Biological hydrogen production: prospects and challenges

Enhanced electron transfer for the improvement of nitrogen removal efficiency and N2O reduction at low temperatures

Low temperature generally restricts biological activity, slowing down electron transfer in biogeochemical cycles and causing a series of environmental problems such as nitrogen pollution. We present a strategy to boost electron transfer in microbial cell at low temperatures via stimulation with low current. It is demonstrated by establishing a constructed wetland system coupled with solar powered microbial electrolysis cell, which enhances microbial activity through external micro currents (18.9 ± 5.5 μA) for removing nitrogen pollution in winter (average temperature from -6.6 to 4.5 °C). We investigated the efficiency of pollutants removal, microbial activity, N2O production and its mechanisms using complexes activity detection, RT-qPCR, incubation, and 15N-18O dual-isotope labeling techniques. The activity of complexes I, II, III, and IV collectively represent the microbial electron transfer rate. Results indicated that the microcurrents increased the activity of complexes II, III and IV by 96 %, 172 %, and 313 %, respectively. The transcription abundance of amoA genes in ammonia oxidation and nirS/K genes in denitrification by 263 % and 51 %, respectively. Consequently, NH4+-N removal efficiency improved from 23 % to 35 %, and NO3--N removal efficiency from 21 % to 31 %. Moreover, microcurrents reduced N2O emission by 44 %. However, external microcurrent stimulation did not alter the microbial production pathway of N2O as determined by the 15N-18O dual isotope labeling technique. The relative abundance of the nitrifying bacteria Nitrosomonas, Nitrosospira, and Nitrospira, as well as the denitrifying bacteria Methylotenera, significantly increased due to microcurrent stimulation. Specifically, Nitrospira exhibited the highest increase of 156 %. Our findings provide a novel way to enhance N removal efficiency and simultaneously reduce N2O emission of biological system like constructed wetlands in winter conditions.

Enhanced photo-fermentative hydrogen production by constructing Rhodobacter capsulatus-ZnO/ZnS hybrid system

This study incorporated ZnO/ZnS nanoparticles with Rhodobacter capsulatus SB1003, forming a hybrid system to promote photo-fermentative hydrogen production. The results indicate that the material's photocatalytic activity and concentration significantly affected hydrogen yield. The addition of ZnO/ZnS exhibited a more significant auxiliary effect than ZnO and achieved an approximately 30% increase in hydrogen production compared to the control group. ZnO/ZnS enhanced the production of extracellular polymers, thereby strengthening the synergy between the nanomaterials and the bacteria. The photogenerated electrons from ZnO/ZnS were utilized by the photosynthetic bacteria. Furthermore, the activity of nitrogenase was enhanced, resulting in improved hydrogen production performance. This study provides insights into hydrogen production by photosynthetic bacteria with the assistance of inorganic semiconductor nanomaterials.

Revalorisation of brewer's spent grain for biotechnological production of hydrogen with Escherichia coli

Introduction

Agro-industrial wastes are generated in huge amounts triggering damages to the environment and human health. Therefore, there is an urgent necessity for its revalorisation into high-value compounds, including biofuels. One such wastes is the brewer's spent grain (BSG), a by-product of the beer industry, which is produced in vast quantities worldwide. The rich-fibre and protein content of BSG makes this waste a valuable resource for biotechnological applications, although the main challenge of this approach is to make the carbohydrates and proteins available for bacterial metabolisation into high-value products. This work aims to optimise a thermal-hydrolysis process to revalorise BSG by bacterial conversion into hydrogen (H2), as a clean energy that can replace fossil fuels.

Methods

A 2k full factorial design method was employed hydrolysation of BSG and showed that temperature and acid concentration are significant factors that affect the extraction of reducing sugars (RS) and proteins. Subsequently, steepest ascent and central composite design (CCD) statistical methods were applied to determine the optimal conditions for hydrolysis.

Results

The optimised hydrolysis condition were 0.047 M H2SO4, 150°C, 30 min and 15% BSG, leading to the theoretical concentrations of 54.8 g RS/L and 20 g/L proteins. However, 5'-hydroxymethylfurfural (HMF) was generated in thermal-hydrolysis conditions at higher temperatures exceeding 132°C. Therefore, a screening of HBSGs fermentation using Escherichia coli was conducted in order to identify the most suitable conditions for maximizing H2, as well as the production of volatile fatty acids (succinate and acetate) and ethanol. Among the tested conditions, HBSG A17 (117°C, 20 min, and 0.1 M H2SO4) yielded the highest H2 production of 48 mmol/L in this work.

Conclusion

This study provides valuable insights into the optimisation of BSG pre-treatment for biotechnological applications, which may help in the selection of the most appropriate hydrolysis conditions based on the desired end product.

Advancements in biohydrogen production - a comprehensive review of technologies, lifecycle analysis, and future scope

The global shift towards sustainable energy sources, necessitated by climate change concerns, has led to a critical review of biohydrogen production (BHP) processes and their potential as a solution to environmental challenges. This review evaluates the efficiency of various reactors used in BHP, focusing on operational parameters such as substrate type, pH, temperature, hydraulic retention time (HRT), and organic loading rate (OLR). The highest yield reported in batch, continuous, and membrane reactors was in the range of 29-40 L H2/L per day at an OLR of 22-120 g/L per day, HRT of 2-3 h and acidic range of 4-6, with the temperature maintained at 37 °C. The highest yield achieved was 208.3 L H2/L per day when sugar beet molasses was used as a substrate with Clostridium at an OLR of 850 g COD/L per day, pH of 4.4, and at 8 h HRT. The integration of artificial intelligence (AI) tools, such as artificial neural networks and support vector machines has emerged as a novel approach for optimizing reactor performance and predicting outcomes. These AI models help in identifying key operational parameters and their optimal ranges, thus enhancing the efficiency and reliability of BHP processes. The review also draws attention to the importance of life cycle and techno-economic analyses in assessing the environmental impact and economic viability of BHP, addressing potential challenges like high operating costs and energy demands during scale-up. Future research should focus on developing more efficient and cost-effective BHP systems, integrating advanced AI techniques for real-time optimization, and conducting comprehensive LCA and TEA to ensure sustainable and economically viable biohydrogen production. By addressing these areas, BHP can become a key component of the transition to sustainable energy sources, contributing to the reduction of greenhouse gas emissions and the mitigation of environmental impacts associated with fossil fuel use.

Hydrogen-producing conditions and mutation mechanisms of a highly efficient mutant strain Ethanoligenens harbinense YR-3

In this study, the optimal hydrogen (H2) production conditions of the high-efficiency H2-producing mutant strain Ethanoligenens harbinense YR-3 (carbon-nitrogen ratio 5.5, phosphate buffer 80 mM, initial pH 6.0, biotin 1.4 mg/L) are obtained by intermittent experiments. The maximum specific H2 production rate of YR-3 (2.85 mol H2/mol glucose) was 1.4 times that of the wild strain ZGX4 (2.04 mol H2/mol glucose). The liquid-phase products are mainly ethanol and acetic acid, indicating that the metabolic pathway has not changed. Two-dimensional electrophoresis and mass spectrometry were used to compare and analyze the protein map differences between YR-3 and ZGX4. The results show that 1,6-fructose diphosphate aldolase and the flavoprotein in hydrogenase are highly expressed. This study will provide a theoretical basis for the genetic modification of high-efficiency H2-producing strains and the improvement of H2 production capacity.

Research on the conversion of biowaste to MCCAs: A review of recent advances in the electrochemical synergistic anaerobic pathway

Medium-chain carboxylic acids (MCCAs) show great promise as commercial chemicals due to their high energy density, significant product value, and wide range of applications. The production of MCCAs from waste biomass through coupling chain extension with anaerobic fermentation represents a new and innovative approach to biomass utilization. This review provides an overview of the principles of MCCAs production through coupled chain extension and anaerobic fermentation, as well as the extracellular electron transfer pathways and microbiological effects involved. Emphasis is placed on the mechanisms, limitations, and microbial interactions in MCCAs production, elucidating metabolic pathways, potential influencing factors, and the cooperative and competitive relationships among various microorganisms. Additionally, this paper delves into a novel technology for the bio-electrocatalytic generation of MCCAs, which promotes electron transfer through the use of different three-dimensional electrodes, various electrical stimulation methods, and hydrogen-assisted approaches. The insights and conclusions from previous studies, as well as the identification of existing challenges, will be valuable for the further development of high-product-selectivity strategies and environmentally friendly treatments.

Biohydrogen from waste feedstocks: An energy opportunity for decarbonization in developing countries

In developing economies, the decarbonization of energy sector has become a global priority for sustainable and cleaner energy system. Biohydrogen production from renewable sources of waste biomass is a good source of energy incentive that reduces the pollution. Biohydrogen has a high calorific value and emits no emissions, producing both energy security and environmental sustainability. Biohydrogen production technologies have become one of the main renewable sources of energy. The present paper entails the role of biohydrogen recovered from waste biomasses like agricultural waste (AW), organic fraction of municipal solid waste (OFMSW), food processing industrial waste (FPIW), and sewage sludge (SS) as a promising solution. The main sources of increasing yield percentage of biohydrogen generation from waste feedstock using different technologies, and process parameters are also emphasized in this review. The production paths for biohydrogen are presented in this review article, and because of advancements in R and D, biohydrogen has gained viability as a biofuel for the future and discusses potential applications in power generation, transportation, and industrial processes, emphasizing the versatility and potential for integration into existing energy infrastructure. The investigation of different biochemical technologies and methods for producing biohydrogen, including anaerobic digestion (AD), dark fermentation (DF), photo fermentation (PF), and integrated dark-photo fermentation (IDPF), has been overviewed. This analysis also discusses future research, investment, and sustainable energy options transitioning towards a low-carbon future, as well as potential problems, economic impediments, and policy-related issues with the deployment of biohydrogen in emerging nations.

Minimal and hybrid hydrogenases are active from archaea

Microbial hydrogen (H2) cycling underpins the diversity and functionality of diverse anoxic ecosystems. Among the three evolutionarily distinct hydrogenase superfamilies responsible, [FeFe] hydrogenases were thought to be restricted to bacteria and eukaryotes. Here, we show that anaerobic archaea encode diverse, active, and ancient lineages of [FeFe] hydrogenases through combining analysis of existing and new genomes with extensive biochemical experiments. [FeFe] hydrogenases are encoded by genomes of nine archaeal phyla and expressed by H2-producing Asgard archaeon cultures. We report an ultraminimal hydrogenase in DPANN archaea that binds the catalytic H-cluster and produces H2. Moreover, we identify and characterize remarkable hybrid complexes formed through the fusion of [FeFe] and [NiFe] hydrogenases in ten other archaeal orders. Phylogenetic analysis and structural modeling suggest a deep evolutionary history of hybrid hydrogenases. These findings reveal new metabolic adaptations of archaea, streamlined H2 catalysts for biotechnological development, and a surprisingly intertwined evolutionary history between the two major H2-metabolizing enzymes.

Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

A mini review on microwave and contemporary based biohydrogen production technologies: a comparison

Hydrogen gas, along with conventional fossil fuels, has been used as a green fuel with enormous potential. Due to the rapid depletion of fossil fuels, a new dimension of hydrogen production technology has arrived to reduce reliance on nonrenewable energy sources. Microwave-based hydrogen production is a more promising and cost-effective technology than other existing green hydrogen production methods such as fermentation and gasification. Microwave heating may be superior to traditional heating due to several advantages such as less power consumption compared to other methods, higher yield, and a higher rate of conversion. Compared to another process for hydrogen production, the microwave-driven process worked efficiently at lower temperatures by providing more than 70% yield. The process of production can be optimized by using properly sized biomass, types of biomass, water flow, temperature, pressure, and reactor size. This method is the most suitable, attractive, and efficient technique for hydrogen production in the presence of a suitable catalyst. Hot spots formed by microwave irradiation would have a substantial impact on the yield and properties of microwave-processed goods. The current techno-economic situation of various technologies for hydrogen production is discussed here, with cost, efficiency, and durability being the most important factors to consider. The present review shows that a cost-competitive hydrogen economy will necessitate continual efforts to increase performance, scale-up, technical prospects, and political backing.

Applying metabolic flux analysis to hydrogen fermentation using a metabolic network constructed for anaerobic mixed cultures

In this study, a mixed-cultural metabolic network for anaerobic digestion that included the concept of a "universal bacterium" was constructed, and metabolic flux analysis (MFA) applying this network was conducted to evaluate the flow of electrons and materials during H2 fermentation under various conditions. The MFA results from two H2 fermenters feeding glucose with (GP) or without (GA) the addition of peptone suggest that hydraulic retention time (HRT) presents a significant impact on hydrogen production, and the reversed trends could be observed at HRTs below and above 4 h. From the MFA results of lactate/acetate-fed H2 fermenter, the highest flux of H2 production is associated with more significant acetate consumption and the following pathways toward the anaplerotic reactions cycle that produces NADH. The occurrence of acetogenesis in the H2 fermenters using various types of bioethanol-fermented residues (BEFRs) was also identified according to the MFA results. By analyzing the MFA results of all 49 sets of data from H2 fermenters via Pearson's correlation, it was revealed that the flux of H2 production positively correlates to the reduction of ferredoxin with pyruvate oxidation, acetate formation, and acetate emission when lactate was produced in the system. On the contrary, negative relationships were found between the flux of H2 production and these three fluxes. The extended application of MFA provides additional information, including the fluxes between intracellular metabolites, and the information has the potential to be used in decision-making systems during the future operation of anaerobic processes by connecting operational parameters.

Sustainable biohydrogen production from lignocellulosic biomass sources - metabolic pathways, production enhancement, and challenges

Hydrogen production from biological processes has been hailed as a promising strategy for generating sustainable energy. Fermentative hydrogen production processes such as dark and photofermentation are considered more sustainable and economical than other biological methods such as biophotolysis. However, these methods have constraints such as low hydrogen yield and conversion efficiency, so practical implementations still need to be made. The present review provides an assessment and feasibility of producing biohydrogen through dark and photofermentation techniques utilizing various lignocellulosic biomass wastes as substrates. Furthermore, this review includes information about the strategies to increase the productivity rate of biohydrogen in an eco-friendly and sustainable manner, like integration of dark and photofermentation techniques, pretreatment of biomass, genetic modification of microorganisms, and application of nanoadditives.

Bioelectrochemical systems (BESs) for agro-food waste and wastewater treatment, and sustainable bioenergy-A review

Producing food by farming and subsequent food manufacturing are central to the world's food supply, accounting for more than half of all production. Production is, however, closely related to the creation of large amounts of organic wastes or byproducts (agro-food waste or wastewater) that negatively impact the environment and the climate. Global climate change mitigation is an urgent need that necessitates sustainable development. For that purpose, proper agro-food waste and wastewater management are essential, not only for waste reduction but also for resource optimization. To achieve sustainability in food production, biotechnology is considered as key factor since its continuous development and broad implementation will potentially benefit ecosystems by turning polluting waste into biodegradable materials; this will become more feasible and common as environmentally friendly industrial processes improve. Bioelectrochemical systems are a revitalized, promising biotechnology integrating microorganisms (or enzymes) with multifaceted applications. The technology can efficiently reduce waste and wastewater while recovering energy and chemicals, taking advantage of their biological elements' specific redox processes. In this review, a consolidated description of agro-food waste and wastewater and its remediation possibilities, using different bioelectrochemical-based systems is presented and discussed together with a critical view of the current and future potential applications.

Monitoring membranes: The exploration of biological bilayers with second harmonic generation

Nature's seemingly controlled chaos in heterogeneous two-dimensional cell membranes stands in stark contrast to the precise, often homogeneous, environment in an experimentalist's flask or carefully designed material system. Yet cell membranes can play a direct role, or serve as inspiration, in all fields of biology, chemistry, physics, and engineering. Our understanding of these ubiquitous structures continues to evolve despite over a century of study largely driven by the application of new technologies. Here, we review the insight afforded by second harmonic generation (SHG), a nonlinear optical technique. From potential measurements to adsorption and diffusion on both model and living systems, SHG complements existing techniques while presenting a large exploratory space for new discoveries.

Ammonia/ammonium removal/recovery from wastewaters using bioelectrochemical systems (BES): A review

This review updates the current research efforts on using BES to recover NH₃/NH₄⁺, highlighting the novel configurations and introducing the working principles and the applications of microbial fuel cell (MFC), microbial electrolysis cell (MEC), microbial desalination cell (MDC), and microbial electrosynthesis cell (MESC) for NH₃/NH₄⁺ removal/recovery. However, commonly studied BES processes for NH₃/NH₄⁺ removal/recovery are energy intensive with external aeration needed for NH₃ stripping being the largest energy input. In such a process bipolar membranes used for yielding a local alkaline pool recovering NH₃ is not cost-effective. This gives a chance to microbial electrosynthesis which turned out to be a potential alternative option to approach circular bioeconomy. Furtherly, the reactor volume and NH₃/NH₄⁺ removal/recovery efficiency has a weakly positive correlation, indicating that there might be other factors controlling the reactor performance that are yet to be investigated.

Biohydrogen production and purification: Focusing on bioelectrochemical systems

Innovative technologies on green hydrogen production become significant as the hydrogen economy has grown globally. Biohydrogen is one of green hydrogen production methods, and microbial electrochemical cells (MECs) can be key to biohydrogen provision. However, MECs are immature for biohydrogen technology due to several limitations including extracellular electron transfer (EET) engineering. Fundamental understanding of EET also needs more works to accelerate MEC commercialization. Interestingly, studies on biohydrogen gas purification are limited although biohydrogen gas mixture requires complex purification for use. To facilitate an MEC-based biohydrogen technology as the green hydrogen supply this review discussed EET kinetics, engineering of EET and direct interspecies electron transfer associated with hydrogen yield and the application of advanced molecular biology for improving EET kinetics. Finally, this article reviewed biohydrogen purification technologies to better understand purification and use appropriate for biohydrogen, focusing on membrane separation.

The role of microbiome in carbon sequestration and environment security during wastewater treatment

Wastewater treatment is an essential aspect of the earth's sustainable future. However, different wastewater treatment methods are responsible for carbon discharge into the environment, raising environmental risks. Hence, such wastewater treatment methods are required that can minimize carbon release without compromising the treatment quality. Microbiome-based carbon sequestration is a potential method for achieving this goal. Limited studies have been carried out to investigate how microbes can capture and utilize CO₂. This review summarizes the approaches including microbial electrolytic carbon capture, microbial electrosynthesis, microbial fuel cell, microalgae cultivation, and constructed wetlands that employ microbes to capture and utilize CO₂. Electroactive Bacteria (EAB) convert carbon dioxide to carbonates and bicarbonates in subsequent steps after organic matter decomposition. Similarly, microbial electrosynthesis (MES) not only helps capture carbon but also produces secondary products (production of polyhydroxyalkanoates by Gram-negative rod Aeromonas hydrophila bacteria) of commercial importance during wastewater treatment. In addition to this, microbial carbon capture cells (MCCs) have been now utilized for energy generation and carbon sequestration at the same time during wastewater treatment. Moreover, microalgae cultivation has also been found to capture CO₂ at a rapid pace while releasing O₂ as a consequence of photosynthesis. Hence, microbe-based wastewater treatment has quite a potential due to two-fold benefits like carbon sequestration and by-product formation.

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.

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.

Vapor-Fed Cathode Microbial Electrolysis Cells with Closely Spaced Electrodes Enables Greatly Improved Performance

Hydrogen can be electrochemically produced in microbial electrolysis cells (MECs) by current generated from bacterial anodes with a small added voltage. MECs typically use a liquid catholyte containing a buffer or salts. However, anions in these catholytes result in charge being balanced predominantly by ions other than hydroxide or protons, leading to anode acidification. To enhance only hydroxide ion transport to the anode, we developed a novel vapor-fed MEC configuration lacking a catholyte with closely spaced electrodes and an anion exchange membrane to limit the acidification. This MEC design produced a record-high sustained current density of 43.1 ± 0.6 A/m² and a H₂ production rate of 72 ± 2 LH₂/L-d (cell voltage of 0.79 ± 0.00 V). There was minimal impact on MEC performance of increased acetate concentrations, solution conductivity, or anolyte buffer capacity at applied voltages up to 1.1 V, as shown by a nearly constant internal resistance of only 6.8 ± 0.3 mΩ m². At applied external voltages >1.1 V, the buffer capacity impacted performance, with current densities increasing from 28.5 ± 0.6 A/m² (20 mM phosphate buffer solution (PBS)) to 51 ± 1 A/m² (100 mM PBS). These results show that a vapor-fed MEC can produce higher and more stable performance than liquid-fed cathodes by enhancing transport of hydroxide ions to the anode.

From pollutant removal to resource recovery: A bibliometric analysis of municipal wastewater research in Europe

Municipal wastewaters are abundant low-strength streams that require adequate treatment and disposal to ensure public and environmental health. This study aims to provide a comprehensive summary of municipal wastewater research in Europe in the 2010s in the form of bibliometric analysis. The work was based on the Science Citation Index Expanded (Web of Science) and carried out using the R-package bibliometrix for bibliometric data analysis and the software VOSviewer for science mapping. Analysing a dataset of 5645 publications, we identified the most influential journals, countries, authors, institutions, and publications, and mapped the co-authorship and keyword co-occurrence networks. Spain had produced the most publications while Switzerland had the highest average citations per publication. China was the most collaborative country from outside of Europe. Analysis of the most cited articles revealed the popularity of micropollutant removal in European municipal wastewater research. The keyword analysis visualized a paradigm shift from pollutant removal towards resource recovery and circular economy. We found that current challenges of resource recovery from municipal wastewater come from both technical and non-technical (e.g., environmental, economic, and social) aspects. We also discussed future research opportunities that can tackle these challenges.

New Insights into Controlling Homoacetogenesis in the Co-digestion of Coffee Waste: Effect of Operational Conditions and Characterization of Microbial Communities

In this research batch reactors were operated with coffee processing waste and autochthonous microbial consortium, and a taxonomic and functional analysis was performed for phase I of stabilization of maximum H₂ production and for phase II of maximum H₂ consumption. During phase I, the reactor's operating conditions were pH 4.84 to 8.18, headspace 33.18% to 66.82%, and pulp and husk from 6.95 to 17.05 g/L. These assays continued for phase II, with initial pH conditions of 5.8-8.1, headspace of 33.18-66.82%, and pulp and husk remaining from phase I. The highest homoacetogenesis was observed in assay 5 with pH 7.7, 40% headspace, and 15 g/L of pulp and husk (initial concentrations of phase I). A relative abundance of Clostridium 41%, Lactobacillus 20% and Acetobacter 14% was observed in phase I. In phase II, there was a change in relative abundance of 21%, 63%, and 1%, respectively, and functional genes involved with autotrophic (formyltetrahydrofolate synthase) and heterotrophic (enolase) homoacetogenesis, butanol (3-hydroxybutyryl-CoA dehydrogenase), and propionic acid (propionate CoA-transferase) were identified. This study provides a new and amplified insight into the physicochemical and microbiological factors, which can be used to propose adequate operational conditions to maximize the bioenergy production and reduce homoacetogenesis in biological reactors.

Significance of Hydrogen as Economic and Environmentally Friendly Fuel

The major demand of energy in today’s world is fulfilled by the fossil fuels which are not renewable in nature and can no longer be used once exhausted. In the beginning of the 21st century, the limitation of the fossil fuels, continually growing energy demand, and growing impact of green-house gas emissions on the environment were identified as the major challenges with current energy infrastructure all over the world. The energy obtained from fossil fuel is cheap due to its established infrastructure; however, these possess serious issues, as mentioned above, and cause bad environmental impact. Therefore, renewable energy resources are looked to as contenders which may fulfil most energy requirements. Among them, hydrogen is considered as the most environmentally friendly fuel. Hydrogen is clean, sustainable fuel and it has promise as a future energy carrier. It also has the ability to substitute the present energy infrastructure which is based on fossil fuel. This is seen and projected as a solution for the above-mentioned problems including rise in global temperature and environmental degradation. Environmental and economic aspects are the important factors to be considered to establish hydrogen infrastructure. This article describes the various aspects of hydrogen including production, storage, and applications with a focus on fuel cell based electric vehicles. Their environmental as well as economic aspects are also discussed herein.

Microbial electrolysis cells for return flow: Simultaneous nitrogen and carbon removal

The concentration of solids in secondary sludge before anaerobic digestion in a wastewater treatment plant, bring about the production of a return flow, which contains high concentrations of all the common pollutant parameters. This return flow could unfavourably affect the performance of the processes and effluent quality of the waterline. Here, we report the utilisation of three similar microbial electrolysis cells reactors that performs simultaneous carbon and nitrogen removal to reduce the impact of the return flow in the plant. The result of the batch-fed (72 h) experiment showed COD and total nitrogen removal efficiencies that reached 90% and 80%, respectively, supporting the premise that return flows are suitable substrates for a bioelectrochemical treatment. The three reactors followed similar trends, showing good replicability and confirming the potential of MECs as a feasible technology for return flow treatment. Furthermore, when cathodic conversion efficiency was higher than 80%, the pure hydrogen production allows to recover the electric energy consumption, indicating that the system could be theoretically energy neutral.

Nitrogen, phosphorus and high CO2 modulate photosynthesis, biomass and lipid production in the green alga Chlorella vulgaris

Climate change could impact nutrient bioavailability in aquatic environment. To understand the interaction of nutrient bioavailability and elevated CO₂, Chlorella vulgaris cells were grown in ambient air or 5% CO₂ in different concentrations of nitrogen and phosphorus in a photobioreactor. The chlorophyll content, photosynthesis and respiration rates increased in 5% CO₂ to support higher biomass production. The nutrient limitation in the growth media resulted in reduced photosynthetic rates of the algal cells and their PSI, PSII, and whole chain electron transport rates and biomass production. Conversely, their lipid content increased partly due to upregulation of expression of several lipid biosynthesis genes. The order of downregulation of photosynthesis and upregulation in lipid production due to nutrient limitation was in the order of N > P. The N-50 and 5% CO₂ culture had only 10% reduction in biomass and 32% increase in lipids having 85% saturated fat required for efficient biofuel production. This growth condition is ideal for generation of biodiesel required to reduce the consumption of fossil fuel and combat global warming.

Effect of silica-core gold-shell nanoparticles on the kinetics of biohydrogen production and pollutant hydrogenation via organic acid photofermentation over enhanced near-infrared illumination

A biological photoinduced fermentation process provides an alternative to traditional hydrogen productions. In this study, biohydrogen production was investigated at near IR region coupled to a near-field enhancement by silica-core gold-shell nanoparticles (NPs) over a range of acetate concentrations (5-40 mM) and light intensities (11-160 W/m2). The kinetic data were modeled using modified Monod equations containing light intensity effects. The yields of H2 and CO2 produced per acetate were determined as 2.31 mol-H2/mol-Ac and 0.83 mol-CO2/mol-Ac and increased to 4.38 mmol-H2/mmol-Ma and 2.62 mmol-CO2/mmol-Ma when malate was used. Maximum increases in H2 and CO2 productions by 115% and 113% were observed by adding NPs without affecting the bacterial growth rates (6.1-8.2 mg-DCM/L/hour) while the highest hydrogen production rate was determined as 0.81 mmol/L/hour. Model simulations showed that the energy conversion efficiency increased with NPs concentration but decreased with the intensity. Complete hydrogenation application was demonstrated with toxic 2-chlorobiphenyl using Pd catalysts.

Biohydrogen from Microalgae: Production and Applications

The need to safeguard our planet by reducing carbon dioxide emissions has led to a significant development of research in the field of alternative energy sources. Hydrogen has proved to be the most promising molecule, as a fuel, due to its low environmental impact. Even if various methods already exist for producing hydrogen, most of them are not sustainable. Thus, research focuses on the biological sector, studying microalgae, and other microorganisms’ ability to produce this precious molecule in a natural way. In this review, we provide a description of the biochemical and molecular processes for the production of biohydrogen and give a general overview of one of the most interesting technologies in which hydrogen finds application for electricity production: fuel cells.

Molecular mechanism of ethanol-H2 co-production fermentation in anaerobic acidogenesis: Challenges and perspectives

Ethanol-type fermentation (ETF) is one of three fermentation types during the acidogenesis of the anaerobic biological treatment. Ethanoligenens, a representative genus of ETF, displays acidophilic, autoaggregative, and ethanol-H₂ co-producing characteristics and facilitates subsequent methanogenesis. Here, the latest advances in the molecular mechanisms of the metabolic regulation of ethanol-H₂ co-producing bacteria based on multi-omics studies were comprehensively reviewed. Comparative genomics demonstrated a low genetic similarity between Ethanoligenens and other hydrogen-producing genera. FeFe‑hydrogenases (FeFe-H₂ases) and pyruvate ferredoxin oxidoreductase (PFOR) played critical roles in the ethanol-H₂ co-metabolic pathway of Ethanoligenens. Global transcriptome analysis revealed that highly expressed [FeFe]-H₂ases and ferredoxins drove hydrogen production by Ethanoligenens at low pH conditions (4.0-4.5). Quantitative proteomic analysis also proved that this genus resists acetic acid-induced intracellular acidification through the up-regulated expression of pyrimidine metabolism related proteins. The autoaggregation of Ethanoligenen facilitated its granulation with acetate-oxidizing bacteria in co-culture systems and mitigated a fast pH drop, providing a new approach for solving a pH imbalance and improving hydrogen production. In-depth studies of the regulatory mechanism underlying ethanol-H₂ co-production metabolism and the syntrophic interactions of ethanol-H₂ co-producing Ethanoligenens with other microorganisms will provide insights into the improvement of bioenergy recovery in anaerobic biotechnology. The coupling of ETF with other biotechnologies, which based on the regulation of electron flow direction, syntrophic interaction, and metabolic flux, can be potential strategies to enhance the cascade recovery of energy and resources.

Metabolic pathways for microalgal biohydrogen production: Current progress and future prospectives

Microalgal biohydrogen (bioH₂) has attracted global interest owing to its potential carbon-free source of sustainable renewable energy. Most of previous reviews which focused on microalgal bioH₂, have shown unclear differentiation among the metabolic pathways. In this review, investigation of all different metabolic pathways for microalgal bioH₂ production along with discussion on the recent research work of last 5-years have been considered. The major factors (such as light, vital nutrients, microalgal cell density, and substrate bioavailability) are highlighted. Moreover, effect of various pretreatment approaches on the constituent's bioaccessibility is reported. Microbial electrolysis cells as a new strategy for bioH₂ production is stated. Comparison between the operation conditions of various bioreactors and economic feasibility is also emphasized. Genetic, metabolic engineering, and synthetic biology as recent technologies improved the microalgal bioH₂ production through inactivation of uptake hydrogenase (H₂ase), inhibition of the competing pathways in polysaccharide synthesis, and improving the O₂ tolerant H₂ase.

Rapid recruitment of hydrogen-producing biofilms for hydrogen production in a moving bed biofilm reactor by a sequential immobilization and deoxygenization approach

To reduce start-up time and enhance hydrogen production efficiency, a sequential immobilization and deoxygenization (SIDO) strategy for hydrogen production was investigated in continuous-flow moving bed biofilm reactors (MBBRs). The pre-immobilization process accelerated the initial enrichment of hydrogen-producing bacteria (HPB) and promoted the biofilm formation, which contribute to higher hydrogen production efficiency in SIDO-MBBRs compared to a non-immobilized reactor. A similar deoxygenization effect was achieved by inoculation with Pseudomonas aeruginosa compared with N₂ sparging, and the P. aeruginosa pre-immobilized MBBR (Pse-MBBR) showed a higher H₂ yield in the initial stage of operation. Microbial community analysis found a higher abundance of putative HPB in the range of 82.82-96.56%, with the predominant populations in the SIDO-MBBR assigned to genera Clostridium and Enterobacter. The results suggest that the SIDO-MBBR is an effective approach for rapid recruitment of HPB and start-up of fermentative hydrogen production.

Thermoanaerobacterium sp. Strain RBIITD as a dominant species in accelerating thermophilic dark fermentation start up through pH and substrate concentration regulation

In this work, accelerated start-up of biological hydrogen production system fed with glucose and molasses at 55 °C by regulating pH and COD concentration was investigated in two groups. Then three reactors in each group were compared: controlling pH, controlling pH with COD, and controlling the COD. The reactors in group A presented best hydrogen yield of 1.84 mol H₂/mol glucose·day and worked stably at the 8th day. The highest hydrogen yield in group B was 2.13 mol H₂/mol molasses·day and steadily at the 11th day. It proved that controlling two key parameters of the inflow pH (8.0) and substrate concentration (4000 mg COD/L) could realize fast start-up of hydrogen production reactor. This study demonstrated that Thermoanaerobacterium sp. strain RBIITD could produce hydrogen and provide a new avenue for biological hydrogen production by dark fermentation using cheap substrate towards a more sustainable and feasible technology.

Algae-Bacteria Consortia as a Strategy to Enhance H2 Production

Biological hydrogen production by microalgae is a potential sustainable, renewable and clean source of energy. However, many barriers limiting photohydrogen production in these microorganisms remain unsolved. In order to explore this potential and make biohydrogen industrially affordable, the unicellular microalga Chlamydomonas reinhardtii is used as a model system to solve barriers and identify new approaches that can improve hydrogen production. Recently, Chlamydomonas–bacteria consortia have opened a new window to improve biohydrogen production. In this study, we review the different consortia that have been successfully employed and analyze the factors that could be behind the improved H₂ production.

Roasted coffee wastes as a substrate for Escherichia coli to grow and produce hydrogen

After brewing roasted coffee, spent coffee grounds (SCGs) are generated being one of the daily wastes emerging in dominant countries with high rate and big quantity. Escherichia coli BW25113 wild-type strain, mutants with defects in hydrogen (H2)-producing/oxidizing four hydrogenases (Hyd) (ΔhyaB ΔhybC, ΔhycE, ΔhyfG) and septuple mutant (ΔhyaB ΔhybC ΔhycA ΔfdoG ΔldhA ΔfrdC ΔaceE) were investigated by measuring change of external pH, bacterial growth and H2 production during the utilization of SCG hydrolysate. In wild type, H2 was produced with rate of 1.28 mL H2 (g sugar)−1 h−1 yielding 30.7 mL H2 (g sugar)−1 or 2.75 L (kg SCG)−1 during 24 h. In septuple mutant, H2 production yield was 72 mL H2 (g sugar)−1 with rate of 3 mL H2 (g sugar)−1 h−1. H2 generation was absent in hycE single mutant showing the main role of Hyd-3 in H2 production. During utilization of SCG wild type, specific growth rate was 0.72 ± 0.01 h−1 with biomass yield of 0.3 g L−1. Genetic modifications and control of external parameters during growth could lead to prolonged and enhanced microbiological H2 production by organic wastes, which will aid more efficiently global sustainable energy needs resulting in diversification of mobile and fixed energy sources.

Improving methane productivity of waste activated sludge by ultrasound and alkali pretreatment in microbial electrolysis cell and anaerobic digestion coupled system

In order to enhance the productivity of methane from the waste activated sludge (WAS), a coupled system of microbial electrolysis cell (MEC) and anaerobic digestion (AD) was proposed. In this study, alkali, ultrasound-alkali, high-temperature coupled microaeration (TM) were applied as pretreatment methods to disintegrate the WAS flocs and break bacterial cell. After ultrasound-alkali pretreatment, the maximum accumulated concentration of VFAs and SCOD increased by 6.4 and 13.8 times compared with the initial concentration, which were 2.8 and 2.6 times of alkali pretreatment, and 2.1 and 2.1 times of TM pretreatment. Then, the pretreated sludge was transferred into MEC-AD coupled reactors and control group of AD reactors. The results showed that, methane production rate was enhanced to 0.15 m³ CH₄/m³ reactor/d in the coupled reactors, which was improved by 3 times compared with control AD (0.05 m³ CH₄/m³ reactor/d). The methane yield of MEC-AD coupled reactors achieved 808 ± 8 mL, which were increased by 97.0% ± 1.85% compared to control AD (410 mL). Using MEC can promote the rate of organics degradation and methane yield. The MEC-AD coupled system realized a good performance on the treatment of WAS and improved the efficiency of methane production.

Recent advancement and strategy on bio-hydrogen production from photosynthetic microalgae

Recently, ensuring energy security is a key challenge to political and economic strength in the world. Bio-hydrogen production from microalgae is the promising alternative source for potential renewable and self-sustainability energy but still in the initial phase of development. Practically and sustainability of microalgae hydrogen production is still debatable. The genetic engineering and metabolic pathway engineering of hydrogenase and nitrogenase play a key role to enhance hydrogen production. Microalgae have photosynthetic efficiency and synthesize huge carbohydrate biomass, used as 4th generation feedstock to generate bio-hydrogen. Recent genetically modified strains of microalgae are the attractive source for enhancing bio-hydrogen production in the future. The potential of hydrogen production from microRNAs are gaining great interest of researcher. The main objective of this review is attentive discussed recent approaches on new molecular genetics engineering and metabolic pathway developments, modern photo-bioreactors efficiency, economic assessment, limitations and knowledge gap of bio-hydrogen production from microalgae.

Acetic acid is key for synergetic hydrogen production in Chlamydomonas-bacteria co-cultures

This study is a proof of concept for the synergetic biohydrogen production in alga-bacteria co-cultures. Algal hydrogen photoproduction was obtained in sugar-containing media only when the green alga Chlamydomonas reinhardtii was co-cultured with Pseudomonas putida (40.8 ml H₂·L^-1), Escherichia coli (35.1 ml H₂·L^-1) and Rhizobium etli (16.1 ml H₂·L^-1). Hydrogen photo-production in these co-cultures was not only linked to the induction of hypoxia, but to the ability of the bacteria to produce acetic acid from sugars. Synergetic hydrogen production was achieved by integrating the photobiological and fermentative production in Chlamydomonas and Escherichia coli co-cultures supplemented with glucose, which resulted in 60% more H₂ production than the sum of the respective monocultures. This cooperation relied on the ability of the alga to consume the excreted bacterial acetic acid, which benefited both bacterial and algal hydrogen production. This knowledge may open new possibilities for the biohydrogen production from industrial wastes.

The complete genome sequence of Ethanoligenens harbinense reveals the metabolic pathway of acetate-ethanol fermentation: A novel understanding of the principles of anaerobic biotechnology

Ethanol-type fermentation is one of three main fermentation types in the acidogenesis of anaerobic treatment systems. Non-spore-forming Ethanoligenens is as a typical genus capable of ethanol-type fermentation in mixed culture (i.e. acetate-ethanol fermentation). This genus can produce ethanol, acetate, CO₂, and H₂ using carbohydrates, and has application potential in anaerobic bioprocesses. Here, the complete genome sequences and methylome of Ethanoligenens harbinense strains with different autoaggregative and coaggregative abilities were obtained using the PacBio single-molecule real-time sequencing platform. The genome size of E. harbinense strains was about 2.97-3.10 Mb with 55.5% G+C content. 3020-3153 genes were annotated, most of which were methylated at specific sites or motifs. The methylation types included 6mA, 4mC, and unknown types. Comparative genomic analysis demonstrated low levels of genetic similarity between E. harbinense and other well-known hydrogen-producing bacteria (i.e., Clostridium and Thermoanaerobacter) in phylogenesis. Hydrogen production of E. harbinense was catalyzed by genes that encode [FeFe]‑hydrogenases and that were synthesized by three maturases of [FeFe]-H₂ase. The metabolic mechanism of H₂-ethanol co-production fermentation, catalyzed by pyruvate ferredoxin oxidoreductase was proposed. This study provides genetic and evolutionary information of a model genus for the further investigation of the metabolic pathway and regulatory network of ethanol-type fermentation and anaerobic bioprocesses for waste or wastewater treatment.

Hydrogen and Propane Production From Butyric Acid Photoreforming Over Pt-TiO2

Photocatalysis is a promising technology from economic, energetic, and ecological points of view because it takes advantage of solar light. Hence, it is one of the investigated green routes to produce hydrogen from renewable energy resources. Butyric acid (BA) is largely present in wastewater and as an intermediate product in anaerobic digestion and therefore it is an inexpensive resource, which can be converted to valuable chemicals. In this work, photoreforming of butyric acid (BAPR) under UV light in aqueous suspensions of platinum-modified titanium dioxide-based catalysts is reported for the first time. Titania nanotubes (TNT) synthesized and calcined at different temperatures (300, 400, 500°C) and commercial TiO₂ (P25), decorated with platinum nanoparticles, have been tested and characterized through different techniques including X-ray powder diffraction, UV-vis diffuse reflectance and photoluminescence spectroscopy, transmission electron microscopy, BET and porosimetry analysis. The main identified products of the BAPR were H₂, propane, CO₂ and several organic acids (e.g., pentanoic and 3-methylhexanoic acid). It has been found that the morphology and crystallinity of the photocatalysts affected dramatically their optical properties and, consequently, the reaction rate and the product distribution. Specifically, the highest conversion of BA (X_BA) and selectivity toward H₂ (S_H2) was recorded with P25-Pt (X_BA = 26.9%, S_H2 = 47.2% after 8 h of irradiation). TNT-400-Pt showed the highest selectivity toward propane (S_C3H8 = 16.1%) with X_BA = 23.4% and S_H2 = 36.2%. The activity results in conjunction with the characterization of the catalysts highlighted that the main factor affecting the activity in terms of X_BA and generation of H₂ was the crystallinity, and in particular the presence of rutile phase in TiO₂, whereas S_C3H8 appears to increase when the electron-holes recombination is lower.

The Use of Acidic Hydrolysates after Furfural Production from Sugar Waste Biomass as a Fermentation Medium in the Biotechnological Production of Hydrogen

This study investigates a simultaneous processing of sugar beet pulp (SBP) for furfural, hydrogen and methane production using various pretreatment methods. In the experiments, sugar beet pulp was first subjected to thermal and thermochemical pretreatment at 140 °C. Then hydrolysates from these operations were investigated for their potential for methane and hydrogen production in batch tests. The experiments showed that thermal pretreatment of SBP resulted in the highest biogas and methane yields of 945 dm3/kg volatile solids (VS) and 374 dm3 CH4/kg VS, respectively, and a moderate hydrogen production of 113 dm3 H2/kg VS, which corresponded to a calculated energy production of 142 kWh/t; however, only low amount of furfural was obtained (1.63 g/L). Conversely, the highest furfural yield of 12 g/L was achieved via thermochemical pretreatment of SBP; however, biogas production from hydrolysate was much lower (215 dm3/kg VS) and contained only 67 dm3/kg VS of hydrogen. Meanwhile, in the experiment with lower amounts of sulfuric acid (2%) used for pretreatment, a moderate furfural production of 4 g/L was achieved with as high as 220 dm3/kg VS of hydrogen and the corresponding energy yield of 75 kWh/t.

Food waste treatment with a leachate bed reactor: Effects of inoculum to substrate ratio and reactor design

This study evaluated the effects of different inoculum to substrate ratios (ISRs) (5, 10, 15%) on hydrolysis and acidogenesis of food waste in a conventional leachate bed reactor (LBR-C) and a novel fractionalized LBR (LBR-F). At ISR of 10%, LBR-C experienced clogging and thus the solid removal and VFA production reduced significantly. Without any clogging events at high ISR of 10%, LBR-F achieved significantly higher (p < 0.05) VS removal of 91%, hydrolysis yield of 837 g cumulative sCOD/kg volatile solids (VS), and VFA yield of 669 g COD/kg VS. Hydrogen yield was as high as 20 m³/ton food waste in LBR-F. Energy balance indicated that the LBR-F can be energy-positive for food waste treatment with net energy benefit of ∼8 kWh/ton food waste treated. Considering the high VFA yield, the LBR-F can also be a promising food waste fermentation system for the biorefinery platform.

Induction of cathodic voltage reversal and hydrogen peroxide synthesis in a serially stacked microbial fuel cell

We developed an innovative strategy to address the inhibition of anode-respiring bacteria due to voltage reversal in serially stacked microbial fuel cells by inducing cathodic voltage reversal and H₂O₂ production. When platinum-coated carbon (Pt/C) cathodes were employed (stacked MFC_Pt/C) and the MFC was operated with acetate medium, the last unit (MFC 4) caused a voltage reversal of -0.8 V with a substantial anode overpotential of 1.22 V. After replacing the Pt/C cathode with a Pt-free carbon gas diffusion electrode in MFC 4, an electrode overpotential, approximately 0.5 V, was shifted from the anode to the cathode, inducing cathodic voltage reversal. Under cathodic voltage reversal, MFC 4 generated H₂O₂ at a production rate of 117 mg H₂O₂/m²-h. Hence, under cathodic voltage reversal induced by Pt-free cathodes, due to less anode polarization, the anode-respiring activity can largely be sustained in a stacked MFC that treats organic wastewater consistently and the quality of treated wastewater may be improved with energy-efficient and on-site generated H₂O₂.

Granular activated carbon supplementation alters the metabolic flux of Clostridium butyricum for enhanced biohydrogen production

In this work, the effects of granular activated carbon (GAC) supplementation on hydrogen fermentation were investigated in Clostridium butyricum. Maximum hydrogen production rate and ultimate hydrogen volume increased up to 1.7 and 1.9 fold, respectively, with 1 g/L GAC supplementation. Indicators of stable hydrogen production, such as acetate and butyrate formation increased with increasing GAC concentration. To identify the factors for hydrogen production enhancement, transcriptome analysis was conducted. Functional genes related to hydrogen production increased by GAC supplementation (1.75 fold for pfor and 1.48 fold for oxct). On the other hand, functional genes related to hydrogen consumption decreased (1.78 fold for ldh, 0.67 fold for adh, 1.64 fold for hbd, 2.4 fold for crt, and 0.71 fold for buk). Considered together, these results suggested that GAC supplementation increased biohydrogen production by changing the metabolic flux associated with hydrogen production and consumption pathways.

Metabolic Engineering of Microalgae for Biofuel Production

Microalgae are considered as promising cell factories for the production of various types of biofuels, including bioethanol, biodiesel, and biohydrogen by using carbon dioxide and sunlight. In spite of unique advantages of these microorganisms, the commercialization of microalgal biofuels has been hindered by poor economic features. Metabolic engineering is among the most promising strategies put forth to overcome this challenge. In this chapter, metabolic pathways involved in lipid and hydrogen production by microalgae are reviewed and discussed. Moreover, metabolic and genetic engineering approaches investigated for improving the rate of lipid (as a feedstock for biodiesel production) and biohydrogen synthesis are presented. Finally, genetic engineering tools and approaches employed for engineering microalgal metabolic pathways are elaborated. A thorough step-by-step protocol for reconstructing the metabolic pathway of various microorganisms including microalgae is also presented.

Characteristics of hydrogen-producing enrichment cultures from marine sediment using macroalgae Laminaria japonica as a feedstock

This study aimed to investigate the characteristics of hydrogen production by mixed cultures using Laminaria japonica hydrolysates. The hydrolysates of L. japonica were prepared by pretreatment methods, including heat (100°C or 121°C) and acid (HCl or H2SO4) pretreatments. The mixed cultures could produce hydrogen using L. japonica as a substrate, with the highest cumulative hydrogen production of 825 ± 14 mL/L from HCl-pretreated L. japonica. High-throughput sequencing of the 16S rRNA gene revealed that the microbial community in the hydrolysate of HCl-pretreated L. japonica was the most diverse among all the samples, with a Shannon diversity index of 5.253. The mixed culture from HCl-pretreated L. japonica and those from heat-pretreated (100°C and 121°C) L. japonica occupied different regions in a principal component analysis (PCA) plot. The dominant population in the hydrolysate of HCl-pretreated L. japonica was represented by hydrogen-producing bacteria, Clostridium spp. and Bacillus spp. The results suggested that L. japonica was an optimal feedstock for hydrogen production. The acid (HCl) pretreatment method could effectively enhance the hydrogen production from L. japonica.