Microoxic Niches within the Thylakoid Stroma of Air-Grown Chlamydomonas reinhardtii Protect [FeFe]-Hydrogenase and Support Hydrogen Production under Fully Aerobic Environment

Strategies to enhance biohydrogen production from microalgae: A comprehensive review

Microalgae represent a promising renewable feedstock for the sustainable production of biohydrogen. Their high growth rates and ability to fix carbon utilizing just sunlight, water, and nutrients make them well-suited for this application. Recent advancements have focused on improving microalgal hydrogen yields and cultivation methods. This review aims to summarize recent developments in microalgal cultivation techniques and genetic engineering strategies for enhanced biohydrogen production. Specific areas of focus include novel microalgal species selection, immobilization methods, integrated hybrid systems, and metabolic engineering. Studies related to microalgal strain selection, cultivation methods, metabolic engineering, and genetic manipulations were compiled and analyzed. Promising microalgal species with high hydrogen production capabilities such as Synechocystis sp., Anabaena variabilis, and Chlamydomonas reinhardtii have been identified. Immobilization techniques like encapsulation in alginate and integration with dark fermentation have led to improved hydrogen yields. Metabolic engineering through modulation of hydrogenase activity and photosynthetic pathways shows potential for enhanced biohydrogen productivity. Considerable progress has been made in developing microalgal systems for biohydrogen. However, challenges around process optimization and scale-up remain. Future work involving metabolic modeling, photobioreactor design, and genetic engineering of electron transfer pathways could help realize the full potential of this renewable technology.

Deciphering the structure, function, and mechanism of lysine acetyltransferase cGNAT2 in cyanobacteria

Lysine acetylation is a conserved regulatory posttranslational protein modification that is performed by lysine acetyltransferases (KATs). By catalyzing the transfer of acetyl groups to substrate proteins, KATs play critical regulatory roles in all domains of life; however, no KATs have yet been identified in cyanobacteria. Here, we tested all predicted KATs in the cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) and demonstrated that A1596, which we named cyanobacterial Gcn5-related N-acetyltransferase (cGNAT2), can catalyze lysine acetylation in vivo and in vitro. Eight amino acid residues were identified as the key residues in the putative active site of cGNAT2, as indicated by structural simulation and site-directed mutagenesis. The loss of cGNAT2 altered both growth and photosynthetic electron transport in Syn7002. In addition, quantitative analysis of the lysine acetylome identified 548 endogenous substrates of cGNAT2 in Syn7002. We further demonstrated that cGNAT2 can acetylate NAD(P)H dehydrogenase J (NdhJ) in vivo and in vitro, with the inability to acetylate K89 residues, thus decreasing NdhJ activity and affecting both growth and electron transport in Syn7002. In summary, this study identified a KAT in cyanobacteria and revealed that cGNAT2 regulates growth and photosynthesis in Syn7002 through an acetylation-mediated mechanism.

A PSII photosynthetic control is activated in anoxic cultures of green algae following illumination

Photosynthetic hydrogen production from microalgae is considered to have potential as a renewable energy source. Yet, the process has two main limitations holding it back from scaling up; (i) electron loss to competing processes, mainly carbon fixation and (ii) sensitivity to O₂ which diminishes the expression and the activity of the hydrogenase enzyme catalyzing H₂ production. Here we report a third, hitherto unknown challenge: We found that under anoxia, a slow-down switch is activated in photosystem II (PSII), diminishing the maximal photosynthetic productivity by three-fold. Using purified PSII and applying in vivo spectroscopic and mass spectrometric techniques on Chlamydomonas reinhardtii cultures, we show that this switch is activated under anoxia, within 10 s of illumination. Furthermore, we show that the recovery to the initial rate takes place following 15 min of dark anoxia, and propose a mechanism in which, modulation in electron transfer at the acceptor site of PSII diminishes its output. Such insights into the mechanism broaden our understanding of anoxic photosynthesis and its regulation in green algae and inspire new strategies to improve bio-energy yields.

Photosystem I light-harvesting proteins regulate photosynthetic electron transfer and hydrogen production

Linear electron flow (LEF) and cyclic electron flow (CEF) compete for light-driven electrons transferred from the acceptor side of photosystem I (PSI). Under anoxic conditions, such highly reducing electrons also could be used for hydrogen (H2) production via electron transfer between ferredoxin and hydrogenase in the green alga Chlamydomonas reinhardtii. Partitioning between LEF and CEF is regulated through PROTON-GRADIENT REGULATION5 (PGR5). There is evidence that partitioning of electrons also could be mediated via PSI remodeling processes. This plasticity is linked to the dynamics of PSI-associated light-harvesting proteins (LHCAs) LHCA2 and LHCA9. These two unique light-harvesting proteins are distinct from all other LHCAs because they are loosely bound at the PSAL pole. Here, we investigated photosynthetic electron transfer and H2 production in single, double, and triple mutants deficient in PGR5, LHCA2, and LHCA9. Our data indicate that lhca2 and lhca9 mutants are efficient in photosynthetic electron transfer, that LHCA2 impacts the pgr5 phenotype, and that pgr5/lhca2 is a potent H2 photo-producer. In addition, pgr5/lhca2 and pgr5/lhca9 mutants displayed substantially different H2 photo-production kinetics. This indicates that the absence of LHCA2 or LHCA9 impacts H2 photo-production independently, despite both being attached at the PSAL pole, pointing to distinct regulatory capacities.

Synthetic biology for improved hydrogen production in Chlamydomonas reinhardtii

Hydrogen is a clean alternative to fossil fuels. It has applications for electricity generation and transportation and is used for the manufacturing of ammonia and steel. However, today, H₂ is almost exclusively produced from coal and natural gas. As such, methods to produce H₂ that do not use fossil fuels need to be developed and adopted. The biological manufacturing of H₂ may be one promising solution as this process is clean and renewable. Hydrogen is produced biologically via enzymes called hydrogenases. There are three classes of hydrogenases namely [FeFe], [NiFe] and [Fe] hydrogenases. The [FeFe] hydrogenase HydA1 from the model unicellular algae Chlamydomonas reinhardtii has been studied extensively and belongs to the A1 subclass of [FeFe] hydrogenases that have the highest turnover frequencies amongst hydrogenases (21,000 ± 12,000 H₂ s⁻¹ for CaHydA from Clostridium acetobutyliticum). Yet to date, limitations in C. reinhardtii H₂ production pathways have hampered commercial scale implementation, in part due to O₂ sensitivity of hydrogenases and competing metabolic pathways, resulting in low H₂ production efficiency. Here, we describe key processes in the biogenesis of HydA1 and H₂ production pathways in C. reinhardtii. We also summarize recent advancements of algal H₂ production using synthetic biology and describe valuable tools such as high‐throughput screening (HTS) assays to accelerate the process of engineering algae for commercial biological H₂ production.

Pyruvate:ferredoxin oxidoreductase and low abundant ferredoxins support aerobic photomixotrophic growth in cyanobacteria

The decarboxylation of pyruvate is a central reaction in the carbon metabolism of all organisms. It is catalyzed by the pyruvate:ferredoxin oxidoreductase (PFOR) and the pyruvate dehydrogenase (PDH) complex. Whereas PFOR reduces ferredoxin, the PDH complex utilizes NAD⁺. Anaerobes rely on PFOR, which was replaced during evolution by the PDH complex found in aerobes. Cyanobacteria possess both enzyme systems. Our data challenge the view that PFOR is exclusively utilized for fermentation. Instead, we show, that the cyanobacterial PFOR is stable in the presence of oxygen in vitro and is required for optimal photomixotrophic growth under aerobic and highly reducing conditions while the PDH complex is inactivated. We found that cells rely on a general shift from utilizing NAD(H)- to ferredoxin-dependent enzymes under these conditions. The utilization of ferredoxins instead of NAD(H) saves a greater share of the Gibbs-free energy, instead of wasting it as heat. This obviously simultaneously decelerates metabolic reactions as they operate closer to their thermodynamic equilibrium. It is common thought that during evolution, ferredoxins were replaced by NAD(P)H due to their higher stability in an oxidizing atmosphere. However, the utilization of NAD(P)H could also have been favored due to a higher competitiveness because of an accelerated metabolism.

Emerging technologies for sustainable production of biohydrogen production from microalgae: A state-of-the-art review of upstream and downstream processes

Biohydrogen (BioH₂) is considered as one of the most environmentally friendly fuels and a strong candidate to meet the future demand for a sustainable source of energy. Presently, the production of BioH₂ from photosynthetic organisms has raised a lot of hopes in the fuel industry. Moreover, microalgal-based BioH₂ synthesis not only helps to combat current global warming by capturing greenhouse gases but also plays a key role in wastewater treatment. Hence, this manuscript provides a state-of-the-art review of the upstream and downstream BioH₂ production processes. Different metabolic routes such as direct and indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis are covered in detail. Upstream processes (e.g. growth techniques, growth media) also have a great impact on BioH₂ productivity and economics, which is also explored. Technical and scientific obstacles of microalgae BioH₂ systems are finally addressed, allowing the technology to become more innovative and commercial.

Photosynthetic hydrogen production: Novel protocols, promising engineering approaches and application of semi-synthetic hydrogenases

Photosynthetic production of molecular hydrogen (H₂ ) by cyanobacteria and green algae is a potential source of renewable energy. These organisms are capable of water biophotolysis by taking advantage of photosynthetic apparatus that links water oxidation at Photosystem II and reduction of protons to H₂ downstream of Photosystem I. Although the process has a theoretical potential to displace fossil fuels, photosynthetic H₂ production in its current state is not yet efficient enough for industrial applications due to a number of physiological, biochemical, and engineering barriers. This article presents a short overview of the metabolic pathways and enzymes involved in H₂ photoproduction in cyanobacteria and green algae and our present understanding of the mechanisms of this process. We also summarize recent advances in engineering photosynthetic cell factories capable of overcoming the major barriers to efficient and sustainable H₂ production.

Phylloquinone is the principal Mehler reaction site within photosystem I in high light

Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O2•-) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.

Bi-directional electron transfer between H2 and NADPH mitigates light fluctuation responses in green algae

The metabolism of green algae has been the focus of much research over the last century. These photosynthetic organisms can thrive under various conditions and adapt quickly to changing environments by concomitant usage of several metabolic apparatuses. The main electron coordinator in their chloroplasts, nicotinamide adenine dinucleotide phosphate (NADPH), participates in many enzymatic activities and is also responsible for inter-organellar communication. Under anaerobic conditions, green algae also accumulate molecular hydrogen (H2), a promising alternative for fossil fuels. However, to scale-up its accumulation, a firm understanding of its integration in the photosynthetic apparatus is still required. While it is generally accepted that NADPH metabolism correlates to H2 accumulation, the mechanism of this collaboration is still vague and relies on indirect measurements. Here, we investigated this connection in Chlamydomonas reinhardtii using simultaneous measurements of both dissolved gases concentration, NADPH fluorescence and electrochromic shifts at 520-546 nm. Our results indicate that energy transfer between H2 and NADPH is bi-directional and crucial for the maintenance of redox balance under light fluctuations. At light onset, NADPH consumption initially eventuates in H2 evolution, which initiates the photosynthetic electron flow. Later on, as illumination continues the majority of NADPH is diverted to the Calvin-Benson-Bassham cycle. Dark onset triggers re-assimilation of H2, which produces NADPH and so, enables initiation of dark fermentative metabolism.

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.

Water oxidation by photosystem II is the primary source of electrons for sustained H2 photoproduction in nutrient-replete green algae

Photosynthetic H₂ production in the green alga Chlamydomonas reinhardtii is catalyzed by O₂-sensitive [FeFe]-hydrogenases, which accept electrons from photosynthetically reduced ferredoxin and reduce protons to H₂. Since the process occurs downstream of photosystem I, the contribution of photosystem II (PSII) in H₂ photoproduction has long been a subject of debate. Indeed, water oxidation by PSII results in O₂ accumulation in chloroplasts, which inhibits H₂ evolution. Therefore, clear evidence for direct water biophotolysis resulting in simultaneous H₂ and O₂ releases in algae has never been presented. This paper demonstrates that sustained H₂ photoproduction in C. reinhardtii is directly linked to PSII-dependent water oxidation and brings insights into regulation of PSII activity and H₂ production by CO₂/HCO₃^– under microoxic conditions.

The unicellular green alga Chlamydomonas reinhardtii is capable of photosynthetic H₂ production. H₂ evolution occurs under anaerobic conditions and is difficult to sustain due to 1) competition between [FeFe]-hydrogenase (H₂ase), the key enzyme responsible for H₂ metabolism in algae, and the Calvin–Benson–Bassham (CBB) cycle for photosynthetic reductants and 2) inactivation of H₂ase by O₂ coevolved in photosynthesis. Recently, we achieved sustainable H₂ photoproduction by shifting algae from continuous illumination to a train of short (1 s) light pulses, interrupted by longer (9 s) dark periods. This illumination regime prevents activation of the CBB cycle and redirects photosynthetic electrons to H₂ase. Employing membrane-inlet mass spectrometry and H218O, we now present clear evidence that efficient H₂ photoproduction in pulse-illuminated algae depends primarily on direct water biophotolysis, where water oxidation at the donor side of photosystem II (PSII) provides electrons for the reduction of protons by H₂ase downstream of photosystem I. This occurs exclusively in the absence of CO₂ fixation, while with the activation of the CBB cycle by longer (8 s) light pulses the H₂ photoproduction ceases and instead a slow overall H₂ uptake is observed. We also demonstrate that the loss of PSII activity in DCMU-treated algae or in PSII-deficient mutant cells can be partly compensated for by the indirect (PSII-independent) H₂ photoproduction pathway, but only for a short (<1 h) period. Thus, PSII activity is indispensable for a sustained process, where it is responsible for more than 92% of the final H₂ yield.

Membrane Inlet Mass Spectrometry: A Powerful Tool for Algal Research

Since the first great oxygenation event, photosynthetic microorganisms have continuously shaped the Earth's atmosphere. Studying biological mechanisms involved in the interaction between microalgae and cyanobacteria with the Earth's atmosphere requires the monitoring of gas exchange. Membrane inlet mass spectrometry (MIMS) has been developed in the early 1960s to study gas exchange mechanisms of photosynthetic cells. It has since played an important role in investigating various cellular processes that involve gaseous compounds (O2, CO2, NO, or H2) and in characterizing enzymatic activities in vitro or in vivo. With the development of affordable mass spectrometers, MIMS is gaining wide popularity and is now used by an increasing number of laboratories. However, it still requires an important theory and practical considerations to be used. Here, we provide a practical guide describing the current technical basis of a MIMS setup and the general principles of data processing. We further review how MIMS can be used to study various aspects of algal research and discuss how MIMS will be useful in addressing future scientific challenges.

Green Catalysts: Applied and Synthetic Photosynthesis

The biological process of photosynthesis was critical in catalyzing the oxygenation of Earth’s atmosphere 2.5 billion years ago, changing the course of development of life on Earth. Recently, the fields of applied and synthetic photosynthesis have utilized the light-driven protein–pigment supercomplexes central to photosynthesis for the photocatalytic production of fuel and other various valuable products. The reaction center Photosystem I is of particular interest in applied photosynthesis due to its high stability post-purification, non-geopolitical limitation, and its ability to generate the greatest reducing power found in nature. These remarkable properties have been harnessed for the photocatalytic production of a number of valuable products in the applied photosynthesis research field. These primarily include photocurrents and molecular hydrogen as fuels. The use of artificial reaction centers to generate substrates and reducing equivalents to drive non-photoactive enzymes for valuable product generation has been a long-standing area of interest in the synthetic photosynthesis research field. In this review, we cover advances in these areas and further speculate synthetic and applied photosynthesis as photocatalysts for the generation of valuable products.

Mechanistic insights into pH-dependent H2 photoproduction in bisulfite-treated Chlamydomonas cells

BACKGROUND

Bisulfite addition is an important H₂ photoproduction strategy that removes O₂ and activates hydrogenase. The pH values of cell cultures can change the ratio of bisulfite to sulfite, which may affect H₂ photoproduction. However, little is known regarding the pH effect of bisulfite addition on H₂ photoproduction and relevant underlying mechanism.

RESULTS

Here, changes in H₂ photoproduction with different initial extracellular pH values showed a parabolic distribution and a pH of 8.0 is an optimal value for H₂ photoproduction in Chlamydomonas reinhardtii cells treated with bisulfite. Compared to the growth pH (pH 7.3), increased photoproduction of H₂ at this optimal pH was primarily caused by a relatively high residual activity of photosystem II (PSII), which provides a relatively plentiful source of electrons for H₂ photoproduction. Such increased H₂ photoproduction was most likely a result of decreased the ratio of bisulfite to sulfite, consistent with the result that the toxicity of bisulfite on PSII was much more than that of sulfite. This possibility was corroborated by the result that treatment with a combination of 7 mM bisulfite and 6 mM sulfite further enhanced H₂ photoproduction compared with 13 mM bisulfite alone.

CONCLUSIONS

Collectively, our findings provide novel mechanistic insights into pH-dependent H₂ photoproduction in C. reinhardtii cells treated with bisulfite, and demonstrate that sulfite addition is another important strategy for H₂ photoproduction, just like bisulfite addition.

Overproduction of the Flv3B flavodiiron, enhances the photobiological hydrogen production by the nitrogen-fixing cyanobacterium Nostoc PCC 7120

Background

The ability of some photosynthetic microorganisms, particularly cyanobacteria and microalgae, to produce hydrogen (H₂) is a promising alternative for renewable, clean-energy production. However, the most recent, related studies point out that much improvement is needed for sustainable cyanobacterial-based H₂ production to become economically viable. In this study, we investigated the impact of induced O₂-consumption on H₂ photoproduction yields in the heterocyte-forming, N₂-fixing cyanobacterium Nostoc PCC7120.

Results

The flv3B gene, encoding a flavodiiron protein naturally expressed in Nostoc heterocytes, was overexpressed. Under aerobic and phototrophic growth conditions, the recombinant strain displayed a significantly higher H₂ production than the wild type. Nitrogenase activity assays indicated that flv3B overexpression did not enhance the nitrogen fixation rates. Interestingly, the transcription of the hox genes, encoding the NiFe Hox hydrogenase, was significantly elevated, as shown by the quantitative RT-PCR analyses.

Conclusion

We conclude that the overproduced Flv3B protein might have enhanced O₂-consumption, thus creating conditions inducing hox genes and facilitating H₂ production. The present study clearly demonstrates the potential to use metabolic engineered cyanobacteria for photosynthesis driven H₂ 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.

O2 sensitivity and H2 production activity of hydrogenases-A review

Hydrogenases are metalloproteins capable of catalyzing the interconversion between molecular hydrogen and protons and electrons. The iron-sulfur clusters within the enzyme enable rapid relay of electrons which are either consumed or generated at the active site. Their unparalleled catalytic efficiency has attracted attention, especially for potential use in H₂ production and/or fuel cell technologies. However, there are limitations to using hydrogenases, especially due to their high O₂ sensitivity. The subclass, called [FeFe] hydrogenases, are particularly more vulnerable to O₂ but proficient in H₂ production. In this review, we provide an overview of mechanistic and protein engineering studies focused on understanding and enhancing O₂ tolerance of the enzyme. The emphasis is on ongoing studies that attempt to overcome O₂ sensitivity of the enzyme while it catalyzes H₂ production in an aerobic environment. We also discuss pioneering attempts to utilize the enzyme in biological H₂ production and other industrial processes, as well as our own perspective on future applications.

Elimination of the flavodiiron electron sink facilitates long-term H2 photoproduction in green algae

BACKGROUND

The development of renewable and sustainable biofuels to cover the future energy demand is one of the most challenging issues of our time. Biohydrogen, produced by photosynthetic microorganisms, has the potential to become a green biofuel and energy carrier for the future sustainable world, since it provides energy without CO₂ emission. The recent development of two alternative protocols to induce hydrogen photoproduction in green algae enables the function of the O₂-sensitive [FeFe]-hydrogenases, located at the acceptor side of photosystem I, to produce H₂ for several days. These protocols prevent carbon fixation and redirect electrons toward H₂ production. In the present work, we employed these protocols to a knockout Chlamydomonas reinhardtii mutant lacking flavodiiron proteins (FDPs), thus removing another possible electron competitor with H₂ production.

RESULTS

The deletion of the FDP electron sink resulted in the enhancement of H₂ photoproduction relative to wild-type C. reinhardtii. Additionally, the lack of FDPs leads to a more effective obstruction of carbon fixation even under elongated light pulses.

CONCLUSIONS

We demonstrated that the rather simple adjustment of cultivation conditions together with genetic manipulation of alternative electron pathways of photosynthesis results in efficient re-routing of electrons toward H₂ photoproduction. Furthermore, the introduction of a short recovery phase by regular switching from H₂ photoproduction to biomass accumulation phase allows to maintain cell fitness and use photosynthetic cells as long-term H₂-producing biocatalysts.

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.

Flavodiiron-Mediated O2 Photoreduction Links H2 Production with CO2 Fixation during the Anaerobic Induction of Photosynthesis

Some microalgae, such as Chlamydomonas reinhardtii, harbor a highly flexible photosynthetic apparatus capable of using different electron acceptors, including carbon dioxide (CO₂), protons, or oxygen (O₂), allowing survival in diverse habitats. During anaerobic induction of photosynthesis, molecular O₂ is produced at photosystem II, while at the photosystem I acceptor side, the reduction of protons into hydrogen (H₂) by the plastidial [FeFe]-hydrogenases primes CO₂ fixation. Although the interaction between H₂ production and CO₂ fixation has been studied extensively, their interplay with O₂ produced by photosynthesis has not been considered. By simultaneously measuring gas exchange and chlorophyll fluorescence, we identified an O₂ photoreduction mechanism that functions during anaerobic dark-to-light transitions and demonstrate that flavodiiron proteins (Flvs) are the major players involved in light-dependent O₂ uptake. We further show that Flv-mediated O₂ uptake is critical for the rapid induction of CO₂ fixation but is not involved in the creation of the micro-oxic niches proposed previously to protect the [FeFe]-hydrogenase from O₂ By studying a mutant lacking both hydrogenases (HYDA1 and HYDA2) and both Flvs (FLVA and FLVB), we show that the induction of photosynthesis is strongly delayed in the absence of both sets of proteins. Based on these data, we propose that Flvs are involved in an important intracellular O₂ recycling process, which acts as a relay between H₂ production and CO₂ fixation.

Green Algal Hydrogenase Activity Is Outcompeted by Carbon Fixation before Inactivation by Oxygen Takes Place

Photoproduction of hydrogen by green algae is considered a transitory release valve of excess reducing power and a potential carbon-free source of sustainable energy. It is generally accepted that the transitory production of hydrogen is governed by fast inactivation of hydrogenase by oxygen. However, our data suggest that photosynthetic electron loss to competing processes, mainly carbon fixation, stops hydrogen production, supports hydrogen uptake, and precedes the inevitable inactivation by oxygen. Here, we show that when transitioning from dark anaerobiosis to light, hydrogen production ceases within 2 min, regardless of the presence of oxygen. Simultaneous monitoring of the active hydrogenase pool size shows that it remains entirely intact up to 4 min after illumination and is inactivated only later. Thus, our data reveal a window of 4 min in which the hydrogenase pool is not being degraded by oxygen. Furthermore, we show that electron loss, prominently to carbon fixation, outcompetes hydrogen production and leads to hydrogen uptake. Indeed, supplying additional reducing power to hydrogenase at the cessation point regenerates the accumulation of hydrogen. Our results imply the fast cessation of hydrogen production is governed by electron loss rather than oxygen inactivation, which takes place minutes later.

Water-splitting-based, sustainable and efficient H2 production in green algae as achieved by substrate limitation of the Calvin-Benson-Bassham cycle

BACKGROUND

Photobiological H₂ production has the potential of becoming a carbon-free renewable energy source, because upon the combustion of H₂, only water is produced. The [Fe-Fe]-type hydrogenases of green algae are highly active, although extremely O₂-sensitive. Sulphur deprivation is a common way to induce H₂ production, which, however, relies substantially on organic substrates and imposes a severe stress effect resulting in the degradation of the photosynthetic apparatus.

RESULTS

We report on the establishment of an alternative H₂ production method by green algae that is based on a short anaerobic induction, keeping the Calvin-Benson-Bassham cycle inactive by substrate limitation and preserving hydrogenase activity by applying a simple catalyst to remove the evolved O₂. Cultures remain photosynthetically active for several days, with the electrons feeding the hydrogenases mostly derived from water. The amount of H₂ produced is higher as compared to the sulphur-deprivation procedure and the process is photoautotrophic.

CONCLUSION

Our protocol demonstrates that it is possible to sustainably use algal cells as whole-cell catalysts for H₂ production, which enables industrial application of algal biohydrogen production.

Microalgal hydrogen production - A review

Bio-hydrogen from microalgae including cyanobacteria has attracted commercial awareness due to its potential as an alternative, reliable and renewable energy source. Photosynthetic hydrogen production from microalgae can be interesting and promising options for clean energy. Advances in hydrogen-fuel-cell technology may attest an eco-friendly way of biofuel production, since, the use of H₂ to generate electricity releases only water as a by-product. Progress in genetic/metabolic engineering may significantly enhance the photobiological hydrogen production from microalgae. Manipulation of competing metabolic pathways by modulating the certain key enzymes such as hydrogenase and nitrogenase may enhance the evolution of H₂ from photoautotrophic cells. Moreover, biological H₂ production at low operating costs is requisite for economic viability. Several photobioreactors have been developed for large-scale biomass and hydrogen production. This review highlights the recent technological progress, enzymes involved and genetic as well as metabolic engineering approaches towards sustainable hydrogen production from microalgae.

Comparative analyses of H2 photoproduction in magnesium- and sulfur-starved Chlamydomonas reinhardtii cultures

Magnesium (Mg)-deprived Chlamydomonas reinhardtii cells are capable to sustain hydrogen (H₂ ) photoproduction at relatively high photosystem II (PSII) activity levels for an extended time period as compared with sulfur (S)-deprived cells. Herein, we present a comparative study of H₂ photoproduction induced by Mg and S shortage to unravel the specific rearrangements of the photosynthetic machinery and cell metabolism occurring under the two deprivation protocols. The exhaustive analysis of photosynthetic activity and regulatory pathways, respiration and starch metabolism revealed the specific rearrangements of the photosynthetic machinery and cellular metabolism, which occur under the two deprivation conditions. The obtained results allowed us to conclude that the expanded time period of H₂ production upon Mg-deprivation is due to the less harmful effects that Mg-depletion has on viability and metabolic performance of the cells. Unlike S-deprivation, the photosynthetic light and dark reactions in Mg-deprived cells remained active over the whole H₂ production period. However, the elevated PSII activity in Mg-deprived cells was counteracted by the operation of pathways for O₂ consumption that maintain anaerobic conditions in the presence of active water splitting.