Overexpression of flv3 improves photosynthesis in the cyanobacterium Synechocystis sp. PCC6803 by enhancement of alternative electron flow

Chassis engineering for high light tolerance in microalgae and cyanobacteria

Oxygenic photosynthesis in microalgae and cyanobacteria is considered an important chassis to accelerate energy transition and mitigate global warming. Currently, cultivation systems for photosynthetic microbes for large-scale applications encountered excessive light exposure stress. High light stress can: affect photosynthetic efficiency, reduce productivity, limit cell growth, and even cause cell death. Deciphering photoprotection mechanisms and constructing high-light tolerant chassis have been recent research focuses. In this review, we first briefly introduce the self-protection mechanisms of common microalgae and cyanobacteria in response to high light stress. These mechanisms mainly include: avoiding excess light absorption, dissipating excess excitation energy, quenching excessive high-energy electrons, ROS detoxification, and PSII repair. We focus on the species-specific differences in these mechanisms as well as recent advancements. Then, we review engineering strategies for creating high-light tolerant chassis, such as: reducing the size of the light-harvesting antenna, optimizing non-photochemical quenching, optimizing photosynthetic electron transport, and enhancing PSII repair. Finally, we propose a comprehensive exploration of mechanisms: underlying identified high light tolerant chassis, identification of new genes pertinent to high light tolerance using innovative methodologies, harnessing CRISPR systems and artificial intelligence for chassis engineering modification, and introducing plant photoprotection mechanisms as future research directions.

Nutraceutical prospects of genetically engineered cyanobacteria- technological updates and significance

Genetically engineered cyanobacterial strains that have improved growth rate, biomass productivity, and metabolite productivity could be a better option for sustainable bio-metabolite production. The global demand for biobased metabolites with nutraceuticals and health benefits has increased due to their safety and plausible therapeutic and nutritional utility. Cyanobacteria are solar-powered green cellular factories that can be genetically tuned to produce metabolites with nutraceutical and pharmaceutical benefits. The present review discusses biotechnological endeavors for producing bioprospective compounds from genetically engineered cyanobacteria and discusses the challenges and troubleshooting faced during metabolite production. This review explores the cyanobacterial versatility, the use of engineered strains, and the techno-economic challenges associated with scaling up metabolite production from cyanobacteria. Challenges to produce cyanobacterial bioactive compounds with remarkable nutraceutical values have been discussed. Additionally, this review also summarises the challenges and future prospects of metabolite production from genetically engineered cyanobacteria as a sustainable approach.

Engineering RNA polymerase to construct biotechnological host strains of cyanobacteria

Application of cyanobacteria for bioproduction, bioremediation and biotransformation is being increasingly explored. Photoautotrophs are carbon-negative by default, offering a direct pathway to reducing emissions in production systems. More robust and versatile host strains are needed for constructing production strains that would function as efficient and carbon-neutral cyanofactories. We have tested if the engineering of sigma factors, regulatory units of the bacterial RNA polymerase, could be used to generate better host strains of the model cyanobacterium Synechocystis sp. PCC 6803. Overexpressing the stress-responsive sigB gene under the strong psbA2 promoter (SigB-oe) led to improved tolerance against heat, oxidative stress and toxic end-products. By targeting transcription initiation in the SigB-oe strain, we could simultaneously activate a wide spectrum of cellular protective mechanisms, including carotenoids, the HspA heat shock protein, and highly activated non-photochemical quenching. Yellow fluorescent protein was used to test the capacity of the SigB-oe strain to produce heterologous proteins. In standard conditions, the SigB-oe strain reached a similar production as the control strain, but when cultures were challenged with oxidative stress, the production capacity of SigB-oe surpassed the control strain. We also tested the production of growth-rate-controlled host strains via manipulation of RNA polymerase, but post-transcriptional regulation prevented excessive overexpression of the primary sigma factor SigA, and overproduction of the growth-restricting SigC factor was lethal. Thus, more research is needed before cyanobacteria growth can be manipulated by engineering RNA polymerase.

Advances in light system engineering across the phototrophic spectrum

Current work in photosynthetic engineering is progressing along the lines of cyanobacterial, microalgal, and plant research. These are interconnected through the fundamental mechanisms of photosynthesis and advances in one field can often be leveraged to improve another. It is worthwhile for researchers specializing in one or more of these systems to be aware of the work being done across the entire research space as parallel advances of techniques and experimental approaches can often be applied across the field of photosynthesis research. This review focuses on research published in recent years related to the light reactions of photosynthesis in cyanobacteria, eukaryotic algae, and plants. Highlighted are attempts to improve photosynthetic efficiency, and subsequent biomass production. Also discussed are studies on cross-field heterologous expression, and related work on augmented and novel light capture systems. This is reviewed in the context of translatability in research across diverse photosynthetic organisms.

Extra O2 evolution reveals an O2-independent alternative electron sink in photosynthesis of marine diatoms

Following the principle of oxygenic photosynthesis, electron transport in the thylakoid membranes (i.e., light reaction) generates ATP and NADPH from light energy, which is subsequently utilized for CO2 fixation in the Calvin-Benson-Bassham cycle (i.e., dark reaction). However, light and dark reactions could discord when an alternative electron flow occurs with a rate comparable to the linear electron flow. Here, we quantitatively monitored O2 and total dissolved inorganic carbon (DIC) during photosynthesis in the pennate diatom Phaeodactylum tricornutum, and found that evolved O2 was larger than the consumption of DIC, which was consistent with 14CO2 measurements in literature. In our measurements, the stoichiometry of O2 evolution to DIC consumption was always around 1.5 during photosynthesis at different DIC concentrations. The same stoichiometry was observed in the cells grown under different CO2 concentrations and nitrogen sources except for the nitrogen-starved cells showing O2 evolution 2.5 times larger than DIC consumption. An inhibitor to nitrogen assimilation did not affect the extra O2 evolution. Further, the same physiological phenomenon was observed in the centric diatom Thalassiosira pseudonana. Based on the present dataset, we propose that the marine diatoms possess the metabolic pathway(s) functioning as the O2-independent electron sink under steady state photosynthesis that reaches nearly half of electron flux of the Calvin-Benson-Bassham cycle.

ppGpp accumulation reduces the expression of the global nitrogen homeostasis-modulating NtcA regulon by affecting 2-oxoglutarate levels

The cyanobacterium Synechococcus elongatus PCC 7942 accumulates alarmone guanosine tetraphosphate (ppGpp) under stress conditions, such as darkness. A previous study observed that artificial ppGpp accumulation under photosynthetic conditions led to the downregulation of genes involved in the nitrogen assimilation system, which is activated by the global nitrogen regulator NtcA, suggesting that ppGpp regulates NtcA activity. However, the details of this mechanism have not been elucidated. Here, we investigate the metabolic responses associated with ppGpp accumulation by heterologous expression of the ppGpp synthetase RelQ. The pool size of 2-oxoglutarate (2-OG), which activates NtcA, is significantly decreased upon ppGpp accumulation. De novo 13C-labeled CO2 assimilation into the Calvin-Benson-Bassham cycle and glycolytic intermediates continues irrespective of ppGpp accumulation, whereas the labeling of 2-OG is significantly decreased under ppGpp accumulation. The low 2-OG levels in the RelQ overexpression cells could be because of the inhibition of metabolic enzymes, including aconitase, which are responsible for 2-OG biosynthesis. We propose a metabolic rearrangement by ppGpp accumulation, which negatively regulates 2-OG levels to maintain carbon and nitrogen balance.

Electron transport in cyanobacterial thylakoid membranes: Are cyanobacteria simple models for photosynthetic organisms?

Cyanobacteria are structurally the simplest oxygenic phototrophs, which makes it difficult to understand the regulation of photosynthesis because the photosynthetic and respiratory processes share the same thylakoid membranes and cytosolic space. This review aimed to summarise the molecular mechanisms and in vivo activities of electron transport in cyanobacterial thylakoid membranes based on the latest progress in photosynthesis research in cyanobacteria. Photosynthetic linear electron transport for CO2 assimilation has the dominant electron flux in the thylakoid membranes. The capacity of O2 photoreduction in cyanobacteria is comparable to the photosynthetic CO2 assimilation, which is mediated by flavodiiron proteins. Additionally, cyanobacterial thylakoid membranes harbour the significant electron flux of respiratory electron transport through a homologue of respiratory complex I, which is also recognized as the part of cyclic electron transport chain if it is coupled with photosystem I in the light. Further, O2-independent alternative electron transports through hydrogenase and nitrate reductase function with reduced ferredoxin as the electron donor. Whereas all these electron transports are recently being understood one by one, the complexity as the whole regulatory system remains to be uncovered in near future.

Changes in intracellular energetic and metabolite states due to increased galactolipid levels in Synechococcus elongatus PCC 7942

The lipid composition of thylakoid membranes is conserved from cyanobacteria to green plants. However, the biosynthetic pathways of galactolipids, the major components of thylakoid membranes, are known to differ substantially between cyanobacteria and green plants. We previously reported on a transformant of the unicellular rod-shaped cyanobacterium Synechococcus elongatus PCC 7942, namely SeGPT, in which the synthesis pathways of the galactolipids monogalactosyldiacylglycerol and digalactosyldiacylglycerol are completely replaced by those of green plants. SeGPT exhibited increased galactolipid content and could grow photoautotrophically, but its growth rate was slower than that of wild-type S. elongatus PCC 7942. In the present study, we investigated pleiotropic effects that occur in SeGPT and determined how its increased lipid content affects cell proliferation. Microscopic observations revealed that cell division and thylakoid membrane development are impaired in SeGPT. Furthermore, physiological analyses indicated that the bioenergetic state of SeGPT is altered toward energy storage, as indicated by increased levels of intracellular ATP and glycogen. We hereby report that we have identified a new promising candidate as a platform for material production by modifying the lipid synthesis system in this way.

Light-Driven Synthetic Biology: Progress in Research and Industrialization of Cyanobacterial Cell Factory

Light-driven synthetic biology refers to an autotrophic microorganisms-based research platform that remodels microbial metabolism through synthetic biology and directly converts light energy into bio-based chemicals. This technology can help achieve the goal of carbon neutrality while promoting green production. Cyanobacteria are photosynthetic microorganisms that use light and CO₂ for growth and production. They thus possess unique advantages as "autotrophic cell factories". Various fuels and chemicals have been synthesized by cyanobacteria, indicating their important roles in research and industrial application. This review summarized the progresses and remaining challenges in light-driven cyanobacterial cell factory. The choice of chassis cells, strategies used in metabolic engineering, and the methods for high-value CO₂ utilization will be discussed.

The influence of electron utilization pathways on photosystem I photochemistry in Synechocystis sp. PCC 6803

The capacity of cyanobacteria to adapt to highly dynamic photon flux and nutrient availability conditions results from controlled management and use of reducing power, and is a major contributing factor to the efficiency of photosynthesis in aquatic environments. The response to changing conditions includes modulating gene expression and protein-protein interactions that serve to adjust the use of electron flux and mechanisms that control photosynthetic electron transport (PET). In this regard, the photochemical activity of photosystem I (PSI) reaction centers can support balancing of cyclic (CEF) and linear electron flow (LEF), and the coupling of redox carriers for use by electron utilization pathways. Therefore, changes in the utilization of reducing power might be expected to result in compensating changes at PSI as a means to support balance of electron flux. To understand this functional relationship, we investigated the properties of PSI and its photochemical activity in cells that lack flavodiiron 1 catalyzed oxygen reduction activity (ORR1). In the absence of ORR1, the oxygen evolution and consumption rates declined together with a shift in the oligomeric form of PSI towards monomers. The effect of these changes on PSI energy and electron transfer properties was examined in isolated trimer and monomer fractions of PSI reaction centers. Collectively, the results demonstrate that PSI photochemistry is modulated through coordination with the depletion of electron demand in the absence of ORR1.

Emerging Trends in Genetic Engineering of Microalgae for Commercial Applications

Recently, microalgal biotechnology has received increasing interests in producing valuable, sustainable and environmentally friendly bioproducts. The development of economically viable production processes entails resolving certain limitations of microalgal biotechnology, and fast evolving genetic engineering technologies have emerged as new tools to overcome these limitations. This review provides a synopsis of recent progress, current trends and emerging approaches of genetic engineering of microalgae for commercial applications, including production of pharmaceutical protein, lipid, carotenoids and biohydrogen, etc. Photochemistry improvement in microalgae and CO2 sequestration by microalgae via genetic engineering were also discussed since these subjects are closely entangled with commercial production of the above mentioned products. Although genetic engineering of microalgae is proved to be very effective in boosting performance of production in laboratory conditions, only limited success was achieved to be applicable to industry so far. With genetic engineering technologies advancing rapidly and intensive investigations going on, more bioproducts are expected to be produced by genetically modified microalgae and even much more to be prospected.

Metabolomics-based engineering for biofuel and bio-based chemical production in microalgae and cyanobacteria: A review

Metabolomics, an essential tool in modern synthetic biology based on the design-build-test-learn platform, is useful for obtaining a detailed understanding of cellular metabolic mechanisms through comprehensive analyses of the metabolite pool size and its dynamic changes. Metabolomics is critical to the design of a rational metabolic engineering strategy by determining the rate-limiting reaction and assimilated carbon distribution in a biosynthetic pathway of interest. Microalgae and cyanobacteria are promising photosynthetic producers of biofuels and bio-based chemicals, with high potential for developing a bioeconomic society through bio-based carbon neutral manufacturing. Metabolomics technologies optimized for photosynthetic organisms have been developed and utilized in various microalgal and cyanobacterial species. This review provides a concise overview of recent achievements in photosynthetic metabolomics, emphasizing the importance of microalgal and cyanobacterial cell factories that satisfy industrial requirements.

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

Cyanobacteria are widely-diverse, environmentally crucial photosynthetic prokaryotes of great interests for basic and applied science. Work to date has focused mostly on the three non-nitrogen fixing unicellular species Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002, which have been selected for their genetic and physiological interests summarized in this review. Extensive "omics" data sets have been generated, and genome-scale models (GSM) have been developed for the rational engineering of these cyanobacteria for biotechnological purposes. We presently discuss what should be done to improve our understanding of the genotype-phenotype relationships of these models and generate robust and predictive models of their metabolism. Furthermore, we also emphasize that because Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002 represent only a limited part of the wide biodiversity of cyanobacteria, other species distantly related to these three models, should be studied. Finally, we highlight the need to strengthen the communication between academic researchers, who know well cyanobacteria and can engineer them for biotechnological purposes, but have a limited access to large photobioreactors, and industrial partners who attempt to use natural or engineered cyanobacteria to produce interesting chemicals at reasonable costs, but may lack knowledge on cyanobacterial physiology and metabolism.

Metabolic Engineering for Carotenoid Production Using Eukaryotic Microalgae and Prokaryotic Cyanobacteria

Eukaryotic microalgae and prokaryotic cyanobacteria are diverse photosynthetic organisms that produce various useful compounds. Due to their rapid growth and efficient biomass production from carbon dioxide and solar energy, microalgae and cyanobacteria are expected to become cost-effective, sustainable bioresources in the future. These organisms also abundantly produce various carotenoids, but further improvement in carotenoid productivity is needed for a successful commercialization. Metabolic engineering via genetic manipulation and mutational breeding is a powerful tool for generating carotenoid-rich strains. This chapter focuses on carotenoid production in microalgae and cyanobacteria, as well as strategies and potential target genes for metabolic engineering. Recent achievements in metabolic engineering that improved carotenoid production in microalgae and cyanobacteria are also reviewed.

Engineering biology approaches for food and nutrient production by cyanobacteria

As photoautotrophic organisms, cyanobacteria capture and store solar energy in the form of biomass. Cyanobacterial biomass has been an important component of diet and nutrition in several regions for centuries. Synthetic biology strategies are currently being applied to increase the yield and productivity of cyanobacterial biomass by optimizing solar energy utilization and CO₂ fixation rates for carbon storage. Likewise, engineering cyanobacteria as cellular factories to synthesize carbohydrates, amino acids, proteins, lipids and fatty acids is providing an attractive way to sustainably produce food and nutrients for human consumption. In this review, we have summarized recent progress in both aspects and prospective trends under development.

A Chlorophyte Alga Utilizes Alternative Electron Transport for Primary Photoprotection

The green alga Desmodesmus armatus is an emerging biofuel platform that produces high amounts of lipids and biomass in mass culture. We observed D. armatus in light-limiting, excess-light, and sinusoidal-light environments to investigate its photoacclimation behaviors and the mechanisms by which it dissipates excess energy. Chlorophyll a/b ratios and the functional absorption cross section of PSII suggested a constitutively small light-harvesting antenna size relative to other green algae. In situ and ex situ measurements of photo-physiology revealed that nonphotochemical quenching is not a significant contributor to photoprotection; however, cells do not suffer substantial photoinhibition despite its near absence. We performed membrane inlet mass spectrometry analysis to show that D. armatus has a very high capacity for alternative electron transport (AET) measured as light-dependent oxygen consumption. Up to 90% of electrons generated at PSII can be dissipated by AET in a water-water cycle during growth in rapidly fluctuating light environments, like those found in industrial-scale photobioreactors. This work highlights the diversity of photoprotective mechanisms present in algal systems, indicating that nonphotochemical quenching is not necessarily required for effective photoprotection in some algae, and suggests that engineering AET may be an attractive target for increasing the biomass productivity of some strains.

Providing an Additional Electron Sink by the Introduction of Cyanobacterial Flavodiirons Enhances Growth of A. thaliana Under Various Light Intensities

The ability of plants to maintain photosynthesis in a dynamically changing environment is of central importance for their growth. As the photosynthetic machinery is a sensitive and early target of adverse environmental conditions as those typically found in the field, photosynthetic efficiency is not always optimal. Cyanobacteria, algae, mosses, liverworts and gymnosperms produce flavodiiron proteins (Flvs), a class of electron sinks not represented in angiosperms; these proteins act to mitigate the photoinhibition of photosystem I under high or fluctuating light. Here, genes specifying two cyanobacterial Flvs have been expressed in the chloroplasts of Arabidopsis thaliana in an attempt to improve plant growth. Co-expression of Flv1 and Flv3 enhanced the efficiency of light utilization, boosting the plant's capacity to accumulate biomass as the growth light intensity was raised. The Flv1/Flv3 transgenics displayed an increased production of ATP, an acceleration of carbohydrate metabolism and a more pronounced partitioning of sucrose into starch. The results suggest that Flvs are able to establish an efficient electron sink downstream of PSI, thereby ensuring efficient photosynthetic electron transport at moderate to high light intensities. The expression of Flvs thus acts to both protect photosynthesis and to control the ATP/NADPH ratio; together, their presence is beneficial for the plant's growth potential.

Simultaneous increases in the levels of compatible solutes by cost-effective cultivation of Synechocystis sp. PCC 6803

Synechocystis sp. PCC 6803, a cyanobacterium widely used for basic research, is often cultivated in a synthetic medium, BG-11, in the presence of 4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid (HEPES) or 2-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid buffer. Owing to the high cost of HEPES buffer (96.9% of the total cost of BG-11 medium), the biotechnological application of BG-11 is limited. In this study, we cultured Synechocystis sp. PCC 6803 cells in BG-11 medium without HEPES buffer and examined the effects on the primary metabolism. Synechocystis sp. PCC 6803 cells could grow in BG-11 medium without HEPES buffer after adjusting for nitrogen sources and light intensity; the production rate reached 0.54 g cell dry weight·L^-1 ·day^-1 , exceeding that of commercial cyanobacteria and Synechocystis sp. PCC 6803 cells cultivated under other conditions. The exclusion of HEPES buffer markedly altered the metabolites in the central carbon metabolism; particularly, the levels of compatible solutes, such as sucrose, glucosylglycerol, and glutamate were increased. Although the accumulation of sucrose and glucosylglycerol under high salt conditions is antagonistic to each other, these metabolites accumulated simultaneously in cells grown in the cost-effective medium. Because these metabolites are used in industrial feedstocks, our results reveal the importance of medium composition for the production of metabolites using cyanobacteria.

Malic Enzyme Facilitates d-Lactate Production through Increased Pyruvate Supply during Anoxic Dark Fermentation in Synechocystis sp. PCC 6803

d-Lactate is one of the most valuable compounds for manufacturing biobased polymers. Here, we have investigated the significance of endogenous malate dehydrogenase (decarboxylating) (malic enzyme, ME), which catalyzes the oxidative decarboxylation of malate to pyruvate, in d-lactate biosynthesis in the cyanobacterium Synechocystis sp. PCC6803. d-Lactate levels were increased by 2-fold in ME-overexpressing strains, while levels in ME-deficient strains were almost equivalent to those in the host strain. Dynamic metabolomics revealed that overexpression of ME led to increased turnover rates in malate and pyruvate metabolism; in contrast, deletion of ME resulted in increased pool sizes of glycolytic intermediates, probably due to sequential feedback inhibition, initially triggered by malate accumulation. Finally, both the loss of the acetate kinase gene and overexpression of endogenous d-lactate dehydrogenase, concurrent with ME overexpression, resulted in the highest production of d-lactate (26.6 g/L) with an initial cell concentration of 75 g-DCW/L after 72 h fermentation.

Heterologous Expression of Ethanol Synthesis Pathway in Glycogen Deficient Synechococcus elongatus PCC 7942 Resulted in Enhanced Production of Ethanol and Exopolysaccharides

In this study, the Synechococcus elongatus PCC 7942 (hereafter S. elongatus) was engineered by the glgC knockout as well as the insertion of the pdc-adh genes from two different microorganisms. The insertion of pdc-adh genes increased the ethanol synthesis with further improvement in the productivity upon the destruction of glycogen synthesis pathway and the supplementation of cofactor. The abolition of glycogen synthesis pathway led to a considerable increase of the engineered S. elongatus metabolites involved in the ethanol synthesis pathway. Moreover, the studies on cofactor addition highlighted the importance of Mg+2, Zn+2, thiamine pyrophosphate, and NADP+ in ethanol synthesis. The yields of 3856 mg/L ethanol and 109.5 µg/108 cells exopolysaccharides were obtained in the engineered S. elongatus using a photo-bioreactor under optimized conditions. This enhanced production in ethanol and exopolysaccharides are attributed to the flux of carbon from glycogen synthesis pathway and proper availability of essential components.

Single-Stage Astaxanthin Production Enhances the Nonmevalonate Pathway and Photosynthetic Central Metabolism in Synechococcus sp. PCC 7002

The natural pigment astaxanthin is widely used in aquaculture, pharmaceutical, nutraceutical, and cosmetic industries due to superior antioxidant properties. The green alga Haematococcus pluvialis is currently used for commercial production of astaxanthin pigment. However, slow growing H. pluvialis requires a complex two-stage stress-induced process with high light intensity leading to increased contamination risks. In contrast, the fast-growing euryhaline cyanobacterium Synechococcus sp. PCC 7002 (Synechococcus 7002) is able to reach high density under stress-free phototrophic conditions, and is therefore a promising metabolic engineering platform for astaxanthin production. In the present study, genes encoding β-carotene hydroxylase and β-carotene ketolase, from the marine bacterium Brevundimonas sp. SD212, are integrated into the endogenous plasmid of Synechococcus 7002, and then expressed to biosynthesize astaxanthin. Although Synechococcus 7002 does not inherently produce astaxanthin, the recombinant ZW strain yields 3 mg/g dry cell weight astaxanthin from CO₂ as the sole carbon source, with significantly higher astaxanthin content than previous cyanobacteria reports. Synechococcus 7002 astaxanthin productivity reached 3.35 mg/L/day after just 2 days in a continuous autotrophic process, which is comparable to the best H. pluvialis astaxanthin productivities when factoring in growth times. Metabolomics analysis reveals increases in fractions of hexose-, pentose-, and triose phosphates along with intermediates involved in the nonmevalonate pathway. Dynamic metabolomics analysis of ¹³C labeled metabolites clearly indicates flux enhancements in the Calvin cycle and glycolysis resulting from the overexpression of astaxanthin biosynthetic genes. This study suggests that cyanobacteria may enhance central metabolism as well as the nonmevalonate pathway in an attempt to replenish depleted pigments such as β-carotene and zeaxanthin.

Evaluation of New Genetic Toolkits and Their Role for Ethanol Production in Cyanobacteria

Since the public awareness for climate change has risen, increasing scientific effort has been made to find and develop alternative resources and production processes to reduce the dependency on petrol-based fuels and chemicals of our society. Among others, the biotechnological fuel production, as for example fermenting sugar-rich crops to ethanol, is one of the main strategies. For this purpose, various classical production systems like Escherichia coli or Saccharomyces cerevisiae are used and have been optimized via genetic modifications. Despite the progress made, this strategy competes for nutritional resources and agricultural land. To overcome this problem, various attempts were made for direct photosynthetic driven ethanol synthesis with different microalgal species including cyanobacteria. However, compared to existing platforms, the development of cyanobacteria as photoautotrophic cell factories has just started, and accordingly, the ethanol yield of established production systems is still unreached. This is mainly attributed to low ethanol tolerance levels of cyanobacteria and there is still potential for optimizing the cyanobacteria towards alternative gene expression systems. Meanwhile, several improvements were made by establishing new toolboxes for synthetic biology offering new possibilities for advanced genetic modifications of cyanobacteria. Here, current achievements and innovations of those new molecular tools are discussed.

Overexpression of plastid terminal oxidase in Synechocystis sp. PCC 6803 alters cellular redox state

Cyanobacteria are the most ancient organisms performing oxygenic photosynthesis, and they are the ancestors of plant plastids. All plastids contain the plastid terminal oxidase (PTOX), while only certain cyanobacteria contain PTOX. Many putative functions have been discussed for PTOX in higher plants including a photoprotective role during abiotic stresses like high light, salinity and extreme temperatures. Since PTOX oxidizes PQH₂ and reduces oxygen to water, it is thought to protect against photo-oxidative damage by removing excess electrons from the plastoquinone (PQ) pool. To investigate the role of PTOX we overexpressed rice PTOX fused to the maltose-binding protein (MBP-OsPTOX) in Synechocystis sp. PCC 6803, a model cyanobacterium that does not encode PTOX. The fusion was highly expressed and OsPTOX was active, as shown by chlorophyll fluorescence and P₇₀₀ absorption measurements. The presence of PTOX led to a highly oxidized state of the NAD(P)H/NAD(P)⁺ pool, as detected by NAD(P)H fluorescence. Moreover, in the PTOX overexpressor the electron transport capacity of PSI relative to PSII was higher, indicating an alteration of the photosystem I (PSI) to photosystem II (PSII) stoichiometry. We suggest that PTOX controls the expression of responsive genes of the photosynthetic apparatus in a different way from the PQ/PQH₂ ratio.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Production of Industrial Chemicals from CO2 by Engineering Cyanobacteria

As photosynthetic prokaryotes, cyanobacteria can directly convert CO₂ to organic compounds and grow rapidly using sunlight as the sole source of energy. The direct biosynthesis of chemicals from CO₂ and sunlight in cyanobacteria is therefore theoretically more attractive than using glucose as carbon source in heterotrophic bacteria. To date, more than 20 different target chemicals have been synthesized from CO₂ in cyanobacteria. However, the yield and productivity of the constructed strains is about 100-fold lower than what can be obtained using heterotrophic bacteria, and only a few products reached the gram level. The main bottleneck in optimizing cyanobacterial cell factories is the relative complexity of the metabolism of photoautotrophic bacteria. In heterotrophic bacteria, energy metabolism is integrated with the carbon metabolism, so that glucose can provide both energy and carbon for the synthesis of target chemicals. By contrast, the energy and carbon metabolism of cyanobacteria are separated. First, solar energy is converted into chemical energy and reducing power via the light reactions of photosynthesis. Subsequently, CO₂ is reduced to organic compounds using this chemical energy and reducing power. Finally, the reduced CO₂ provides the carbon source and chemical energy for the synthesis of target chemicals and cell growth. Consequently, the unique nature of the cyanobacterial energy and carbon metabolism determines the specific metabolic engineering strategies required for these organisms. In this chapter, we will describe the specific characteristics of cyanobacteria regarding their metabolism of carbon and energy, summarize and analyze the specific strategies for the production of chemicals in cyanobacteria, and propose metabolic engineering strategies which may be most suitable for cyanobacteria.

Evolutionary engineering of salt-resistant Chlamydomonas sp. strains reveals salinity stress-activated starch-to-lipid biosynthesis switching

The aim of this study was to improve biomass production of the green microalga Chlamydomonas sp. JSC4 under high salinity conditions. For this purpose, heavy ion beam-coupled mutagenesis and evolutionary engineering were performed using JSC4 as the parent strain. After long-term and continuous cultivation with high salinity, salt-resistant strains that grow well even in the presence of 7% sea salt were successfully obtained. Transcriptional analysis revealed inactivation of starch-to-lipid biosynthesis switching, which resulted in delayed starch degradation and decreased lipid content in the salt-resistant strains. Cellular aggregation and hypertrophy during high salinity were relieved in these strains, indicating strong resistance to salt stress. These results suggest that high salinity stress, not the salinity condition itself, is important for activating lipid accumulation mechanisms in microalgae.

Metabolic engineering of Synechococcus elongatus PCC 7942 for improvement of 1,3-propanediol and glycerol production based on in silico simulation of metabolic flux distribution

Background

Production directly from carbon dioxide by engineered cyanobacteria is one of the promising technologies for sustainable future. Previously, we have successfully achieved 1,3-propanediol (1,3-PDO) production using Synechococcus elongatus PCC 7942 with a synthetic metabolic pathway. The strain into which the synthetic metabolic pathway was introduced produced 3.48 mM (0.265 g/L) 1,3-PDO and 14.3 mM (1.32 g/L) glycerol during 20 days of incubation. In this study, the productivities of 1,3-PDO were improved by gene disruption selected by screening with in silico simulation.

Methods

First, a stoichiometric metabolic model was applied to prediction of cellular metabolic flux distribution in a 1,3-PDO-producing strain of S. elongatus PCC 7942. A genome-scale model of S. elongatus PCC 7942 constructed by Knoop was modified by the addition of a synthetic metabolic pathway for 1,3-PDO production. Next, the metabolic flux distribution predicted by metabolic flux balance analysis (FBA) was used for in silico simulation of gene disruption. As a result of gene disruption simulation, NADPH dehydrogenase 1 (NDH-1) complexes were found by screening to be the most promising candidates for disruption to improve 1,3-PDO production. The effect of disruption of the gene encoding a subunit of the NDH-1 complex was evaluated in the 1,3-PDO-producing strain.

Results and Conclusions

During 20 days of incubation, the ndhF1-null 1,3-PDO-producing strain showed the highest titers: 4.44 mM (0.338 g/L) 1,3-PDO and 30.3 mM (2.79 g/L) glycerol. In this study, we successfully improved 1,3-PDO productivity on the basis of in silico simulation of gene disruption.

Electronic supplementary material

The online version of this article (10.1186/s12934-017-0824-4) contains supplementary material, which is available to authorized users.

Dynamic metabolic profiling together with transcription analysis reveals salinity-induced starch-to-lipid biosynthesis in alga Chlamydomonas sp. JSC4

Biodiesel production using microalgae would play a pivotal role in satisfying future global energy demands. Understanding of lipid metabolism in microalgae is important to isolate oleaginous strain capable of overproducing lipids. It has been reported that reducing starch biosynthesis can enhance lipid accumulation. However, the metabolic mechanism controlling carbon partitioning from starch to lipids in microalgae remains unclear, thus complicating the genetic engineering of algal strains. We here used “dynamic” metabolic profiling and essential transcription analysis of the oleaginous green alga Chlamydomonas sp. JSC4 for the first time to demonstrate the switching mechanisms from starch to lipid synthesis using salinity as a regulator, and identified the metabolic rate-limiting step for enhancing lipid accumulation (e.g., pyruvate-to-acetyl-CoA). These results, showing salinity-induced starch-to-lipid biosynthesis, will help increase our understanding of dynamic carbon partitioning in oleaginous microalgae. Moreover, we successfully determined the changes of several key lipid-synthesis-related genes (e.g., acetyl-CoA carboxylase, pyruvate decarboxylase, acetaldehyde dehydrogenase, acetyl-CoA synthetase and pyruvate ferredoxin oxidoreductase) and starch-degradation related genes (e.g., starch phosphorylases), which could provide a breakthrough in the marine microalgal production of biodiesel.

High-Speed Scanning for the Quantitative Evaluation of Glycogen Concentration in Bioethanol Feedstock Synechocystis sp. PCC6803 Using a Near-Infrared Hyperspectral Imaging System with a New Near-Infrared Spectral Camera

In the present study, the high-speed quantitative evaluation of glycogen concentration accumulated in bioethanol feedstock Synechocystis sp. PCC6803 was performed using a near-infrared (NIR) imaging system with a hyperspectral NIR spectral camera named Compovision. The NIR imaging system has a feature for high-speed and wide area monitoring and the two-dimensional scanning speed is almost 100 times faster than the general NIR imaging systems for the same pixel size. For the quantitative analysis of glycogen concentration, partial least squares regression (PLSR) and moving window PLSR (MWPLSR) were performed with the information of glycogen concentration measured by high performance liquid chromatography (HPLC) and the calibration curves for the concentration within the Synechocystis sp. PCC6803 cell were constructed. The results had high accuracy for the quantitative estimation of glycogen concentration as the best squared correlation coefficient R² was bigger than 0.99 and a root mean square error (RMSE) was less than 2.9%. The present results proved not only the potential for the applicability of NIR spectroscopy to the high-speed quantitative evaluation of glycogen concentration in the bioethanol feedstock but also the expansivity of the NIR imaging instrument to in-line or on-line product evaluation on a factory production line of bioethanol in the future.

Moderate Heat Stress Stimulates Repair of Photosystem II During Photoinhibition in Synechocystis sp. PCC 6803

Examination of the effects of high temperature on the photoinhibition of photosystem II (PSII) in the cyanobacterium Synechocystis sp. PCC 6803 revealed that the extent of photoinhibition of PSII was lower at moderately high temperatures (35-42 °C) than at 30 °C. Photodamage to PSII, as determined in the presence of chloramphenicol, which blocks the repair of PSII, was accelerated at the moderately high temperatures but the effects of repair were greater than those of photodamage. The synthesis de novo of the D1 protein, which is essential for the repair of PSII, was enhanced at 38 °C. Electron transport and the synthesis of ATP were also enhanced at 38 °C, while levels of reactive oxygen species fell. Inhibition of the Calvin-Benson cycle with glycolaldehyde abolished the enhancement of repair of PSII at 38 °C, suggesting that an increase in the activity of the Calvin-Benson cycle might be required for the enhancement of repair at moderately high temperatures. The synthesis de novo of metabolic intermediates of the Calvin-Benson cycle, such as 3-phosphoglycerate, was also enhanced at 38 °C. We propose that moderate heat stress might enhance the repair of PSII by stimulating the synthesis of ATP and depressing the production of reactive oxygen species, via the stimulation of electron transport and suppression of the accumulation of excess electrons on the acceptor side of photosystem I, which might be driven by an increase in the activity of the Calvin-Benson cycle.

The Flavodiiron Protein Flv3 Functions as a Homo-Oligomer During Stress Acclimation and is Distinct from the Flv1/Flv3 Hetero-Oligomer Specific to the O2 Photoreduction Pathway

The flavodiiron proteins (FDPs) Flv1 and Flv3 in cyanobacteria function in photoreduction of O2 to H2O, without concomitant formation of reactive oxygen species, known as the Mehler-like reaction. Both Flv1 and Flv3 are essential for growth under fluctuating light (FL) intensities, providing protection for PSI. Here we compared the global transcript profiles of the wild type (WT), Δflv1 and Δflv1/Δflv3 grown under constant light (GL) and FL. In the WT, FL induced the largest down-regulation in transcripts involved in carbon-concentrating mechanisms (CCMs), while those of the nitrogen assimilation pathways increased as compared with GL. Already under GL the Δflv1/Δflv3 double mutant demonstrated a partial down-regulation of transcripts for CCM and nitrogen metabolism, while in FL conditions the transcripts for nitrogen assimilation were strongly down-regulated. Many alterations were specific only for Δflv1/Δflv3, and not detected in Δflv1, suggesting that certain transcripts are affected primarily because of the lack of flv3 By constructing the strains overproducing solely either Flv1 or Flv3, we demonstrate that the homo-oligomers of these proteins also function in acclimation of cells to FL, by catalyzing reactions with as yet unidentified components, while the presence of both Flv1 and Flv3 is a prerequisite for the Mehler-like reaction and thus the electron transfer to O2 Considering the low expression of flv1, it is unlikely that the Flv1 homo-oligomer is present in the WT.

Improved sugar-free succinate production by Synechocystis sp. PCC 6803 following identification of the limiting steps in glycogen catabolism

Succinate produced by microorganisms can replace currently used petroleum-based succinate but typically requires mono- or poly-saccharides as a feedstock. The cyanobacterium Synechocystis sp. PCC6803 can produce organic acids such as succinate from CO₂ not supplemented with sugars under dark anoxic conditions using an unknown metabolic pathway. The TCA cycle in cyanobacteria branches into oxidative and reductive routes. Time-course analyses of the metabolome, transcriptome and metabolic turnover described here revealed dynamic changes in the metabolism of Synechocystis sp. PCC6803 cultivated under dark anoxic conditions, allowing identification of the carbon flow and rate-limiting steps in glycogen catabolism. Glycogen biosynthesized from CO₂ assimilated during periods of light exposure is catabolized to succinate via glycolysis, the anaplerotic pathway, and the reductive TCA cycle under dark anoxic conditions. Expression of the phosphoenolpyruvate (PEP) carboxylase gene (ppc) was identified as a rate-limiting step in succinate biosynthesis and this rate limitation was alleviated by ppc overexpression, resulting in improved succinate excretion. The sugar-free succinate production was further enhanced by the addition of bicarbonate. In vivo labeling with NaH¹³CO₃ clearly showed carbon incorporation into succinate via the anaplerotic pathway. Bicarbonate is in equilibrium with CO₂. Succinate production by Synechocystis sp. PCC6803 therefore holds significant promise for CO₂ capture and utilization.

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The cyanobacterium Synechocystis produces succinate under dark anoxic condition.

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Multi-omics revealed dynamic change in metabolism under dark anoxic condition.

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Carbon flow and rate-limiting steps in glycogen catabolism was elucidated.

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PEP carboxylase gene overexpression improved Synechocystis succinate production.

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Bicarbonate addition to medium dramatically improved succinate production.