Newly discovered Synechococcus sp. PCC 11901 is a robust cyanobacterial strain for high biomass production

Cyanobacteria fouling in photobioreactors: current status and future perspectives for prevention

Cyanobacteria biomass sources have the potential to contribute to the replacement of fossil fuels and to the reduction in global warming by sustainable conversion of atmospheric CO2 into biofuels and high-value chemicals. Cyanobacteria cultivation in photobioreactors (PBRs) results in biofouling on their transparent inner walls, which reduces photosynthetic efficiency and productivity. While cyanobacteria biofouling in PBRs is recognized as a significant operating challenge, this review draws attention to the lack of studies on antifouling strategies for PBRs involving cyanobacteria and discusses several areas related to cyanobacteria fouling mechanisms on PBR materials, which require further investigation. These include an in-depth analysis of conditioning films, the role of pili and EPS in gliding and adhesion, potential revisions to existing theoretical models for predicting adhesion, and material properties that affect cyanobacteria adhesion. We use knowledge from marine, medical, and industrial biofouling management to help identify strategies to combat cyanobacteria fouling in PBRs, and we review the applicability of various bioinspired physical and chemical strategies, as well as genetic engineering approaches to prevent cyanobacteria biofilm formation in PBRs.

Photosynthetic sorbitol production in Synechococcus sp. PCC 7002 is enhanced by addressing phosphatase promiscuity, nutrient availability and Calvin cycle bottlenecks

Cyanobacteria represent promising biocatalysts for producing carbohydrates, including sorbitol, a naturally-occurring, fermentable sugar alcohol with conventional uses as a sweetener, pharmaceutical additive, and biodegradable plasticizer. Previously, Synechocystis sp. PCC 6803 was engineered to produce sorbitol, reaching a final titer of 2.3 g/L after 18 days. To improve upon this performance, sorbitol production was herein engineered in the faster growing strain Synechococcus sp. PCC 7002. Upon introducing the sorbitol biosynthetic pathway, up to 500 mg/L sorbitol was initially produced after seven days. However, due to the initial use of two highly promiscuous sugar phosphatase variants, this also resulted in the unwanted co-production of ribose and growth inhibition due to depletion of ribose-5-phosphate from the Calvin cycle. This off-target effect was ultimately mitigated via the discovery that mannitol-1-phosphate phosphatase from Eimeria tenella also dephosphorylates sorbitol-6-phosphate to sorbitol with greater specificity, leading to improved growth and sorbitol production. Next, two bottleneck enzymes in the Calvin cycle, namely fructose-bisphosphate aldolase (FBA) and bifunctional fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase (BiBPase), were overexpressed both individually and in combination, resulting in sorbitol production up to 1.3 g/L. Finally, upon optimizing the culture media to address nutrient limitation, the final strain produced up to 3.6 g/L sorbitol in nine days, respectively representing 1.5- and 3-fold increases in titer and productivity relative to previously-engineered Synechocystis sp. PCC 6803.

Introduction of a phenylalanine sink in fast growing cyanobacterium Synechococcus elongatus PCC 11801 leads to improved PSII efficiency, linear electron transport, and carbon fixation

Cyanobacteria are investigated for fundamental photosynthesis research and sustainable production of valuable biochemicals. However, low product titer and biomass productivities are major bottlenecks to the economical scale-up. Recent studies have shown that the introduction of a metabolic sink, such as sucrose, 2,3-butanediol, and 2-phenyl ethanol, in cyanobacteria improves carbon fixation by relieving the "sink" limitation of photosynthesis. However, the impact of light intensity on the behavior of this sink-derived enhancement in carbon fixation is not well understood and is necessary for translation to outdoor cultivation. Here, using random mutagenesis, we engineered Synechococcus elongatus PCC 11801 to overproduce 1.24 g L-1 phenylalanine (Phe) in 3 days, identified L531W in the TolC protein as an important driver of Phe efflux, and investigated the effect of light intensity on total carbon fixation. We found that low light results in competition between biomass and Phe, whereas under excess light, a higher flux of fixed carbon is directed to the Phe sink. The introduction of the Phe sink improves the quantum yields of photosystem I and II with a concomitant increase in the total electron flow leading to nearly 70% increase in carbon fixation at high light in the mutant strain. Additionally, the cyclic electron flow decreased, which has implications for the ATP/NADPH production ratio. Our data highlight how light intensity affects the sink-derived enhancement in carbon fixation, the role of CEF to balance the source-sink demand for ATP and NADPH, and the enhancement of inorganic carbon fixation in cyanobacteria with an engineered sink.

Towards high atom economy in whole-cell redox biocatalysis: up-scaling light-driven cyanobacterial ene-reductions in a flat panel photobioreactor

Light-driven biotransformations in recombinant cyanobacteria benefit from the atom-efficient regeneration of reaction equivalents like NADPH from water and light by oxygenic photosynthesis. The self-shading of photosynthetic cells throughout the reaction volume, along with the need for extended light paths, limits adequate light supply and significantly restricts the potential for upscaling. Here, we present a flat panel photobioreactor (1 cm optical path length) as a scalable system to provide efficient illumination at high cell densities. The genes of five ene-reductases from different classes were expressed in Synechocystis sp. PCC 6803. The strains were characterised in the light-driven reduction of a set of prochiral substrates. With specific activities up to 150 U gCDW -1 under standard conditions in small-scale reactions, the recombinant strains harbouring the ene-reductases TsOYE C25G I67T and OYE3 showed the highest specific activities observed so far in photobiotransformations and were selected for the up-scale in the flat panel photobioreactor in 120 mL-scale. The strain producing OYE3 exhibited a specific activity as high as 56.1 U gCDW -1. The corresponding volumetric productivity of 1 g L-1 h-1 compares favourably to other photosynthesis-driven processes. This setup facilitated the conversion of 50 mM over approximately 8 hours to an isolated yield of 87%. The atom economy of 88% compares favourably to the use of the sacrificial co-substrates glucose and formic acid with 49% and 78%, respectively. Determination of the complete E-Factor of 203 including water reveals that the volumetric yield and water required for cultivation are crucial for the sustainability. In summary, our results point out key factors for the sustainability of light-driven whole-cell biotransformations, and provide a solid basis for future optimisation and up-scale campaigns of photosynthesis-driven bioproduction.

Engineering of the fast-growing cyanobacterium Synechococcus sp. PCC 11901 to synthesize astaxanthin

BACKGROUND

Astaxanthin is a red pigment required by feed, nutraceutical, and cosmetic industries for its pigmentation and antioxidant properties. This carotenoid is one of the main high-value products that can nowadays be derived from microalgae cultivation, raising important industrial interest. However, state-of-the-art astaxanthin production is the cultivation of the green alga Haematococcus pluvialis (or lacustris), which faces high costs and low production yield. Hence, alternative and efficient sources for astaxanthin need to be developed, and novel biotechnological solutions must be found. The recently discovered cyanobacterium, Synechococcus sp. PCC 11901 is a promising photosynthetic platform for the large-scale production of high-value products, but its potential has yet to be thoroughly tested.

RESULTS

In this study, the cyanobacterium Synechococcus sp. PCC 11901 was engineered for the first time to our knowledge to produce astaxanthin, a high-value ketocarotenoid, by expressing recombinant β-ketolase (bKT) and a β-hydroxylase enzymes (CtrZ). During photoautotrophic growth, the bKT-CtrZ transformed strain (called BC) accumulated astaxanthin to above 80% of the total carotenoid. Moreover, BC cells grew faster than wild-type (WT) cells in high light and continuous bubbling with CO2-enriched air. The engineered strain reached stationary phase after only 4 days of growth in an airlift 80-mL photobioreactor, producing 7 g/L of dry biomass, and accumulated ~ 10 mg/L/day of astaxanthin, which is more than other CO2-consuming multi-engineered systems. In addition, BC cells were cultivated in a 330-L photobioreactor to link lab-scale experiments to the industrial scale-up.

CONCLUSIONS

The astaxanthin volumetric productivity achieved, 10 mg/L/day, exceeds that previously reported for Haematococcus pluvialis, the standard microalgal species nowadays used at the industrial level for astaxanthin production, or for other microalgal strains engineered to produce ketocarotenoids. Overall, this work identifies a new route to produce astaxanthin on an industrial scale.

Current insights into molecular mechanisms of environmental stress tolerance in Cyanobacteria

The photoautotrophic nature of cyanobacteria, coupled with their fast growth and relative ease of genetic manipulation, makes these microorganisms very promising factories for the sustainable production of bio-products from atmospheric carbon dioxide. However, both in nature and in cultivation, cyanobacteria go through different abiotic stresses such as high light (HL) stress, heavy metal stress, nutrient limitation, heat stress, salt stress, oxidative stress, and alcohol stress. In recent years, significant improvement has been made in identifying the stress-responsive genes and the linked pathways in cyanobacteria and developing genome editing tools for their manipulation. Metabolic pathways play an important role in stress tolerance; their modification is also a very promising approach to adapting to stress conditions. Several synthetic as well as systems biology approaches have been developed to identify and manipulate genes regulating cellular responses under different stresses. In this review, we summarize the impact of different stresses on metabolic processes, the small RNAs, genes and heat shock proteins (HSPs) involved, changes in the metabolome and their adaptive mechanisms. The developing knowledge of the adaptive behaviour of cyanobacteria may also be utilised to develop better stress-responsive strains for various applications.

Perspectives of cyanobacterial cell factories

Cyanobacteria are prokaryotic photosynthetic microorganisms that can generate, in addition to biomass, useful chemicals and proteins/enzymes, essentially from sunlight, carbon dioxide, and water. Selected aspects of cyanobacterial production (isoprenoids and high-value proteins) and scale-up methods suitable for product generation and downstream processing are addressed in this review. The work focuses on the challenge and promise of specialty chemicals and proteins production, with isoprenoid products and biopharma proteins as study cases, and the challenges encountered in the expression of recombinant proteins/enzymes, which underline the essence of synthetic biology with these microorganisms. Progress and the current state-of-the-art in these targeted topics are emphasized.

Unlocking the potential of cyanobacteria: a high-throughput strategy for enhancing biocatalytic performance through genetic optimization

Cyanobacteria show promise as hosts for whole-cell biocatalysis. Their photoautotrophic metabolism can be leveraged for a sustainable production process. Despite advancements, performance still lags behind heterotrophic hosts. A key challenge is the limited ability to overexpress recombinant enzymes, which also hinders their biocatalytic efficiency. To address this, we generated large-scale expression libraries and developed a high-throughput method combining fluorescence-activated cell sorting (FACS) and deep sequencing in Synechocystis sp. PCC 6803 (Syn. 6803) to screen and optimize its genetic background. We apply this approach to enhance expression and biocatalyst performance for three enzymes: the ketoreductase LfSDR1M50, enoate reductase YqjM, and Baeyer-Villiger monooxygenase (BVMO) CHMOmut. Diverse genetic combinations yielded significant improvements: optimizing LfSDR1M50 expression showed a 17-fold increase to 39.2 U gcell dry weight (CDW)-1. In vivo activity of Syn. YqjM was improved 16-fold to 58.7 U gCDW-1 and, for Syn. CHMOmut, a 1.5-fold increase to 7.3 U gCDW-1 was achieved by tailored genetic design. Thus, this strategy offers a pathway to optimize cyanobacteria as expression hosts, paving the way for broader applications in other cyanobacteria strains and larger libraries.

Cyanobacteria newly isolated from marine volcanic seeps display rapid sinking and robust, high-density growth

Cyanobacteria are photosynthetic organisms that play important roles in carbon cycling and are promising bioproduction chassis. Here, we isolate two novel cyanobacteria with 4.6Mbp genomes, UTEX 3221 and UTEX 3222, from a unique marine environment with naturally elevated CO₂. We describe complete genome sequences for both isolates and, focusing on UTEX 3222 due to its planktonic growth in liquid, characterize biotechnologically relevant growth and biomass characteristics. UTEX 3222 outpaces other fast-growing model strains on a solid medium. It can double every 2.35 hours in a liquid medium and grows to high density (>31 g/L biomass dry weight) in batch culture, nearly double that of Synechococcus sp. PCC 11901, whose high-density growth was recently reported. In addition, UTEX 3222 sinks readily, settling more quickly than other fast-growing strains, suggesting favorable economics of harvesting UTEX 3222 biomass. These traits may make UTEX 3222 a compelling choice for marine carbon dioxide removal (CDR) and photosynthetic bioproduction from CO₂. Overall, we find that bio-prospecting in environments with naturally elevated CO₂ may uncover novel CO₂-metabolizing organisms with unique characteristics.

IMPORTANCE

Cyanobacteria provide a potential avenue for both biomanufacturing and combatting climate change via high-efficiency photosynthetic carbon sequestration. This study identifies novel photosynthetic organisms isolated from a unique geochemical environment and describes their genomes, growth behavior in culture, and biochemical composition. These cyanobacteria appear to make a tractable research model, and cultures are made publicly available alongside information about their culture and maintenance. Application of these organisms to carbon sequestration and/or biomanufacturing is discussed, including unusual, rapid settling characteristics of the strains relevant to scaled culture.

Genome-Scale Metabolic Model Reconstruction and Investigation into the Fluxome of the Fast-Growing Cyanobacterium Synechococcus sp. PCC 11901

The ability to convert atmospheric CO2 and light into biomass and value-added chemicals makes cyanobacteria a promising resource microbial host for biotechnological applications. A newly discovered fastest-growing cyanobacterial strain, Synechococcus sp. PCC 11901, has been reported to have the highest biomass accumulation rate, making it a preferred target host for producing renewable fuels, value-added biochemicals, and natural products. System-level knowledge of an organism is imperative to understand the metabolic potential of the strain, which can be attained by developing genome-scale metabolic models (GEMs). We present the first genome-scale metabolic model of Synechococcus sp. PCC 11901 (iRS840), which contains 840 genes, 1001 reactions, and 944 metabolites. The model has been optimized and validated under different trophic modes, i.e., autotrophic and mixotrophic, by conducting an in vivo growth experiment. The robustness of the metabolic network was evaluated by changing the biomass coefficient of the model, which showed a higher sensitivity toward pigments under the photoautotrophic condition, whereas under the heterotrophic condition, amino acids were found to be more influential. Furthermore, it was discovered that PCC 11901 synthesizes succinyl-CoA via succinic semialdehyde due to its imperfect TCA cycle. Subsequent flux balance analysis (FBA) revealed a quantum yield of 0.16 in silico, which is higher compared to that of PCC 6803. Under mixotrophic conditions (with glycerol and carbon dioxide), the flux through the Calvin cycle increased compared to autotrophic conditions. This model will be useful for gaining insights into the metabolic potential of PCC 11901 and developing effective metabolic engineering strategies for product development.

A toolbox to engineer the highly productive cyanobacterium Synechococcus sp. PCC 11901

Synechococcus sp. PCC 11901 (PCC 11901) is a fast-growing marine cyanobacterial strain that has a capacity for sustained biomass accumulation to very high cell densities, comparable to that achieved by commercially relevant heterotrophic organisms. However, genetic tools to engineer PCC 11901 for biotechnology applications are limited. Here we describe a suite of tools based on the CyanoGate MoClo system to unlock the engineering potential of PCC 11901. First, we characterized neutral sites suitable for stable genomic integration that do not affect growth even at high cell densities. Second, we tested a suite of constitutive promoters, terminators, and inducible promoters including a 2,4-diacetylphloroglucinol (DAPG)-inducible PhlF repressor system, which has not previously been demonstrated in cyanobacteria and showed tight regulation and a 228-fold dynamic range of induction. Lastly, we developed a DAPG-inducible dCas9-based CRISPR interference (CRISPRi) system and a modular method to generate markerless mutants using CRISPR-Cas12a. Based on our findings, PCC 11901 is highly responsive to CRISPRi-based repression and showed high efficiencies for single insertion (31% to 81%) and multiplex double insertion (25%) genome editing with Cas12a. We envision that these tools will lay the foundations for the adoption of PCC 11901 as a robust model strain for engineering biology and green biotechnology.

Bio-Sprayed/Threaded Microalgae Remain Viable and Indistinguishable from Controls

Microalgae are increasingly playing a significant role in many areas of research and development. Recent studies have demonstrated their ability to aid wound healing by their ability to generate oxygen, aiding the healing process. Bearing this in mind, the capability to spray/spin deposit microalgae in suspension (solution) or compartmentalize living microalgae within architectures such as fibers/scaffolds and beads, would have significance as healing mechanisms for addressing a wide range of wounds. Reconstructing microalgae-bearing architectures as either scaffolds or beads could be generated via electric field (bio-electrospraying and cell electrospinning) and non-electric field (aerodynamically assisted bio-jetting/threading) driven technologies. However, before studying the biomechanical properties of the generated living architectures, the microalgae exposed to these techniques must be interrogated from a molecular level upward first, to establish these techniques, have no negative effects brought on the processed microalgae. Therefore these studies, demonstrate the ability of both these jetting and threading technologies to directly handle living microalgae, in suspension or within a polymeric suspension, safely, and form algae-bearing architectures such as beads and fibers/scaffolds.

EXCRETE workflow enables deep proteomics of the microbial extracellular environment

Extracellular proteins play a significant role in shaping microbial communities which, in turn, can impact ecosystem function, human health, and biotechnological processes. Yet, for many ubiquitous microbes, there is limited knowledge regarding the identity and function of secreted proteins. Here, we introduce EXCRETE (enhanced exoproteome characterization by mass spectrometry), a workflow that enables comprehensive description of microbial exoproteomes from minimal starting material. Using cyanobacteria as a case study, we benchmark EXCRETE and show a significant increase over current methods in the identification of extracellular proteins. Subsequently, we show that EXCRETE can be miniaturized and adapted to a 96-well high-throughput format. Application of EXCRETE to cyanobacteria from different habitats (Synechocystis sp. PCC 6803, Synechococcus sp. PCC 11901, and Nostoc punctiforme PCC 73102), and in different cultivation conditions, identified up to 85% of all potentially secreted proteins. Finally, functional analysis reveals that cell envelope maintenance and nutrient acquisition are central functions of the predicted cyanobacterial secretome. Collectively, these findings challenge the general belief that cyanobacteria lack secretory proteins and suggest that multiple functions of the secretome are conserved across freshwater, marine, and terrestrial species.

Turning food waste to microbial lipid towards a superb economic and environmental sustainability: An innovative integrated biological route

With the drastic growth of the economic and population, the global energy requirement is on the rise, and massive human and material resources have been put into the development of alternative and renewable energy sources. Biodiesel has been recognized as a green and sustainable alternative energy, but the raw materials-associated source and cost makes it difficult to achieve large-scale commercial production. Microbial lipids (ML) produced by oleaginous microbes have attracted more and more topics as feedstocks for biodiesel production because of their unique advantages (fast growth cycle, small footprint and so on). However, there are still many problems and challenges ahead towards commercialization of ML-based biodiesel, especially the cost of feedstock for ML production. Food waste (FW) rich in organic matters and nutrients is an excellent and almost zero-cost feedstock for ML production. However, current biological routes of FW-based ML production have some defects, which make it impossible to achieve full industrialization at present. Therefore, this review intends to provide a critical and comprehensive analysis of current biological routes of FW-based ML production with the focus on the challenges and solutions forward. The biological routes towards future FW-based ML production must be able to concurrently achieve economic feasibility and environmental sustainability. On this condition, an innovative integrated biological route for FW-based ML production has thus been put forward, which is also elucidated on its economic and environmental sustainability. Moreover, the prospective advantages, limitations and challenges for future scale-up of FW-based ML production have also been outlined, together with the perspectives and directions forward.

What is holding back cyanobacterial research and applications? A survey of the cyanobacterial research community

Cyanobacteria are a diverse group of prokaryotic organisms that have been the subject of intense basic research, resulting in a wealth of knowledge about fundamental cellular processes such as photosynthesis. However, the translation of that research towards industry-relevant applications is still limited. To understand the reasons for this contradictory situation, we conducted a quantitative survey among researchers in the cyanobacterial community, a set of individual interviews with established researchers, and a literature analysis. Our results show that the community seems to be committed to embracing cyanobacterial diversity and promoting collaboration. Additionally, participants expressed a strong desire to develop standardized protocols for research and establish larger consortia to accelerate progress. The results of the survey highlight the need for a more integrated approach to cyanobacterial research that encompasses both basic and applied aspects. Based on the survey and interview results as well as our literature analysis, we highlight areas for potential improvement, strategies to enhance cyanobacterial research, and open questions that demand further exploration. Addressing these challenges should accelerate the development of industrial applications based on cyanobacterial research.

A quantitative description of light-limited cyanobacterial growth using flux balance analysis

The metabolism of phototrophic cyanobacteria is an integral part of global biogeochemical cycles, and the capability of cyanobacteria to assimilate atmospheric CO2 into organic carbon has manifold potential applications for a sustainable biotechnology. To elucidate the properties of cyanobacterial metabolism and growth, computational reconstructions of genome-scale metabolic networks play an increasingly important role. Here, we present an updated reconstruction of the metabolic network of the cyanobacterium Synechocystis sp. PCC 6803 and its quantitative evaluation using flux balance analysis (FBA). To overcome limitations of conventional FBA, and to allow for the integration of experimental analyses, we develop a novel approach to describe light absorption and light utilization within the framework of FBA. Our approach incorporates photoinhibition and a variable quantum yield into the constraint-based description of light-limited phototrophic growth. We show that the resulting model is capable of predicting quantitative properties of cyanobacterial growth, including photosynthetic oxygen evolution and the ATP/NADPH ratio required for growth and cellular maintenance. Our approach retains the computational and conceptual simplicity of FBA and is readily applicable to other phototrophic microorganisms.

Development of Leptolyngbya sp. BL0902 into a model organism for synthetic biological research in filamentous cyanobacteria

Cyanobacteria have great potential in CO2-based bio-manufacturing and synthetic biological studies. The filamentous cyanobacterium, Leptolyngbya sp. strain BL0902, is comparable to Arthrospira (Spirulina) platensis in commercial-scale cultivation while proving to be more genetically tractable. Here, we report the analyses of the whole genome sequence, gene inactivation/overexpression in the chromosome and deletion of non-essential chromosomal regions in this strain. The genetic manipulations were performed via homologous double recombination using either an antibiotic resistance marker or the CRISPR/Cpf1 editing system for positive selection. A desD-overexpressing strain produced γ-linolenic acid in an open raceway photobioreactor with the productivity of 0.36 g·m-2·d-1. Deletion mutants of predicted patX and hetR, two genes with opposite effects on cell differentiation in heterocyst-forming species, were used to demonstrate an analysis of the relationship between regulatory genes in the non-heterocystous species. Furthermore, a 50.8-kb chromosomal region was successfully deleted in BL0902 with the Cpf1 system. These results supported that BL0902 can be developed into a stable photosynthetic cell factory for synthesizing high value-added products, or used as a model strain for investigating the functions of genes that are unique to filamentous cyanobacteria, and could be systematically modified into a genome-streamlined chassis for synthetic biological purposes.

Disrupted H2 synthesis combined with methyl viologen treatment inhibits photosynthetic electron flow to synergistically enhance glycogen accumulation in the cyanobacterium Synechocystis sp. PCC 6803

Under nitrogen deprivation (-N), cyanobacterium Synechocystis sp. PCC 6803 exhibits growth arrest, reduced protein content, and remarkably increased glycogen accumulation. However, producing glycogen under this condition requires a two-step process with cell transfer from normal to -N medium. Metabolic engineering and chemical treatment for rapid glycogen accumulation can bypass the need for two-step cultivation. For example, recent studies indicate that individually disrupting hydrogen (H2) or poly(3-hydroxybutyrate) (PHB) synthesis, or treatment with methyl viologen (MV), effectively increases glycogen accumulation in Synechocystis. Here we explore the effects of disrupted H2 or poly(3-hydroxybutyrate) synthesis, together with MV treatment to on enhanced glycogen accumulation in Synechocystis grown in normal medium. Wild-type cells without MV treatment exhibited low glycogen content of less than 6% w/w dry weight (DW). Compared with wild type, disrupting PHB synthesis combined with MV treatment did not increase glycogen content. Disrupted H₂ production without MV treatment yielded up to 11% w/w DW glycogen content. Interestingly, when combined, disrupted H2 production with MV treatment synergistically enhanced glycogen accumulation to 51% and 59% w/w DW within 3 and 7 days, respectively. Metabolomic analysis suggests that MV treatment mediated the conversion of proteins into glycogen. Metabolomic and transcriptional-expression analysis suggests that disrupted H2 synthesis under MV treatment positively influenced glycogen synthesis. Disrupted H₂ synthesis under MV treatment significantly increased NADPH levels. This increased NADPH content potentially contributed to the observed enhancements in antioxidant activity against MV-induced oxidants, O2 evolution, and metabolite substrates levels for glycogen synthesis in normal medium, ultimately leading to enhanced glycogen accumulation in Synechocystis. KEY MESSAGE: Combining disrupted hydrogen-gas synthesis and the treatment by photosynthesis electron-transport inhibitor significantly enhance glycogen production in cyanobacteria.

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.

Application of Cyanobacteria as Chassis Cells in Synthetic Biology

Synthetic biology is an exciting new area of research that combines science and engineering to design and build new biological functions and systems. Predictably, with the development of synthetic biology, more efficient and economical photosynthetic microalgae chassis will be successfully constructed, making it possible to break through laboratory research into large-scale industrial applications. The synthesis of a range of biochemicals has been demonstrated in cyanobacteria; however, low product titers are the biggest barrier to the commercialization of cyanobacterial biotechnology. This review summarizes the applied improvement strategies from the perspectives of cyanobacteria chassis cells and synthetic biology. The harvest advantages of cyanobacterial products and the latest progress in improving production strategies are discussed according to the product status. As cyanobacteria synthetic biology is still in its infancy, apart from the achievements made, the difficulties and challenges in the application and development of cyanobacteria genetic tool kits in biochemical synthesis, environmental monitoring, and remediation were assessed.

Engineering highly productive cyanobacteria towards carbon negative emissions technologies

Cyanobacteria are a diverse and ecologically important group of photosynthetic prokaryotes that contribute significantly to the global carbon cycle through the capture of CO2 as biomass. Cyanobacterial biotechnology could play a key role in a sustainable bioeconomy through negative emissions technologies (NETs), such as carbon sequestration or bioproduction. However, the primary issues of low productivities and high infrastructure costs currently limit the commercialisation of such applications. The isolation of several fast-growing strains and recent advancements in molecular biology tools now offer promising new avenues for improving yields, including metabolic engineering approaches guided by high-throughput screening and metabolic models. Furthermore, emerging research on engineering coculture communities could help to develop more robust culturing systems to support broader NET applications.

Microbial Catalysis for CO2 Sequestration: A Geobiological Approach

One of the greatest threats facing the planet is the continued increase in excess greenhouse gasses, with CO2 being the primary driver due to its rapid increase in only a century. Excess CO2 is exacerbating known climate tipping points that will have cascading local and global effects including loss of biodiversity, global warming, and climate migration. However, global reduction of CO2 emissions is not enough. Carbon dioxide removal (CDR) will also be needed to avoid the catastrophic effects of global warming. Although the drawdown and storage of CO2 occur naturally via the coupling of the silicate and carbonate cycles, they operate over geological timescales (thousands of years). Here, we suggest that microbes can be used to accelerate this process, perhaps by orders of magnitude, while simultaneously producing potentially valuable by-products. This could provide both a sustainable pathway for global drawdown of CO2 and an environmentally benign biosynthesis of materials. We discuss several different approaches, all of which involve enhancing the rate of silicate weathering. We use the silicate mineral olivine as a case study because of its favorable weathering properties, global abundance, and growing interest in CDR applications. Extensive research is needed to determine both the upper limit of the rate of silicate dissolution and its potential to economically scale to draw down significant amounts (Mt/Gt) of CO2 Other industrial processes have successfully cultivated microbial consortia to provide valuable services at scale (e.g., wastewater treatment, anaerobic digestion, fermentation), and we argue that similar economies of scale could be achieved from this research.

Fast-growing cyanobacterial chassis for synthetic biology application

Carbon neutrality by 2050 has become one of the most urgent challenges the world faces today. To address the issue, it is necessary to develop and promote new technologies related with CO2 recycling. Cyanobacteria are the only prokaryotes performing oxygenic photosynthesis, capable of fixing CO2 into biomass under sunlight and serving as one of the most important primary producers on earth. Notably, recent progress on synthetic biology has led to utilizing model cyanobacteria such as Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 as chassis for "light-driven autotrophic cell factories" to produce several dozens of biofuels and various fine chemicals directly from CO2. However, due to the slow growth rate and low biomass accumulation in the current chassis, the productivity for most products is still lower than the threshold necessary for large-scale commercial application, raising the importance of developing high-efficiency cyanobacterial chassis with fast growth and/or higher biomass accumulation capabilities. In this article, we critically reviewed recent progresses on identification, systems biology analysis, and engineering of fast-growing cyanobacterial chassis. Specifically, fast-growing cyanobacteria identified in recent years, such as S. elongatus UTEX 2973, S. elongatus PCC 11801, S. elongatus PCC 11802 and Synechococcus sp. PCC 11901 was comparatively analyzed. In addition, the progresses on their recent application in converting CO2 into chemicals, and genetic toolboxes developed for these new cyanobacterial chassis were discussed. Finally, the article provides insights into future challenges and perspectives on the synthetic biology application of cyanobacterial chassis.

Photosynthetic production of α-farnesene by engineered Synechococcus elongatus UTEX 2973 from carbon dioxide

Cyanobacteria are the prospective biosolar cell factories to produce a range of bioproducts through CO2 sequestration. Farnesene is a sesquiterpene with an array of applications in biofuels, pest management, cosmetics, flavours and fragrances. This is the first time a codon-optimized farnesene synthase (AFS) gene is engineered into the genomic neutral site of Synechococcus elongatus UTEX 2973 for farnesene synthesis through its endogenous methylerythritol phosphate (MEP) pathway, rendering UTEX AFS strain. Similarly, bottleneck gene(s) of the MEP pathway, 1-deoxy-D-xylulose-5-phosphate synthase (dxs) and/or fusion of isopentenyl diphosphate isomerase and farnesyl diphosphate synthase (idispA) were engineered engendering UTEX AFS::dxs, UTEX AFS::idispA and UTEX AFS::dxs::idispA strains. UTEX AFS::dxs::idispA achieves farnesene productivity of 2.57 mg/L/day, the highest among engineered cyanobacterial strains studied so far. It demonstrates farnesene production, which is 31.3-times higher than the UTEX AFS strain. Moreover, the engineered strains show similar productivity over a three-month period, stipulating the genetic stability of the strains.

Marine cyanobacterial biomass is an efficient feedstock for fungal bioprocesses

BACKGROUND

Marine cyanobacteria offer many sustainability advantages, such as the ability to fix atmospheric CO2, very fast growth and no dependence on freshwater for culture. Cyanobacterial biomass is a rich source of sugars and proteins, two essential nutrients for culturing any heterotroph. However, no previous study has evaluated their application as a feedstock for fungal bioprocesses.

RESULTS

In this work, we cultured the marine cyanobacterium Synechococcus sp. PCC 7002 in a 3-L externally illuminated bioreactor with working volume of 2 L with a biomass productivity of ~ 0.8 g L-1 day-1. Hydrolysis of the biomass with acids released proteins and hydrolyzed glycogen while hydrolysis of the biomass with base released only proteins but did not hydrolyze glycogen. Among the different acids tested, treatment with HNO3 led to the highest release of proteins and glucose. Cyanobacterial biomass hydrolysate (CBH) prepared in HNO3 was used as a medium to produce cellulase enzyme by the Penicillium funiculosum OAO3 strain while CBH prepared in HCl and treated with charcoal was used as a medium for citric acid by Aspergillus tubingensis. Approximately 50% higher titers of both products were obtained compared to traditional media.

CONCLUSIONS

These results show that the hydrolysate of marine cyanobacteria is an effective source of nutrients/proteins for fungal bioprocesses.

Cyanobacterial myxoxanthophylls: biotechnological interventions and biological implications

Cyanobacteria safeguard their photosynthetic machinery from oxidative damage caused by adverse environmental factors such as high-intensity light. Together with many photoprotective compounds, they contain myxoxanthophylls, a rare group of glycosidic carotenoids containing a high number of conjugated double bonds. These carotenoids have been shown to: have strong photoprotective effects, contribute to the integrity of the thylakoid membrane, and upregulate in cyanobacteria under a variety of stress conditions. However, their metabolic potential has not been fully utilized in the stress biology of cyanobacteria and the pharmaceutical industry due to a lack of mechanistic understanding and their insufficient biosynthesis. This review summarizes current knowledge on: biological function, genetic regulation, biotechnological production, and pharmaceutical potential of myxoxanthophyll, with a focus on strain engineering and parameter optimization strategies for increasing their cellular content. The summarized knowledge can be utilized in cyanobacterial metabolic engineering to improve the stress tolerance of useful strains and enhance the commercial-scale synthesis of myxoxanthophyll for pharmaceutical uses.

The diversity and ecological significance of microbial traits potentially involved in B12 biosynthesis in the global ocean

Cobalamin (B12), an essential nutrient and growth cofactor for many living organisms on Earth, can be fully synthesized only by selected prokaryotes in nature. Therefore, microbial communities related to B12 biosynthesis could serve as an example subsystem to disentangle the underlying ecological mechanisms balancing the function and taxonomic make-up of complex functional assemblages. By anchoring microbial traits potentially involved in B12 biosynthesis, we depict the biogeographic patterns of B12 biosynthesis genes and the taxa harboring them in the global ocean, despite the limitations of detecting de novo B12 synthesizers via metagenomes alone. Both the taxonomic and functional composition of B12 biosynthesis genes were strongly shaped by depth, differentiating the epipelagic zones from the mesopelagic layers. Functional genes related to B12 biosynthesis were relatively stably distributed across different oceans, but the taxa harboring them varied considerably, showing clear functional redundancy among microbial systems. Microbial taxa carrying B12 biosynthesis genes in the surface water were influenced by environmental factors such as temperature, oxygen, and nitrate. However, the composition of functional genes was only weakly associated with these environmental factors. Null model analyses demonstrated that determinism governed the variations in B12 biosynthesis genes, whereas a higher degree of stochasticity was associated with taxonomic variations. Significant associations were observed between the chlorophyll a concentration and B12 biosynthesis, confirming its importance in primary production in the global ocean. The results of this study reveal an essential ecological mechanism governing the assembly of microbes in nature: the environment selects for function rather than taxonomy; functional redundancy underlies stochastic community assembly.

Genomic Insights on the Carbon-Negative Workhorse: Systematical Comparative Genomic Analysis on 56 Synechococcus Strains

Synechococcus, a type of ancient photosynthetic cyanobacteria, is crucial in modern carbon-negative synthetic biology due to its potential for producing bioenergy and high-value products. With its high biomass, fast growth rate, and established genetic manipulation tools, Synechococcus has become a research focus in recent years. Abundant germplasm resources have been accumulated from various habitats, including temperature and salinity conditions relevant to industrialization. In this study, a comprehensive analysis of complete genomes of the 56 Synechococcus strains currently available in public databases was performed, clarifying genetic relationships, the adaptability of Synechococcus to the environment, and its reflection at the genomic level. This was carried out via pan-genome analysis and a detailed comparison of the functional gene groups. The results revealed an open-genome pattern, with 275 core genes and variable genome sizes within these strains. The KEGG annotation and orthology composition comparisons unveiled that the cold and thermophile strains have 32 and 84 unique KO functional units in their shared core gene functional units, respectively. Each KO functional unit reflects unique gene families and pathways. In terms of salt tolerance and comparative genomics, there are 65 unique KO functional units in freshwater-adapted strains and 154 in strictly marine strains. By delving into these aspects, our understanding of the metabolic potential of Synechococcus was deepened, promoting the development and industrial application of cyanobacterial biotechnology.

Engineering Cyanobacterial Cell Factories for Photosynthetic Production of Fructose

Fructose is an important monosaccharide product widely applied in the food, medicine, and chemical industries. Currently, fructose is mainly manufactured with plant biomass-sourced polysaccharides through multiple steps of digestion, conversion, separation, and purification. The development of cyanobacterial metabolic engineering provides an attractive alternative route for the one-step direct production of fructose utilizing carbon dioxide and solar energy. In this work, we developed a paradigm for engineering cyanobacterial chassis cells into efficient cell factories for the photosynthetic production of fructose. In a representative cyanobacterial strain, Synechococcus elongatus PCC 7942, knockout of fructokinase effectively activated the synthesis and secretion of fructose in hypersaline conditions, independent of any heterologous transporters. The native sucrose synthesis pathway was identified as playing a primary role in fructose synthesis. Through combinatory optimizations on the levels of metabolism, physiology, and cultivation, the fructose yield of the Synechococcus cell factories was stepwise improved to 3.9 g/L. Such a paradigm was also adopted to engineer another Synechococcus strain, the marine species Synechococcus sp. PCC 7002, and facilitated an even higher fructose yield of over 6 g/L. Finally, the fructose synthesized and secreted by the cyanobacterial photosynthetic cell factories was successfully extracted and prepared from the culture broth in the form of products with 86% purity through multistep separation-purification operations. This work demonstrated a paradigm for systematically engineering cyanobacteria for photosynthetic production of desired metabolites, and it also confirmed the feasibility and potential of cyanobacterial photosynthetic biomanufacturing as a simple and efficient route for fructose production.

Present and future potential role of toxin-producing Synechococcus in the tropical region

As anthropogenic induced temperature rises and nutrient loadings increase in fresh and brackish environments, the ecological function of the phytoplankton community is expected to favour the picocyanobacteria, of the genus Synechococcus. Synechococcus is already a ubiquitous cyanobacterium found in both freshwater and marine environments, notwithstanding that the toxigenic species still remains unexplored in many freshwaters. Their fast growth rate and their ability to produce toxins make Synechococcus a potential dominant player in harmful algal blooms under climate change scenarios. This study examines the responses of a novel toxin-producing Synechococcus (i.e., one belonging to a freshwater clade; the other belonging to a brackish clade) to environmental changes that reflect climate change effects. We conducted a series of controlled experiments under present and predicted future temperatures, as well as under various N and P nutrients loadings. Our findings highlight how Synechococcus can be altered by the differing reactions to increasing temperature and nutrients, which resulted in considerable variations in cell abundance, growth rate, death rate, cellular stoichiometry and toxin production. Synechococcus had the highest growth observed at 28 °C, and further increases in temperature resulted in a decline for both fresh and brackish waters. Cellular stoichiometry was also altered, where more nitrogen (N) per cell was required, and the plasticity of N:P was more severe for the brackish clade. However, Synechococcus become more toxic under future scenario. Anatoxin-a (ATX) saw the greatest spike when temperature was at 34 °C especially under P-enrichment conditions. In contrast, Cylindrospermopsin (CYN) was promoted at the lowest tested temperature (25 °C) and under N-limitation. Overall, both temperature and external nutrients are the dominant control over Synechococcus toxins production. A model was also created to assess Synechococcus toxicity to zooplankton grazing. Zooplankton grazing was reduced by two folds under nutrient limitation, but temperature accounted for very insignificant change.

Isotopically non-stationary 13 C metabolic flux analysis of two closely related fast-growing cyanobacteria, Synechococcus elongatus PCC 11801 and 11802

Synechococcus elongatus PCC 11801 and 11802 are closely related cyanobacterial strains that are fast-growing and tolerant to high light and temperature. These strains hold significant promise as chassis for photosynthetic production of chemicals from carbon dioxide. A detailed quantitative understanding of the central carbon pathways would be a reference for future metabolic engineering studies with these strains. We conducted isotopic non-stationary 13 C metabolic flux analysis to quantitively assess the metabolic potential of these two strains. This study highlights key similarities and differences in the central carbon flux distribution between these and other model/non-model strains. The two strains demonstrated a higher Calvin-Benson-Bassham (CBB) cycle flux coupled with negligible flux through the oxidative pentose phosphate pathway and the photorespiratory pathway and lower anaplerosis fluxes under photoautotrophic conditions. Interestingly, PCC 11802 shows the highest CBB cycle and pyruvate kinase flux values among those reported in cyanobacteria. The unique tricarboxylic acid (TCA) cycle diversion in PCC 11801 makes it ideal for the large-scale production of TCA cycle-derived chemicals. Additionally, dynamic labeling transients were measured for intermediates of amino acid, nucleotide, and nucleotide sugar metabolism. Overall, this study provides the first detailed metabolic flux maps of S. elongatus PCC 11801 and 11802, which may aid metabolic engineering efforts in these strains.

Global translational control by the transcriptional repressor TrcR in the filamentous cyanobacterium Anabaena sp. PCC 7120

Transcriptional and translational regulations are important mechanisms for cell adaptation to environmental conditions. In addition to house-keeping tRNAs, the genome of the filamentous cyanobacterium Anabaena sp. strain PCC 7120 (Anabaena) has a long tRNA operon (trn operon) consisting of 26 genes present on a megaplasmid. The trn operon is repressed under standard culture conditions, but is activated under translational stress in the presence of antibiotics targeting translation. Using the toxic amino acid analog β-N-methylamino-L-alanine (BMAA) as a tool, we isolated and characterized several BMAA-resistance mutants from Anabaena, and identified one gene of unknown function, all0854, named as trcR, encoding a transcription factor belonging to the ribbon-helix-helix (RHH) family. We provide evidence that TrcR represses the expression of the trn operon and is thus the missing link between the trn operon and translational stress response. TrcR represses the expression of several other genes involved in translational control, and is required for maintaining translational fidelity. TrcR, as well as its binding sites, are highly conserved in cyanobacteria, and its functions represent an important mechanism for the coupling of the transcriptional and translational regulations in cyanobacteria.

Unlocking the potentials of cyanobacterial photosynthesis for directly converting carbon dioxide into glucose

Glucose is the most abundant monosaccharide, serving as an essential energy source for cells in all domains of life and as an important feedstock for the biorefinery industry. The plant-biomass-sugar route dominates the current glucose supply, while the direct conversion of carbon dioxide into glucose through photosynthesis is not well studied. Here, we show that the potential of Synechococcus elongatus PCC 7942 for photosynthetic glucose production can be unlocked by preventing native glucokinase activity. Knocking out two glucokinase genes causes intracellular accumulation of glucose and promotes the formation of a spontaneous mutation in the genome, which eventually leads to glucose secretion. Without heterologous catalysis or transportation genes, glucokinase deficiency and spontaneous genomic mutation lead to a glucose secretion of 1.5 g/L, which is further increased to 5 g/L through metabolic and cultivation engineering. These findings underline the cyanobacterial metabolism plasticities and demonstrate their applications for supporting the direct photosynthetic production of glucose.

Cyanotoxins and Other Bioactive Compounds from the Pasteur Cultures of Cyanobacteria (PCC)

In tribute to the bicentenary of the birth of Louis Pasteur, this report focuses on cyanotoxins, other natural products and bioactive compounds of cyanobacteria, a phylum of Gram-negative bacteria capable of carrying out oxygenic photosynthesis. These microbes have contributed to changes in the geochemistry and the biology of Earth as we know it today. Furthermore, some bloom-forming cyanobacterial species are also well known for their capacity to produce cyanotoxins. This phylum is preserved in live cultures of pure, monoclonal strains in the Pasteur Cultures of Cyanobacteria (PCC) collection. The collection has been used to classify organisms within the Cyanobacteria of the bacterial kingdom and to investigate several characteristics of these bacteria, such as their ultrastructure, gas vacuoles and complementary chromatic adaptation. Thanks to the ease of obtaining genetic and further genomic sequences, the diversity of the PCC strains has made it possible to reveal some main cyanotoxins and to highlight several genetic loci dedicated to completely unknown natural products. It is the multidisciplinary collaboration of microbiologists, biochemists and chemists and the use of the pure strains of this collection that has allowed the study of several biosynthetic pathways from genetic origins to the structures of natural products and, eventually, their bioactivity.

Impact of engineering the ATP synthase rotor ring on photosynthesis in tobacco chloroplasts

The chloroplast ATP synthase produces the ATP needed for photosynthesis and plant growth. The trans-membrane flow of protons through the ATP synthase rotates an oligomeric assembly of c subunits, the c-ring. The ion-to-ATP ratio in rotary F1F0-ATP synthases is defined by the number of c-subunits in the rotor c-ring. Engineering the c-ring stoichiometry is, therefore, a possible route to manipulate ATP synthesis by the ATP synthase and hence photosynthetic efficiency in plants. Here, we describe the construction of a tobacco (Nicotiana tabacum) chloroplast atpH (chloroplastic ATP synthase subunit c gene) mutant in which the c-ring stoichiometry was increased from 14 to 15 c-subunits. Although the abundance of the ATP synthase was decreased to 25% of wild-type (WT) levels, the mutant lines grew as well as WT plants and photosynthetic electron transport remained unaffected. To synthesize the necessary ATP for growth, we found that the contribution of the membrane potential to the proton motive force was enhanced to ensure a higher proton flux via the c15-ring without unwanted low pH-induced feedback inhibition of electron transport. Our work opens avenues to manipulate plant ion-to-ATP ratios with potentially beneficial consequences for photosynthesis.

Dynamics of a harvested cyanobacteria-fish model with modified Holling type Ⅳ functional response

In this paper, considering the aggregation effect and Allee effect of cyanobacteria populations and the harvesting of both cyanobacteria and fish by human beings, a new cyanobacteria-fish model with two harvesting terms and a modified Holling type Ⅳ functional response function is proposed. The main purpose of this paper is to further elucidate the influence of harvesting terms on the dynamic behavior of a cyanobacteria-fish model. Critical conditions for the existence and stability of several interior equilibria are given. The economic equilibria and the maximum sustainable total yield problem are also studied. The model exhibits several bifurcations, such as transcritical bifurcation, saddle-node bifurcation, Hopf bifurcation and Bogdanov-Takens bifurcation. It is concluded from a biological perspective that the survival mode of cyanobacteria and fish can be determined by the harvesting terms. Finally, concrete examples of our model are given through numerical simulations to verify and enrich the theoretical results.

Light and carbon: Synthetic biology toward new cyanobacteria-based living biomaterials

Cyanobacteria are ideal candidates to use in developing carbon neutral and carbon negative technologies; they are efficient photosynthesizers and amenable to genetic manipulation. Over the past two decades, researchers have demonstrated that cyanobacteria can make sustainable, useful biomaterials, many of which are engineered living materials. However, we are only beginning to see such technologies applied at an industrial scale. In this review, we explore the ways in which synthetic biology tools enable the development of cyanobacteria-based biomaterials. First we give an overview of the ecological and biogeochemical importance of cyanobacteria and the work that has been done using cyanobacteria to create biomaterials so far. This is followed by a discussion of commonly used cyanobacteria strains and synthetic biology tools that exist to engineer cyanobacteria. Then, three case studies-bioconcrete, biocomposites, and biophotovoltaics-are explored as potential applications of synthetic biology in cyanobacteria-based materials. Finally, challenges and future directions of cyanobacterial biomaterials are discussed.

Recent developments in the production and utilization of photosynthetic microorganisms for food applications

The growing use of photosynthetic microorganisms for food and food-related applications is driving related biotechnology research forward. Increasing consumer acceptance, high sustainability, demand of eco-friendly sources for food, and considerable global economic concern are among the main factors to enhance the focus on the novel foods. In the cases of not toxic strains, photosynthetic microorganisms not only provide a source of sustainable nutrients but are also potentially healthy. Several published studies showed that microalgae are sources of accessible protein and fatty acids. More than 400 manuscripts were published per year in the last 4 years. Furthermore, industrial approaches utilizing these microorganisms are resulting in new jobs and services. This is in line with the global strategy for bioeconomy that aims to support sustainable development of bio-based sectors. Despite the recognized potential of the microalgal biomass value chain, significant knowledge gaps still exist especially regarding their optimized production and utilization. This review highlights the potential of microalgae and cyanobacteria for food and food-related applications as well as their market size. The chosen topics also include advanced production as mixed microbial communities, production of high-value biomolecules, photoproduction of terpenoid flavoring compounds, their utilization for sustainable agriculture, application as source of nutrients in space, and a comparison with heterotrophic microorganisms like yeast to better evaluate their advantages over existing nutrient sources. This comprehensive assessment should stimulate further interest in this highly relevant research topic.

Current Metabolic Engineering Strategies for Photosynthetic Bioproduction in Cyanobacteria

Cyanobacteria are photosynthetic microorganisms capable of using solar energy to convert CO2 and H2O into O2 and energy-rich organic compounds, thus enabling sustainable production of a wide range of bio-products. More and more strains of cyanobacteria are identified that show great promise as cell platforms for the generation of bioproducts. However, strain development is still required to optimize their biosynthesis and increase titers for industrial applications. This review describes the most well-known, newest and most promising strains available to the community and gives an overview of current cyanobacterial biotechnology and the latest innovative strategies used for engineering cyanobacteria. We summarize advanced synthetic biology tools for modulating gene expression and their use in metabolic pathway engineering to increase the production of value-added compounds, such as terpenoids, fatty acids and sugars, to provide a go-to source for scientists starting research in cyanobacterial metabolic engineering.

Chemical Triggering Cyanobacterial Glycogen Accumulation: Methyl Viologen Treatment Increases Synechocystis sp. PCC 6803 Glycogen Storage by Enhancing Levels of Gene Transcript and Substrates in Glycogen Synthesis

Two-stage cultivation is effective for glycogen production by cyanobacteria. Cells were first grown under adequate nitrate supply (BG11) to increase biomass and subsequently transferred to nitrogen deprivation (-N) to stimulate glycogen accumulation. However, the two-stage method is time-consuming and requires extensive energy. Thus, one-stage cultivation that enables both cell growth and glycogen accumulation is advantageous. Such one-stage method could be achieved using a chemical triggering glycogen storage. However, there is a limited study on such chemicals. Here, nine compounds previously reported to affect cyanobacterial cellular functions were examined in Synechocystis sp. PCC 6803. 2-Phenylethanol, phenoxyethanol, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and methyl viologen can stimulate glycogen accumulation. The oxidative stress agent, methyl viologen significantly increased glycogen levels up to 57% and 69% [w/w dry weight (DW)] under BG11 and -N cultivation, respectively. One-stage cultivation where methyl viologen was directly added to the pre-grown culture enhanced glycogen storage to 53% (w/w DW), compared to the 10% (w/w DW) glycogen level of the control cells without methyl viologen. Methyl viologen treatment reduced the contents of total proteins (including phycobiliproteins) but caused increased transcript levels of glycogen synthetic genes and elevated levels of metabolite substrates for glycogen synthesis. Metabolomic results suggested that upon methyl viologen treatment, proteins degraded to amino acids, some of which could be used as a carbon source for glycogen synthesis. Results of oxygen evolution and metabolomic analysis suggested that photosynthesis and carbon fixation were not completely inhibited upon methyl viologen treatment, and these two processes may partially generate upstream metabolites required for glycogen synthesis.

Introduction to Cyanobacteria

Cyanobacteria are highly interesting microbes with the capacity for oxygenic photosynthesis. They fulfill an important purpose in nature but are also potent biocatalysts. This chapter gives a brief overview of this diverse phylum and shortly addresses the functions these organisms have in the natural ecosystems. Further, it introduces the main topics covered in this volume, which is dealing with the development and application of cyanobacteria as solar cell factories for the production of chemicals including potential fuels. We discuss cyanobacteria as industrial workhorses, present established chassis strains, and give an overview of the current target products. Genetic engineering strategies aiming at the photosynthetic efficiency as well as approaches to optimize carbon fluxes are summarized. Finally, main cultivation strategies are sketched.

Temperature-induced zeaxanthin overproduction in Synechococcus elongatus PCC 7942

The exogenous crtZ gene from Brevundimonas sp. SD212, coding for a 3,3' β-car hydroxylase, was expressed in Synechococcus elongatus PCC 7942 under the control of a temperature-inducible promoter in an attempt to engineer the carotenoid metabolic pathway, to increase the content of zeaxanthin and its further hydroxylated derivatives caloxanthin and nostoxanthin. These molecules are of particular interest due to their renowned antioxidant properties. Cultivation of the engineered strain S7942Z-Ti at 35 °C, a temperature which is well tolerated by the wild-type strain and at which the inducible expression system is activated, led to a significant redistribution of the relative carotenoid content. β-Carotene decreased to about 10% of the pool that is an excess of a threefold decrease with respect to the control, and concomitantly, zeaxanthin became the dominant carotenoid accounting for about half of the pool. As a consequence, zeaxanthin and its derivatives caloxanthin and nostoxanthin collectively accounted for about 90% of the accumulated carotenoids. Yet, upon induction of CrtZ expression at 35 °C the S7942Z-Ti strain displayed a substantial growth impairment accompanied, initially, by a relative loss of carotenoids and successively by the appearance of chlorophyll degradation products which can be interpreted as markers of cellular stress. These observations suggest a limit to the exploitation of Synechococcus elongatus PCC 7942 for biotechnological purposes aimed at increasing the production of hydroxylated carotenoids.

Carbon-negative synthetic biology: challenges and emerging trends of cyanobacterial technology

Global warming and climate instability have spurred interest in using renewable carbon resources for the sustainable production of chemicals. Cyanobacteria are ideal cellular factories for carbon-negative production of chemicals owing to their great potentials for directly utilizing light and CO₂ as sole energy and carbon sources, respectively. However, several challenges in adapting cyanobacterial technology to industry, such as low productivity, poor tolerance, and product harvesting difficulty, remain. Synthetic biology may finally address these challenges. Here, we summarize recent advances in the production of value-added chemicals using cyanobacterial cell factories, particularly in carbon-negative synthetic biology and emerging trends in cyanobacterial applications. We also propose several perspectives on the future development of cyanobacterial technology for commercialization.

Genomic analysis and biochemical profiling of an unaxenic strain of Synechococcus sp. isolated from the Peruvian Amazon Basin region

Cyanobacteria are diverse photosynthetic microorganisms able to produce a myriad of bioactive chemicals. To make possible the rational exploitation of these microorganisms, it is fundamental to know their metabolic capabilities and to have genomic resources. In this context, the main objective of this research was to determine the genome features and the biochemical profile of Synechococcus sp. UCP002. The cyanobacterium was isolated from the Peruvian Amazon Basin region and cultured in BG-11 medium. Growth parameters, genome features, and the biochemical profile of the cyanobacterium were determined using standardized methods. Synechococcus sp. UCP002 had a specific growth rate of 0.086 ± 0.008 μ and a doubling time of 8.08 ± 0.78 h. The complete genome of Synechococcus sp. UCP002 had a size of ∼3.53 Mb with a high coverage (∼200x), and its quality parameters were acceptable (completeness = 99.29%, complete and single-copy genes = 97.5%, and contamination = 0.35%). Additionally, the cyanobacterium had six plasmids ranging from 24 to 200 kbp. The annotated genome revealed ∼3,422 genes, ∼ 3,374 protein-coding genes (with ∼41.31% hypothetical protein-coding genes), two CRISPR Cas systems, and 61 non-coding RNAs. Both the genome and plasmids had the genes for prokaryotic defense systems. Additionally, the genome had genes coding the transcription factors of the metalloregulator ArsR/SmtB family, involved in sensing heavy metal pollution. The biochemical profile showed primary nutrients, essential amino acids, some essential fatty acids, pigments (e.g., all-trans-β-carotene, chlorophyll a, and phycocyanin), and phenolic compounds. In conclusion, Synechococcus sp. UCP002 shows biotechnological potential to produce human and animal nutrients and raw materials for biofuels and could be a new source of genes for synthetic biological applications.

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.

Interactive effects of CO2 , temperature, irradiance, and nutrient limitation on the growth and physiology of the marine cyanobacterium Synechococcus (Cyanophyceae)

The marine cyanobacterium Synechococcus elongatus was grown in a continuous culture system to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO₂ (pCO2) on its growth and physiological characteristics. Cells were grown on a 14:10 h light:dark cycle at all combinations of low and high irradiance (50 and 300 μmol photons ⋅ m^-2 ⋅ s^-1 , respectively), low and high pCO₂ (400 and 1000 ppmv, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 20-45°C in 5°C increments. The maximum growth rate was ~4.5 · d^-1 at 30-35°C. Under nutrient-replete conditions, growth rates at most temperatures and irradiances were about 8% slower at a pCO₂ of 1000 ppmv versus 400 ppmv. The single exception was 45°C and high irradiance. Under those conditions, growth rates were ~45% higher at 1000 ppmv. Cellular carbon:nitrogen ratios were independent of temperature at a fixed relative growth rate but higher at high irradiance than at low irradiance. Initial slopes of photosynthesis-irradiance curves were higher at all temperatures under nutrient-replete versus nitrate-limited conditions; they were similar at all temperatures under high and low irradiance, except at 20°C, when they were suppressed at high irradiance. A model of phytoplankton growth in which cellular carbon was allocated to structure, storage, or the light or dark reactions of photosynthesis accounted for the general patterns of cell composition and growth rate. Allocation of carbon to the light reactions of photosynthesis was consistently higher at low versus high light and under nutrient-replete versus nitrate-limited conditions.

Synthetic biology in marine cyanobacteria: Advances and challenges

The current economic and environmental context requests an accelerating development of sustainable alternatives for the production of various target compounds. Biological processes offer viable solutions and have gained renewed interest in the recent years. For example, photosynthetic chassis organisms are particularly promising for bioprocesses, as they do not require biomass-derived carbon sources and contribute to atmospheric CO2 fixation, therefore supporting climate change mitigation. Marine cyanobacteria are of particular interest for biotechnology applications, thanks to their rich diversity, their robustness to environmental changes, and their metabolic capabilities with potential for therapeutics and chemicals production without requiring freshwater. The additional cyanobacterial properties, such as efficient photosynthesis, are also highly beneficial for biotechnological processes. Due to their capabilities, research efforts have developed several genetic tools for direct metabolic engineering applications. While progress toward a robust genetic toolkit is continuously achieved, further work is still needed to routinely modify these species and unlock their full potential for industrial applications. In contrast to the understudied marine cyanobacteria, genetic engineering and synthetic biology in freshwater cyanobacteria are currently more advanced with a variety of tools already optimized. This mini-review will explore the opportunities provided by marine cyanobacteria for a greener future. A short discussion will cover the advances and challenges regarding genetic engineering and synthetic biology in marine cyanobacteria, followed by a parallel with freshwater cyanobacteria and their current genetic availability to guide the prospect for marine species.

Chemicals Affecting Cyanobacterial Poly(3-hydroxybutyrate) Accumulation: 2-Phenylethanol Treatment Combined with Nitrogen Deprivation Synergistically Enhanced Poly(3-hydroxybutyrate) Storage in Synechocystis sp. PCC6803 and Anabaena sp. TISTR8076

Various photoautotrophic cyanobacteria increase the accumulation of bioplastic poly(3-hydroxybutyrate) (PHB) under nitrogen deprivation (-N) for energy storage. Several metabolic engineering enhanced cyanobacterial PHB accumulation, but these strategies are not applicable in non-gene-transformable strains. Alternatively, stimulating PHB levels by chemical exposure is desirable because it might be applied to various cyanobacterial strains. However, the study of such chemicals is still limited. Here, 19 compounds previously reported to affect bacterial cellular processes were evaluated for their effect on PHB accumulation in Synechocystis sp. PCC6803, where 3-(3,4-dichlorophenyl)-1,1-dimethylurea, methyl viologen, arsenite, phenoxyethanol and 2-phenylethanol were found to increase PHB accumulation. When cultivated with optimal nitrate supply, Synechocystis contained less than 0.5% [w/w dry weight (DW)] PHB, while cultivation under -N conditions increased the PHB content to 7% (w/w DW). Interestingly, the -N cultivation combined with 2-phenylethanol exposure reduced the Synechocystis protein content by 27% (w/w DW) but significantly increased PHB levels up to 33% (w/w DW), the highest ever reported photoautotrophic cyanobacterial PHB accumulation in a wild-type strain. Results from transcriptomic and metabolomic analysis suggested that under 2-phenylethanol treatment, Synechocystis proteins were degraded to amino acids, which might be subsequently utilized as the source of carbon and energy for PHB biosynthesis. 2-Phenylethanol treatment also increased the levels of metabolites required for Synechocystis PHB synthesis (acetyl-CoA, acetoacetyl-CoA, 3-hydroxybutyryl-CoA and NADPH). Additionally, under -N, the exposure to phenoxyethanol and 2-phenylethanol increased the PHB levels of Anabaena sp. from 0.4% to 4.1% and 6.6% (w/w DW), respectively. The chemicals identified in this study might be applicable for enhancing PHB accumulation in other cyanobacteria.

A droplet-based microfluidic platform enables high-throughput combinatorial optimization of cyanobacterial cultivation

Cyanobacteria are fast-growing, genetically accessible, photoautotrophs. Therefore, they have attracted interest as sustainable production platforms. However, the lack of techniques to systematically optimize cultivation parameters in a high-throughput manner is holding back progress towards industrialization. To overcome this bottleneck, here we introduce a droplet-based microfluidic platform capable of one- (1D) and two-dimension (2D) screening of key parameters in cyanobacterial cultivation. We successfully grew three different unicellular, biotechnologically relevant, cyanobacteria: Synechocystis sp. PCC 6803, Synechococcus elongatus UTEX 2973 and Synechococcus sp. UTEX 3154. This was followed by a highly-resolved 1D screening of nitrate, phosphate, carbonate, and salt concentrations. The 1D screening results suggested that nitrate and/or phosphate may be limiting nutrients in standard cultivation media. Finally, we use 2D screening to determine the optimal N:P ratio of BG-11. Application of the improved medium composition in a high-density cultivation setup led to an increase in biomass yield of up to 15.7%. This study demonstrates that droplet-based microfluidics can decrease the volume required for cyanobacterial cultivation and screening up to a thousand times while significantly increasing the multiplexing capacity. Going forward, microfluidics have the potential to play a significant role in the industrial exploitation of cyanobacteria.