Advances and challenges in photosynthetic hydrogen production

Co-expression of auxiliary genes enhances the activity of a heterologous O2-tolerant hydrogenase in the cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria bear great biotechnological potential as photosynthetic cell factories. In particular, hydrogenases are promising with respect to light-driven H2 production as well as H2-driven redox biocatalysis. Their utilization relies on effective strain design as well as a balanced synthesis and maturation of heterologous enzymes. In a previous study, the soluble O2-tolerant hydrogenase complex from Cupriavidus necator (CnSH) could be introduced into the model cyanobacterium Synechocystis sp. PCC 6803. Due to its O2-tolerance, it was indeed active under photoautotrophic growth conditions. However, the specific activity was rather low indicating that further engineering is required, for which we followed a two-step approach. First, we optimized the CnSH multigene expression in Synechocystis by applying different regulatory elements. Although corresponding protein levels and specific CnSH activity increased, the apparent rise in enzyme levels did not fully translate into activity increase. Second, the entire set of hyp genes, encoding CnSH maturases, was co-expressed in Synechocystis to investigate, if CnSH maturation was limiting. Indeed, the native CnSH maturation apparatus promoted functional CnSH synthesis, enabling a threefold higher H2 oxidation activity compared to the parental strain. Our results suggest that a fine balance between heterologous hydrogenase and maturase expression is required to ensure high specific activity over an extended time period.

Chromosome-level genome provides novel insights into the starch metabolism regulation and evolutionary history of Tetraselmis helgolandica

INTRODUCTION

Tetraselmis helgolandica is a marine microalga belonging to the Chlorophyta phylum. It is widely distributed in the coastal waters of Asia and is commonly used as aquatic feed. T. helgolandica is characterized by its large size, preference for starch accumulation, low temperature tolerance, presence of flagella, and strong motility. However, research on T. helgolandica is limited, and its genome data remains unavailable.

OBJECTIVE

We generated a high-quality, chromosome-scale genome of T. helgolandica. Through comparative genomics, we uncovered the genome characteristics and evolutionary history of T. helgolandica. Additionally, by integrating transcriptome data, we elucidated how the light-dark rhythm enhances the high starch production.

METHODS

We utilized long-read sequencing data and high-throughput chromosome conformation capture data from the Oxford Nanopore platform to construct a high-quality genome of T. helgolandica. Genome annotation was performed using multiple databases, and comparative genomic analysis was conducted with nine species, including Arabidopsis thaliana, to reveal the evolutionary history. Finally, we combined transcriptome data to elucidate the molecular mechanisms underlying the high starch yield.

RESULTS

Circadian rhythm significantly promote starch accumulation and increase amylose content. The chromosome-scale genome revealed it shares a common ancestor with other green algae approximately 1,017 million years ago. This relatively ancient divergence underscores its evolutionary distinction within the green lineage. It may possess a more complex protein modification mechanism and a more fully developed Golgi apparatus. Circadian rhythm broadly up-regulates key enzymes involved in starch synthesis, including GBSS and Starch Synthase, while down-regulating SS IIIa. This regulation enhances starch accumulation and increases the amylose content.

CONCLUSION

This study provided a high-quality genome of T. helgolandica and revealed the potential mechanism by which the circadian rhythm promotes starch accumulation and increases the amylose ratio. The genome of T. helgolandica will serve as an important resource for evolutionary research and transgenic platform development.

Outlook on Synthetic Biology-Driven Hydrogen Production: Lessons from Algal Photosynthesis Applied to Cyanobacteria

Photobiological hydrogen production offers a sustainable route to clean energy by harnessing solar energy through photosynthetic microorganisms. The pioneering sulfur-deprivation technique developed by Melis and colleagues in the green alga Chlamydomonas reinhardtii successfully enabled sustained hydrogen production by downregulating photosystem II (PSII) activity to reduce oxygen evolution, creating anaerobic conditions necessary for hydrogenase activity. Inspired by this approach, we present the project of the European consortium PhotoSynH2, which builds on these biological insights and employs synthetic biology to replicate and enhance this strategy in cyanobacteria, specifically, Synechocystis sp. PCC 6803. By genetically engineering precise downregulation of PSII, we aim to reduce oxygen evolution without the unintended effects associated with nutrient deprivation, enabling efficient hydrogen production. Additionally, re-engineering endogenous respiration to continuously replenish glycogen consumed during respiration allows matching oxygen production with consumption, maintaining anaerobic conditions conducive to hydrogen production. This review discusses how focusing on molecular-level processes and leveraging advanced genetic tools can lead to a new methodology that potentially offers improved results over traditional approaches. By redirecting electron flow and optimizing redox pathways, we seek to enhance hydrogen production efficiency in cyanobacteria. Our approach demonstrates how harnessing photosynthesis through synthetic biology can contribute to scalable and sustainable hydrogen production, addressing the growing demand for renewable energy and advancing toward a carbon-neutral future.

Acidogenic fermentation of Ulva in a fed-batch reactor system: tubular versus foliose biomass

The present study proposes a biorefinery of the macroalgae Ulva, focusing on evaluating two different morphologies of the species (foliose and tubular) during acidogenic fermentation in fed-batch reactors. Stage 1 of the study evaluates lyophilised foliose and tubular Ulva, whilst Stage 2 analyses the impact of ulvan extraction on volatile fatty acids yield and changes in carbohydrate availability. Acetic, propionic, and butyric acids were produced from each substrate, with peak concentrations of total VFAs recorded at 2179.5 mg HAc/L (foliose Ulva) and 2029.3 mg HAc/L (tubular Ulva) when ulvan was present. After ulvan extraction, the acidogenic fermentation of the foliose morphotype was negatively affected, reaching at most 315.3 mg HAc/L. In contrast, the extraction showed no influence on the tubular morphotype, peaking at 2165.0 mg HAc/L. Additional variations were noted in the availability of carbohydrates in each substrate during the acidogenic fermentation process. The ulvan-extracted tubular morphotype exhibited the highest peak in carbohydrate concentration (9.8 g glucose/L), whilst the ulvan-extracted foliose morphotype yielded up to 8.5 g glucose/L. This study highlights the biorefinery potential of Ulva biomass, proposing a multiple cascading approach linking multiple energy and biomolecule applications to maximise the valorisation of the biomass.

Predicting biomass conversion and COD removal in wastewater treatment by phototrophic bacteria with interpretable machine learning

Photosynthetic bacteria (PSB) excel in wastewater treatment by removing pollutants and generating biomass but are challenging to optimize due to complex operational and environmental interactions. Neural Ordinary Differential Equations, Elastic Net, Stacking, and Categorical Boosting were applied as artificial intelligence methods to predict chemical oxygen demand (COD) removal efficiency, biomass productivity, biomass yield, and energy yield. Among these, the Stacking model demonstrated superior predictive performance across all targets. Interpretable machine learning methods were employed to identify key features and establish their workable ranges, which included dissolved oxygen (0.3-2.8 mg L⁻1), illuminance (2995.3-6000.0 lux), and light energy (20.0-40.0 kWh) for COD removal efficiency; organic loading rate (OLR, 5.7-7.5 g COD L⁻1 d⁻1), hydraulic retention time (HRT, 0.2-3.2 d), and COD concentration (5.3-10.1 g L⁻1) for biomass productivity; COD/N ratio (609.0-800.0), OLR (0.1-2.4 g COD L⁻1 d⁻1), and illuminance (2661-6000 lux) for biomass yield; and pH (6.5-7.9) and HRT (1.2-2.6 d) for energy yield. The two-dimensional partial dependence plots revealed that optimal interactions between two key input features resulted in COD removal efficiency >72%, biomass productivity >28 g L⁻1 d⁻1, biomass yield> 0.96 g CODbiomass g CODremoved⁻1, energy yield> 0.49 g kWh⁻1. This work advances the understanding of PSB optimization in wastewater treatment through a combination of advanced machine learning and interpretability analysis, offering potential for more efficient resource recovery and process optimization.

Engineering a Photoautotrophic Microbial Coculture toward Enhanced Biohydrogen Production

The application of synthetic phototrophic microbial consortia holds promise for sustainable bioenergy production. Nevertheless, strategies for the efficient construction and regulation of such consortia remain challenging. Applying tools of genetic engineering, this study successfully constructed a synthetic community of phototrophs using Rhodopseudomonas palustris (R. palustris) and an engineered strain of Synechocystis sp PCC6803 for acetate production (Synechocystis_acs), enabling the production of biohydrogen and fatty acids during nitrogen and carbon dioxide fixation. Elemental balance confirmed carbon capture and nitrogen fixation into the consortium. The strategy of circadian illumination effectively limited oxygen levels in the system, ensuring the activity of the nitrogenase in R. palustris, despite oxygenic photosynthesis happening in Synechocystis. When infrared light was introduced into the circadian illumination, the production of H2 (9.70 μmol mg-1) and fatty acids (especially C16 and C18) was significantly enhanced. Proteomic analysis indicated acetate exchange and light-dependent regulation of metabolic activities. Infrared illumination significantly stimulated the expression of proteins coding for nitrogen fixation, carbohydrate metabolism, and transporters in R. palustris, while constant white light led to the most upregulation of photosynthesis-related proteins in Synechocystis_acs. This study demonstrated the successful construction and light regulation of a phototrophic community, enabling H2 and fatty acid production through carbon and nitrogen fixation.

Photosynthetic Bacteria: Light-Responsive Biomaterials for Anti-Tumor Photodynamic Therapy

Photodynamic therapy (PDT) is a promising noninvasive tumor treatment modality that relies on generating reactive oxygen species (ROS) and requires an adequate oxygen supply to the target tissue. However, hypoxia is a common feature of solid tumors and profoundly restricts the anti-tumor efficacy of PDT. In recent years, scholars have focused on exploring nanomaterial-based strategies for oxygen supplementation and integrating non-oxygen-consuming treatment approaches to overcome the hypoxic limitations of PDT. Some scholars have harnessed the photosynthetic oxygen production of cyanobacteria under light irradiation to overcome tumor hypoxia and engineered them as carriers of photosensitizers instead of inorganic nanomaterials, resulting in photosynthetic bacteria (PSB) attracting significant attention. Recent studies have shown that light-triggered PSB can exhibit additional properties, such as photosynthetic hydrogen production, ROS generation, and photothermal conversion, facilitating their use as promising light-responsive biomaterials for enhancing the anti-tumor efficacy of PDT. Therefore, understanding PSB can provide new insights and ideas for future research. This review mainly introduces the characteristics of PSB and recent research on light-triggered PSB in anti-tumor PDT to enrich our knowledge in this area. Finally, the challenges and prospects of using PSB to enhance the anti-tumor efficacy of PDT were also discussed.

Reactivation and long-term stabilization of the [NiFe] Hox hydrogenase of Synechocystis sp. PCC6803 by glutathione after oxygen exposure

Hydrogenases are key enzymes forming or consuming hydrogen. The inactivation of these transition metal biocatalysts with oxygen limits their biotechnological applications. Oxygen-sensitive hydrogenases are distinguished from oxygen-insensitive (tolerant) ones by their initial hydrogen turnover rates influenced by oxygen. Several hydrogenases, such as the oxygen-sensitive bidirectional [NiFe] Hox hydrogenase (Hox) of the unicellular cyanobacterium Synechocystis sp. PCC6803, are reactivated after oxygen-induced deactivation by redox mechanisms. In cyanobacteria, the glutathione (GSH) redox buffer majorly controls intracellular redox potentials. The relationship between Hox turnover rates and the redox potential in its natural reaction environment is not fully understood. We thus determined hydrogen oxidation rates as activities of Hox in cell-free extracts of Synechocystis using benzyl viologen as artificial electron acceptor. We found that GSH modulates Hox hydrogen oxidation rates under oxygen-free conditions. After oxygen exposure, it influences the maximal turnover rate and aids in the reactivation of Hox. Moreover, GSH stabilizes the long-term Hox activity under anoxic conditions and attenuates oxygen-induced deactivation of Hox in a concentration-dependent manner, probably by fostering reactivation. Conversely, oxidized GSH (GSSG) negatively affects Hox activity and oxygen insensitivity. Using Blue Native PAGE followed by mass spectrometry, we showed that oxygen affects Hox complex integrity. The in silico predicted structure of the Hox complex and complexome analyses reveal the formation of various Hox subcomplexes under different conditions. Our findings refine our current classification of oxygen-hydrogenase interactions beyond sensitive and insensitive, which is particularly important for understanding hydrogenase function under physiological conditions in future.

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

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

Cryo-EM structure of a photosystem I variant containing an unusual plastoquinone derivative in its electron transfer chain

Photosystem I (PS I) is a light-driven oxidoreductase responsible for converting photons into chemical bond energy. Its application for renewable energy was revolutionized by the creation of the MenB deletion (ΔmenB) variant in the cyanobacterium Synechocystis sp. PCC 6803, in which phylloquinone is replaced by plastoquinone-9 with a low binding affinity. This permits its exchange with exogenous quinones covalently coupled to dihydrogen catalysts that bind with high affinity, thereby converting PS I into a stable solar fuel catalyst. Here, we reveal the 2.03-Å-resolution cryo-EM structure of a recent MenB variant of PS I. The quinones and their binding environment are analyzed in the context of previous biophysical data, thereby enabling a protocol to solve future PS I hybrids and constructs from this genetically tractable cyanobacterium.

Structure, function, and assembly of PSI in thylakoid membranes of vascular plants

The photosynthetic apparatus is formed by thylakoid membrane-embedded multiprotein complexes that carry out linear electron transport in oxygenic photosynthesis. The machinery is largely conserved from cyanobacteria to land plants, and structure and function of the protein complexes involved are relatively well studied. By contrast, how the machinery is assembled in thylakoid membranes remains poorly understood. The complexes participating in photosynthetic electron transfer are composed of many proteins, pigments, and redox-active cofactors, whose temporally and spatially highly coordinated incorporation is essential to build functional mature complexes. Several proteins, jointly referred to as assembly factors, engage in the biogenesis of these complexes to bring the components together in a step-wise manner, in the right order and time. In this review, we focus on the biogenesis of the terminal protein supercomplex of the photosynthetic electron transport chain, PSI, in vascular plants. We summarize our current knowledge of the assembly process and the factors involved and describe the challenges associated with resolving the assembly pathway in molecular detail.

CHAMP: A Centrifugal Microfluidics-Based CRISPR/Cas12b-Combined Real-Time LAMP One-Pot Method for Mycoplasma pneumoniae Infection Diagnosis

The Mycoplasma pneumoniae outbreak poses health risks to community residents. However, it still has limitations for current clinical diagnostic methods (qPCR nucleic acid assay or IgM immunoassay), including specialized handling, expensive equipment, prolonged turnaround time, and false positives and negatives, highlighting the need to improve clinical diagnostic methods. Herein, we present a novel centrifugal microfluidics-based method for rapidly diagnosing M. pneumoniae infections (CHAMP system). This user-friendly method combines CRISPR/Cas12b and real-time loop-mediated isothermal amplification (LAMP) in a one-pot reaction, offering high sensitivity, specificity, and simplicity for methodology. By adding fully automated nucleic acid magnetic bead-extracted samples to a prepackaged centrifugal microfluidics chip, 48 samples can be automated tested simultaneously within 15 to 60 min at 60 °C. 427 clinical nasopharyngeal swab specimens were used for validation, demonstrating good positive and negative predictive values and good diagnostic sensitivity, specificity, and significant time savings. This method is particularly suitable for detecting low nucleic acid copies of M. pneumoniae samples.

Enhancing strategies of photosynthetic hydrogen production from microalgae: Differences in hydrogen production between prokaryotic and eukaryotic algae

Hydrogen production through the metabolic bypass of microalgae photosynthesis is an environmentally friendly method. This review examines the genetic differences in hydrogen production between prokaryotic and eukaryotic microalgae. Additionally, the pathways for enhancing microalgae-based photosynthetic hydrogen generation are summarized. The main strategies for enhancing microalgal hydrogen production involve inhibiting the oxygen-generating process of photosynthesis and promoting the oxygen tolerance of hydrogenase. Future research is needed to explore the regulation of physiological metabolism through quorum sensing in microalgae to enhance photosynthetic hydrogen production. Moreover, effective evaluation of carbon emissions and sequestration across the entire photosynthetic hydrogen production process is crucial for determining the sustainability of microalgae-based production approaches through comprehensive lifecycle assessment. This review elucidates the prospects and challenges associated with photosynthetic hydrogen production by microalgae.

Synergistic enhancement of pulsed light-induced H2 photoproduction in Chlamydomonas cells by optimal sulfite concentration and light waveform

Pulsed light illumination stands out as a noteworthy technique for photosynthetic H2 production, playing a crucial role in eliminating O2 and activating hydrogenase enzymes. However, further improvements are essential to make H2 photoproduction suitable for future commercial applications. In our study, we observed a distinct enhancement in pulsed light-induced H2 photoproduction in the unicellular green alga Chlamydomonas reinhardtii when treated with the optimal concentration of the mild O2 scavenger Na2SO3. This improvement was a result of reduced O2 content, increased hydrogenase enzyme activity, and suppressed H2-uptake activity. Furthermore, our findings indicate that exposing Na2SO3-treated C. reinhardtii to optimal light waveform continues to significantly boost pulsed light-induced H2 photoproduction, attributed to the alleviation of impaired photosystem II activity. Altogether, the combined application of optimal sulfite concentration and light waveform effectively enhances pulsed light-induced photosynthetic H2 production in the green alga C. reinhardtii.

Photosynthesis and hydrogen energy for sustainability: harnessing the sun for a greener future

At the dawn of the 21st century, the rapid expansion of manufacturing plants and the widespread destruction of natural habitats significantly contributed to accelerating global warming. This phenomenon has led to severe droughts, irreversible agricultural damage, and substantial challenges in securing food supplies for the burgeoning global population. The alarming surge in atmospheric carbon dioxide concentrations underscores the urgent need to embrace clean energy technologies. To date, the primary goal of mankind is to develop innovative approaches to return Earth's ecology to its pre-industrial condition, as a century ago. The special issue (SI) in the International Journal of Hydrogen Energy presents a collection of papers on photosynthetic and biomimetic hydrogen (H2) production, presented at the 'Photosynthesis and Hydrogen Energy Research for Sustainability - 2023' conference, held in Istanbul, Turkey, from 3-9 July 2023 (https://phrs-conference.com). The event was supported by the International Society of Photosynthesis Research (ISPR) and the International Association for Hydrogen Energy (IAHE). SI aims to deliver the latest insights into sustainable energy, with a particular emphasis on Biohydrogen and Artificial Photosynthesis. At the conference, nine promising young investigators were honoured with awards. Included herein are photographs capturing the conference's congenial atmosphere. We cordially invite you to the 12th International Meeting of 'Photosynthesis and Hydrogen Energy Research for Sustainability - 2024', honouring esteemed researchers John Allen (UK), Eva-Mari Aro (Finland), Ibrahim Dincer (Canada), Kazunari Domen (Japan), Elizabeth Gantt (USA), Andrey Rubin (Russia), and scheduled to take place in Turkey (13-19 October 2024).

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

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

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.

HSP90.2 promotes CO2 assimilation rate, grain weight and yield in wheat

Wheat fixes CO₂ by photosynthesis into kernels to nourish humankind. Improving the photosynthesis rate is a major driving force in assimilating atmospheric CO₂ and guaranteeing food supply for human beings. Strategies for achieving the above goal need to be improved. Here, we report the cloning and mechanism of CO₂ ASSIMILATION RATE AND KERNEL-ENHANCED 1 (CAKE1) from durum wheat (Triticum turgidum L. var. durum). The cake1 mutant displayed a lower photosynthesis rate with smaller grains. Genetic studies identified CAKE1 as HSP90.2-B, encoding cytosolic molecular chaperone folding nascent preproteins. The disturbance of HSP90.2 decreased leaf photosynthesis rate, kernel weight (KW) and yield. Nevertheless, HSP90.2 over-expression increased KW. HSP90.2 recruited and was essential for the chloroplast localization of nuclear-encoded photosynthesis units, for example PsbO. Actin microfilaments docked on the chloroplast surface interacted with HSP90.2 as a subcellular track towards chloroplasts. A natural variation in the hexaploid wheat HSP90.2-B promoter increased its transcription activity, enhanced photosynthesis rate and improved KW and yield. Our study illustrated an HSP90.2-Actin complex sorting client preproteins towards chloroplasts to promote CO₂ assimilation and crop production. The beneficial haplotype of Hsp90.2 is rare in modern varieties and could be an excellent molecular switch promoting photosynthesis rate to increase yield in future elite wheat varieties.

Light-dependent biohydrogen production: Progress and perspectives

The fourth industrial revolution anticipates energy to be sustainable, renewable and green. Hydrogen (H₂) is one of the green forms of energy and is deemed a possible solution to climate change. Light-dependent H₂ production is a promising method derived from nature's most copious resources: solar energy, water and biomass. Reduced environmental impacts, absorption of carbon dioxide, relative efficiency, and cost economics made it an eye-catching approach. However, low light conversion efficiency, limited ability to utilize complex carbohydrates, and the O₂ sensitivity of enzymes result in low yield. Isolation of efficient H₂ producers, development of microbial consortia having a synergistic impact, genetically improved strains, regulating bidirectional hydrogenase activity, physiological parameters, immobilization, novel photobioreactors, and additive strategies are summarized for their possibilities to augment the processes of bio-photolysis and photo-fermentation. The challenges and future perspectives have been addressed to explore a sustainable way forward in a bio-refinery approach.

Cyanobacteria as whole-cell factories: current status and future prospectives

Cyanobacteria as phototrophic microorganisms bear great potential to produce chemicals from sustainable resources such as light and CO₂. Most studies focus on either strain engineering or tackling metabolic constraints. Recently gained knowledge on internal electron and carbon fluxes and their regulation provides new opportunities to efficiently channel cellular resources toward product formation. Concomitantly, novel photobioreactor concepts are developed to ensure sufficient light supply. This review summarizes the newest developments in the field of cyanobacterial engineering to finally establish photosynthesis-based production processes. A holistic approach tackling genetic, metabolic, and biochemical engineering in parallel is considered essential to turn their application into an ecoefficient and economically feasible option for a future green bioeconomy.

Harnessing photosynthesis to produce electricity using cyanobacteria, green algae, seaweeds and plants

The conversion of solar energy into electrical current by photosynthetic organisms has the potential to produce clean energy. Life on earth depends on photosynthesis, the major mechanism for biological conversion of light energy into chemical energy. Indeed, billions of years of evolution and adaptation to extreme environmental habitats have resulted in highly efficient light-harvesting and photochemical systems in the photosynthetic organisms that can be found in almost every ecological habitat of our world. In harnessing photosynthesis to produce green energy, the native photosynthetic system is interfaced with electrodes and electron mediators to yield bio-photoelectrochemical cells (BPECs) that transform light energy into electrical power. BPECs utilizing plants, seaweeds, unicellular photosynthetic microorganisms, thylakoid membranes or purified complexes, have been studied in attempts to construct efficient and non-polluting BPECs to produce electricity or hydrogen for use as green energy. The high efficiency of photosynthetic light-harvesting and energy production in the mostly unpolluting processes that make use of water and CO2 and produce oxygen beckons us to develop this approach. On the other hand, the need to use physiological conditions, the sensitivity to photoinhibition as well as other abiotic stresses, and the requirement to extract electrons from the system are challenging. In this review, we describe the principles and methods of the different kinds of BPECs that use natural photosynthesis, with an emphasis on BPECs containing living oxygenic photosynthetic organisms. We start with a brief summary of BPECs that use purified photosynthetic complexes. This strategy has produced high-efficiency BPECs. However, the lifetimes of operation of these BPECs are limited, and the preparation is laborious and expensive. We then describe the use of thylakoid membranes in BPECs which requires less effort and usually produces high currents but still suffers from the lack of ability to self-repair damage caused by photoinhibition. This obstacle of the utilization of photosynthetic systems can be significantly reduced by using intact living organisms in the BPEC. We thus describe here progress in developing BPECs that make use of cyanobacteria, green algae, seaweeds and higher plants. Finally, we discuss the future challenges of producing high and longtime operating BPECs for practical use.