Perspectives and advances of biological H2 production in microorganisms

Sustainable hydrogen production via microalgae: Technological advancements, economic indicators, environmental aspects, challenges, and policy implications

Hydrogen has emerged as an alternative energy source to meet the increasing global energy demand, depleting fossil fuels and environmental issues resulting from fossil fuel consumption. Microalgae-based biomass is gaining attention as a potential source of hydrogen production due to its green energy carrier properties, high energy content, and carbon-free combustion. This review examines the hydrogen production process from microalgae, including the microalgae cultivation technological process for biomass production, and the three main routes of biomass-to-hydrogen production: thermochemical conversion, photo biological conversion, and electrochemical conversion. The current progress of technological options in the three main routes is presented, with the various strains of microalgae and operating conditions of the processes. Furthermore, the economic and environmental perspectives of biomass-to-hydrogen from microalgae are evaluated, and critical operational parameters are used to assess the feasibility of scaling up biohydrogen production for commercial industrial-scale applications. The key finding is the thermochemical conversion process is the most feasible process for biohydrogen production, compared to the pyrolysis process. In the photobiological and electrochemical process, pure hydrogen can be achieved, but further process development is required to enhance the production yield. In addition, the high production cost is the main challenge in biohydrogen production. The cost of biohydrogen production for direct bio photolysis it cost around $7.24 kg-1; for indirect bio photolysis it costs around $7.54 kg-1 and for fermentation, it costs around $7.61 kg-1. Therefore, comprehensive studies and efforts are required to make biohydrogen production from microalgae applications more economical in the future.

Bioconversion of volatile fatty acids from organic wastes to produce high-value products by photosynthetic bacteria: A review

Anaerobic fermentation of organic waste to produce volatile fatty acids (VFAs) production is a relatively mature technology. VFAs can be used as a cheap and readily available carbon source by photosynthetic bacteria (PSB) to produce high value-added products, which are widely used in various applications. To better enhance the VFAs obtained from organic wastes for PSB to produce high value-added products, a comprehensive review is needed, which is currently not available. This review systematically summarizes the current status of microbial proteins, H2, poly-β-hydroxybutyrate (PHB), coenzyme Q10 (CoQ10), and 5-aminolevulinic acid (ALA) production by PSB utilizing VFAs as a carbon resource. Meanwhile, the metabolic pathways involved in the H2, PHB, CoQ10, and 5-ALA production by PSB were deeply explored. In addition, a systematic resource utilization pathway for PSB utilizing VFAs from anaerobic fermentation of organic wastes to produce high value-added products was proposed. Finally, the current challenges and priorities for future research were presented, such as the screening of efficient PSB strains, conducting large-scale experiments, high-value product separation, recovery, and purification, and the mining of metabolic pathways for the VFA utilization to generate high value-added products by PSB.

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.

Thermal Design of a Biohydrogen Production System Driven by Integrated Gasification Combined Cycle Waste Heat Using Dynamic Simulation

Utilizing biological processes for hydrogen production via gasification is a promising alternative method to coal gasification. The present study proposes a dynamic simulation model that uses a one-dimensional heat-transfer analysis method to simulate a biohydrogen production system. The proposed model is based on an existing experimental design setup. It is used to simulate a biohydrogen production system driven by the waste heat from an integrated gasification combined cycle (IGCC) power plant equipped with carbon capture and storage technologies. The data from the simulated results are compared with the experimental measurement data to validate the developed model’s reliability. The results show good agreement between the experimental data and the developed model. The relative root-mean-square error for the heat storage, feed-mixing, and bioreactor tanks is 1.26%, 3.59%, and 1.78%, respectively. After the developed model’s reliability is confirmed, it is used to simulate and optimize the biohydrogen production system inside the IGCC power plant. The bioreactor tank’s time constant can be improved when reducing the operating volume of the feed-mixing tank by the scale factors of 0.75 and 0.50, leading to a 15.76% and 31.54% faster time constant, respectively, when compared with the existing design.

A review on biological recycling in agricultural waste-based biohydrogen production: Recent developments

Hydrogen has become a research highlight by virtue of its clean energy production technology and high energy content. The technology of biohydrogen production from biological waste via fermentation has lower costs, provides environment-friendly methods regarding energy balance, and creates a pathway for sustainable utilization of massive agricultural waste. However, biohydrogen production is generally limited by lower productivity. Many studies have been conducted aimed at improving biohydrogen production efficiency. Hence, this review is intended to describe improving routes for biohydrogen production from agricultural waste and highlights recent advances in these approaches. In addition, the critical factors affecting biohydrogen production, including the pretreatment method, substrate resource, fermentation conditions, and bioreactor design, were also comprehensively discussed along with challenges and future prospects.

Bioprocesses for the recovery of bioenergy and value-added products from wastewater: A review

Wastewater and activated sludge present a major challenge worldwide. Wastewater generated from large and small-scale industries, laundries, human residential areas and other sources is emerging as a main problem in sanitation and maintenance of smart/green cities. During the last decade, different technologies and processes have been developed to recycle and purify the wastewater. Currently, identification and fundamental consideration of development of more advanced microbial-based technologies that enable wastewater treatment and simultaneous resource recovery to produce bioenergy, biofuels and other value-added compounds (organic acids, fatty acids, bioplastics, bio-pesticides, bio-surfactants and bio-flocculants etc.) became an emerging topic. In the last several decades, significant development of bioprocesses and techniques for the extraction and recovery of mentioned valuable molecules and compounds from wastewater, waste biomass or sludge has been made. This review presents different microbial-based process routes related to resource recovery and wastewater application for the production of value-added products and bioenergy. Current process limitations and insights for future research to promote more efficient and sustainable routes for this under-utilized and continually growing waste stream are also discussed.

Microalgal Production of Biofuels Integrated with Wastewater Treatment

Human civilization will need to reduce its impacts on air and water quality and reduce its use of fossil fuels in order to advance towards a more sustainable future. Using microalgae to treat wastewater as well as simultaneously produce biofuels is one of the approaches for a sustainable future. The manufacture of biofuels from microalgae is one of the next-generation biofuel solutions that has recently received a lot of interest, as it can remove nutrients from the wastewater whilst capturing carbon dioxide from the atmosphere. The resulting biomass are employed to generate biofuels, which can run fuel cell vehicles of zero emission, power combustion engines and power plants. By cultivating microalgae in wastewater, eutrophication can be prevented, thereby enhancing the quality of the effluent. Thus, by combining wastewater treatment and biofuel production, the cost of the biofuels, as well as the environmental hazards, can be minimized, as there is a supply of free and already available nutrients and water. In this article, the steps involved to generate the various biofuels through microalgae are detailed.

Post-translational Modifications of Serine/Threonine and Histidine Kinases and Their Roles in Signal Transductions in Synechocystis Sp. PCC 6803

Cyanobacterium Synechocystis sp. PCC 6803, a popular model organism for researches in photosynthesis and biofuel production, contains plant-like photosynthetic machineries which significantly contribute to global carbon fixation. There are 12 eukaryotic-type Ser/Thr kinases (SpkA-L) and 49 His kinases (Hik1-49) of two-component systems in the genome of Synechocystis sp. PCC 6803. They are the key regulators in sensing and transmitting stimuli including light- and glucose-mediate signal transduction. Proteomic studies were able to identify all the kinases. The majority of kinases no matter whether they have a predicted transmembrane domain were identified in the membrane fractions. Six Ser/Thr kinases (SpkA-D, F and G) and ten His kinases (Hik4, 12, 14, 21, 26-27, 29, 36, 43, and 46) were identified to have one or more of the three types of post-translational modifications: phosphorylation, acetylation, and thiol oxidation. Interestingly, SpkG has the phosphorylatable threonine residue that was aligned with the phosphorylated threonine residue in the activation loop of human CDK7, demonstrating conserved phosphorylation between cyanobacterial and human kinases. Transcriptomics and proteomics revealed differential expression of the kinases in heterotrophic and photoheterotrophic compared with photoautotrophic conditions, indicating their roles in regulating the growth modes of cyanobacteria. In summary, this review focuses on the discussions on post-transcriptional modifications, transcriptomic, and proteomic studies of Ser/Thr and His kinases. This together with our published review in 2019 present a complete story of an overview of sequences, domain architectures, and biochemical and physiological functions of cyanobacterial kinases with adequate details in the context of high throughput systems. We also emphasize the importance of discovering upstream molecules and substrates to understand the exact functions of the kinases in vivo. As an attempt, a model is proposed in which Hik31, His33, Sll1334, and IcfG are hypothesized to be critical for switching between autotrophic and heterotrophic modes based on the results from the phenotypes of the gene knockout strains combined with their post-translational modifications, and gene expression profiles.

Biohydrogen and poly-β-hydroxybutyrate production by winery wastewater photofermentation: Effect of substrate concentration and nitrogen source

The applicability and convenience of biohydrogen and poly-β-hydroxybutyrate production through single-stage photofermentation of winery wastewater is demonstrated in the present study. Experiments are conducted using a purple non-sulfur bacteria mixed consortium, subject to variable nutrient conditions, to analyze the effect of initial chemical oxygen demand and the available nitrogen source on the metabolic response. Results show that winery wastewater is a promising substrate for photofermentation processes, despite the presence of inhibiting compounds such as phenolics. Nonetheless, the initial chemical oxygen demand must be carefully controlled to maximize hydrogen production. Up to 468 mL L^-1 of hydrogen and 203 mg L^-1 of poly-β-hydroxybutyrate can be produced starting from an initial chemical oxygen demand of 1500 mg L^-1. The used nitrogen source may direct substrate transformation through different metabolic pathways. Interestingly, the maximum production of both hydrogen and poly-β-hydroxybutyrate occurred when glutamate was used as the nitrogen source.

Continuous production of biohydrogen from brewery effluent using co-culture of mutated Rhodobacter M 19 and Enterobacter aerogenes

This study investigated the biohydrogen production from brewery effluents using free and immobilized co-culture of mutated Rhodobacter M 19 and Enterobacter aerogenes obtained from random mutagenesis with ultra violet (UV) and ethidium bromide (EtBr) treatment. The best mutant for biohydrogen production was screened based on the sugar utilization efficiency. Maximum hydrogen production of 87% was achieved with immobilized EtBr mutated co-culture. The mutant immobilized strains showed around 30% enhanced hydrogen production than wild strains at pH 6.9. Gompertz and Richard's model were used to fit the augmenting biohydrogen production and Logistics equation determines the fitness of biomass growth data. The maximal biomass concentration of co-cultures strains was 3.145 g/L with carrying capacity coefficient 0.137 h^-1. Gompertz model showed the best fit with minimal error in predicting the biohydrogen potential.

Absolute quantification of selected photosynthetic electron transfer proteins in Chlamydomonas reinhardtii in the presence and absence of oxygen

The absolute amount of plastocyanin (PC), ferredoxin-NADP⁺-oxidoreductase (FNR), hydrogenase (HYDA1), and ferredoxin 5 (FDX5) were quantified in aerobic and anaerobic Chlamydomonas reinhardtii whole cells using purified (recombinant) proteins as internal standards in a mass spectrometric approach. Quantified protein amounts were related to the estimated amount of PSI. The ratios of PC to FNR to HYDA1 to FDX5 in aerobic cells were determined to be 1.4:1.2:0.003:0. In anaerobic cells, the ratios changed to 1.1:1.3:0.019:0.027 (PC:FNR:HYDA1:FDX5). Employing sodium dithionite and methyl viologen as electron donors, the specific activity of hydrogenase in whole cells was calculated to be 382 ± 96.5 μmolH₂ min^-1 mg^-1. Importantly, these data reveal an about 70-fold lower abundance of HYDA1 compared to FNR. Despite this great disproportion between both proteins, which might further enhance the competition for electrons, the alga is capable of hydrogen production under anaerobic conditions, thus pointing to an efficient channeling mechanism of electrons from FDX1 to the HYDA1.

Enhancement of microalgal growth and biocomponent-based transformations for improved biofuel recovery: A review

Microalgal biomass has received much attention as feedstock for biofuel production due to its capacity to accumulate a substantial amount of biocomponents (including lipid, carbohydrate, and protein), high growth rate, and environmental benefit. However, commercial realization of microalgal biofuel is a challenge due to its low biomass production and insufficient technology for complete utilization of biomass. Recently, advanced strategies have been explored to overcome the challenges of conventional approaches and to achieve maximum possible outcomes in terms of growth. These strategies include a combination of stress factors; co-culturing with other microorganisms; and addition of salts, flue gases, and phytohormones. This review summarizes the recent progress in the application of single and combined abiotic stress conditions to stimulate microalgal growth and its biocomponents. An innovative schematic model is presented of the biomass-energy conversion pathway that proposes the transformation of all potential biocomponents of microalgae into biofuels.

Presence of a [3Fe-4S] cluster in a PsaC variant as a functional component of the photosystem I electron transfer chain in Synechococcus sp. PCC 7002

A site-directed C14G mutation was introduced into the stromal PsaC subunit of Synechococcus sp. strain PCC 7002 in vivo in order to introduce an exchangeable coordination site into the terminal F_B [4Fe-4S] cluster of Photosystem I (PSI). Using an engineered PSI-less strain (psaAB deletion), psaC was deleted and replaced with recombinant versions controlled by a strong promoter, and the psaAB deletion was complemented. Modified PSI accumulated at lower levels in this strain and supported slower photoautotrophic growth than wild type. As-isolated PSI complexes containing PsaC_C14G showed resonances with g values of 2.038 and 2.007 characteristic of a [3Fe-4S]¹⁺ cluster. When the PSI complexes were illuminated at 15 K, these resonances partially disappeared and two new sets of resonances appeared. The majority set had g values of 2.05, 1.95, and 1.85, characteristic of F_A^-, and the minority set had g values of 2.11, 1.90, and 1.88 from F_B' in the modified site. The S = 1/2 spin state of the latter implied the presence of a thiolate as the terminal ligand. The [3Fe-4S] clusters could be partially reconstituted with iron, producing a larger population of [4Fe-4S] clusters. Rates of flavodoxin reduction were identical in PSI complexes isolated from wild type and the PsaC_C14G variant strain; this implied equivalent capacity for forward electron transfer in PSI complexes that contained [3Fe-4S] and [4Fe-4S] clusters. The development of this cyanobacterial strain is a first step toward translation of in vitro PSI-based biosolar molecular wire systems in vivo and provides new insights into the formation of Fe/S clusters.

Hydrogen photoproduction in green algae Chlamydomonas reinhardtii under magnesium deprivation

Mg deprivation results in the sustained H₂ formation in Chlamydomonas reinhardtii.

Deletion of Proton Gradient Regulation 5 (PGR5) and PGR5-Like 1 (PGRL1) proteins promote sustainable light-driven hydrogen production in Chlamydomonas reinhardtii due to increased PSII activity under sulfur deprivation

Continuous hydrogen photo-production under sulfur deprivation was studied in the Chlamydomonas reinhardtii pgr5 pgrl1 double mutant and respective single mutants. Under medium light conditions, the pgr5 exhibited the highest performance and produced about eight times more hydrogen than the wild type, making pgr5 one of the most efficient hydrogen producer reported so far. The pgr5 pgrl1 double mutant showed an increased hydrogen burst at the beginning of sulfur deprivation under high light conditions, but in this case the overall amount of hydrogen produced by pgr5 pgrl1 as well as pgr5 was diminished due to photo-inhibition and increased degradation of PSI. In contrast, the pgrl1 was effective in hydrogen production in both high and low light. Blocking photosynthetic electron transfer by DCMU stopped hydrogen production almost completely in the mutant strains, indicating that the main pathway of electrons toward enhanced hydrogen production is via linear electron transport. Indeed, PSII remained more active and stable in the pgr mutant strains as compared to the wild type. Since transition to anaerobiosis was faster and could be maintained due to an increased oxygen consumption capacity, this likely preserves PSII from photo-oxidative damage in the pgr mutants. Hence, we conclude that increased hydrogen production under sulfur deprivation in the pgr5 and pgrl1 mutants is caused by an increased stability of PSII permitting sustainable light-driven hydrogen production in Chlamydomonas reinhardtii.

The diurnal logic of the expression of the chloroplast genome in Chlamydomonas reinhardtii

Chloroplasts are derived from cyanobacteria and have retained a bacterial-type genome and gene expression machinery. The chloroplast genome encodes many of the core components of the photosynthetic apparatus in the thylakoid membranes. To avoid photooxidative damage and production of harmful reactive oxygen species (ROS) by incompletely assembled thylakoid protein complexes, chloroplast gene expression must be tightly regulated and co-ordinated with gene expression in the nucleus. Little is known about the control of chloroplast gene expression at the genome-wide level in response to internal rhythms and external cues. To obtain a comprehensive picture of organelle transcript levels in the unicellular model alga Chlamydomonas reinhardtii in diurnal conditions, a qRT-PCR platform was developed and used to quantify 68 chloroplast, 21 mitochondrial as well as 71 nuclear transcripts in cells grown in highly controlled 12 h light/12 h dark cycles. Interestingly, in anticipation of dusk, chloroplast transcripts from genes involved in transcription reached peak levels first, followed by transcripts from genes involved in translation, and finally photosynthesis gene transcripts. This pattern matches perfectly the theoretical demands of a cell “waking up” from the night. A similar trend was observed in the nuclear transcripts. These results suggest a striking internal logic in the expression of the chloroplast genome and a previously unappreciated complexity in the regulation of chloroplast genes.

Plastidial Expression of Type II NAD(P)H Dehydrogenase Increases the Reducing State of Plastoquinones and Hydrogen Photoproduction Rate by the Indirect Pathway in Chlamydomonas reinhardtii1

Biological conversion of solar energy into hydrogen is naturally realized by some microalgae species due to a coupling between the photosynthetic electron transport chain and a plastidial hydrogenase. While promising for the production of clean and sustainable hydrogen, this process requires improvement to be economically viable. Two pathways, called direct and indirect photoproduction, lead to sustained hydrogen production in sulfur-deprived Chlamydomonas reinhardtii cultures. The indirect pathway allows an efficient time-based separation of O₂ and H₂ production, thus overcoming the O₂ sensitivity of the hydrogenase, but its activity is low. With the aim of identifying the limiting step of hydrogen production, we succeeded in overexpressing the plastidial type II NAD(P)H dehydrogenase (NDA2). We report that transplastomic strains overexpressing NDA2 show an increased activity of nonphotochemical reduction of plastoquinones (PQs). While hydrogen production by the direct pathway, involving the linear electron flow from photosystem II to photosystem I, was not affected by NDA2 overexpression, the rate of hydrogen production by the indirect pathway was increased in conditions, such as nutrient limitation, where soluble electron donors are not limiting. An increased intracellular starch was observed in response to nutrient deprivation in strains overexpressing NDA2. It is concluded that activity of the indirect pathway is limited by the nonphotochemical reduction of PQs, either by the pool size of soluble electron donors or by the PQ-reducing activity of NDA2 in nutrient-limited conditions. We discuss these data in relation to limitations and biotechnological improvement of hydrogen photoproduction in microalgae.

Trends in biohydrogen production: major challenges and state-of-the-art developments

Hydrogen has shown enormous potential to be an alternative fuel of the future. Hydrogen production technology has gained much attention in the last few decades due to advantages such as its high conversion efficiency, recyclability and non-polluting nature. Over the last few decades, biological hydrogen production has shown great promise for generating large scale sustainable energy to meet ever increasing global energy demands. Various microorganisms, namely bacteria, cyanobacteria, and algae which are capable of producing hydrogen from water, solar energy, and a variety of organic substrates, are explored and studied in detail. Current biohydrogen production technologies, however, face two major challenges such as low-yield and high production cost. Advances have been made in recent years in biohydrogen research to improve the hydrogen yield through process modifications, physiological manipulations, through metabolic and genetic engineering. Recently, cell immobilization such as microbes trapping with nanoparticles within the bioreactor has shown an increase in hydrogen production. This review critically evaluated various biological hydrogen production technologies, key challenges, and recent advancements in biohydrogen research and development.

Increased photosystem II stability promotes H2 production in sulfur-deprived Chlamydomonas reinhardtii

Photobiological H2 production is an attractive option for renewable solar fuels. Sulfur-deprived cells of Chlamydomonas reinhardtii have been shown to produce hydrogen with the highest efficiency among photobiological systems. We have investigated the photosynthetic reactions during sulfur deprivation and H2 production in the wild-type and state transition mutant 6 (Stm6) mutant of Chlamydomonas reinhardtii. The incubation period (130 h) was dissected into different phases, and changes in the amount and functional status of photosystem II (PSII) were investigated in vivo by electron paramagnetic resonance spectroscopy and variable fluorescence measurements. In the wild type it was found that the amount of PSII is decreased to 25% of the original level; the electron transport from PSII was completely blocked during the anaerobic phase preceding H2 formation. This block was released during the H2 production phase, indicating that the hydrogenase withdraws electrons from the plastoquinone pool. This partly removes the block in PSII electron transport, thereby permitting electron flow from water oxidation to hydrogenase. In the Stm6 mutant, which has higher respiration and H2 evolution than the wild type, PSII was analogously but much less affected. The addition of the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea revealed that ∼80% of the H2 production was inhibited in both strains. We conclude that (i) at least in the earlier stages, most of the electrons delivered to the hydrogenase originate from water oxidation by PSII, (ii) a faster onset of anaerobiosis preserves PSII from irreversible photoinhibition, and (iii) mutants with enhanced respiratory activity should be considered for better photobiological H2 production.

Metabolic engineering for enhanced hydrogen production: a review

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

Nicotinamide, NAD(P)(H), and Methyl-Group Homeostasis Evolved and Became a Determinant of Ageing Diseases: Hypotheses and Lessons from Pellagra

Compartmentalized redox faults are common to ageing diseases. Dietary constituents are catabolized to NAD(H) donating electrons producing proton-based bioenergy in coevolved, cross-species and cross-organ networks. Nicotinamide and NAD deficiency from poor diet or high expenditure causes pellagra, an ageing and dementing disorder with lost robustness to infection and stress. Nicotinamide and stress induce Nicotinamide-N-methyltransferase (NNMT) improving choline retention but consume methyl groups. High NNMT activity is linked to Parkinson's, cancers, and diseases of affluence. Optimising nicotinamide and choline/methyl group availability is important for brain development and increased during our evolution raising metabolic and methylome ceilings through dietary/metabolic symbiotic means but strict energy constraints remain and life-history tradeoffs are the rule. An optimal energy, NAD and methyl group supply, avoiding hypo and hyper-vitaminoses nicotinamide and choline, is important to healthy ageing and avoids utilising double-edged symbionts or uncontrolled autophagy or reversions to fermentation reactions in inflammatory and cancerous tissue that all redistribute NAD(P)(H), but incur high allostatic costs.

Semi-continuous photo-fermentative H2 production by Rhodobacter sphaeroides: effect of decanting volume ratio

In this study, a semi-continuous operation of photo-fermentative H2-producing reactor was attempted at various decanting volume ratios (DVR, decanting volume per day/total working volume, %), ranging 30-70%, using Rhodobacter sphaeroides KD131. H2 production was not efficient with showing low H2 yields of 0.2 and 0.5 mol H2/mol succinate(added) at 30% and 40% DVR, respectively. The low performance ascribed to the fact that over 70% of substrate electrons were diverted towards cell growth under these conditions. Meanwhile, cell growth was limited at DVR≥50%; therefore, higher H2 yields (>2.0 mol H2/mol succinateadded) were observed. Both the highest H2 yield of 3.7 mol H2/mol succinateadded and production rate of 1494 mL H2/L-reactor/d were achieved at 60% DVR. The content of soluble microbial products (SMPs) was measured, which accounted for 3-15% of substrate electrons. It was found that the largest (65-75%) portion of SMPs comprised low molecular-weight (<3 kDa).

Time-course global expression profiles of Chlamydomonas reinhardtii during photo-biological H₂ production

We used a microarray study in order to compare the time course expression profiles of two Chlamydomonas reinhardtii strains, namely the high H₂ producing mutant stm6glc4 and its parental WT strain during H₂ production induced by sulfur starvation. Major cellular reorganizations in photosynthetic apparatus, sulfur and carbon metabolism upon H₂ production were confirmed as common to both strains. More importantly, our results pointed out factors which lead to the higher H₂ production in the mutant including a higher starch accumulation in the aerobic phase and a lower competition between the H₂ase pathway and alternative electron sinks within the H₂ production phase. Key candidate genes of interest with differential expression pattern include LHCSR3, essential for efficient energy quenching (qE). The reduced LHCSR3 protein expression in mutant stm6glc4 could be closely related to the high-light sensitive phenotype. H₂ measurements carried out with the LHCSR3 knock-out mutant npq4 however clearly demonstrated that a complete loss of this protein has almost no impact on H₂ yields under moderate light conditions. The nuclear gene disrupted in the high H₂ producing mutant stm6glc4 encodes for the mitochondrial transcription termination factor (mTERF) MOC1, whose expression strongly increases during -S-induced H₂ production in WT strains. Studies under phototrophic high-light conditions demonstrated that the presence of functional MOC1 is a prerequisite for proper LHCSR3 expression. Furthermore knock-down of MOC1 in a WT strain was shown to improve the total H₂ yield significantly suggesting that this strategy could be applied to further enhance H₂ production in other strains already displaying a high H₂ production capacity. By combining our array data with previously published metabolomics data we can now explain some of the phenotypic characteristics which lead to an elevated H₂ production in stm6glc4.

Hydrogen production by Rhodopseudomonas palustris WP 3-5 in a serial photobioreactor fed with hydrogen fermentation effluent

In this study, a lab-scale serial photobioreactor composed of three column reactors was constructed and continuously operated to investigate several parameters influencing photohydrogen production when using the synthetic wastewater and the anaerobic hydrogen fermentation effluents as the influents. The results indicated that better hydrogen production rate was obtained when the serial photobioreactor was operated under cellular recycling at a short HRT of 8h. The serial photobioreactor maintained high hydrogen content ca. 80% in the produced gas and 0.4× dilution ratio was the suitable ratio for hydrogen production. When the photobioreactor fed with the real wastewater (Effluent 1) containing 100 mg/L NH4Cl, Column 1 reactor successfully reduced ammonia concentration to about 60 mg/L for cell synthesis, resulting in a steady hydrogen production in the following two column reactors. The average hydrogen production rate was 205 mL-H2/L/d.

Hydrogen production: two stage processes for waste degradation

The dark fermentation process generates hydrogen by biological means. It presents two main advantages: fulfilling requirements for mild operational conditions and gaining benefit from the residual biomass. The process itself may be seen as a pre-treatment step in a complete stabilisation chain, with the aim of attaining the valorisation of residual biomass. However, increasing the yield of H2 production is an imperative task. In this manuscript, a review of recent work in the field of fermentative hydrogen production is presented. As dark fermentation has a maximum yield of 33% (on sugars), a description is also presented of possible second stage processes for the degradation of dark fermentation effluents. Alternatives considered were photofermentation and bioelectrochemical systems (BES) as processes capable of converting fermentation sub-products into H2. Anaerobic digestion as a final stabilisation stage was also considered owing to the wide application of this technology in the treatment of bio-wastes.

Multiple facets of anoxic metabolism and hydrogen production in the unicellular green alga Chlamydomonas reinhardtii

Many microbes in the soil environment experience micro-oxic or anoxic conditions for much of the late afternoon and night, which inhibit or prevent respiratory metabolism. To sustain the production of energy and maintain vital cellular processes during the night, organisms have developed numerous pathways for fermentative metabolism. This review discusses fermentation pathways identified for the soil-dwelling model alga Chlamydomonas reinhardtii, its ability to produce molecular hydrogen under anoxic conditions through the activity of hydrogenases, and the molecular flexibility associated with fermentative metabolism that has only recently been revealed through the analysis of specific mutant strains.

Rewiring hydrogenase-dependent redox circuits in cyanobacteria

Hydrogenases catalyze the reversible reaction 2H(+) + 2e(-) ↔ H(2) with an equilibrium constant that is dependent on the reducing potential of electrons carried by their redox partner. To examine the possibility of increasing the photobiological production of hydrogen within cyanobacterial cultures, we expressed the [FeFe] hydrogenase, HydA, from Clostridium acetobutylicum in the non-nitrogen-fixing cyanobacterium Synechococcus elongatus sp. 7942. We demonstrate that the heterologously expressed hydrogenase is functional in vitro and in vivo, and that the in vivo hydrogenase activity is connected to the light-dependent reactions of the electron transport chain. Under anoxic conditions, HydA activity is capable of supporting light-dependent hydrogen evolution at a rate > 500-fold greater than that supported by the endogenous [NiFe] hydrogenase. Furthermore, HydA can support limited growth solely using H(2) and light as the source of reducing equivalents under conditions where Photosystem II is inactivated. Finally, we demonstrate that the addition of exogenous ferredoxins can modulate redox flux in the hydrogenase-expressing strain, allowing for greater hydrogen yields and for dark fermentation of internal energy stores into hydrogen gas.

Next-generation biofuels: Survey of emerging technologies and sustainability issues

Next-generation biofuels, such as cellulosic bioethanol, biomethane from waste, synthetic biofuels obtained via gasification of biomass, biohydrogen, and others, are currently at the center of the attention of technologists and policy makers in search of the more sustainable biofuel of tomorrow. To set realistic targets for future biofuel options, it is important to assess their sustainability according to technical, economical, and environmental measures. With this aim, the review presents a comprehensive overview of the chemistry basis and of the technology related aspects of next generation biofuel production, as well as it addresses related economic issues and environmental implications. Opportunities and limits are discussed in terms of technical applicability of existing and emerging technology options to bio-waste feedstock, and further development forecasts are made based on the existing social-economic and market situation, feedstock potentials, and other global aspects. As the latter ones are concerned, the emphasis is placed on the opportunities and challenges of developing countries in adoption of this new industry.

Hydrogen cycling by the unicellular marine diazotroph Crocosphaera watsonii strain WH8501

The hydrogen (H₂) cycle associated with the dinitrogen (N₂) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H₂ production and acetylene (C₂H₂) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H₂ production was coincident with the maximum rates of C₂H₂ reduction. Theoretical stoichiometry for N₂ fixation predicts an equimolar ratio of H₂ produced to N₂ fixed. However, the maximum rate of net H₂ production observed was 0.09 nmol H₂ μg chlorophyll a (chl a)⁻¹ h⁻¹) compared to the N₂ fixation rate of 5.5 nmol N₂ μg chl a⁻¹ h⁻¹, with an H₂ production/N₂ fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H₂ production was hypothesized to be a result of H₂ reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H₂ production rates ∼40-fold to a maximum rate of 3.5 nmol H₂ μg chl a⁻¹ h⁻¹. We conclude that the reassimilation of H₂ by C. watsonii is highly efficient (> 98%) and hypothesize that the tight coupling between H₂ production and consumption is a consequence of fixing N₂ at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H₂ cycle provides unique insight into N₂ fixation and associated metabolic processes in C. watsonii.

The interplay of proton, electron, and metabolite supply for photosynthetic H2 production in Chlamydomonas reinhardtii

To obtain a detailed picture of sulfur deprivation-induced H(2) production in microalgae, metabolome analyses were performed during key time points of the anaerobic H(2) production process of Chlamydomonas reinhardtii. Analyses were performed using gas chromatography coupled to mass spectrometry (GC/MS), two-dimensional gas chromatography combined with time-of-flight mass spectrometry (GCxGC-TOFMS), lipid and starch analysis, and enzymatic determination of fermentative products. The studies were designed to provide a detailed metabolite profile of the solar Bio-H(2) production process. This work reports on the differential analysis of metabolic profiles of the high H(2)-producing strain Stm6Glc4 and the wild-type cc406 (WT) before and during the H(2) production phase. Using GCxGC-TOFMS analysis the number of detected peaks increased from 128 peaks, previously detected by GC/MS techniques, to ∼1168. More detailed analysis of the anaerobic H(2) production phase revealed remarkable differences between wild-type and mutant cells in a number of metabolic pathways. Under these physiological conditions the WT produced up to 2.6 times more fatty acids, 2.2 times more neutral lipids, and up to 4 times more fermentation products compared with Stm6Glc4. Based on these results, specific metabolic pathways involving the synthesis of fatty acids, neutral lipids, and fermentation products during anaerobiosis in C. reinhardtii have been identified as potential targets for metabolic engineering to further enhance substrate supply for the hydrogenase(s) in the chloroplast.

Hydrogen production by the unicellular, diazotrophic cyanobacterium Cyanothece sp. strain ATCC 51142 under conditions of continuous light

We report on the hydrogen production properties of the unicellular, diazotrophic cyanobacterium Cyanothece sp. strain ATCC 51142. This organism has a versatile metabolism and can grow in the presence or absence of combined nitrogen and can grow photosynthetically or mixotrophically and heterotrophically in the presence of glycerol. The strain produces a bidirectional hydrogenase (encoded by the hox genes), an uptake hydrogenase (hupLS), and nitrogenase (nifHDK). We demonstrated hydrogen production by both the hydrogenase and the nitrogenase under appropriate metabolic conditions. The highest rates of hydrogen production were produced under nitrogen-fixing conditions when cells were grown and incubated under continuous light conditions, in either the presence or absence of glycerol. Under such nitrogen-fixing conditions, we have achieved rates of 300 micromol H(2)/mg chloramphenicol (Chl)/hr during the first 24 h of incubation. The levels of H(2) measured were dependent upon the incubation conditions, such as sparging with argon, which generated anaerobic conditions. We demonstrated that the same conditions led to high levels of H(2) production and N(2) fixation, indicating that low-oxygen conditions favor nitrogenase activity for both processes. The levels of hydrogen produced by the hydrogenase are much lower, typically 5 to 10 micromol H(2)/mg Chl/hr. Hydrogenase activity was dependent upon electron transport through photosystem II (PS II), whereas nitrogenase activity was more dependent on PS I, as well as on respiration. Although cells do not double under the incubation conditions when sparged with argon to provide a low-oxygen environment, the cells are metabolically active, and hydrogen production can be inhibited by the addition of chloramphenicol to inhibit protein synthesis.

Towards solar fuels from water and CO2

Solar fuels from water and CO2 are a topic of current large scientific and industrial interest. Research advances on bioroutes, concentrated solar thermal and low-temperature conversion using semiconductors and a photoelectrocatalytic (PEC) approach, are critically discussed and compared in an attempt to define challenges and current limits and to identify the priorities on which focus research and development (R&D). The need to produce fuels that are easy to transport and store, which can be integrated into the existing energy infrastructure, is emphasized. The role of solar fuels produced from CO2 in comparison with solar H2 is analyzed. Solar fuels are complementary to solar to electrical energy conversion, but they still need intensified R&D before possible commercialization.

Photosynthetic hydrogen production by a hybrid complex of photosystem I and [NiFe]-hydrogenase

Nature provides key components for generating fuels from renewable resources in the form of enzymatic nanomachines which catalyze crucial steps in biological energy conversion, for example, the photosynthetic apparatus, which transforms solar power into chemical energy, and hydrogenases, capable of generating molecular hydrogen. As sunlight is usually used to synthesize carbohydrates, direct generation of hydrogen from light represents an exception in nature. On the molecular level, the crucial step for conversion of solar energy into H(2) lies in the efficient electronic coupling of photosystem I and hydrogenase. Here we show the stepwise assembly of a hybrid complex consisting of photosystem I and hydrogenase on a solid gold surface. This device gave rise to light-induced H(2) evolution. Hydrogen production is possible at far higher potential and thus lower energy compared to those of previously described (bio)nanoelectronic devices that did not employ the photosynthesis apparatus. The successful demonstration of efficient solar-to-hydrogen conversion may serve as a blueprint for the establishment of this system in a living organism with the paramount advantage of self-replication.