Function of the chloroplast hydrogenase in the microalga Chlamydomonas: the role of hydrogenase and state transitions during photosynthetic activation in anaerobiosis

Light-induced H2 generation in a photosystem I-O2-tolerant [FeFe] hydrogenase nanoconstruct

The fusion of hydrogenases and photosynthetic reaction centers (RCs) has proven to be a promising strategy for the production of sustainable biofuels. Type I (iron-sulfur-containing) RCs, acting as photosensitizers, are capable of promoting electrons to a redox state that can be exploited by hydrogenases for the reduction of protons to dihydrogen (H2). While both [FeFe] and [NiFe] hydrogenases have been used successfully, they tend to be limited due to either O2 sensitivity, binding specificity, or H2 production rates. In this study, we fuse a peripheral (stromal) subunit of Photosystem I (PS I), PsaE, to an O2-tolerant [FeFe] hydrogenase from Clostridium beijerinckii using a flexible [GGS]4 linker group (CbHydA1-PsaE). We demonstrate that the CbHydA1 chimera can be synthetically activated in vitro to show bidirectional activity and that it can be quantitatively bound to a PS I variant lacking the PsaE subunit. When illuminated in an anaerobic environment, the nanoconstruct generates H2 at a rate of 84.9 ± 3.1 µmol H2 mgchl-1 h-1. Further, when prepared and illuminated in the presence of O2, the nanoconstruct retains the ability to generate H2, though at a diminished rate of 2.2 ± 0.5 µmol H2 mgchl-1 h-1. This demonstrates not only that PsaE is a promising scaffold for PS I-based nanoconstructs, but the use of an O2-tolerant [FeFe] hydrogenase opens the possibility for an in vivo H2 generating system that can function in the presence of O2.

Mutation of negative regulatory gene CEHC1 encoding an FBXO3 protein results in normoxic expression of HYDA genes in Chlamydomonas reinhardtii

Oxygen is known to prevent hydrogen production in Chlamydomonas, both by inhibiting the hydrogenase enzyme and by preventing the accumulation of HYDA-encoding transcripts. We developed a screen for mutants showing constitutive accumulation of HYDA1 transcripts in the presence of oxygen. A reporter gene required for ciliary motility, placed under the control of the HYDA1 promoter, conferred motility only in hypoxic conditions. By selecting for mutants able to swim even in the presence of oxygen we obtained strains that express the reporter gene constitutively. One mutant identified a gene encoding an F-box only protein 3 (FBXO3), known to participate in ubiquitylation and proteasomal degradation pathways in other eukaryotes. Transcriptome profiles revealed that the mutation, termed cehc1-1 , leads to constitutive expression of HYDA1 and other genes regulated by hypoxia, and of many genes known to be targets of CRR1, a transcription factor in the nutritional copper signaling pathway. CRR1 was required for the constitutive expression of the HYDA1 reporter gene in cehc1-1 mutants. The CRR1 protein, which is normally degraded in Cu-supplemented cells, was stabilized in cehc1-1 cells, supporting the conclusion that CEHC1 acts to facilitate the degradation of CRR1. Our results reveal a novel negative regulator in the CRR1 pathway and possibly other pathways leading to complex metabolic changes associated with response to hypoxia.

Induction of photosynthesis under anoxic condition in Thalassiosira pseudonana and Euglena gracilis: interactions between fermentation and photosynthesis

INTRODUCTION

In their natural environment, microalgae can be transiently exposed to hypoxic or anoxic environments. Whereas fermentative pathways and their interactions with photosynthesis are relatively well characterized in the green alga model Chlamydomonas reinhardtii, little information is available in other groups of photosynthetic micro-eukaryotes. In C. reinhardtii cyclic electron flow (CEF) around photosystem (PS) I, and light-dependent oxygen-sensitive hydrogenase activity both contribute to restoring photosynthetic linear electron flow (LEF) in anoxic conditions.

METHODS

Here we analyzed photosynthetic electron transfer after incubation in dark anoxic conditions (up to 24 h) in two secondary microalgae: the marine diatom Thalassiosira pseudonana and the excavate Euglena gracilis.

RESULTS

Both species showed sustained abilities to prevent over-reduction of photosynthetic electron carriers and to restore LEF. A high and transient CEF around PSI was also observed specifically in anoxic conditions at light onset in both species. In contrast, at variance with C. reinhardtii, no sustained hydrogenase activity was detected in anoxic conditions in both species.

DISCUSSION

Altogether our results suggest that another fermentative pathway might contribute, along with CEF around PSI, to restore photosynthetic activity in anoxic conditions in E. gracilis and T. pseudonana. We discuss the possible implication of the dissimilatory nitrate reduction to ammonium (DNRA) in T. pseudonana and the wax ester fermentation in E. gracilis.

Long- and short-term acclimation of the photosynthetic apparatus to salinity in Chlamydomonas reinhardtii. The role of Stt7 protein kinase

Salt stress triggers an Stt7-mediated LHCII-phosphorylation signaling mechanism similar to light-induced state transitions. However, phosphorylated LHCII, after detaching from PSII, does not attach to PSI but self-aggregates instead. Salt is a major stress factor in the growth of algae and plants. Here, our study mainly focuses on the organization of the photosynthetic apparatus to the long-term responses of Chlamydomonas reinhardtii to elevated NaCl concentrations. We analyzed the physiological effects of salt treatment at a cellular, membrane, and protein level by microscopy, protein profile analyses, transcripts, circular dichroism spectroscopy, chlorophyll fluorescence transients, and steady-state and time-resolved fluorescence spectroscopy. We have ascertained that cells that were grown in high-salinity medium form palmelloids sphere-shaped colonies, where daughter cells with curtailed flagella are enclosed within the mother cell walls. Palmelloid formation depends on the presence of a cell wall, as it was not observed in a cell-wall-less mutant CC-503. Using the stt7 mutant cells, we show Stt7 kinase-dependent phosphorylation of light-harvesting complex II (LHCII) in both short- and long-term treatments of various NaCl concentrations-demonstrating NaCl-induced state transitions that are similar to light-induced state transitions. The grana thylakoids were less appressed (with higher repeat distances), and cells grown in 150 mM NaCl showed disordered structures that formed diffuse boundaries with the flanking stroma lamellae. PSII core proteins were more prone to damage than PSI. At high salt concentrations (100-150 mM), LHCII aggregates accumulated in the thylakoid membranes. Low-temperature and time-resolved fluorescence spectroscopy indicated that the stt7 mutant was more sensitive to salt stress, suggesting that LHCII phosphorylation has a role in the acclimation and protection of the photosynthetic apparatus.

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.

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.

Cytoklepty in the plankton: A host strategy to optimize the bioenergetic machinery of endosymbiotic algae

Endosymbioses have shaped the evolutionary trajectory of life and remain ecologically important. Investigating oceanic photosymbioses can illuminate how algal endosymbionts are energetically exploited by their heterotrophic hosts and inform on putative initial steps of plastid acquisition in eukaryotes. By combining three-dimensional subcellular imaging with photophysiology, carbon flux imaging, and transcriptomics, we show that cell division of endosymbionts (Phaeocystis) is blocked within hosts (Acantharia) and that their cellular architecture and bioenergetic machinery are radically altered. Transcriptional evidence indicates that a nutrient-independent mechanism prevents symbiont cell division and decouples nuclear and plastid division. As endosymbiont plastids proliferate, the volume of the photosynthetic machinery volume increases 100-fold in correlation with the expansion of a reticular mitochondrial network in close proximity to plastids. Photosynthetic efficiency tends to increase with cell size, and photon propagation modeling indicates that the networked mitochondrial architecture enhances light capture. This is accompanied by 150-fold higher carbon uptake and up-regulation of genes involved in photosynthesis and carbon fixation, which, in conjunction with a ca.15-fold size increase of pyrenoids demonstrates enhanced primary production in symbiosis. Mass spectrometry imaging revealed major carbon allocation to plastids and transfer to the host cell. As in most photosymbioses, microalgae are contained within a host phagosome (symbiosome), but here, the phagosome invaginates into enlarged microalgal cells, perhaps to optimize metabolic exchange. This observation adds evidence that the algal metamorphosis is irreversible. Hosts, therefore, trigger and benefit from major bioenergetic remodeling of symbiotic microalgae with potential consequences for the oceanic carbon cycle. Unlike other photosymbioses, this interaction represents a so-called cytoklepty, which is a putative initial step toward plastid acquisition.

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.

The evolutionary conserved iron-sulfur protein TCR controls P700 oxidation in photosystem I

In natural habitats, plants have developed sophisticated regulatory mechanisms to optimize the photosynthetic electron transfer rate at the maximum efficiency and cope with the changing environments. Maintaining proper P700 oxidation at photosystem I (PSI) is the common denominator for most regulatory processes of photosynthetic electron transfers. However, the molecular complexes and cofactors involved in these processes and their function(s) have not been fully clarified. Here, we identified a redox-active chloroplast protein, the triplet-cysteine repeat protein (TCR). TCR shared similar expression profiles with known photosynthetic regulators and contained two triplet-cysteine motifs (CxxxCxxxC). Biochemical analysis indicated that TCR localizes in chloroplasts and has a [3Fe-4S] cluster. Loss of TCR limited the electron sink downstream of PSI during dark-to-light transition. Arabidopsis pgr5-tcr double mutant reduced growth significantly and showed unusual oxidation and reduction of plastoquinone pool. These results indicated that TCR is involved in electron flow(s) downstream of PSI, contributing to P700 oxidation.

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P700 oxidation at photosystem I is important for regulation of photosynthesis

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TCR is a redox active chloroplast protein harboring a 3Fe-4S iron-sulfur cluster

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TCR controls electron flow around photosystem I, contributing to P700 oxidation

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.

Algal photosynthesis converts nitric oxide into nitrous oxide

Nitrous oxide (N₂O), the third most important greenhouse gas in the atmosphere, is produced in great quantities by microalgae, but molecular mechanisms remain elusive. Here we show that the green microalga Chlamydomonas reinhardtii produces N₂O in the light by a reduction of NO driven by photosynthesis and catalyzed by flavodiiron proteins, the dark N₂O production being catalyzed by a cytochrome p450. Both mechanisms of N₂O production are present in chlorophytes, but absent from diatoms. Our study provides an unprecedented mechanistic understanding of N₂O production by microalgae, allowing a better assessment of N₂O-producing hot spots in aquatic environments.

Nitrous oxide (N₂O), a potent greenhouse gas in the atmosphere, is produced mostly from aquatic ecosystems, to which algae substantially contribute. However, mechanisms of N₂O production by photosynthetic organisms are poorly described. Here we show that the green microalga Chlamydomonas reinhardtii reduces NO into N₂O using the photosynthetic electron transport. Through the study of C. reinhardtii mutants deficient in flavodiiron proteins (FLVs) or in a cytochrome p450 (CYP55), we show that FLVs contribute to NO reduction in the light, while CYP55 operates in the dark. Both pathways are active when NO is produced in vivo during the reduction of nitrites and participate in NO homeostasis. Furthermore, NO reduction by both pathways is restricted to chlorophytes, organisms particularly abundant in ocean N₂O-producing hot spots. Our results provide a mechanistic understanding of N₂O production in eukaryotic phototrophs and represent an important step toward a comprehensive assessment of greenhouse gas emission by aquatic ecosystems.

Re-routing photosynthetic energy for continuous hydrogen production in vivo

BACKGROUND

Hydrogen is considered a promising energy vector that can be produced from sustainable resources such as sunlight and water. In green algae, such as Chlamydomonas reinhardtii, photoproduction of hydrogen is catalyzed by the enzyme [FeFe]-hydrogenase (HydA). Although highly efficient, this process is transitory and thought to serve as a release valve for excess reducing power. Up to date, prolonged production of hydrogen was achieved by the deprivation of either nutrients or light, thus, hindering the full potential of photosynthetic hydrogen production. Previously we showed that the enzyme superoxide dismutase (SOD) can enhance HydA activity in vitro, specifically when tied together to a fusion protein.

RESULTS

In this work, we explored the in vivo hydrogen production phenotype of HydA-SOD fusion. We found a sustained hydrogen production, which is dependent on linear electron flow, although other pathways feed it as well. In addition, other characteristics such as slower growth and oxygen production were also observed in Hyd-SOD-expressing algae.

CONCLUSIONS

The Hyd-SOD fusion manages to outcompete the Calvin-Benson cycle, allowing sustained hydrogen production for up to 14 days in non-limiting conditions.

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.

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.

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.

Isolation and characterization of mutants corresponding to the MENA, MENB, MENC and MENE enzymatic steps of 5'-monohydroxyphylloquinone biosynthesis in Chlamydomonas reinhardtii

Phylloquinone (PhQ), or vitamin K₁ , is an essential electron carrier (A₁ ) in photosystem I (PSI). In the green alga Chlamydomonas reinhardtii, which is a model organism for the study of photosynthesis, a detailed characterization of the pathway is missing with only one mutant deficient for MEND having been analyzed. We took advantage of the fact that a double reduction of plastoquinone occurs in anoxia in the A₁ site in the mend mutant, interrupting photosynthetic electron transfer, to isolate four new phylloquinone-deficient mutants impaired in MENA, MENB, MENC (PHYLLO) and MENE. Compared with the wild type and complemented strains for MENB and MENE, the four men mutants grow slowly in low light and are sensitive to high light. When grown in low light they show a reduced photosynthetic electron transfer due to a specific decrease of PSI. Upon exposure to high light for a few hours, PSI becomes almost completely inactive, which leads in turn to lack of phototrophic growth. Loss of PhQ also fully prevents reactivation of photosynthesis after dark anoxia acclimation. In silico analyses allowed us to propose a PhQ biosynthesis pathway in Chlamydomonas that involves 11 enzymatic steps from chorismate located in the chloroplast and in the peroxisome.

Ascorbate accumulation during sulphur deprivation and its effects on photosystem II activity and H2 production of the green alga Chlamydomonas reinhardtii

In nature, H2 production in Chlamydomonas reinhardtii serves as a safety valve during the induction of photosynthesis in anoxia, and it prevents the over-reduction of the photosynthetic electron transport chain. Sulphur deprivation of C. reinhardtii also triggers a complex metabolic response resulting in the induction of various stress-related genes, down-regulation of photosynthesis, the establishment of anaerobiosis and expression of active hydrogenase. Photosystem II (PSII) plays dual role in H2 production because it supplies electrons but the evolved O2 inhibits the hydrogenase. Here, we show that upon sulphur deprivation, the ascorbate content in C. reinhardtii increases about 50-fold, reaching the mM range; at this concentration, ascorbate inactivates the Mn-cluster of PSII, and afterwards, it can donate electrons to tyrozin Z(+) at a slow rate. This stage is followed by donor-side-induced photoinhibition, leading to the loss of charge separation activity in PSII and reaction centre degradation. The time point at which maximum ascorbate concentration is reached in the cell is critical for the establishment of anaerobiosis and initiation of H2 production. We also show that ascorbate influenced H2 evolution via altering the photosynthetic electron transport rather than hydrogenase activity and starch degradation.

Multiple regulatory mechanisms in the chloroplast of green algae: relation to hydrogen production

A complex regulatory network in the chloroplast of green algae provides an efficient tool for maintenance of energy and redox balance in the cell under aerobic and anaerobic conditions. In this review, we discuss the structural and functional organizations of electron transport pathways in the chloroplast, and regulation of photosynthesis in the green microalga Chlamydomonas reinhardtii. The focus is on the regulatory mechanisms induced in response to nutrient deficiency stress and anoxia and especially on the role of a hydrogenase-mediated reaction in adaptation to highly reducing conditions and ATP deficiency in the cell.

Induction of Photosynthetic Carbon Fixation in Anoxia Relies on Hydrogenase Activity and Proton-Gradient Regulation-Like1-Mediated Cyclic Electron Flow in Chlamydomonas reinhardtii

The model green microalga Chlamydomonas reinhardtii is frequently subject to periods of dark and anoxia in its natural environment. Here, by resorting to mutants defective in the maturation of the chloroplastic oxygen-sensitive hydrogenases or in Proton-Gradient Regulation-Like1 (PGRL1)-dependent cyclic electron flow around photosystem I (PSI-CEF), we demonstrate the sequential contribution of these alternative electron flows (AEFs) in the reactivation of photosynthetic carbon fixation during a shift from dark anoxia to light. At light onset, hydrogenase activity sustains a linear electron flow from photosystem II, which is followed by a transient PSI-CEF in the wild type. By promoting ATP synthesis without net generation of photosynthetic reductants, the two AEF are critical for restoration of the capacity for carbon dioxide fixation in the light. Our data also suggest that the decrease in hydrogen evolution with time of illumination might be due to competition for reduced ferredoxins between ferredoxin-NADP(+) oxidoreductase and hydrogenases, rather than due to the sensitivity of hydrogenase activity to oxygen. Finally, the absence of the two alternative pathways in a double mutant pgrl1 hydrogenase maturation factor G-2 is detrimental for photosynthesis and growth and cannot be compensated by any other AEF or anoxic metabolic responses. This highlights the role of hydrogenase activity and PSI-CEF in the ecological success of microalgae in low-oxygen environments.

Algae after dark: mechanisms to cope with anoxic/hypoxic conditions

Chlamydomonas reinhardtii is a unicellular, soil-dwelling (and aquatic) green alga that has significant metabolic flexibility for balancing redox equivalents and generating ATP when it experiences hypoxic/anoxic conditions. The diversity of pathways available to ferment sugars is often revealed in mutants in which the activities of specific branches of fermentative metabolism have been eliminated; compensatory pathways that have little activity in parental strains under standard laboratory fermentative conditions are often activated. The ways in which these pathways are regulated and integrated have not been extensively explored. In this review, we primarily discuss the intricacies of dark anoxic metabolism in Chlamydomonas, but also discuss aspects of dark oxic metabolism, the utilization of acetate, and the relatively uncharacterized but critical interactions that link chloroplastic and mitochondrial metabolic networks.

Protein redox regulation in the thylakoid lumen: the importance of disulfide bonds for violaxanthin de-epoxidase

When exposed to saturating light conditions photosynthetic eukaryotes activate the xanthophyll cycle where the carotenoid violaxanthin is converted into zeaxanthin by the enzyme violaxanthin de-epoxidase (VDE). VDE protein sequence includes 13 cysteine residues, 12 of which are strongly conserved in both land plants and algae. Site directed mutagenesis of Arabidopsis thaliana VDE showed that all these 12 conserved cysteines have a major role in protein function and their mutation leads to a strong reduction of activity. VDE is also shown to be active in its completely oxidized form presenting six disulfide bonds. Redox titration showed that VDE activity is sensitive to variation in redox potential, suggesting the possibility that dithiol/disulfide exchange reactions may represent a mechanism for VDE regulation.

Hydrogen sulfide can inhibit and enhance oxygenic photosynthesis in a cyanobacterium from sulfidic springs

We used microsensors to investigate the combinatory effect of hydrogen sulfide (H2 S) and light on oxygenic photosynthesis in biofilms formed by a cyanobacterium from sulfidic springs. We found that photosynthesis was both positively and negatively affected by H2 S: (i) H2 S accelerated the recovery of photosynthesis after prolonged exposure to darkness and anoxia. We suggest that this is possibly due to regulatory effects of H2 S on photosystem I components and/or on the Calvin cycle. (ii) H2 S concentrations of up to 210 μM temporarily enhanced the photosynthetic rates at low irradiance. Modelling showed that this enhancement is plausibly based on changes in the light-harvesting efficiency. (iii) Above a certain light-dependent concentration threshold H2 S also acted as an inhibitor. Intriguingly, this inhibition was not instant but occurred only after a specific time interval that decreased with increasing light intensity. That photosynthesis is most sensitive to inhibition at high light intensities suggests that H2 S inactivates an intermediate of the oxygen evolving complex that accumulates with increasing light intensity. We discuss the implications of these three effects of H2 S in the context of cyanobacterial photosynthesis under conditions with diurnally fluctuating light and H2 S concentrations, such as those occurring in microbial mats and biofilms.

The involvement of hydrogen-producing and ATP-dependent NADPH-consuming pathways in setting the redox poise in the chloroplast of Chlamydomonas reinhardtii in anoxia

Photosynthetic microalgae are exposed to changing environmental conditions. In particular, microbes found in ponds or soils often face hypoxia or even anoxia, and this severely impacts their physiology. Chlamydomonas reinhardtii is one among such photosynthetic microorganisms recognized for its unusual wealth of fermentative pathways and the extensive remodeling of its metabolism upon the switch to anaerobic conditions. As regards the photosynthetic electron transfer, this remodeling encompasses a strong limitation of the electron flow downstream of photosystem I. Here, we further characterize the origin of this limitation. We show that it stems from the strong reducing pressure that builds up upon the onset of anoxia, and this pressure can be relieved either by the light-induced synthesis of ATP, which promotes the consumption of reducing equivalents, or by the progressive activation of the hydrogenase pathway, which provides an electron transfer pathway alternative to the CO2 fixation cycle.

A security network in PSI photoprotection: regulation of photosynthetic control, NPQ and O2 photoreduction by cyclic electron flow

Cyclic electron flow (CEF) around PSI regulates acceptor-side limitations and has multiple functions in the green alga, Chlamydomonas reinhardtii. Here we draw on recent and historic literature and concentrate on its role in Photosystem I (PSI) photoprotection, outlining causes and consequences of damage to PSI and CEF's role as an avoidance mechanism. We outline two functions of CEF in PSI photoprotection that are both linked to luminal acidification: firstly, its action on Photosystem II with non-photochemical quenching and photosynthetic control and secondly, its action in poising the stroma to overcome acceptor-side limitation by rebalancing NADPH and ATP ratios for carbon fixation.

[FeFe]- and [NiFe]-hydrogenase diversity, mechanism, and maturation

The [FeFe]- and [NiFe]-hydrogenases catalyze the formal interconversion between hydrogen and protons and electrons, possess characteristic non-protein ligands at their catalytic sites and thus share common mechanistic features. Despite the similarities between these two types of hydrogenases, they clearly have distinct evolutionary origins and likely emerged from different selective pressures. [FeFe]-hydrogenases are widely distributed in fermentative anaerobic microorganisms and likely evolved under selective pressure to couple hydrogen production to the recycling of electron carriers that accumulate during anaerobic metabolism. In contrast, many [NiFe]-hydrogenases catalyze hydrogen oxidation as part of energy metabolism and were likely key enzymes in early life and arguably represent the predecessors of modern respiratory metabolism. Although the reversible combination of protons and electrons to generate hydrogen gas is the simplest of chemical reactions, the [FeFe]- and [NiFe]-hydrogenases have distinct mechanisms and differ in the fundamental chemistry associated with proton transfer and control of electron flow that also help to define catalytic bias. A unifying feature of these enzymes is that hydrogen activation itself has been restricted to one solution involving diatomic ligands (carbon monoxide and cyanide) bound to an Fe ion. On the other hand, and quite remarkably, the biosynthetic mechanisms to produce these ligands are exclusive to each type of enzyme. Furthermore, these mechanisms represent two independent solutions to the formation of complex bioinorganic active sites for catalyzing the simplest of chemical reactions, reversible hydrogen oxidation. As such, the [FeFe]- and [NiFe]-hydrogenases are arguably the most profound case of convergent evolution. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

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.

Redox and ATP control of photosynthetic cyclic electron flow in Chlamydomonas reinhardtii: (II) involvement of the PGR5-PGRL1 pathway under anaerobic conditions

In oxygenic photosynthesis, cyclic electron flow around photosystem I denotes the recycling of electrons from stromal electron carriers (reduced nicotinamide adenine dinucleotide phosphate, NADPH, ferredoxin) towards the plastoquinone pool. Whether or not cyclic electron flow operates similarly in Chlamydomonas and plants has been a matter of debate. Here we would like to emphasize that despite the regulatory or metabolic differences that may exist between green algae and plants, the general mechanism of cyclic electron flow seems conserved across species. The most accurate way to describe cyclic electron flow remains to be a redox equilibration model, while the supramolecular reorganization of the thylakoid membrane (state transitions) has little impact on the maximal rate of cyclic electron flow. The maximum capacity of the cyclic pathways is shown to be around 60 electrons transferred per photosystem per second, which is in Chlamydomonas cells treated with 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and placed under anoxic conditions. Part I of this work (aerobic conditions) was published in a previous issue of BBA-Bioenergetics (vol. 1797, pp. 44-51) (Alric et al., 2010).