Solar hydrogen-producing bionanodevice outperforms natural photosynthesis

Novel concepts and engineering strategies for heterologous expression of efficient hydrogenases in photosynthetic microorganisms

Hydrogen is considered one of the key enablers of the transition towards a sustainable and net-zero carbon economy. When produced from renewable sources, hydrogen can be used as a clean and carbon-free energy carrier, as well as improve the sustainability of a wide range of industrial processes. Photobiological hydrogen production is considered one of the most promising technologies, avoiding the need for renewable electricity and rare earth metal elements, the demands for which are greatly increasing due to the current simultaneous electrification and decarbonization goals. Photobiological hydrogen production employs photosynthetic microorganisms to harvest solar energy and split water into molecular oxygen and hydrogen gas, unlocking the long-pursued target of solar energy storage. However, photobiological hydrogen production has to-date been constrained by several limitations. This review aims to discuss the current state-of-the art regarding hydrogenase-driven photobiological hydrogen production. Emphasis is placed on engineering strategies for the expression of improved, non-native, hydrogenases or photosynthesis re-engineering, as well as their combination as one of the most promising pathways to develop viable large-scale hydrogen green cell factories. Herein we provide an overview of the current knowledge and technological gaps curbing the development of photobiological hydrogenase-driven hydrogen production, as well as summarizing the recent advances and future prospects regarding the expression of non-native hydrogenases in cyanobacteria and green algae with an emphasis on [FeFe] hydrogenases.

Solar water splitting Pt-nanoparticle photosystem I thylakoid systems: Catalyst identification, location and oligomeric structure

Photosynthetic conversion of light energy into chemical energy occurs in sheet-like membrane-bound compartments called thylakoids and is mediated by large integral membrane protein-pigment complexes called reaction centers (RCs). Oxygenic photosynthesis of higher plants, cyanobacteria and algae requires the symbiotic linking of two RCs, photosystem II (PSII) and photosystem I (PSI), to split water and assimilate carbon dioxide. Worldwide there is a large research investment in developing RC-based hybrids that utilize the highly evolved solar energy conversion capabilities of RCs to power catalytic reactions for solar fuel generation. Of particular interest is the solar-powered production of H₂, a clean and renewable energy source that can replace carbon-based fossil fuels and help provide for ever-increasing global energy demands. Recently, we developed thylakoid membrane hybrids with abiotic catalysts and demonstrated that photosynthetic Z-scheme electron flow from the light-driven water oxidation at PSII can drive H₂ production from PSI. One of these hybrid systems was created by self-assembling Pt-nanoparticles (PtNPs) with the stromal subunits of PSI that extend beyond the membrane plane in both spinach and cyanobacterial thylakoids. Using PtNPs as site-specific probe molecules, we report the electron microscopic (EM) imaging of oligomeric structure, location and organization of PSI in thylakoid membranes and provide the first direct visualization of photosynthetic Z-scheme solar water-splitting biohybrids for clean H₂ production.

Proximity Labeling Facilitates Defining the Proteome Neighborhood of Photosystem II Oxygen Evolution Complex in a Model Cyanobacterium

Ascorbate peroxidase (APEX)-based proximity labeling coupled with mass spectrometry has a great potential for spatiotemporal identification of proteins proximal to a protein complex of interest. Using this approach is feasible to define the proteome neighborhood of important protein complexes in a popular photosynthetic model cyanobacterium Synechocystis sp. PCC6803 (hereafter named as Synechocystis). To this end, we developed a robust workflow for APEX2-based proximity labeling in Synechocystis and used the workflow to identify proteins proximal to the photosystem II (PS II) oxygen evolution complex (OEC) through fusion APEX2 with a luminal OEC subunit, PsbO. In total, 38 integral membrane proteins (IMPs) and 93 luminal proteins were identified as proximal to the OEC. A significant portion of these proteins are involved in PS II assembly, maturation, and repair, while the majority of the rest were not previously implicated with PS II. The IMPs include subunits of PS II and cytochrome b₆/f, but not of photosystem I (except for PsaL) and ATP synthases, suggesting that the latter two complexes are spatially separated from the OEC with a distance longer than the APEX2 labeling radius. Besides, the topologies of six IMPs were successfully predicted because their lumen-facing regions exclusively contain potential APEX2 labeling sites. The luminal proteins include 66 proteins with a predicted signal peptide and 57 proteins localized also in periplasm, providing important targets to study the regulation and selectivity of protein translocation. Together, we not only developed a robust workflow for the application of APEX2-based proximity labeling in Synechocystis and showcased the feasibility to define the neighborhood proteome of an important protein complex with a short radius but also discovered a set of the proteins that potentially interact with and regulate PS II structure and function.

Advances and challenges in photosynthetic hydrogen production

The vision to replace coal with hydrogen goes back to Jules Verne in 1874. However, sustainable hydrogen production remains challenging. The most elegant approach is to utilize photosynthesis for water splitting and to subsequently save solar energy as hydrogen. Cyanobacteria and green algae are unicellular photosynthetic organisms that contain hydrogenases and thereby possess the enzymatic equipment for photosynthetic hydrogen production. These features of cyanobacteria and algae have inspired artificial and semi-artificial in vitro techniques, that connect photoexcited materials or enzymes with hydrogenases or mimics of these for hydrogen production. These in vitro methods have on their part been models for the fusion of cyanobacterial and algal hydrogenases to photosynthetic photosystem I (PSI) in vivo, which recently succeeded as proofs of principle.

Biophotoelectrochemistry for renewable energy and environmental applications

Biophotoelectrochemistry (BPEC) is an interdisciplinary research field and combines bioelectrochemistry and photoelectrochemistry through the utilization of the catalytic abilities of biomachineries and light harvesters to accomplish the production of energy or chemicals driven by solar energy. The BPEC process may act as a new approach for sustainable green chemistry and waste minimization. This review provides the state-of-the-art introduction of BPEC basics and systems, with a focus on light harvesters and biocatalysts, configurations, photoelectron transfer mechanisms, and the potential applications in energy and environment. Several examples of BPEC applications are discussed including H2 production, CO2 reduction, chemical synthesis, pollution control, and biogeochemical cycle of elements. The challenges about BPEC systems are identified and potential solutions are proposed. The review aims to encourage further research of BPEC toward development of practical BPEC systems for energy and environmental applications.

Development of a universal conductive platform for anchoring photo- and electroactive proteins using organometallic terpyridine molecular wires

The construction of an efficient conductive interface between electrodes and electroactive proteins is a major challenge in the biosensor and bioelectrochemistry fields to achieve the desired nanodevice performance. Concomitantly, metallo-organic terpyridine wires have been extensively studied for their great ability to mediate electron transfer over a long-range distance. In this study, we report a novel stepwise bottom-up approach for assembling bioelectrodes based on a genetically modified model electroactive protein, cytochrome c553 (cyt c553) and an organometallic terpyridine (TPY) molecular wire self-assembled monolayer (SAM). Efficient anchoring of the TPY derivative (TPY-PO(OH)2) onto the ITO surface was achieved by optimising solvent composition. Uniform surface coverage with the electroactive protein was achieved by binding the cyt c553 molecules via the C-terminal His6-tag to the modified TPY macromolecules containing Earth abundant metallic redox centres. Photoelectrochemical characterisation demonstrates the crucial importance of the metal redox centre for the determination of the desired electron transfer properties between cyt and the ITO electrode. Even without the cyt protein, the ITO-TPY nanosystem reported here generates photocurrents whose densities are 2-fold higher that those reported earlier for ITO electrodes functionalised with the photoactive proteins such as photosystem I in the presence of an external mediator, and 30-fold higher than that of the pristine ITO. The universal chemical platform for anchoring and nanostructuring of (photo)electroactive proteins reported in this study provides a major advancement for the construction of efficient (bio)molecular systems requiring a high degree of precise supramolecular organisation as well as efficient charge transfer between (photo)redox-active molecular components and various types of electrode materials.

Protection of Oxygen-Sensitive Enzymes by Peptide Hydrogel

Molecular oxygen (O₂) is a highly reactive oxidizing agent and is harmful to many biological and industrial systems. Although O₂ often interacts via metals or reducing agents, a binding mechanism involving an organic supramolecular structure has not been described to date. In this work, the prominent dipeptide hydrogelator fluorenylmethyloxycarbonyl-diphenylalanine is shown to encage O₂ and significantly limit its diffusion and penetration through the hydrogel. Molecular dynamics simulations suggested that the O₂ binding mechanism is governed by pockets formed between the aromatic rings in the supramolecular structure of the gel, which bind O₂ through hydrophobic interactions. This phenomenon is harnessed to maintain the activity of the O₂-hypersensitive enzyme [FeFe]-hydrogenase, which holds promising potential for utilizing hydrogen gas for sustainable energy applications. Hydrogenase encapsulation within the gel allows hydrogen production following exposure to ambient O₂. This phenomenon may lead to utilization of this low molecular weight gelator in a wide range of O₂-sensitive applications.

Closing the Gap for Electronic Short-Circuiting: Photosystem I Mixed Monolayers Enable Improved Anisotropic Electron Flow in Biophotovoltaic Devices

Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi-artificial energy conversion devices, capable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short-circuiting processes that typically limit the performance of PSI-based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]-hydrogenase confirms the possibility to realize light-induced H2 evolution.

In-vivo quantification of electron flow through photosystem I - Cyclic electron transport makes up about 35% in a cyanobacterium

Photosynthetic electron flow, driven by photosystem I and II, provides chemical energy for carbon fixation. In addition to a linear mode a second cyclic route exists, which only involves photosystem I. The exact contributions of linear and cyclic transport are still a matter of debate. Here, we describe the development of a method that allows quantification of electron flow in absolute terms through photosystem I in a photosynthetic organism for the first time. Specific in-vivo protocols allowed to discern the redox states of plastocyanin, P700 and the FeS-clusters including ferredoxin at the acceptor site of PSI in the cyanobacterium Synechocystis sp. PCC 6803 with the near-infrared spectrometer Dual-KLAS/NIR. P700 absorbance changes determined with the Dual-KLAS/NIR correlated linearly with direct determinations of PSI concentrations using EPR. Dark-interval relaxation kinetics measurements (DIRK_PSI) were applied to determine electron flow through PSI. Counting electrons from hydrogen oxidation as electron donor to photosystem I in parallel to DIRK_PSI measurements confirmed the validity of the method. Electron flow determination by classical PSI yield measurements overestimates electron flow at low light intensities and saturates earlier compared to DIRK_PSI. Combination of DIRK_PSI with oxygen evolution measurements yielded a proportion of 35% of surplus electrons passing PSI compared to PSII. We attribute these electrons to cyclic electron transport, which is twice as high as assumed for plants. Counting electrons flowing through the photosystems allowed determination of the number of quanta required for photosynthesis to 11 per oxygen produced, which is close to published values.

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.

Energy conservation in photosynthetic microorganisms

Photosynthesis is a biological process of energy conversion from solar radiation to useful organic compounds for the photosynthetic organisms themselves. It, thereby, also plays a role of food production for almost all animals on the Earth. The utilization of photosynthesis as an artificial carbon cycle is also attracting a lot of attention regarding its benefits for human life. Hydrogen and biofuels, obtained from photosynthetic microorganisms, such as microalgae and cyanobacteria, will be promising products as energy and material resources. Considering that the efficiency of bioenergy production is insufficient to replace fossil fuels at present, techniques for the industrial utilization of photosynthesis processes need to be developed intensively. Increase in the efficiency of photosynthesis, the yields of target substances, and the growth rates of algae and cyanobacteria must be subjects for efficient industrialization. Here, we overview the whole aspect of the energy production from photosynthesis to biomass production of various photosynthetic microorganisms.

Opportunities toward hydrogen production biotechnologies

Hydrogen is already a major commodity and process intermediate for fertilizer production, petroleum processing, and chemical synthesis. It also offers unrealized potential for energy storage. While biological production offers an expandable and sustainable source, enthusiasm has been dampened by slow research progress. Also, the very low cost of natural gas (the major current hydrogen source) imposes severe economic challenges. This discussion describes process, metabolic, and protein engineering opportunities toward cost-effective biohydrogen production. Recent progress in hydrogenase engineering and photosynthetic bacterial research now suggests a favorable risk versus reward opportunity. Although the risks are still significant, successful technologies would provide important components in an integrated energy portfolio that enables global sustainability.

Generating dihydrogen by tethering an [FeFe]hydrogenase via a molecular wire to the A1A/A1B sites of photosystem I

Photosystem I complexes from the menB deletion mutant of Synechocystis sp. PCC 6803 were previously wired to a Pt nanoparticle via a molecular wire consisting of 15-(3-methyl-1,4-naphthoquinone-2-yl)]pentadecyl sulfide. In the presence of a sacrificial electron donor and an electron transport mediator, the PS I-NQ(CH₂)₁₅S-Pt nanoconstruct generated dihydrogen at a rate of 44.3 µmol of H₂ mg Chl^-1 h^-1 during illumination at pH 8.3. The menB deletion strain contains an interruption in the biosynthetic pathway of phylloquinone, which results in the presence of a displaceable plastoquinone-9 in the A_1A/A_1B sites. The synthesized quinone contains a headgroup identical to the native phylloquinone along with a 15-carbon long tail that is terminated in a thiol. The thiol on the molecular wire is used to bind the Pt nanoparticle. In this short communication, we replaced the Pt nanoparticle with an [FeFe]H₂ase variant from Clostridium acetobutylicum that contains an exposed iron on the distal [4Fe-4S] cluster afforded by mutating the surface exposed Cys97 residue to Gly. The thiol on the molecular wire is then used to coordinate the corner iron atom of the iron-sulfur cluster. When all three components are combined and illuminated in the presence of a sacrificial electron donor and an electron transport mediator, the PS I-NQ(CH₂)₁₅S-[FeFe]H₂ase nanoconstruct generated dihydrogen at a rate of 50.3 ± 9.96 μmol of H₂ mg Chl^-1 h^-1 during illumination at pH 8.3. This successful in vitro experiment sets the stage for assembling a PS I-NQ(CH₂)₁₅S-[FeFe]H₂ase nanoconstruct in vivo in the menB mutant of Synechocystis sp. PCC 6803.

Designing a modified clostridial 2[4Fe-4S] ferredoxin as a redox coupler to directly link photosystem I with a Pt nanoparticle

A methodology previously developed in our laboratory utilized an aliphatic hydrocarbon terminated by thiol groups to tether two redox proteins, i.e., the [4Fe-4S] cluster F_B of photosystem I (PS I) and the distal [4Fe-4S] cluster of a [FeFe]-hydrogenase, to create a biohybrid dihydrogen-generating complex. These studies guided the design of a modified 2[4Fe-4S] cluster ferredoxin from Clostridium pasteurianum (CpFd) containing two externally facing cysteine residues in close proximity to each [4Fe-4S] cluster that replaces the aliphatic hydrocarbon dithiol tether. The advantage of using a protein is the potential to create a coupled dihydrogen-generating system in vivo. The wild-type CpFd_WT and variants CpFd_S11C/D40C, CpFd_P20C/P49C, CpFd_D7S/D36S, CpFd_{S11C/D40C/D7S/D36S} and CpFd_{P20C/P49C/D7S/D36S} were expressed in Escherichia coli and found to contain ~ 8 Fe and ~ 8 S atoms. The absorption spectra of the wild-type and CpFd variants displayed a peak centered at ~ 390 nm characteristic of a S → Fe charge transfer band that diminishes upon reduction with Na-dithionite. Low-temperature X-band EPR studies of the Na-dithionite-reduced wild-type and CpFd variants showed a complex spectrum indicative of two magnetically coupled [4Fe-4S]¹⁺ clusters. EPR-monitored redox titrations of CpFd_WT, CpFd_D7S/D36S, CpFd_S11C/D40C, CpFd_P20C/P49C, CpFd_{S11C/D40C/D7S/D36S} and CpFd_{P20C/P49C/D7S/D36S} revealed redox potentials of - 412 ± 8 mV, - 395 ± 4 mV, - 408 ± 7 mV, - 426 ± 11 mV, - 384 ± 4 mV and - 423 ± 4 mV, respectively. The in vitro PS I-CpFd_{S11C/D40C/D7S/D36S}-Pt nanoparticle complex was the highest performer, generating dihydrogen at a rate of 3.25 μmol H₂ mg Chl^-1 h^-1 or 278.8 mol H₂ mol PS I^-1 h^-1 under continuous illumination.

Interprotein electron transfer biohybrid system for photocatalytic H2 production

Worldwide there is a large research investment in developing solar fuel systems as clean and sustainable sources of energy. The fundamental mechanisms of natural photosynthesis can provide a source of inspiration for these studies. Photosynthetic reaction center (RC) proteins capture and convert light energy into chemical energy that is ultimately used to drive oxygenic water-splitting and carbon fixation. For the light energy to be used, the RC communicates with other donor/acceptor components via a sophisticated electron transfer scheme that includes electron transfer reactions between soluble and membrane bound proteins. Herein, we reengineer an inherent interprotein electron transfer pathway in a natural photosynthetic system to make it photocatalytic for aqueous H₂ production. The native electron shuttle protein ferredoxin (Fd) is used as a scaffold for binding of a ruthenium photosensitizer and H₂ catalytic function is imparted to its partner protein, ferredoxin-NADP⁺-reductase (FNR), by attachment of cobaloxime molecules. We find that this 2-protein biohybrid system produces H₂ in aqueous solutions via light-induced interprotein electron transfer reactions (TON > 2500 H₂/FNR), providing insight about using native protein-protein interactions as a method for fuel generation.

Semi-biological approaches to solar-to-chemical conversion

This review provides an overview of the cross-disciplinary field of semi-artificial photosynthesis, which combines strengths of biocatalysis and artificial photosynthesis to develop new concepts and approaches for solar-to-chemical conversion.

Photocatalytic Hydrogen Generation by Vesicle-Embedded [FeFe]Hydrogenase Mimics: A Mechanistic Study

Artificial photosynthesis—the direct photochemical generation of hydrogen from water—is a promising but scientifically challenging future technology. Because nature employs membranes for photodriven reactions, the aim of this work is to elucidate the effect of membranes on artificial photocatalysis. To do so, a combination of electrochemistry, photocatalysis, and time‐resolved spectroscopy on vesicle‐embedded [FeFe]hydrogenase mimics, driven by a ruthenium tris‐2,2′‐bipyridine photosensitizer, is reported. The membrane effects encountered can be summarized as follows: the presence of vesicles steers the reactivity of the [FeFe]‐benzodithiolate catalyst towards disproportionation, instead of protonation, due to membrane characteristics, such as providing a constant local effective pH, and concentrating and organizing species inside the membrane. The maximum turnover number is limited by photodegradation of the resting state in the catalytic cycle. Understanding these fundamental productive and destructive pathways in complex photochemical systems allows progress towards the development of efficient artificial leaves.

Genetic Engineering, Synthetic Biology and the Light Reactions of Photosynthesis

Applications of synthetic biology to photosynthesis currently range from exchanging photosynthetic proteins to the utilization of photosynthesis as a source of electrons for entirely unrelated reactions.

Z-scheme solar water splitting via self-assembly of photosystem I-catalyst hybrids in thylakoid membranes

Nature's solar energy converters, the Photosystem I (PSI) and Photosystem II (PSII) reaction center proteins, flawlessly manage photon capture and conversion processes in plants, algae, and cyanobacteria to drive oxygenic water-splitting and carbon fixation. Herein, we utilize the native photosynthetic Z-scheme electron transport chain to drive hydrogen production from thylakoid membranes by directional electron transport to abiotic catalysts bound at the stromal end of PSI. Pt-nanoparticles readily self-assemble with PSI in spinach and cyanobacterial membranes as evidenced by light-driven H2 production in the presence of a mediating electron shuttle protein and the sacrificial electron donor sodium ascorbate. EPR characterization confirms placement of the Pt-nanoparticles on the acceptor end of PSI. In the absence of sacrificial reductant, H2 production at PSI occurs via coupling to light-induced PSII O2 evolution as confirmed by correlation of catalytic activity to the presence or absence of the PSII inhibitor DCMU. To create a more sustainable system, first-row transition metal molecular cobaloxime and nickel diphosphine catalysts were found to perform photocatalysis when bound in situ to cyanobacterial thylakoid membranes. Thus, the self-assembly of abiotic catalysts with photosynthetic membranes demonstrates a tenable method for accomplishing solar overall water splitting to generate H2, a renewable and clean fuel. This work benchmarks a significant advance toward improving photosynthetic efficiency for solar fuel production.

Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the "green" process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme's active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel-forming reactions (e.g., H₂ evolution, CO₂ reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer-Villiger oxidation). This Review provides an overview of recent advances in light-driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.

Insights into the binding behavior of native and non-native cytochromes to photosystem I from Thermosynechococcus elongatus

The binding of photosystem I (PS I) from Thermosynechococcus elongatus to the native cytochrome (cyt) c₆ and cyt c from horse heart (cyt c_HH) was analyzed by oxygen consumption measurements, isothermal titration calorimetry (ITC), and rigid body docking combined with electrostatic computations of binding energies. Although PS I has a higher affinity for cyt c_HH than for cyt c₆, the influence of ionic strength and pH on binding is different in the two cases. ITC and theoretical computations revealed the existence of unspecific binding sites for cyt c_HH besides one specific binding site close to P₇₀₀ Binding to PS I was found to be the same for reduced and oxidized cyt c_HH Based on this information, suitable conditions for cocrystallization of cyt c_HH with PS I were found, resulting in crystals with a PS I:cyt c_HH ratio of 1:1. A crystal structure at 3.4-Å resolution was obtained, but cyt c_HH cannot be identified in the electron density map because of unspecific binding sites and/or high flexibility at the specific binding site. Modeling the binding of cyt c₆ to PS I revealed a specific binding site where the distance and orientation of cyt c₆ relative to P₇₀₀ are comparable with cyt c₂ from purple bacteria relative to P₈₇₀ This work provides new insights into the binding modes of different cytochromes to PS I, thus facilitating steps toward solving the PS I-cyt c costructure and a more detailed understanding of natural electron transport processes.

In-vivo turnover frequency of the cyanobacterial NiFe-hydrogenase during photohydrogen production outperforms in-vitro systems

Cyanobacteria provide all components for sunlight driven biohydrogen production. Their bidirectional NiFe-hydrogenase is resistant against low levels of oxygen with a preference for hydrogen evolution. However, until now it was unclear if its catalytic efficiency can keep pace with the photosynthetic electron transfer rate. We identified NikKLMQO (sll0381-sll0385) as a nickel transporter, which is required for hydrogen production. ICP-MS measurements were used to quantify hydrogenase molecules per cell. We found 400 to 2000 hydrogenase molecules per cell depending on the conditions. In-vivo turnover frequencies of the enzyme ranged from 62 H2/s in the wild type to 120 H2/s in a mutant during photohydrogen production. These frequencies are above maximum in-vivo photosynthetic electron transfer rates of 47 e-/s (equivalent to 24 H2/s). They are also above those of existing in-vitro systems working with unlimited electron supply and show that in-vivo photohydrogen production is limited by electron delivery to the enzyme.

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.

Translating Divergent Environmental Stresses into a Common Proteome Response through the Histidine Kinase 33 (Hik33) in a Model Cyanobacterium

The histidine kinase Hik33 plays important roles in mediating cyanobacterial response to divergent types of abiotic stresses including cold, salt, high light (HL), and osmotic stresses. However, how these functions are regulated by Hik33 remains to be addressed. Using a hik33-deficient strain (Δhik33) of Synechocystis sp. PCC 6803 (Synechocystis) and quantitative proteomics, we found that Hik33 depletion induces differential protein expression highly like that induced by divergent types of stresses. This typically includes downregulation of proteins in photosynthesis and carbon assimilation that are necessary for cell propagation, and upregulation of heat shock proteins, chaperons, and proteases that are important for cell survival. This observation indicates that depletion of Hik33 alone mimics divergent types of abiotic stresses, and that Hik33 could be important for preventing abnormal stress response in the normal condition. Moreover, we found most proteins of plasmid origin were significantly upregulated in Δhik33, though their biological significance remains to be addressed. Together, the systematically characterized Hik33-regulated cyanobacterial proteome, which is largely involved in stress responses, builds the molecular basis for Hik33 as a general regulator of stress responses.

Plasmon-Enhanced Photocurrent from Photosystem I Assembled on Ag Nanopyramids

Plasmonic metal nanostructures have been known to tune optoelectronic properties of fluorophores. Here, we report the first-ever experimental observation of plasmon-induced photocurrent enhancements from Photosystem I (PSI) immobilized on Fischer patterns of silver nanopyramids (Ag-NP). To this end, the plasmonic peaks of Ag-NP were tuned to match the PSI absorption peaks at ∼450 and ∼680 nm wavelengths. Specifically, the plasmon-enhanced photocurrents indicate enhancement factors of ∼6.5 and ∼5.8 as compared to PSI assembly on planar Ag substrates for nominal excitation wavelengths of 660 and 470 nm, respectively. The comparable enhancement factors from both 470 and 660 nm excitations, in spite of a significantly weaker plasmon absorption peak at ∼450 nm for the Ag-NP structures, can be rationalized by previously reported excessive plasmon-induced fluorescence emission losses from PSI in the red region as compared to the blue region of the excitation wavelengths.

Zeaxanthin-dependent nonphotochemical quenching does not occur in photosystem I in the higher plant Arabidopsis thaliana

Nonphotochemical quenching (NPQ) is the process that protects the photosynthetic apparatus of plants and algae from photodamage by dissipating as heat the energy absorbed in excess. Studies on NPQ have almost exclusively focused on photosystem II (PSII), as it was believed that NPQ does not occur in photosystem I (PSI). Recently, Ballottari et al. [Ballottari M, et al. (2014) Proc Natl Acad Sci USA 111:E2431-E2438], analyzing PSI particles isolated from an Arabidopsis thaliana mutant that accumulates zeaxanthin constitutively, have reported that this xanthophyll can efficiently induce chlorophyll fluorescence quenching in PSI. In this work, we have checked the biological relevance of this finding by analyzing WT plants under high-light stress conditions. By performing time-resolved fluorescence measurements on PSI isolated from Arabidopsis thaliana WT in dark-adapted and high-light-stressed (NPQ) states, we find that the fluorescence kinetics of both PSI are nearly identical. To validate this result in vivo, we have measured the kinetics of PSI directly on leaves in unquenched and NPQ states; again, no differences were observed. It is concluded that PSI does not undergo NPQ in biologically relevant conditions in Arabidopsis thaliana The possible role of zeaxanthin in PSI photoprotection is discussed.

Enzymes as modular catalysts for redox half-reactions in H2-powered chemical synthesis: from biology to technology

The present study considers the ways in which redox enzyme modules are coupled in living cells for linking reductive and oxidative half-reactions, and then reviews examples in which this concept can be exploited technologically in applications of coupled enzyme pairs. We discuss many examples in which enzymes are interfaced with electronically conductive particles to build up heterogeneous catalytic systems in an approach which could be termed synthetic biochemistry We focus on reactions involving the H+/H2 redox couple catalysed by NiFe hydrogenase moieties in conjunction with other biocatalysed reactions to assemble systems directed towards synthesis of specialised chemicals, chemical building blocks or bio-derived fuel molecules. We review our work in which this approach is applied in designing enzyme-modified particles for H2-driven recycling of the nicotinamide cofactor NADH to provide a clean cofactor source for applications of NADH-dependent enzymes in chemical synthesis, presenting a combination of published and new work on these systems. We also consider related photobiocatalytic approaches for light-driven production of chemicals or H2 as a fuel. We emphasise the techniques available for understanding detailed catalytic properties of the enzymes responsible for individual redox half-reactions, and the importance of a fundamental understanding of the enzyme characteristics in enabling effective applications of redox biocatalysis.

Trophic Mode-Dependent Proteomic Analysis Reveals Functional Significance of Light-Independent Chlorophyll Synthesis in Synechocystis sp. PCC 6803

The photosynthetic model organism Synechocystis sp. PCC 6803 can grow in different trophic modes, depending on the availability of light and exogenous organic carbon source. However, how the protein profile changes to facilitate the cells differentially propagate in different modes has not been comprehensively investigated. Using isobaric labeling-based quantitative proteomics, we simultaneously identified and quantified 45% Synechocystis proteome across four different trophic modes, i.e., autotrophic, heterotrophic, photoheterotrophic, and mixotrophic modes. Among the 155 proteins that are differentially expressed across four trophic modes, proteins involved in nitrogen assimilation and light-independent chlorophyll synthesis are dramatically upregulated in the mixotrophic mode, concomitant with a dramatic increase of P_II phosphorylation that senses carbon and nitrogen assimilation status. Moreover, functional study using a mutant defective in light-independent chlorophyll synthesis revealed that this pathway is important for chlorophyll accumulation under a cycled light/dark illumination regime, a condition mimicking day/night cycles in certain natural habitats. Collectively, these results provide the most comprehensive information on trophic mode-dependent protein expression in cyanobacterium, and reveal the functional significance of light-independent chlorophyll synthesis in trophic growth.

In vitro kinetics of P700+ reduction of Thermosynechococcus elongatus trimeric Photosystem I complexes by recombinant cytochrome c 6 using a Joliot-type LED spectrophotometer

The reduction rate of photo-oxidized Photosystem I (PSI) with various natural and artificial electron donors have been well studied by transient absorption spectroscopy. The electron transfer rate from various donors to P₇₀₀⁺ has been measured for a wide range of photosynthetic organisms encompassing cyanobacteria, algae, and plants. PSI can be a limiting component due to tedious extraction and purification methods required for this membrane protein. In this report, we have determined the in vivo, intracellular cytochrome c ₆ (cyt c ₆)/PSI ratio in Thermosynechococcus elongatus (T.e.) using quantitative Western blot analysis. This information permitted the determination of P₇₀₀⁺ reduction kinetics via recombinant cyt c ₆ in a physiologically relevant ratio (cyt c ₆: PSI) with a Joliot-type, LED-driven, pump-probe spectrophotometer. Dilute PSI samples were tested under varying cyt c ₆ concentration, temperature, pH, and ionic strength, each of which shows similar trends to the reported literature utilizing much higher PSI concentrations with laser-based spectrophotometer. Our results do however indicate kinetic differences between actinic light sources (laser vs. LED), and we have attempted to resolve these effects by varying our LED light intensity and duration. The standardized configuration of this spectrophotometer will also allow a more uniform kinetic analysis of samples in different laboratories. We can conclude that our findings from the LED-based system display an added total protein concentration effect due to multiple turnover events of P₇₀₀⁺ reduction by cyt c ₆ during the longer illumination regime.

Hybrid bio-photo-electro-chemical cells for solar water splitting

Photoelectrochemical water splitting uses solar power to decompose water to hydrogen and oxygen. Here we show how the photocatalytic activity of thylakoid membranes leads to overall water splitting in a bio-photo-electro-chemical (BPEC) cell via a simple process. Thylakoids extracted from spinach are introduced into a BPEC cell containing buffer solution with ferricyanide. Upon solar-simulated illumination, water oxidation takes place and electrons are shuttled by the ferri/ferrocyanide redox couple from the thylakoids to a transparent electrode serving as the anode, yielding a photocurrent density of 0.5 mA cm⁻². Hydrogen evolution occurs at the cathode at a bias as low as 0.8 V. A tandem cell comprising the BPEC cell and a Si photovoltaic module achieves overall water splitting with solar to hydrogen efficiency of 0.3%. These results demonstrate the promise of combining natural photosynthetic membranes and man-made photovoltaic cells in order to convert solar power into hydrogen fuel.

Photoelectrochemical water splitting uses solar power to decompose water to hydrogen and oxygen. Here, the authors integrate thylakoid membranes extracted from spinach into a bio-photo-electro-chemical cell capable of overall water splitting without the need for any sacrificial reagents.

Oxygen-Evolving Porous Glass Plates Containing the Photosynthetic Photosystem II Pigment-Protein Complex

The development of artificial photosynthesis has focused on the efficient coupling of reaction at photoanode and cathode, wherein the production of hydrogen (or energy carriers) is coupled to the electrons derived from water-splitting reactions. The natural photosystem II (PSII) complex splits water efficiently using light energy. The PSII complex is a large pigment-protein complex (20 nm in diameter) containing a manganese cluster. A new photoanodic device was constructed incorporating stable PSII purified from a cyanobacterium Thermosynechococcus vulcanus through immobilization within 20 or 50 nm nanopores contained in porous glass plates (PGPs). PSII in the nanopores retained its native structure and high photoinduced water splitting activity. The photocatalytic rate (turnover frequency) of PSII in PGP was enhanced 11-fold compared to that in solution, yielding a rate of 50-300 mol e(-)/(mol PSII·s) with 2,6-dichloroindophenol (DCIP) as an electron acceptor. The PGP system realized high local concentrations of PSII and DCIP to enhance the collisional reactions in nanotubes with low disturbance of light penetration. The system allows direct visualization/determination of the reaction inside the nanotubes, which contributes to optimize the local reaction condition. The PSII/PGP device will substantively contribute to the construction of artificial photosynthesis using water as the ultimate electron source.

Challenges and opportunities for hydrogen production from microalgae

The global population is predicted to increase from ~7.3 billion to over 9 billion people by 2050. Together with rising economic growth, this is forecast to result in a 50% increase in fuel demand, which will have to be met while reducing carbon dioxide (CO2 ) emissions by 50-80% to maintain social, political, energy and climate security. This tension between rising fuel demand and the requirement for rapid global decarbonization highlights the need to fast-track the coordinated development and deployment of efficient cost-effective renewable technologies for the production of CO2 neutral energy. Currently, only 20% of global energy is provided as electricity, while 80% is provided as fuel. Hydrogen (H2 ) is the most advanced CO2 -free fuel and provides a 'common' energy currency as it can be produced via a range of renewable technologies, including photovoltaic (PV), wind, wave and biological systems such as microalgae, to power the next generation of H2 fuel cells. Microalgae production systems for carbon-based fuel (oil and ethanol) are now at the demonstration scale. This review focuses on evaluating the potential of microalgal technologies for the commercial production of solar-driven H2 from water. It summarizes key global technology drivers, the potential and theoretical limits of microalgal H2 production systems, emerging strategies to engineer next-generation systems and how these fit into an evolving H2 economy.