A comprehensive and quantitative review of dark fermentative biohydrogen production

Anaerobic co-digestion of microalgal biomass, sugarcane vinasse, and residual glycerol from biodiesel using simplex-centroid mixture design: methane potential, synergic effect, and microbial diversity

Microalgal biomass (MB) is a promising feedstock for bioenergy production. Nonetheless, the cell recalcitrance and the low C/N ratio limit the methane yield during anaerobic digestion. As an alternative to overcome these challenges, MB co-digestion with different feedstocks has been proposed. Thus, this study evaluated the anaerobic co-digestion (AcoD) of MB cultivated in wastewater with sugarcane vinasse (VIN) and residual glycerol from biodiesel production (GLY). Batch tests were conducted using augmented simplex-centroid mixture design to investigate the impact of AcoD on methane production (SMP), synergistic effects, and the influence on microbial community. When compared to MB digestion, 150 NmL CH4.g-1VS, binary and ternary AcoD achieved SMP increases from 120 to 337%. The combination of 16.7:16.7:66.7 (MB:VIN:GLY) showed the highest SMP for a ternary mixture (631 NmL CH4.g-1VS). Optimal synergies ranged from 1.3 to 1.4 and were primarily found for the MB:GLY AcoD. Acetoclastic Methanosaeta genus was predominant, regardless the combination between substrates. Despite the largest SMP obtained from the MB:GLY AcoD, other ternary mixtures were also highly synergetic and therefore had strong potential as a strategic renewable energy source.

Comparative analysis of hydrogen production and bacterial communities in mesophilic and thermophilic consortia using multiple inoculum sources

This study investigates the hydrogen (H2) production performance and bacterial communities in mesophilic (37 °C) and thermophilic (50 °C) H2-producing consortia derived from different inoculum sources and utilizing food waste as a substrate. This study found notable variations in H2 production characteristics among these consortia. Among the mesophilic consortia (MC), the W-MC obtained with wetland (W) as the inoculum source exhibited the highest hydrogen production (3900 mL·L-1 and 117 mL·L-1·h-1), while among the thermophilic consortia (TC), the FP-TC obtained with forest puddle sediment (FP) as the inoculum source showed the highest performance (2112 mL·L-1 and 127 mL·L-1·h-1). This study reveals that the choice of inoculum source plays a crucial role in determining hydrogen production efficiency. Furthermore, the bacterial community analysis demonstrated varying microbial diversity and richness in different inoculum sources. Clostridium, a well-known H2-producing bacterium, was found in both mesophilic and thermophilic consortia and showed a positive correlation with H2 production. Other bacteria, such as Sporanaerobacter, Caproiciproducens, and Caldibacillus, also exhibited significant correlations with H2 production, suggesting their potential roles in the process. The study highlights the complex interactions between bacterial communities and hydrogen production performance, shedding light on the critical factors influencing this renewable energy source. Overall, this study contributes to our understanding of the microbial ecology and the factors affecting hydrogen production in different temperature conditions, which can have practical implications for optimizing biohydrogen production processes using organic waste substrates.

Nickel-iron doped on granular activated carbon for efficient immobilization in biohydrogen production

Nickel-iron doped granular activated carbon (GAC-N) was used to enhance immobilization in biohydrogen production. The effect of the sludge ratio to GAC-N, ranged 1:0.5-4, was studied. The optimum hydrogen yield (HY) of 1.64 ± 0.04 mol H2/mol sugar consumed and hydrogen production rate (HPR) of 45.67 ± 1.00 ml H2/L.h was achieved at a ratio of 1:1. Immobilization study was performed at 2 d HRT with a stable HY of 2.94 ± 0.16 mol H2/mol sugar consumed (HPR of 83.10 ± 4.61 ml H2/L.h), shorten biohydrogen production from 66 d to 26 d, incrementing HY by 57.30 %. The Monod model resulted in the optimum initial sugar, maximum specific growth rate, specific growth rate, and cell growth saturation coefficient at 20 g/L, 2.05 h-1, 1.98 h-1 and 6.96 g/L, respectively. The dominant bacteria identified was Thermoanaerobacterium spp. The GAC-N showed potential as a medium for immobilization to improve biohydrogen production.

Biogeochemical transformation of mercury driven by microbes involved in anaerobic digestion of municipal wastewater

Anaerobic digestion (AD) with municipal wastewater contained heavy metal mercury (Hg) highly affects the utilization of activated sludge, and poses severe threat to the health of human beings. However, the biogeochemical transformation of Hg during AD remains unclear. Here, we investigated the biogeochemical transformation and environmental characteristics of Hg and the variations of dominant microbes during AD. The results showed that Hg(II) methylation is dominant in the early stage of AD, while methylmercury (MeHg) demethylation dominates in the later stage. Dissolved total Hg (DTHg) in the effluent sludge decreased with time, while THg levels enhanced to varying degrees at the final stage. Sulfate significant inhibits MeHg formation, reduces bioavailability of Hg(II) by microbes and thus inhibits Hg(II) methylation. Microbial community analysis reveals that strains in Methanosarcina and Aminobacterium from the class of Methanomicrobia, rather than Deltaproteobacteria, may be directly related to Hg(II) methylation and MeHg demethylation. Overall, this research provide insights into the biogeochemical transformation of Hg in the anaerobic digestion of municipal wastewater treatment. This work is beneficial for scientific treatment of municipal wastewater and effluent sludge, thus reducing the risk of MeHg to human beings.

Optimization of hydrogen production in Enterobacter aerogenes by Complex I peripheral fragments destruction and maeA overexpression

As a concentrated energy source with high added value, hydrogen has great development prospects, with special emphasis on sustainable microbial production as a replacement for traditional fossil fuels. In this study, λ-Red recombination was used to alter the activity of Complex I by single and combined knockout of nuoE, nuoF and nuoG. In addition, the conversion of malic to pyruvic acid was promoted by overexpressing the maeA gene, which could increase the content of NADH and formic acid in the bacterial cells. Compared to the original strain, hydrogen production was 65% higher in the optimized strain IAM1183-EFG/M, in which the flux of the formic acid pathway was increased by 257%, the flux of the NADH pathway was increased by 13%, and the content of metabolites also changed significantly. In further bioreactor, the total hydrogen production of the scale-up IAM1183-EFG/M after 44 h of fermentation was 4.76 L, which increased by 18% compared with the starting strain. This study provides a new direction for future exploration of microbial hydrogen production by combinatorial modification of multiple genes.

Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Volatile fatty acids (VFA) are intermediary degradation products during anaerobic digestion (AD) that are subsequently converted to methanogenic substrates, such as hydrogen (H2), carbon dioxide (CO2), and acetic acid (CH3COOH). The final step of AD is the conversion of these methanogenic substrates into biogas, a mixture of methane (CH4) and CO2. In arrested AD (AAD), the methanogenic step is suppressed to inhibit VFA conversion to biogas, making VFA the main product of AAD, with CO2 and H2. VFA recovered from the AAD fermentation can be further converted to sustainable biofuels and bioproducts. Although this concept is known, commercialization of the AAD concept has been hindered by low VFA titers and productivity and lack of cost-effective separation methods for recovering VFA. This article reviews the different techniques used to rewire AD to AAD and the current state of the art of VFA production with AAD, emphasizing recent developments made for increasing the production and separation of VFA from complex organic materials. Finally, this paper discusses VFA production by AAD could play a pivotal role in producing sustainable jet fuels from agricultural biomass and wet organic waste materials.

Carbon-based photocatalysts for hydrogen production: A review

Future energy crises and environmental deterioration may only be avoided by converting solar energy into sustainable, safe, cost-effective, and environmentally friendly technologies such as water splitting. Many researchers and governments throughout the globe have stressed the imperative need for affordable, environmental benign, resistive to corrosion, and earth-abundant nanostructured photocatalysts. This has led scientists to look for a green and cost-effective way to generate energy. As a result, the significance of photo catalyst engineering and reactor design difficulties connected to the performance of the photocatalytic reactions, as well as the examination and analysis of photocatalyst behaviors for adaptable and cost effective H2 production, is emphasized and summarized. The carbon-based materials have an appealing band structure, strong chemical stability, is plentiful on Earth, and is relatively easy to produce, making them suitable for hydrogen production. As example, graphene oxide (GO) with the oxygenated functional groups and graphene and its counterparts, including Graphene quantum dots (GQDs), GO, reduce graphene oxide (rGO), have been demonstrated to be ideal nanocomposite materials due to their superior properties and distribution in matrix and CNTs with excellent electronic transmission efficiency, low cost, stability, and environmental friendly are a great alternative of electron mediators for photocatalytic devices to boost light absorptivity for efficient hydrogen generation but some of them have limited photocatalytic activity due to their low sunlight usage efficiency, therefore the numerous methods, such as doping ions, constructing heterostructure, and functionalizing carbon-based materials, have recently been proven to promote the photocatalytic activity of them. The pore structure of carbon material functions as an acceptor of photogenerated electrons, improved the photocatalyst's specific surface area. Generally low-dimensional carbon materials demonstrated immense promise as highly efficient, low-cost, and environmentally friendly catalysts for hydrogen generation as an energy source. This article reviews the recent research progress on carbon-based materials for hydrogen evolution for the first time. It commences with a quick overview of the present state of affairs and fundamental concepts of hydrogen production in carbon-based nanomaterials for use in this field. We anticipate that this study will inspire readers to expand the use of carbon-based materials in H₂ generation in a more environmentally friendly way.

Formate-driven H2 production by whole cells of Thermoanaerobacter kivui

Background

In times of global warming there is an urgent need to replace fossil fuel-based energy vectors by less carbon dioxide (CO2)-emitting alternatives. One attractive option is the use of molecular hydrogen (H2) since its combustion emits water (H2O) and not CO2. Therefore, H2 is regarded as a non-polluting fuel. The ways to produce H2 can be diverse, but steam reformation of conventional fossil fuel sources is still the main producer of H2 gas up to date. Biohydrogen production via microbes could be an alternative, environmentally friendly and renewable way of future H2 production, especially when the flexible and inexpensive C1 compound formate is used as substrate.

Results

In this study, the versatile compound formate was used as substrate to drive H2 production by whole cells of the thermophilic acetogenic bacterium Thermoanaerobacter kivui which harbors a highly active hydrogen-dependent CO2 reductase (HDCR) to oxidize formate to H2 and CO2 and vice versa. Under optimized reaction conditions, T. kivui cells demonstrated the highest H2 production rates (qH2 = 685 mmol g−1 h−1) which were so far reported in the literature for wild-type organisms. Additionally, high yields (Y(H2/formate)) of 0.86 mol mol−1 and a hydrogen evolution rate (HER) of 999 mmol L−1 h−1 were observed. Finally, stirred-tank bioreactor experiments demonstrated the upscaling feasibility of the applied whole cell system and indicated the importance of pH control for the reaction of formate-driven H2 production.

Conclusions

The thermophilic acetogenic bacterium T. kivui is an efficient biocatalyst for the oxidation of formate to H2 (and CO2). The existing genetic tool box of acetogenic bacteria bears further potential to optimize biohydrogen production in future and to contribute to a future sustainable formate/H2 bio-economy.

Machine learning technology in biohydrogen production from agriculture waste: Recent advances and future perspectives

Agricultural waste biomass has shown great potential to deliver green energy produced by biochemical and thermochemical conversion processes to mitigate future energy crises. Biohydrogen has become more interested in carbon-free and high-energy dense fuels among different biofuels. However, it is challenging to develop models based on experience or theory for precise predictions due to the complexity of biohydrogen production systems and the limitations of human perception. Recent advancements in machine learning (ML) may open up new possibilities. For this reason, this critical study offers a thorough understanding of ML's use in biohydrogen production. The most recent developments in ML-assisted biohydrogen technologies, including biochemical and thermochemical processes, are examined in depth. This review paper also discusses the prediction of biohydrogen production from agricultural waste. Finally, the techno-economic and scientific obstacles to ML application in agriculture waste biomass-based biohydrogen production are summarized.

Recent Advances in Membrane-Based Biogas and Biohydrogen Upgrading

Biogas and biohydrogen, due to their renewable nature and zero carbon footprint, are considered two of the gaseous biofuels that will replace conventional fossil fuels. Biogas from anaerobic digestion must be purified and converted into high-quality biomethane prior to use as a vehicle fuel or injection into natural gas networks. Likewise, the enrichment of biohydrogen from dark fermentation requires the removal of CO2, which is the main pollutant of this new gaseous biofuel. Currently, the removal of CO2 from both biogas and biohydrogen is carried out by means of physical/chemical technologies, which exhibit high operating costs and corrosion problems. Biological technologies for CO2 removal from biogas, such as photosynthetic enrichment and hydrogenotrophic enrichment, are still in an experimental development phase. In this context, membrane separation has emerged as the only physical/chemical technology with the potential to improve the performance of CO2 separation from both biogas and biohydrogen, and to reduce investment and operating costs, as a result of the recent advances in the field of nanotechnology and materials science. This review will focus on the fundamentals, potential and limitations of CO2 and H2 membrane separation technologies. The latest advances on membrane materials for biogas and biohydrogen purification will be systematically reviewed.

Debottlenecking the biological hydrogen production pathway of dark fermentation: insight into the impact of strain improvement

Confronted with the exhaustion of the earth’s fossil fuel reservoirs, bio-based process to produce renewable energy is receiving significant interest. Hydrogen is considered as an attractive energy carrier that can replace fossil fuels in the future mainly due to its high energy content, recyclability and environment-friendly nature. Biological hydrogen production from renewable biomass or waste materials by dark fermentation is a promising alternative to conventional routes since it is energy-saving and reduces environmental pollution. However, the current yield and evolution rate of fermentative hydrogen production are still low. Strain improvement of the microorganisms employed for hydrogen production is required to make the process competitive with traditional production methods. The present review summarizes recent progresses on the screening for highly efficient hydrogen-producing strains using various strategies. As the metabolic pathways for fermentative hydrogen production have been largely resolved, it is now possible to engineer the hydrogen-producing strains by rational design. The hydrogen yields and production rates by different genetically modified microorganisms are discussed. The key limitations and challenges faced in present studies are also proposed. We hope that this review can provide useful information for scientists in the field of fermentative hydrogen production.

Hydrogen can be generated by microorganisms.

Dark fermentation is efficient for biological hydrogen production.

Strain improvement is critical to enhancing hydrogen-producing ability.

Microalgae-bacterial granular consortium: Striding towards sustainable production of biohydrogen coupled with wastewater treatment

Anthropogenic activities have drastically affected the environment, leading to increased waste accumulation in atmospheric bodies, including water. Wastewater treatment is an energy-consuming process and typically requires thousands of kilowatt hours of energy. This enormous energy demand can be fulfilled by utilizing the microbial electrolysis route to breakdown organic pollutants in wastewater which produces clean water and biohydrogen as a by-product of the reaction. Microalgae are the promising microorganism for the biohydrogen production, and it has been investigated that the interaction between microalgae and bacteria can be used to boost the yield of biohydrogen. Consortium of algae and bacteria resulting around 50-60% more biohydrogen production compared to the biohydrogen production of algae and bacteria separately. This review summarises the recent development in different microalgae-bacteria granular consortium systems successfully employed for biohydrogen generation. We also discuss the limitations in biohydrogen production and factors affecting its production from wastewater.

Promotion of biological H2 (Bio-H2) production by the nitrogen-fixing anaerobic microbial consortia using humin, a solid-phase humic substance

Dark fermentative biological hydrogen (Bio-H₂) production is expected to be a clean and sustainable H₂ production technology, and the technologies have been studied to increase in the product yield as index. This study achieved high product yields of Bio-H₂ using nitrogen-fixing consortia under nitrogen-deficient conditions with glucose or mannitol as substrate and humin as the extracellular electron mediator: 4.12 mol-H₂/mol-glucose and 3.12 mol-H₂/mol-mannitol. The high Bio-H₂ production was observed under the conditions where both nitrogenase and hydrogenase were active in the presence of humin. Nitrogenase activity was confirmed by acetylene reduction activity and hydrogenase activity by Bio-H₂ production under nitrogenase-inhibiting conditions with NH₄NO₃. [Fe-Fe] hydrogenase detected by a specific PCR and acetate, butyrate, formate, lactate, and pyruvate produced as by-products suggested the involvement of both pyruvate-ferredoxin-oxidoreductase and pyruvate formate lyase pathways in Bio-H₂ production. Humin promoted the Bio-H₂ production beyond the capacity of the consortium, which had reached saturation with the optimum concentrations of glucose and mannitol. Carbon balance suggested the concurrent H₂ consumption by hydrogenotrophic methanogenesis and acetogenesis. Bio-H₂ production of the washed and starved consortium with reduced humin under conditions with or without NH₄NO₃ suggests that humin promoted hydrogenase and nitrogenase activity by donating extracellular electrons. Clostridium and Ruminococcus in the consortia were considered major hydrogen producers. Thus, this study demonstrated the outstanding potential of nitrogen-fixing consortia under nitrogen-deficient conditions with humin as an extracellular electron mediator for dark fermentative Bio-H₂ production with high yields.

Enhanced biohydrogen production with low graphene oxide content using thermophilic bioreactors

Modern society envisions hydrogen (H₂) fuel to drive the transportation, industrial, and domestic sectors. Here, we explore use of graphene oxide nanoparticles (GO NPs) for greatly enhancing bio-H₂ production by a consortium based on Thermoanaerobacterium thermosaccharolyticum spp. Thermophilic batch bioreactors were set up at 60 ^OC and initial pH of 8.5 to assess the effects of GO NPs supplements on biohydrogen production. Under optimal GO NPs loading of 10 mg/L, the supplemented system yielded ∼ 300% higher H₂ yield (6.78 mol H₂/mol sucrose) than control. Such an optimized system offered 73% H₂ purity and 85% conversion efficiency by promoted the desirable acetate fermentation pathway. Miseq Illumina sequencing tests revealed that the optimal levels of GO NPs did not alter the microbial composition of consortium.

Hydrogen Recovery by Mixed Matrix Membranes Made from 6FCl-APAF HPA with Different Contents of a Porous Polymer Network and Their Thermal Rearrangement

Mixed matrix membranes (MMMs) consisting of a blend of a hydroxypolyamide (HPA) matrix and variable loads of a porous polymer network (PPN) were thermally treated to induce the transformation of HPA to polybenzoxazole (β-TR-PBO). Here, the HPA matrix was a hydroxypolyamide having two hexafluoropropyilidene moieties, 6FCl-APAF, while the PPN was prepared by reacting triptycene (TRP) and trifluoroacetophenone (TFAP) in a superacid solution. The most probable size of the PPN particles was 75 nm with quite large distributions. The resulting membranes were analyzed by SEM and AFM. Up to 30% PPN loads, both SEM and AFM images confirmed quite planar surfaces, at low scale, with limited roughness. Membranes with high hydrogen permeability and good selectivity for the gas pairs H2/CH4 and H2/N2 were obtained. For H2/CO2, selectivity almost vanished after thermal rearrangement. In all cases, their hydrogen permeability increased with increasing loads of PPN until around 30% PPN with ulterior fairly abrupt decreasing of permeability for all gases studied. Thermal rearrangement of the MMMs resulted in higher permeabilities but lower selectivities. For all the membranes and gas pairs studied, the balance of permeability vs. selectivity surpassed the 1991 Robeson's upper bound, and approached or even exceeded the 2008 line, for MMMs having 30% PPN loads. In all cases, the HPA-MMMs before thermal rearrangement provided good selectivity versus permeability compromise, similar to their thermally rearranged counterparts but in the zone of high selectivity. For H2/CH4, H2/N2, these nonthermally rearranged MMMs approach the 2008 Robeson's upper bound while H2/CO2 gives selective transport favoring H2 on the 1991 Robeson's bound. Thus, attending to the energy cost of thermal rearrangement, it could be avoided in some cases especially when high selectivity is the target rather than high permeability.

Enhancement of magnetic field on fermentative hydrogen production by Clostridium pasteurianum

Microbial fermentation plays important roles in hydrogen production. Various methods to promote hydrogen production are being developed. Here, different magnetic field intensities (2.7 mT, 3.2 mT and 9.1 mT) were applied to the glucose fermentation system of Clostridium pasteurianum to evaluate the feasibility and effect of statistic magnetic field on hydrogen production. The results showed that the magnetic field intensity of 3.2 mT effectively enhanced the hydrogen production. The total glucose consumption reached 0.64 ± 0.010 mmol, the maximum hydrogen yield reached 2.34 ± 0.020 mol H₂/mol glucose, and the maximum hydrogen production rate reached 0.065 ± 0.002 mmol/h. Compared with the control, the maximum biomass, carbon conversion efficiency and energy conversion efficiency were elevated by 366%, 114%, and 26.8%, respectively. Our results provide a new way for promotion of hydrogen production, better understanding of the interaction mechanism between magnetic field and microorganisms and for optimizing the hydrogen production.

Biomethanation of Carbon Monoxide by Hyperthermophilic Artificial Archaeal Co-Cultures

Climate neutral and sustainable energy sources will play a key role in future energy production. Biomethanation by gas to gas conversion of flue gases is one option with regard to renewable energy production. Here, we performed the conversion of synthetic carbon monoxide (CO)-containing flue gases to methane (CH4) by artificial hyperthermophilic archaeal co-cultures, consisting of Thermococcus onnurineus and Methanocaldococcus jannaschii, Methanocaldococcus vulcanius, or Methanocaldococcus villosus. Experiments using both chemically defined and complex media were performed in closed batch setups. Up to 10 mol% CH4 was produced by converting pure CO or synthetic CO-containing industrial waste gases at a high rate using a co-culture of T. onnurineus and M. villosus. These findings are a proof of principle and advance the fields of Archaea Biotechnology, artificial microbial ecosystem design and engineering, industrial waste-gas recycling, and biomethanation.

Continuous dark and photo biohydrogen production in a baffled bioreactor and electrons distribution analysis

This work studied the sequential hydrogen production by dark and photo-fermentation (HPDPF) in continuous baffled bioreactors. Taken enzymatic hydrolysate of corn stover as initial carbon source, the influence of hydraulic retention time (HRT) of dark fermentation (DF) and the dilution ratio (DR) of dark fermentation effluents (DFEs) on the hydrogen production performance of the combined fermentation system and electron distribution were investigated. For DF unit, the highest hydrogen production rate (HPR) of 5.24 L/(L·d) was detected at HRT of 18 h, however, the maximum HPR of 4.60 L/(L·d) was obtained from DFEs with HRT of 12 h and DR of 1:0.5 during photo fermentation unit, meanwhile, the electrons in substrate partitioning to H₂ reached the maximum value of 35.69%. In terms of hydrogen yield, the optimum operating conditions of the combined system were HRT of 12 h (DF) and DR of 1:0.5(DFEs), in which the hydrogen yield reached 12.73 L/d.

Nanoparticle-associated single step hydrogen fermentation for the conversion of starch potato waste biomass by thermophilic Parageobacillus thermoglucosidasius

In the present study, starch-based potato peel waste biomass (PWB) was utilized as a potential substrate for hydrogen production via dark fermentation by the thermophillic amylase producing strain Parageobacillus thermoglucosidasius KCTC 33548. Supplementation of Fe₃O₄ nanoparticles (300 mg/L) led to a 4.15-fold increase in hydrogen production as compared to the control. The addition of optimized concentrations of both Fe₃O₄ nanoparticles (300 mg/L) and L-cysteine (250 mg/L) during hydrogen fermentation using pure starch and PWB generated maximum cumulative hydrogen yields of 167 and 71.9 mL with maximum production rates of 2.81 and 1.26 mL/h, respectively. Further, the correlation between Fe₃O₄ and the expression of hydrogenase isoforms and the related hydrogenase activity was explored. The possible mechanisms of the action of Fe₃O₄ on enhanced hydrogenase activity and hydrogen production was elucidated. To our knowledge, there are no such studies reported on enhanced hydrogen production from PWB in a single step.

Sustainable Biological Ammonia Production towards a Carbon-Free Society

A sustainable society was proposed more than 50 years ago. However, it is yet to be realised. For example, the production of ammonia, an important chemical widely used in the agriculture, steel, chemical, textile, and pharmaceutical industries, still depends on fossil fuels. Recently, biological approaches to achieve sustainable ammonia production have been gaining attention. Moreover, unlike chemical methods, biological approaches have a lesser environmental impact because ammonia can be produced under mild conditions of normal temperature and pressure. Therefore, in previous studies, nitrogen fixation by nitrogenase, including enzymatic ammonia production using food waste, has been attempted. Additionally, the production of crops using nitrogen-fixing bacteria has been implemented in the industry as one of the most promising approaches to achieving a sustainable ammonia economy. Thus, in this review, we described previous studies on biological ammonia production and showed the prospects for realising a sustainable society.

Zeolite addition to improve biohydrogen production from dark fermentation of C5/C6-sugars and Sargassum sp. biomass

Thermophilic biohydrogen production by dark fermentation from a mixture (1:1) of C5 (arabinose) and C6 (glucose) sugars, present in lignocellulosic hydrolysates, and from Sargassum sp. biomass, is studied in this work in batch assays and also in a continuous reactor experiment. Pursuing the interest of studying interactions between inorganic materials (adsorbents, conductive and others) and anaerobic bacteria, the biological processes were amended with variable amounts of a zeolite type-13X in the range of zeolite/inoculum (in VS) ratios (Z/I) of 0.065–0.26 g g⁻¹. In the batch assays, the presence of the zeolite was beneficial to increase the hydrogen titer by 15–21% with C5 and C6-sugars as compared to the control, and an increase of 27% was observed in the batch fermentation of Sargassum sp. Hydrogen yields also increased by 10–26% with sugars in the presence of the zeolite. The rate of hydrogen production increased linearly with the Z/I ratios in the experiments with C5 and C6-sugars. In the batch assay with Sargassum sp., there was an optimum value of Z/I of 0.13 g g⁻¹ where the H₂ production rate observed was the highest, although all values were in a narrow range between 3.21 and 4.19 mmol L⁻¹ day⁻¹. The positive effect of the zeolite was also observed in a continuous high-rate reactor fed with C5 and C6-sugars. The increase of the organic loading rate (OLR) from 8.8 to 17.6 kg m⁻³ day⁻¹ of COD led to lower hydrogen production rates but, upon zeolite addition (0.26 g g⁻¹ VS inoculum), the hydrogen production increased significantly from 143 to 413 mL L⁻¹ day⁻¹. Interestingly, the presence of zeolite in the continuous operation had a remarkable impact in the microbial community and in the profile of fermentation products. The effect of zeolite could be related to several properties, including the porous structure and the associated surface area available for bacterial adhesion, potential release of trace elements, ion-exchanger capacity or ability to adsorb different compounds (i.e. protons). The observations opens novel perspectives and will stimulate further research not only in biohydrogen production, but broadly in the field of interactions between bacteria and inorganic materials.

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.

Technical-scale biophotovoltaics for long-term photo-current generation from Synechocystis sp. PCC6803

A carbon-free energy supply is essential to sustain our future. Biophotovoltaics (BPV) provides a promising solution for hydrogen supply by directly coupling light-driven water splitting to hydrogen formation using oxygenic photoautotrophic cyanobacteria. However, BPV is currently limited by its low photon-to-current efficiency, and current experimental setups at a miniaturized scale hinder the rational investigation of the process and thus system optimization. In this article, we developed and optimized a new technical-scale (~250 ml working volume) BPV platform with defined and controllable operating parameters. Factors that interfered with reproducible and stable current output signals were identified and adapted. We found that the classical BG11 medium, used for the cultivation of cyanobacteria and also in many BPV studies, caused severe interferences in the bioelectrochemical experiments. An optimized nBG11 medium guaranteed a low and stable background current in the BPV reactor, regardless of the presence of light and/or mediators. As proof-of-principle, a very high long-term light-dependent current output (peak current of over 20 µA) was demonstrated in the new set-up over 12 days with living Synechocystis sp. PCC6803 cells and validated with appropriate controls. These results report the first reliable BPV platform generating reproducible photocurrent while still allowing quantitative investigation, rational optimization, and scale-up of BPV processes.

Overcoming Energetic Barriers in Acetogenic C1 Conversion

Currently one of the biggest challenges for society is to combat global warming. A solution to this global threat is the implementation of a CO₂-based bioeconomy and a H₂-based bioenergy economy. Anaerobic lithotrophic bacteria such as the acetogenic bacteria are key players in the global carbon and H₂ cycle and thus prime candidates as driving forces in a H₂- and CO₂-bioeconomy. Naturally, they convert two molecules of CO₂via the Wood-Ljungdahl pathway (WLP) to one molecule of acetyl-CoA which can be converted to different C2-products (acetate or ethanol) or elongated to C4 (butyrate) or C5-products (caproate). Since there is no net ATP generation from acetate formation, an electron-transport phosphorylation (ETP) module is hooked up to the WLP. ETP provides the cell with additional ATP, but the ATP gain is very low, only a fraction of an ATP per mol of acetate. Since acetogens live at the thermodynamic edge of life, metabolic engineering to obtain high-value products is currently limited by the low energy status of the cells that allows for the production of only a few compounds with rather low specificity. To set the stage for acetogens as production platforms for a wide range of bioproducts from CO₂, the energetic barriers have to be overcome. This review summarizes the pathway, the energetics of the pathway and describes ways to overcome energetic barriers in acetogenic C1 conversion.

Homologous production, one-step purification, and proof of Na+ transport by the Rnf complex from Acetobacterium woodii, a model for acetogenic conversion of C1 substrates to biofuels

BACKGROUND

Capture and storage of the energy carrier hydrogen as well as of the greenhouse gas carbon dioxide are two major problems that mankind faces currently. Chemical catalysts have been developed, but only recently a group of anaerobic bacteria that convert hydrogen and carbon dioxide to acetate, formate, or biofuels such as ethanol has come into focus, the acetogenic bacteria. These biocatalysts produce the liquid organic hydrogen carrier formic acid from H₂ + CO₂ or even carbon monoxide with highest rates ever reported. The autotrophic, hydrogen-oxidizing, and CO₂-reducing acetogens have in common a specialized metabolism to catalyze CO₂ reduction, the Wood-Ljungdahl pathway (WLP). The WLP does not yield net ATP, but is hooked up to a membrane-bound respiratory chain that enables ATP synthesis coupled to CO₂ fixation. The nature of the respiratory enzyme has been an enigma since the discovery of these bacteria and has been unraveled in this study.

RESULTS

We have produced a His-tagged variant of the ferredoxin:NAD oxidoreductase (Rnf complex) from the model acetogen Acetobacterium woodii, solubilized the enzyme from the cytoplasmic membrane, and purified it by Ni²⁺-NTA affinity chromatography. The enzyme was incorporated into artificial liposomes and catalyzed Na⁺ transport coupled to ferredoxin-dependent NAD reduction. Our results using the purified enzyme do not only verify that the Rnf complex from A. woodii is Na⁺-dependent, they also demonstrate for the first time that this membrane-embedded molecular engine creates a Na⁺ gradient across the membrane of A. woodii which can be used for ATP synthesis.

DISCUSSION

We present a protocol for homologous production and purification for an Rnf complex. The enzyme catalyzed electron-transfer driven Na⁺ export and, thus, our studies provided the long-awaited biochemical proof that the Rnf complex is a respiratory enzyme.

Archaea Biotechnology

Archaea are a domain of prokaryotic organisms with intriguing physiological characteristics and ecological importance. In Microbial Biotechnology, archaea are historically overshadowed by bacteria and eukaryotes in terms of public awareness, industrial application, and scientific studies, although their biochemical and physiological properties show a vast potential for a wide range of biotechnological applications. Today, the majority of microbial cell factories utilized for the production of value-added and high value compounds on an industrial scale are bacterial, fungal or algae based. Nevertheless, archaea are becoming ever more relevant for biotechnology as their cultivation and genetic systems improve. Some of the main advantages of archaeal cell factories are the ability to cultivate many of these often extremophilic organisms under non-sterile conditions, and to utilize inexpensive feedstocks often toxic to other microorganisms, thus drastically reducing cultivation costs. Currently, the only commercially available products of archaeal cell factories are bacterioruberin, squalene, bacteriorhodopsin and diether-/tetraether-lipids, all of which are produced utilizing halophiles. Other archaeal products, such as carotenoids and biohydrogen, as well as polyhydroxyalkanoates and methane are in early to advanced development stages, respectively. The aim of this review is to provide an overview of the current state of Archaea Biotechnology by describing the actual state of research and development as well as the industrial utilization of archaeal cell factories, their role and their potential in the future of sustainable bioprocessing, and to illustrate their physiological and biotechnological potential.

On-line analysis and in situ pH monitoring of mixed acid fermentation by Escherichia coli using combined FTIR and Raman techniques

We introduce an experimental setup allowing continuous monitoring of bacterial fermentation processes by simultaneous optical density (OD) measurements, long-path FTIR headspace monitoring of CO2, acetaldehyde and ethanol, and liquid Raman spectroscopy of acetate, formate, and phosphate anions, without sampling. We discuss which spectral features are best suited for detection, and how to obtain partial pressures and concentrations by integrations and least squares fitting of spectral features. Noise equivalent detection limits are about 2.6 mM for acetate and 3.6 mM for formate at 5 min integration time, improving to 0.75 mM for acetate and 1.0 mM for formate at 1 h integration. The analytical range extends to at least 1 M with a standard deviation of percentage error of about 8%. The measurement of the anions of the phosphate buffer allows the spectroscopic, in situ determination of the pH of the bacterial suspension via a modified Henderson-Hasselbalch equation in the 6-8 pH range with an accuracy better than 0.1. The 4 m White cell FTIR measurements provide noise equivalent detection limits of 0.21 μbar for acetaldehyde and 0.26 μbar for ethanol in the gas phase, corresponding to 3.2 μM acetaldehyde and 22 μM ethanol in solution, using Henry's law. The analytical dynamic range exceeds 1 mbar ethanol corresponding to 85 mM in solution. As an application example, the mixed acid fermentation of Escherichia coli is studied. The production of CO2, ethanol, acetaldehyde, acids such as formate and acetate, and the changes in pH are discussed in the context of the mixed acid fermentation pathways. Formate decomposition into CO2 and H2 is found to be governed by a zeroth-order kinetic rate law, showing that adding exogenous formate to a bioreactor with E. coli is expected to have no beneficial effect on the rate of formate decomposition and biohydrogen production.

Biohydrogen production beyond the Thauer limit by precision design of artificial microbial consortia

Dark fermentative biohydrogen (H₂) production could become a key technology for providing renewable energy. Until now, the H₂ yield is restricted to 4 moles of H₂ per mole of glucose, referred to as the "Thauer limit". Here we show, that precision design of artificial microbial consortia increased the H₂ yield to 5.6 mol mol^-1 glucose, 40% higher than the Thauer limit. In addition, the volumetric H₂ production rates of our defined artificial consortia are superior compared to any mono-, co- or multi-culture system reported to date. We hope this study to be a major leap forward in the engineering of artificial microbial consortia through precision design and provide a breakthrough in energy science, biotechnology and ecology. Constructing artificial consortia with this drawing-board approach could in future increase volumetric production rates and yields of other bioprocesses. Our artificial consortia engineering blueprint might pave the way for the development of a H₂ production bioindustry.

The characterization of arsenic biotransformation microbes in paddy soil after straw biochar and straw amendments

Straw biochar and straw application to paddy soil dramatically altered arsenic (As) biogeochemical cycling in soil-rice system, but it remains unknown how As biotransformation microbes (ABMs) contribute to these processes. In this study, rice pot experiments combining terminal restriction fragment length polymorphism (T-RFLP) analysis and clone library were performed to characterize ABMs. Through linear discriminant analysis (LDA) effect size (LEfSe) and correlation analysis, results revealed that arrA-harbouring iron-reducing bacteria (e.g., Geobacter and Shewanella) and arsC-harbouring Gammaproteobacteria (e.g., fermentative hydrogen-producing and lignin-degrading microorganisms) potentially mediated arsenate [As(V)] reduction under biochar and straw amendments, respectively. Methanogens and sulfate-reducing bacteria (SRB) carrying arsM gene might regulate methylated As concentration in soil-rice system. Network analysis demonstrated that the association among ABMs in rhizosphere was significantly stronger than that in bulk soil. Arsenite [As(III)] methylators carrying arsM gene exhibited much stronger co-occurrence pattern with arsC-harbouring As(V) reducers than with arrA-harbouring As(V) reducers. This study would broaden our insights for the dramatic variation of As biogeochemical cycling in soil-rice system after straw biochar and straw amendments through the activities of ABMs, which could contribute to the safe rice production and high rice yield in As-contaminated fields.

Industrial wastewater to biohydrogen: Possibilities towards successful biorefinery route

The aim of this review is to summarize the modern developments and enhancement strategies reported for improving the biorefinery route of industrial wastewater to biohydrogen. Recent developments towards biohydrogen production chiefly involves culture enrichment, pretreatment of biocatalysts, co culture fermentation, metabolic and genetic engineering, ecobiotechnological approaches and the coupling process of biohydrogen. In addition, an overview of dark fermentation, pathways involved, microbes involved in biohydrogen production, industrial wastewater as substrate have been focused. The utilization of organic residuals of dark fermentation for subsequent value added products are highlighted. More apparently, the two stage coupling process and its possibilities towards biorefinery has been reviewed comprehensively. Moreover, comparative energy and economic aspects of biohydrogen production from industrial wastewater and its prospects towards pilot scale applications are also spotlighted. Though all the enhancement strategies have both benefits and disadvantages, coupling process is considered as the most successful biorefinery route for biohydrogen production.

Aerobic nitrogen-fixing bacteria for hydrogen and ammonium production: current state and perspectives

Biological nitrogen fixation (BNF) is accomplished through the action of the oxygen-sensitive enzyme nitrogenase. One unique caveat of this reaction is the inclusion of hydrogen gas (H₂) evolution as a requirement of the reaction mechanism. In the absence of nitrogen gas as a substrate, nitrogenase will reduce available protons to become a directional ATP-dependent hydrogenase. Aerobic nitrogen-fixing microbes are of particular interest, because these organisms have evolved to perform these reactions with oxygen-sensitive enzymes in an environment surrounded by oxygen. The ability to maintain a functioning nitrogenase in aerobic conditions facilitates the application of these organisms under conditions where most anaerobic nitrogen fixers are excluded. In recent years, questions related to the potential yields of the nitrogenase-derived products ammonium and H₂ have grown more approachable to experimentation based on efforts to construct increasingly more complicated strains of aerobic nitrogen fixers such as the obligate aerobe Azotobacter vinelandii. This mini-review provides perspectives of recent and historical efforts to understand and quantify the yields of ammonium and H₂ that can be obtained through the model aerobe A. vinelandii, and outstanding questions that remain to be answered to fully realize the potential of nitrogenase in these applications with model aerobic bacteria.

Methods for quantification of growth and productivity in anaerobic microbiology and biotechnology

Anaerobic microorganisms (anaerobes) possess a fascinating metabolic versatility. This characteristic makes anaerobes interesting candidates for physiological studies and utilizable as microbial cell factories. To investigate the physiological characteristics of an anaerobic microbial population, yield, productivity, specific growth rate, biomass production, substrate uptake, and product formation are regarded as essential variables. The determination of those variables in distinct cultivation systems may be achieved by using different techniques for sampling, measuring of growth, substrate uptake, and product formation kinetics. In this review, a comprehensive overview of methods is presented, and the applicability is discussed in the frame of anaerobic microbiology and biotechnology.

Nanoengineered cellulosic biohydrogen production via dark fermentation: A novel approach

The insights of nanotechnology for cellulosic biohydrogen production through dark fermentation are reviewed. Lignocellulosic biomass to sugar generation is a complex process and covers the most expensive part of cellulose to sugar production technology. In this context, the impacts of nanomaterial on lignocellulosic biomass to biohydrogen production process have been reviewed. In addition, the feasibility of nanomaterials for implementation in each step of the cellulosic biohydrogen production is discussed for economic viability of the process. Numerous aspects such as possible replacement of chemical pretreatment method using nanostructured materials, use of immobilized enzyme for a fast rate of reaction and its reusability along with long viability of microbial cells and hydrogenase enzyme for improving the productivity are the highlights of this review. It is found that various types of nanostructured materials e.g. metallic nanoparticles (Fe°, Ni, Cu, Au, Pd, Au), metal oxide nanoparticles (Fe₂O₃, F₃O₄, NiCo₂O₄, CuO, NiO, CoO, ZnO), nanocomposites (Si@CoFe₂O₄, Fe₃O₄/alginate) and graphene-based nanomaterials can influence different parameters of the process and therefore may perhaps be utilized for cellulosic biohydrogen production_. The emphasis has been given on the cost issue and synthesis sustainability of nanomaterials for making the biohydrogen technology cost effective. Finally, recent advancements and feasibility of nanomaterials as the potential solution for improved cellulose conversion to the biohydrogen production process have been discussed, and this is likely to assist in developing an efficient, economical and sustainable biohydrogen production technology.

A Review of the Enhancement of Bio-Hydrogen Generation by Chemicals Addition

Bio-hydrogen production (BHP) produced from renewable bio-resources is an attractive route for green energy production, due to its compelling advantages of relative high efficiency, cost-effectiveness, and lower ecological impact. This study reviewed different BHP pathways, and the most important enzymes involved in these pathways, to identify technological gaps and effective approaches for process intensification in industrial applications. Among the various approaches reviewed in this study, a particular focus was set on the latest methods of chemicals/metal addition for improving hydrogen generation during dark fermentation (DF) processes; the up-to-date findings of different chemicals/metal addition methods have been quantitatively evaluated and thoroughly compared in this paper. A new efficiency evaluation criterion is also proposed, allowing different BHP processes to be compared with greater simplicity and validity.

Enhancing Hydrogen Production from Chlorella sp. Biomass by Pre-Hydrolysis with Simultaneous Saccharification and Fermentation (PSSF)

Simultaneous saccharification and fermentation (SSF) and pre-hydrolysis with SSF (PSSF) were used to produce hydrogen from the biomass of Chlorella sp. SSF was conducted using an enzyme mixture consisting of 80 filter paper unit (FPU) g-biomass−1 of cellulase, 92 U g-biomass−1 of amylase, and 120 U g-biomass−1 of glucoamylase at 35 °C for 108 h. This yielded 170 mL-H2 g-volatile-solids−1 (VS), with a productivity of 1.6 mL-H2 g-VS−1 h−1. Pre-hydrolyzing the biomass at 50 °C for 12 h resulted in the production of 1.8 g/L of reducing sugars, leading to a hydrogen yield (HY) of 172 mL-H2 g-VS−1. Using PSSF, the fermentation time was shortened by 36 h in which a productivity of 2.4 mL-H2 g-VS−1 h−1 was attained. To the best of our knowledge, the present study is the first report on the use of SSF and PSSF for hydrogen production from microalgal biomass, and the HY obtained in the study is by far the highest yield reported. Our results indicate that PSSF is a promising process for hydrogen production from microalgal biomass.

Food waste fermentation in a leach bed reactor: Reactor performance, and microbial ecology and dynamics

Food waste fermentation was investigated in a leach bed reactor operated at acidic, neutral and alkaline conditions. Highest solids reduction of 87% was obtained at pH 7 in 14 days of reaction time with minimum mixing. The concentration of volatile fatty acids increased to 28.6 g COD/L under pH 7, while the highest butyric acid of 16 g COD/L was obtained at pH 6. Bacterial community structure was narrowed down to Bifidobacterium and Clostridium at pH 6, while Bacteroides and Dysgonomonas were identified as main players at both pH 7 and 8. Bacterial populations in the food residue generally reflected those in the leachate, but some bacteria were selectively enriched in the leachate or the food residue. Bacterial community dynamics suggested that biodegradable food waste was first fermented by one of dominant players (e.g., Clostridium) and the other degraded resistant dietary fibers later (e.g., Bifidobacterium, Bacteroides, Dysgonomonas).

Complex Multimeric [FeFe] Hydrogenases: Biochemistry, Physiology and New Opportunities for the Hydrogen Economy

Hydrogenases are key enzymes of the energy metabolism of many microorganisms. Especially in anoxic habitats where molecular hydrogen (H₂) is an important intermediate, these enzymes are used to expel excess reducing power by reducing protons or they are used for the oxidation of H₂ as energy and electron source. Despite the fact that hydrogenases catalyze the simplest chemical reaction of reducing two protons with two electrons it turned out that they are often parts of multimeric enzyme complexes catalyzing complex chemical reactions with a multitude of functions in the metabolism. Recent findings revealed multimeric hydrogenases with so far unknown functions particularly in bacteria from the class Clostridia. The discovery of [FeFe] hydrogenases coupled to electron bifurcating subunits solved the enigma of how the otherwise highly endergonic reduction of the electron carrier ferredoxin can be carried out and how H₂ production from NADH is possible. Complexes of [FeFe] hydrogenases with formate dehydrogenases revealed a novel enzymatic coupling of the two electron carriers H₂ and formate. These novel hydrogenase enzyme complex could also contribute to biotechnological H₂ production and H₂ storage, both processes essential for an envisaged economy based on H₂ as energy carrier.

Clostridial whole cell and enzyme systems for hydrogen production: current state and perspectives

Strictly anaerobic bacteria of the Clostridium genus have attracted great interest as potential cell factories for molecular hydrogen production purposes. In addition to being a useful approach to this process, dark fermentation has the advantage of using the degradation of cheap agricultural residues and industrial wastes for molecular hydrogen production. However, many improvements are still required before large-scale hydrogen production from clostridial metabolism is possible. Here we review the literature on the basic biological processes involved in clostridial hydrogen production, and present the main advances obtained so far in order to enhance the hydrogen productivity, as well as suggesting some possible future prospects.

Efforts toward optimization of aerobic biohydrogen reveal details of secondary regulation of biological nitrogen fixation by nitrogenous compounds in Azotobacter vinelandii

Biological nitrogen fixation (BNF) through the enzyme nitrogenase is performed by a unique class of organisms known as diazotrophs. One interesting facet of BNF is that it produces molecular hydrogen (H₂) as a requisite by-product. In the absence of N₂ substrate, or under conditions that limit access of N₂ to the enzyme through modifications of amino acids near the active site, nitrogenase activity can be redirected toward a role as a dedicated hydrogenase. In free-living diazotrophs, nitrogenases are tightly regulated to minimize BNF to meet only the growth requirements of the cell, and are often accompanied by uptake hydrogenases that oxidize the H₂ by-product to recover the electrons from this product. The wild-type strain of Azotobacter vinelandii performs all of the tasks described above to minimize losses of H₂ while also growing as an obligate aerobe. Individual alterations to A. vinelandii have been demonstrated that disrupt key aspects of the N₂ reduction cycle, thereby diverting resources and energy toward the production of H₂. In this work, we have combined three approaches to override the primary regulation of BNF and redirect metabolism to drive biological H₂ production by nitrogenase in A. vinelandii. The resulting H₂-producing strain was further utilized as a surrogate to study secondary, post-transcriptional regulation of BNF by several key nitrogen-containing metabolites. The improvement in yields of H₂ that were achieved through various combinations of these three approaches was compared and is presented along with the insights into inhibition of BNF by several nitrogen compounds that are common in various waste streams. The findings indicate that both ammonium and nitrite hinder BNF through this secondary inhibition, but urea and nitrate do not. These results provide essential details to inform future biosynthetic approaches to yield nitrogen products that do not inadvertently inhibit BNF.

Bioremediation Strategies Aimed at Stimulating Chlorinated Solvent Dehalogenation Can Lead to Microbially-Mediated Toluene Biogenesis

In situ bioremediation practices that include subsurface addition of fermentable electron donors to stimulate reductive dechlorination by anaerobic bacteria have become widely employed to combat chlorinated solvent contamination in groundwater. At a contaminated site located near Baton Rouge, Louisiana (USA), toluene was transiently observed in groundwater at concentrations that sometimes far exceeded the US drinking water maximum contaminant level (MCL) of 1 mg/L after a fermentable substrate (agricultural feed grade cane molasses) was injected into the subsurface with the intent of providing electron donors for reductive dechlorination. Here, we present data that demonstrate that indigenous microorganisms can biologically produce toluene by converting phenylacetic acid, phenylalanine, phenyllactate, and phenylpyruvate to toluene. When grown in defined medium with phenylacetic acid at concentrations ≤350 mg/L, the molar ratio between toluene accumulated and phenylacetic acid supplied was highly correlated ( R² ≥ 0.96) with a toluene yield exceeding 0.9:1. Experiments conducted using ¹³C labeled compounds (phenylacetic acid-2-¹³C and l-phenylalanine-3-¹³C) resulted in production of toluene-α-¹³C, confirming that toluene was synthesized from these precursors by two independently developed enrichment cultures. Results presented here suggest that monitoring of aromatic hydrocarbons is warranted during enhanced bioremediation activities where electron donors are introduced to stimulate anaerobic biotransformation of chlorinated solvents.

Metabolic reconstruction and experimental verification of glucose utilization in Desulfurococcus amylolyticus DSM 16532

Desulfurococcus amylolyticus DSM 16532 is an anaerobic and hyperthermophilic crenarchaeon known to grow on a variety of different carbon sources, including monosaccharides and polysaccharides. Furthermore, D. amylolyticus is one of the few archaea that are known to be able to grow on cellulose. Here, we present the metabolic reconstruction of D. amylolyticus’ central carbon metabolism. Based on the published genome, the metabolic reconstruction was completed by integrating complementary information available from the KEGG, BRENDA, UniProt, NCBI, and PFAM databases, as well as from available literature. The genomic analysis of D. amylolyticus revealed genes for both the classical and the archaeal version of the Embden-Meyerhof pathway. The metabolic reconstruction highlighted gaps in carbon dioxide-fixation pathways. No complete carbon dioxide-fixation pathway such as the reductive citrate cycle or the dicarboxylate-4-hydroxybutyrate cycle could be identified. However, the metabolic reconstruction indicated that D. amylolyticus harbors all genes necessary for glucose metabolization. Closed batch experimental verification of glucose utilization by D. amylolyticus was performed in chemically defined medium. The findings from in silico analyses and from growth experiments are discussed with respect to physiological features of hyperthermophilic organisms.

Efficient whole cell biocatalyst for formate-based hydrogen production

BACKGROUND

Molecular hydrogen (H₂) is an attractive future energy carrier to replace fossil fuels. Biologically and sustainably produced H₂ could contribute significantly to the future energy mix. However, biological H₂ production methods are faced with multiple barriers including substrate cost, low production rates, and low yields. The C1 compound formate is a promising substrate for biological H₂ production, as it can be produced itself from various sources including electrochemical reduction of CO₂ or from synthesis gas. Many microbes that can produce H₂ from formate have been isolated; however, in most cases H₂ production rates cannot compete with other H₂ production methods.

RESULTS

We established a formate-based H₂ production method utilizing the acetogenic bacterium Acetobacterium woodii. This organism can use formate as sole energy and carbon source and possesses a novel enzyme complex, the hydrogen-dependent CO₂ reductase that catalyzes oxidation of formate to H₂ and CO₂. Cell suspensions reached specific formate-dependent H₂ production rates of 71 mmol g_protein^-1 h^-1 (30.5 mmol g_CDW^-1 h^-1) and maximum volumetric H₂ evolution rates of 79 mmol L^-1 h^-1. Using growing cells in a two-step closed batch fermentation, specific H₂ production rates reached 66 mmol g_CDW^-1 h^-1 with a volumetric H₂ evolution rate of 7.9 mmol L^-1 h^-1. Acetate was the major side product that decreased the H₂ yield. We demonstrate that inhibition of the energy metabolism by addition of a sodium ionophore is suitable to completely abolish acetate formation. Under these conditions, yields up to 1 mol H₂ per mol formate were achieved. The same ionophore can be used in cultures utilizing formate as specific switch from a growing phase to a H₂ production phase.

CONCLUSIONS

Acetobacterium woodii reached one of the highest formate-dependent specific H₂ productivity rates at ambient temperatures reported so far for an organism without genetic modification and converted the substrate exclusively to H₂. This makes this organism a very promising candidate for sustainable H₂ production and, because of the reversibility of the A. woodii enzyme, also a candidate for reversible H₂ storage.

Microalgal cultivation for value-added products: a critical enviro-economical assessment

The present review focuses on the cultivation of algal biomass for generating value-added products (VAP) and to assess their economic benefits and harmful environmental impact. Additionally, the impact of bioreactor designs on the yield of microalgal biomass for VAP is also considered. All these factors are discussed in relation to the impact of microalgae production on the bio-economy sector of commercial biotechnology.