land for plantation, does not require synthetic fertilisers to grow and acts as a carbon sink. The cellulose and hemicellulose fractions of wood are renewable sources of sugars that can be used for fermentative production of gaseous biofuels. However, widespread use of wood as a gaseous biofuel feedstock is constrained due to the recalcitrant nature of wood to enzymatic hydrolysis. Pretreatment makes cellulose and hemicellulose accessible to microbial enzymes to produce fermentable sugars. Here we review wood composition, its structure and different pretreatment techniques in the context of their effects on deconstruction of wood to improve hydrolysis and fermentative gaseous fuel production. The anaerobic digestion of pretreated wood for biogas and dark fermentation for biohydrogen production are discussed with reference to gas yields. Key advancements in lab-scale research are described for pretreatments and for pure, co- and mixed culture fermentations. Limitations to yield improvements are identified and future perspectives and prospects of gaseous biofuel production from woody biomass are discussed, with reference to new developments in engineered biocatalysts and process integration.

Hydrogen Dark Fermentation for Degradation of Solid and Liquid Food Waste

The constant increase in the amount of food waste accumulating in landfills and discharged into the water reservoirs causes environment pollution and threatens human health. Solid and liquid food wastes include fruit, vegetable, and meat residues, alcohol bard, and sewage from various food enterprises. These products contain high concentrations of biodegradable organic compounds and represent an inexpensive and renewable substrate for the hydrogen fermentation. The goal of the work was to study the efficiency of hydrogen obtaining and decomposition of solid and liquid food waste via fermentation by granular microbial preparation (GMP). The application of GMP improved the efficiency of the dark fermentation of food waste. Hydrogen yields reached 102 L/kg of solid waste and 2.3 L/L of liquid waste. The fermentation resulted in the 91-fold reduction in the weight of the solid waste, while the concentration of organics in the liquid waste decreased 3-fold. Our results demonstrated the potential of granular microbial preparations in the production of hydrogen via dark fermentation. Further development of this technology may help to clean up the environment and reduce the reliance on fossil fuels by generating green hydrogen via recycling of household and industrial organic wastes.

One-step acquirement of superior microbial communities from mesophilic digested sludge to upgrade anaerobic digestion

Anaerobic digestion is a promising waste-to-energy alternative technology. However, the efficiency upgrading for conventional mesophilic digestion of organic solid waste is always indispensable. Employing hyperthermophilic or thermophilic microbial community is one of the viable upgrading alternatives. Given the unavailability of the superior microbial communities, mesophilic digested sludge was used as inoculum, and instantly controlled at 70 °C and 55 °C for acclimation of hyperthermophilic and thermophilic inocula, respectively. Waste activated sludge was continuously and synchronously fed into two digesters. After one round, thermophilic digester achieved stable biogas production rate at 0.22 L L^-1 d^-1, with a methane proportion over 60%, whereas fluctuation was observed in the hyperthermophilic digester, and approximately triple time was needed to reach a relatively stable biogas production rate 0.12 L L^-1 d^-1. Nevertheless, higher hydrolysis ratio 24.4% was observed in the hyperthermophilic digester despite the lower biogas production. Therefore, methanogenesis step limited the whole anaerobic process for the hyperthermophilic digestion, and digestion at 70 °C was appropriate as a pre-fermentation stage to enhanced hydrolysis. The genus Methanothrix proportion in the thermophilic digester gradually decreased, while another acetoclastic genus Methanosarcina ultimately was acclimated to the dominant methanogen. In addition to Methanothrix, hydrogenotrophic archaea became competitive in the hyperthermophilic digester, with Methanothermobacter dominant at 22.6%. The genus Psychrobacter, affiliated to the phylum Proteobacteria could survive better than the others at 70 °C, with a final proportion of 62.5%.

Establishment and differential performance of hyperthermophilic microbial community during anaerobic self-degradation of waste activated sludge

Hyperthermophilic anaerobic digestion, especially at 70 °C, has drawn wide attention. In order to acquire the inoculum and digestion characteristics, batch acclimation and continuous operation experiments were conducted under hyperthermophilic (70 °C), thermophilic (55 °C) and mesophilic (35 °C) conditions, respectively. Archaea at each temperature was successfully enriched from the sole-source waste activated sludge (WAS). Hyperthermophilic digestion achieved higher archaea diversity, close to the Shannon index 2.23 for the thermophilic digestion, but the population were not improved, at a 16S rRNA genes 5.99 × 10⁵ copies mL^-1. Hydrogenotrophic methanogens, Methanospirillum and Methanothermobacter, dominated in the hyperthermophilic digester, accounting for 27.15%, while the primary phylum Firmicutes was promoted to 36.31%, with the proteolytic genus Coprothermobacter in Firmicutes at 19.50%. Refractory organic fractions were converted more with a higher digestion temperature, which was demonstrated by the fact that the COD/VS increased to 5.8, 5.2 and 4.2 at 70 °C, 55 °C and 35 °C, respectively, at the end of batch acclimation. In addition, the most solubilization for the dominant fraction protein in the WAS occurred at 70 °C as well. Similar hydrolysis ratio, over 10%, and specific hydrolysis rate, around 0.025 g COD (g VSS·d)^-1, were achieved at 70 °C and 55 °C. The higher hydrolysis for hyperthermophilic digestion even resulted in a higher methane yield than that for the mesophilic digestion. Nevertheless, contrary to higher hydrolysis, methanogenesis limited hyperthermophilic digestion in WAS degradation, with an ultimate methane yield 71.2 mL g^-1 VS_added, despite an almost complete VFA conversion through the continuous operation.

Seed Pretreatment for Increased Hydrogen Production Using Mixed-Culture Systems with Advantages over Pure-Culture Systems

Hydrogen is an important source of energy and is considered as the future energy carrier post-petroleum era. Nowadays hydrogen production through various methods is being explored and developed to minimize the production costs. Biological hydrogen production has remained an attractive option, highly economical despite low yields. The mixed-culture systems use undefined microbial consortia unlike pure-cultures that use defined microbial species for hydrogen production. This review summarizes mixed-culture system pretreatments such as heat, chemical (acid, alkali), microwave, ultrasound, aeration, and electric current, amongst others, and their combinations to improve the hydrogen yields. The literature representation of pretreatments in mixed-culture systems is as follows: 45–50% heat-treatment, 15–20% chemical, 5–10% microwave, 10–15% combined and 10–15% other treatment. In comparison to pure-culture mixed-culture offers several advantages, such as technical feasibility, minimum inoculum steps, minimum media supplements, ease of operation, and the fact it works on a wide spectrum of low-cost easily available organic wastes for valorization in hydrogen production. In comparison to pure-culture, mixed-culture can eliminate media sterilization (4 h), incubation step (18–36 h), media supplements cost ($4–6 for bioconversion of 1 kg crude glycerol (CG)) and around 10–15 Millijoule (MJ) of energy can be decreased for the single run.

Mixed fermentation of Aspergillus niger and Candida shehatae to produce bioethanol with ionic-liquid-pretreated bagasse

In this study, bagasse was pretreated with ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and 1% NaOH solution for initial activation of bagasse. A mixed fermentation of treated bagasse by Aspergillus niger and Candida shehatae showed the optimal conditions with the addition of C. shehatae 12 h later at a 1:1 proportion to A. niger. To further improve the ethanol production and obtain optimal fermentation conditions, a Plackett-Burman design was applied to screen the significant formulation and process variables. The optimal ethanol fermentation conditions with IL pretreated bagasse were determined using response surface methodology by Box-Behnken design. Three variables "initial pH, (NH4)2SO4, fermentation time" were regarded as significant factors in the optimization study. The resulting optimum fermentation conditions for bioethanol was identified as: initial pH of 5.89, (NH4)2SO4 concentration of 0.40 g/50 mL, and fermentation time of 3.60 days. The verification experimental ethanol concentration was 8.14 g/L, which agreed with the predicted value. An enhancement of approximately 153.58% compared with initial fermentation conditions in ethanol production was found using optimized conditions. It demonstrated that optimization methodology had a positive effect on the improvement of ethanol production. Under the optimal fermentation medium and conditions, the ethanol production with IL-pretreated bagasse and untreated bagasse was 8.14 g/L and 5.03 g/L, respectively, which exhibited 62% increase, compared to initial conditions with production of 3.21 g/L and 2.67 g/L, respectively, which displayed 20% increase. Both under optimal and original fermentation conditions, compared to the fermentation medium with untreated bagasse, all the results indicated that IL-pretreated bagasse resulted in higher ethanol production than untreated bagasse, demonstrating that IL-pretreated bagasse successfully increased the ethanol production in the mixed fermentation by A. niger and C. shehatae.

The chemostat metabolite spectra of alkaline mixed culture fermentation under mesophilic, thermophilic, and extreme-thermophilic conditions

Alkaline mixed culture fermentation (MCF) is a promising technology for reducing organic waste and producing biochemicals. However, chemostat metabolite spectra that are necessary for constructing a model and analyzing factors are seldom reported. In the present study, the effects of pH on the metabolites distribution in mesophilic (35 °C), thermophilic (55 °C), and extreme-thermophilic (70 °C) alkaline MCF were demonstrated. A chemical oxygen demand balance above 80% was achieved, and the main metabolites included acetate, ethanol, propionate, lactate, and formate. The yields of ethanol and formate increased as pH was increased from 7.5 to higher pH under mesophilic and thermophilic conditions, while the yields of acetate, lactate, and/or propionate decreased. The yields of ethanol, acetate, and formate increased under extreme-thermophilic conditions as pH was increased from 7.5 to 9.0, whereas lactate and hydrogen yields decreased. Low biomass yield under thermophilic and extreme-thermophilic conditions benefited higher metabolite production and minimized the accumulation of sludge.

The diversity and specificity of the extracellular proteome in the cellulolytic bacterium Caldicellulosiruptor bescii is driven by the nature of the cellulosic growth substrate

BACKGROUND

Caldicellulosiruptor bescii is a thermophilic cellulolytic bacterium that efficiently deconstructs lignocellulosic biomass into sugars, which subsequently can be fermented into alcohols, such as ethanol, and other products. Deconstruction of complex substrates by C. bescii involves a myriad of highly abundant, substrate-specific extracellular solute binding proteins (ESBPs) and carbohydrate-active enzymes (CAZymes) containing carbohydrate-binding modules (CBMs). Mass spectrometry-based proteomics was employed to investigate how these substrate recognition proteins and enzymes vary as a function of lignocellulosic substrates.

RESULTS

Proteomic analysis revealed several key extracellular proteins that respond specifically to either C5 or C6 mono- and polysaccharides. These include proteins of unknown functions (PUFs), ESBPs, and CAZymes. ESBPs that were previously shown to interact more efficiently with hemicellulose and pectin were detected in high abundance during growth on complex C5 substrates, such as switchgrass and xylan. Some proteins, such as Athe_0614 and Athe_2368, whose functions are not well defined were predicted to be involved in xylan utilization and ABC transport and were significantly more abundant in complex and C5 substrates, respectively. The proteins encoded by the entire glucan degradation locus (GDL; Athe_1857, 1859, 1860, 1865, 1867, and 1866) were highly abundant under all growth conditions, particularly when C. bescii was grown on cellobiose, switchgrass, or xylan. In contrast, the glycoside hydrolases Athe_0609 (Pullulanase) and 0610, which both possess CBM20 and a starch binding domain, appear preferential to C5/complex substrate deconstruction. Some PUFs, such as Athe_2463 and 2464, were detected as highly abundant when grown on C5 substrates (xylan and xylose), also suggesting C5-substrate specificity.

CONCLUSIONS

This study reveals the protein membership of the C. bescii secretome and demonstrates its plasticity based on the complexity (mono-/disaccharides vs. polysaccharides) and type of carbon (C5 vs. C6) available to the microorganism. The presence or increased abundance of extracellular proteins as a response to specific substrates helps to further elucidate C. bescii's utilization and conversion of lignocellulosic biomass to biofuel and other valuable products. This includes improved characterization of extracellular proteins that lack discrete functional roles and are poorly/not annotated.

Influence of acidogenic headspace pressure on methane production under schematic of diversion of acidogenic off-gas to methanogenic reactor

This study investigated the effects of 12.6psi (T1), 6.3psi (T2), 3.3psi (T3) and ambient (T4) headspace pressure on the metabolic pathways in the acidogenic leach bed reactor (LBR) and overall methane recovery during two-phase anaerobic digestion of food waste. Diversion of biogas from LBR enhanced COD and soluble product generation in T2, T3 and T4 whereas, high pressure (T1) resulted in comparatively higher lactate production and low protein degradation. A pressure of 3-6psi (T2 and T3) improved the production of COD by ∼22-36%, soluble products by ∼9-43%, volatile solid reduction by ∼14-19%, and CH₄ production by ∼10-31% compared with control. Besides, ∼3-6psi headspace pressure positively influenced the composition of soluble products resulting in enhanced methane recovery adding advantage to the two-phase system. A headspace pressure of ∼3-6psi is recommended to enhance the hydrolysis-acidogenesis; however, the actual hydrogen concentration should be considered.

Biohydrogen production from xylose by fresh and digested activated sludge at 37, 55 and 70 °C

Two heat-treated inocula, fresh and digested activated sludge from the same municipal wastewater treatment plant, were compared for their H₂ production via dark fermentation at mesophilic (37 °C), thermophilic (55 °C) and hyperthermophilic (70 °C) conditions using xylose as the substrate. At both 37 and 55 °C, the fresh activated sludge yielded more H₂ than the digested sludge, whereas at 70 °C, neither of the inocula produced H₂ effectively. A maximum yield of 1.85 mol H₂ per mol of xylose consumed was obtained at 55 °C. H₂ production was linked to acetate and butyrate production, and there was a linear correlation (R² = 0.96) between the butyrate and H₂ yield for the fresh activated sludge inoculum at 55 °C. Approximately 2.4 mol H₂ per mol of butyrate produced were obtained against a theoretical maximum of 2.0, suggesting that H₂ was produced via the acetate pathway prior to switching to the butyrate pathway due to the increased H₂ partial pressure. Clostridia sp. were the prevalent species at both 37 and 55 °C, irrespectively of the inoculum type. Although the two inocula originated from the same plant, different thermophilic microorganisms were detected at 55 °C. Thermoanaerobacter sp., detected only in the fresh activated sludge cultures, may have contributed to the high H₂ yield obtained with such an inoculum.

Simultaneous Decolorization and Biohydrogen Production from Xylose by Klebsiella oxytoca GS-4-08 in the Presence of Azo Dyes with Sulfonate and Carboxyl Groups

Biohydrogen production from the pulp and paper effluent containing rich lignocellulosic material could be achieved by the fermentation process. Xylose, an important hemicellulose hydrolysis product, is used less efficiently as a substrate for biohydrogen production. Moreover, azo dyes are usually added to fabricate anticounterfeiting paper, which further increases the complexity of wastewater. This study reports that xylose could serve as the sole carbon source for a pure culture of Klebsiella oxytoca GS-4-08 to achieve simultaneous decolorization and biohydrogen production. With 2 g liter^-1 of xylose as the substrate, a maximum xylose utilization rate (UR_xyl) and a hydrogen molar yield (HMY) of 93.99% and 0.259 mol of H₂ mol of xylose^-1, respectively, were obtained. Biohydrogen kinetics and electron equivalent (e^- equiv) balance calculations indicated that methyl red (MR) penetrates and intracellularly inhibits both the pentose phosphate pathway and pyruvate fermentation pathway, while methyl orange (MO) acted independently of the glycolysis and biohydrogen pathway. The data demonstrate that biohydrogen pathways in the presence of azo dyes with sulfonate and carboxyl groups were different, but the azo dyes could be completely reduced during the biohydrogen production period in the presence of MO or MR. The feasibility of hydrogen production from industrial pulp and paper effluent by the strain if the xylose is sufficient was also proved and was not affected by toxic substances which usually exist in such wastewater, except for chlorophenol. This study offers a promising energy-recycling strategy for treating pulp and paper wastewaters, especially for those containing azo dyes.IMPORTANCE The pulp and paper industry is a major industry in many developing countries, and the global market of pulp and paper wastewater treatment is expected to increase by 60% between 2012 and 2020. Such wastewater contains large amounts of refractory contaminants, such as lignin, whose reclamation is considered economically crucial and environmentally friendly. Furthermore, azo dyes are usually added in order to fabricate anticounterfeiting paper, which further increases the complexity of the pulp and paper wastewater. This work may offer a better understanding of biohydrogen production from xylose in the presence of azo dyes and provide a promising energy-recycling method for treating pulp and paper wastewater, especially for those containing azo dyes.

Model development and experimental validation of capnophilic lactic fermentation and hydrogen synthesis by Thermotoga neapolitana

The aim of the present study was to develop a kinetic model for a recently proposed unique and novel metabolic process called capnophilic (CO2-requiring) lactic fermentation (CLF) pathway in Thermotoga neapolitana. The model was based on Monod kinetics and the mathematical expressions were developed to enable the simulation of biomass growth, substrate consumption and product formation. The calibrated kinetic parameters such as maximum specific uptake rate (k), semi-saturation constant (kS), biomass yield coefficient (Y) and endogenous decay rate (kd) were 1.30 h(-1), 1.42 g/L, 0.1195 and 0.0205 h(-1), respectively. A high correlation (>0.98) was obtained between the experimental data and model predictions for both model validation and cross validation processes. An increase of the lactate production in the range of 40-80% was obtained through CLF pathway compared to the classic dark fermentation model. The proposed kinetic model is the first mechanistically based model for the CLF pathway. This model provides useful information to improve the knowledge about how acetate and CO2 are recycled back by Thermotoga neapolitana to produce lactate without compromising the overall hydrogen yield.

Hydrogen and lipid production from starch wastewater by co-culture of anaerobic sludge and oleaginous microalgae with simultaneous COD, nitrogen and phosphorus removal

Anaerobic sludge (AS) and microalgae were co-cultured to enhance the energy conversion and nutrients removal from starch wastewater. Mixed ratio, starch concentration and initial pH played critical roles on the hydrogen and lipid production of the co-culture system. The maximum hydrogen production of 1508.3 mL L(-1) and total lipid concentration of 0.36 g L(-1) were obtained under the optimized mixed ratio (algae:AS) of 30:1, starch concentration of 6 g L(-1) and initial pH of 8. The main soluble metabolites in dark fermentation were acetate and butyrate, most of which can be consumed in co-cultivation. When sweet potato starch wastewater was used as the substrate, the highest COD, TN and TP removal and energy conversion efficiencies reached 80.5%, 88.7%, 80.1% and 34.2%, which were 176%, 178%, 200% and 119% higher than that of the control group (dark fermentation), respectively. This research provided a novel approach and achieved efficient simultaneous energy recovery and nutrients removal from starch wastewater by the co-culture system.

Organisms for biofuel production: natural bioresources and methodologies for improving their biosynthetic potentials

In order to relieve the pressure of energy supply and environment contamination that humans are facing, there are now intensive worldwide efforts to explore natural bioresources for production of energy storage compounds, such as lipids, alcohols, hydrocarbons, and polysaccharides. Around the world, many plants have been evaluated and developed as feedstock for bioenergy production, among which several crops have successfully achieved industrialization. Microalgae are another group of photosynthetic autotroph of interest due to their superior growth rates, relatively high photosynthetic conversion efficiencies, and vast metabolic capabilities. Heterotrophic microorganisms, such as yeast and bacteria, can utilize carbohydrates from lignocellulosic biomass directly or after pretreatment and enzymatic hydrolysis to produce liquid biofuels such as ethanol and butanol. Although finding a suitable organism for biofuel production is not easy, many naturally occurring organisms with good traits have recently been obtained. This review mainly focuses on the new organism resources discovered in the last 5 years for production of transport fuels (biodiesel, gasoline, jet fuel, and alkanes) and hydrogen, and available methods to improve natural organisms as platforms for the production of biofuels.

Biohydrogen production and kinetic modeling using sediment microorganisms of Pichavaram mangroves, India

Mangrove sediments host rich assemblages of microorganisms, predominantly mixed bacterial cultures, which can be efficiently used for biohydrogen production through anaerobic dark fermentation. The influence of process parameters such as effect of initial glucose concentration, initial medium pH, and trace metal (Fe²⁺) concentration was investigated in this study. A maximum hydrogen yield of 2.34, 2.3, and 2.6 mol H₂ mol⁻¹ glucose, respectively, was obtained under the following set of optimal conditions: initial substrate concentration—10,000 mg L⁻¹, initial pH—6.0, and ferrous sulphate concentration—100 mg L⁻¹, respectively. The addition of trace metal to the medium (100 mg L⁻¹ FeSO₄·7H₂O) enhanced the biohydrogen yield from 2.3 mol H₂ mol⁻¹ glucose to 2.6 mol H₂ mol⁻¹ glucose. Furthermore, the experimental data was subjected to kinetic analysis and the kinetic constants were estimated with the help of well-known kinetic models available in the literature, namely, Monod model, logistic model and Luedeking-Piret model. The model fitting was found to be in good agreement with the experimental observations, for all the models, with regression coefficient values >0.92.

Thermoanaerobacter pentosaceus sp. nov., an anaerobic, extremely thermophilic, high ethanol-yielding bacterium isolated from household waste

An extremely thermophilic, xylanolytic, spore-forming and strictly anaerobic bacterium, strain DTU01T, was isolated from a continuously stirred tank reactor fed with xylose and household waste. Cells stained Gram-negative and were rod-shaped (0.5–2 µm in length). Spores were terminal with a diameter of approximately 0.5 µm. Optimal growth occurred at 70 °C and pH 7, with a maximum growth rate of 0.1 h−1. DNA G+C content was 34.2 mol%. Strain DTU01T could ferment arabinose, cellobiose, fructose, galactose, glucose, lactose, mannitol, mannose, melibiose, pectin, starch, sucrose, xylan, yeast extract and xylose, but not cellulose, Avicel, inositol, inulin, glycerol, rhamnose, acetate, lactate, ethanol, butanol or peptone. Ethanol was the major fermentation product and a maximum yield of 1.39 mol ethanol per mol xylose was achieved when sulfite was added to the cultivation medium. Thiosulfate, but not sulfate, nitrate or nitrite, could be used as electron acceptor. On the basis of 16S rRNA gene sequence similarity, strain DTU01T was shown to be closely related to Thermoanaerobacter mathranii A3T, Thermoanaerobacter italicus Ab9T and Thermoanaerobacter thermocopriae JT3-3T, with 98–99 % similarity. Despite this, the physiological and phylogenetic differences (DNA G+C content, substrate utilization, electron acceptors, phylogenetic distance and isolation site) allow for the proposal of strain DTU01T as a representative of a novel species within the genus Thermoanaerobacter , for which the name Thermoanaerobacter pentosaceus sp. nov. is proposed, with the type strain DTU01T ( = DSM 25963T = KCTC 4529T = VKM B-2752T = CECT 8142T).

High-yield production of dihydrogen from xylose by using a synthetic enzyme cascade in a cell-free system

Let enzymes work: H2 was produced from xylose and water in one reactor containing 13 enzymes (red). By using a novel polyphosphate xylulokinase (XK), xylose was converted into H2 and CO2 with approaching 100 % of the theoretical yield. The findings suggest that cell-free biosystems could produce H2 from biomass xylose at low cost. Xu5P = xylulose 5-phosphate, G6P = glucose 6-phosphate.

Metabolic engineering for enhanced hydrogen production: a review

Hydrogen gas exhibits potential as a sustainable fuel for the future. Therefore, many attempts have been made with the aim of producing high yields of hydrogen gas through renewable biological routes. Engineering of strains to enhance the production of hydrogen gas has been an active area of research for the past 2 decades. This includes overexpression of hydrogen-producing genes (native and heterologous), knockout of competitive pathways, creation of a new productive pathway, and creation of dual systems. Interestingly, genetic mutations in 2 different strains of the same species may not yield similar results. Similarly, 2 different studies on hydrogen productivities may differ largely for the same mutation and on the same species. Consequently, here we analyzed the effect of various genetic modifications on several species, considering a wide range of published data on hydrogen biosynthesis. This article includes a comprehensive metabolic engineering analysis of hydrogen-producing organisms, namely Escherichia coli, Clostridium, and Enterobacter species, and in addition, a short discussion on thermophilic and halophilic organisms. Also, apart from single-culture utilization, dual systems of various organisms and associated developments have been discussed, which are considered potential future targets for economical hydrogen production. Additionally, an indirect contribution towards hydrogen production has been reviewed for associated species.

Enhancement of fermentative hydrogen production in an extreme-thermophilic (70°C) mixed-culture environment by repeated batch cultivation

Repeated batch cultivation was applied to enrich hydrogen fermentative microflora under extreme-thermophilic (70°C) environment. Initial inoculums received from a hydrogen producing reactor fed with organic fraction of household solid wastes. In total seven transfers was conducted and maximum hydrogen yield reached 296 ml H(2)/g (2.38 mol/mol) glucose and 252 ml H(2)/g (2.03 mol/mol) for 1 and 2 g/l glucose medium, respectively. It was found that hydrogen production was firstly decreased and got increased gradually from third generation. Acetate was found to be the main metabolic by-product in all batch cultivation. Furthermore, the diversity of bacterial community got decreased after repeated batch cultivation. It was proved that repeated batch cultivation was a good method to enhance the hydrogen production by enriching the mixed cultures of dominant species.

Optimization of fermentative biohydrogen production by response surface methodology using fresh leachate as nutrient supplement

Fresh compost leachate was used as a nutrients source to facilitate anaerobic fermentative hydrogen production from glucose inoculated with mixed culture. The optimum condition for hydrogen production was predicted by response surface methodology (RSM). The model showed the maximum cumulative hydrogen volume (469.74 mL) and molar hydrogen yield (1.60 mol H2/mol glucose) could be achieved at 6174.93 mg/L glucose and 3383.20 mg COD/L leachate. According to the predicted optimal condition, four tests were carried out to validate the predicted values and evaluate the leachate's effect on co-fermentation with juice wastewater. A maximum cumulative hydrogen volume of 587.05 ± 15.08 mL was obtained in co-fermentation test, and the molar hydrogen yield reached 2.06 ± 0.06 mol H2/mol glucose. The co-fermentation of fresh leachate and glucose/juice wastewater was a combination of acetic acid and butyric acid type-fermentation. The results demonstrated that leachate can serve as a nutrients source for biohydrogen production.

Performance and microbial community analysis of two-stage process with extreme thermophilic hydrogen and thermophilic methane production from hydrolysate in UASB reactors

The two-stage process for extreme thermophilic hydrogen and thermophilic methane production from wheat straw hydrolysate was investigated in up-flow anaerobic sludge bed (UASB) reactors. Specific hydrogen and methane yields of 89 ml-H(2)/g-VS (190 ml-H(2)/g-sugars) and 307 ml-CH(4)/g-VS, respectively were achieved simultaneously with the overall VS removal efficiency of 81% by operating with total hydraulic retention time (HRT) of 4 days . The energy conversion efficiency was dramatically increased from only 7.5% in the hydrogen stage to 87.5% of the potential energy from hydrolysate, corresponding to total energy of 13.4 kJ/g-VS. Dominant hydrogen-producing bacteria in the H(2)-UASB reactor were Thermoanaerobacter wiegelii, Caldanaerobacter subteraneus, and Caloramator fervidus. Meanwhile, the CH(4)-UASB reactor was dominated with methanogens of Methanosarcina mazei and Methanothermobacter defluvii. The results from this study suggest the two stage anaerobic process can be effectively used for energy recovery and for stabilization of hydrolysate at anaerobic conditions.

Enhanced bioenergy recovery from rapeseed plant in a biorefinery concept

The present study investigated the utilization of the whole rapeseed plant (seed and straw) for multi-biofuels production in a biorefinery concept. Results showed that bioethanol production from straw was technically feasible with ethanol yield of 0.15 g ethanol/g dry straw after combined alkaline peroxide and stream pretreatment. The byproducts (rapeseed cake, glycerol, hydrolysate and stillage) were evaluated for hydrogen and methane production. In batch experiments, the energy yields from each feedstock for, either methane production alone or for both hydrogen and methane, were similar. However, results from continuous experiments demonstrated that the two-stage hydrogen and methane fermentation process could work stably at organic loading rate up to 4.5 gVS/(Ld), while the single-stage methane production process failed. The energy recovery efficiency from rapeseed plant increased from 20% in the conventional biodiesel process to 60% in the biorefinery concept, by utilization of the whole rapeseed plant for biodiesel, bioethanol, biohydrogen and methane production.

Stable coexistence of two Caldicellulosiruptor species in a de novo constructed hydrogen-producing co-culture

BACKGROUND

Mixed culture enrichments have been used frequently for biohydrogen production from different feedstock. In spite of the several advantages offered by those cultures, they suffer poor H2 yield. Constructing defined co-cultures of known H2 producers may offer a better performance than mixed-population enrichments, while overcoming some of the limitations of pure cultures based on synergies among the microorganisms involved.

RESULTS

The extreme thermophiles Caldicellulosiruptor saccharolyticus DSM 8903 and C. kristjanssonii DSM 12137 were combined in a co-culture for H2 production from glucose and xylose in a continuous-flow stirred tank reactor. The co-culture exhibited a remarkable stability over a period of 70 days under carbon-sufficient conditions, with both strains coexisting in the system at steady states of different dilution rates, as revealed by species-specific quantitative PCR assays. The two strains retained their ability to stably coexist in the reactor even when glucose was used as the sole growth-limiting substrate. Furthermore, H2 yields on glucose exceeded those of either organism alone under the same conditions, alluding to a synergistic effect of the two strains on H2 production. A maximum H2 yield of 3.7 mol (mol glucose)(-1) was obtained by the co-culture at a dilution rate of 0.06 h(-1); a higher yield than that reported for any mixed culture to date. A reproducible pattern of population dynamics was observed in the co-culture under both carbon and non-carbon limited conditions, with C. kristjanssonii outgrowing C. saccharolyticus during the batch start-up phase and prevailing at higher dilution rates. A basic continuous culture model assuming the ability of C. saccharolyticus to enhance the growth of C. kristjanssonii could mimic the pattern of population dynamics observed experimentally and provide clues to the nature of interaction between the two strains. As a proof, the cell-free growth supernatant of C. saccharolyticus was found able to enhance the growth of C. kristjanssonii in batch culture through shortening its lag phase and increasing its maximum biomass concentration by ca. 18%.

CONCLUSIONS

This study provides experimental evidence on the stable coexistence of two closely related organisms isolated from geographically-distant habitats under continuous operation conditions, with the production of H2 at high yields. An interspecies interaction is proposed as the reason behind the remarkable ability of the two Caldicellulosiruptor strains to coexist in the system rather than only competing for the growth-limiting substrate.

Extreme thermophilic biohydrogen production from wheat straw hydrolysate using mixed culture fermentation: effect of reactor configuration

Hydrogen production from hemicellulose-rich wheat straw hydrolysate was investigated in continuously-stirred tank reactor (CSTR), up-flow anaerobic sludge bed (UASB) reactor, and anaerobic filter (AF) reactor. The CSTR was operated at an hydraulic retention time (HRT) of 3 days, and the UASB and AF reactors were operated at 1 day HRT, using mixed extreme thermophiles at 70 °C. The highest hydrogen production yield of 212.0±6.6 mL-H₂/g-sugars, corresponding to a hydrogen production rate of 821.4±25.5 mL-H₂/dL was achieved with the UASB reactor. Lowering the HRT to 2.5 days caused cell mass washout in the CSTR, while the UASB and AF reactors gave fluctuating and reducing hydrogen production at a 0.5-day HRT. The original rate and yield were recovered when the HRT was increased back to 1 day. These results demonstrate that reactor configuration is an important factor for enhancing and stabilizing H₂ production.

Biohydrogen production from wheat straw hydrolysate by dark fermentation using extreme thermophilic mixed culture

Hydrolysate was tested as substrate for hydrogen production by extreme thermophilic mixed culture (70 degrees C) in both batch and continuously fed reactors. Hydrogen was produced at hydrolysate concentrations up to 25% (v/v), while no hydrogen was produced at hydrolysate concentration of 30% (v/v), indicating that hydrolysate at high concentrations was inhibiting the hydrogen fermentation process. In addition, the lag phase for hydrogen production was strongly influenced by the hydrolysate concentration, and was prolonged from approximately 11 h at the hydrolysate concentrations below 20% (v/v) to 38 h at the hydrolysate concentration of 25% (v/v). The maximum hydrogen yield as determined in batch assays was 318.4 +/- 5.2 mL-H(2)/g-sugars (14.2 +/- 0.2 mmol-H(2)/g-sugars) at the hydrolysate concentration of 5% (v/v). Continuously fed, and the continuously stirred tank reactor (CSTR), operating at 3 day hydraulic retention time (HRT) and fed with 20% (v/v) hydrolysate could successfully produce hydrogen. The hydrogen yield and production rate were 178.0 +/- 10.1 mL-H(2)/g-sugars (7.9 +/- 0.4 mmol H(2)/g-sugars) and 184.0 +/- 10.7 mL-H(2)/day L(reactor) (8.2 +/- 0.5 mmol-H(2)/day L(reactor)), respectively, corresponding to 12% of the chemical oxygen demand (COD) from sugars. Additionally, it was found that toxic compounds, furfural and hydroxymethylfurfural (HMF), contained in the hydrolysate were effectively degraded in the CSTR, and their concentrations were reduced from 50 and 28 mg/L, respectively, to undetectable concentrations in the effluent. Phylogenetic analysis of the mixed culture revealed that members involved hydrogen producers in both batch and CSTR reactors were phylogenetically related to the Caldanaerobacter subteraneus, Thermoanaerobacter subteraneus, and Thermoanaerobacterium thermosaccharolyticum.

A selection that reports on protein-protein interactions within a thermophilic bacterium

Many proteins can be split into fragments that exhibit enhanced function upon fusion to interacting proteins. While this strategy has been widely used to create protein-fragment complementation assays (PCAs) for discovering protein-protein interactions within mesophilic organisms, similar assays have not yet been developed for studying natural and engineered protein complexes at the temperatures where thermophilic microbes grow. We describe the development of a selection for protein-protein interactions within Thermus thermophilus that is based upon growth complementation by fragments of Thermotoga neapolitana adenylate kinase (AK(Tn)). Complementation studies with an engineered thermophile (PQN1) that is not viable above 75 degrees C because its adk gene has been replaced by a Geobacillus stearothermophilus ortholog revealed that growth could be restored at 78 degrees C by a vector that coexpresses polypeptides corresponding to residues 1-79 and 80-220 of AK(Tn). In contrast, PQN1 growth was not complemented by AK(Tn) fragments harboring a C156A mutation within the zinc-binding tetracysteine motif unless these fragments were fused to Thermotoga maritima chemotaxis proteins that heterodimerize (CheA and CheY) or homodimerize (CheX). This enhanced complementation is interpreted as arising from chemotaxis protein-protein interactions, since AK(Tn)-C156A fragments having only one polypeptide fused to a chemotaxis protein did not complement PQN1 to the same extent. This selection increases the maximum temperature where a PCA can be used to engineer thermostable protein complexes and to map protein-protein interactions.