Fatty acid production in genetically modified cyanobacteria

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.

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.

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.

Machine learning model of the catalytic efficiency and substrate specificity of acyl-ACP thioesterase variants generated from natural and in vitro directed evolution

Modulating the catalytic activity of acyl-ACP thioesterase (TE) is an important biotechnological target for effectively increasing flux and diversifying products of the fatty acid biosynthesis pathway. In this study, a directed evolution approach was developed to improve the fatty acid titer and fatty acid diversity produced by E. coli strains expressing variant acyl-ACP TEs. A single round of in vitro directed evolution, coupled with a high-throughput colorimetric screen, identified 26 novel acyl-ACP TE variants that convey up to a 10-fold increase in fatty acid titer, and generate altered fatty acid profiles when expressed in a bacterial host strain. These in vitro-generated variant acyl-ACP TEs, in combination with 31 previously characterized natural variants isolated from diverse phylogenetic origins, were analyzed with a random forest classifier machine learning tool. The resulting quantitative model identified 22 amino acid residues, which define important structural features that determine the catalytic efficiency and substrate specificity of acyl-ACP TE.

Metabolic engineering to produce palmitic acid or palmitoleic acid in an oleic acid-producing Corynebacterium glutamicum strain

Focusing on the differences in the catalytic properties of two type I fatty acid synthases FasA and FasB, the fasA gene was disrupted in an oleic acid-producing Corynebacterium glutamicum strain. The resulting oleic acid-requiring strain whose fatty acid synthesis depends only on FasB exhibited almost exclusive production (217 mg/L) of palmitic acid (C16:0) from 1% glucose under the conditions supplemented with the minimum concentration of sodium oleate for growth. Plasmid-mediated amplification of fasB led to a 1.47-fold increase in palmitic acid production (320 mg/L), while fasB disruption resulted in no fatty acid production, with excretion of malonic acid (30 mg/L). Next, aiming at conversion of the palmitic acid producer to a producer of palmitoleic acid (POA, C16:1Δ9), we introduced the Pseudomonas nitroreducens Δ9-desaturase genes desBC into the palmitic acid producer. Although this resulted in failure, we noticed the emergence of suppressor mutants that exhibited the oleic acid-non-requiring phenotype. Production experiments revealed that one such mutant M-1 undoubtedly produced POA (17 mg/L) together with palmitic acid (173 mg/L). Whole genomic analysis and subsequent genetic analysis identified the suppressor mutation of strain M-1 as a loss-of-function mutation for the DtxR protein, a global regulator of iron metabolism. Considering that DesBC are both iron-containing enzymes, we investigated the conditions for increased iron availability to improve the DesBC-dependent conversion ratio of palmitic acid to POA. Eventually, supplementation of both hemin and the iron chelator protocatechuic acid in the engineered strain dramatically enhanced POA production to 161 mg/L with a conversion ratio of 80.1%. Cellular fatty acid analysis revealed that the POA-producing cells were really equipped with unnatural membrane lipids comprised predominantly of palmitic acid (85.1% of total cellular fatty acids), followed by non-native POA (12.4%).

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.

Marinobacter: A case study in bioelectrochemical chassis evaluation

The junction of bioelectrochemical systems and synthetic biology opens the door to many potentially groundbreaking technologies. When developing these possibilities, choosing the correct chassis organism can save a great deal of engineering effort and, indeed, can mean the difference between success and failure. Choosing the correct chassis for a specific application requires a knowledge of the metabolic potential of the candidate organisms, as well as a clear delineation of the traits, required in the application. In this review, we will explore the metabolic and electrochemical potential of a single genus, Marinobacter. We will cover its strengths, (salt tolerance, biofilm formation and electrochemical potential) and weaknesses (insufficient characterization of many strains and a less developed toolbox for genetic manipulation) in potential synthetic electromicrobiology applications. In doing so, we will provide a roadmap for choosing a chassis organism for bioelectrochemical systems.

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.

Production of Fatty Acids and Derivatives Using Cyanobacteria

Fatty acids and their derivatives are highly valuable chemicals that can be produced through chemical or enzymatic processes using plant lipids. This may compete with human food sources. Therefore, there has been an urge to create a new method for synthesizing these chemicals. One approach is to use microbial cells, specifically cyanobacteria, as a factory platform. Engineering may need to be implemented in order to allow a cost-competitive production and to enable a production of a variety of different fatty acids and derivatives. In this chapter, we explain in details the importance of fatty acids and their derivatives, including fatty aldehydes, fatty alcohols, hydrocarbons, fatty acid methyl esters, and hydroxy fatty acids. The production of these chemicals using cyanobacterial native metabolisms together with strategies to engineer them are also explained. Moreover, recent examples of fatty acid and fatty acid derivative production from engineered cyanobacteria are gathered and reported. Commercial opportunities to manufacture fatty acids and derivatives are also discussed in this chapter. Altogether, it is clear that fatty acids and their derivatives are important chemicals, and with recent advancements in genetic engineering, a cyanobacterial platform for bio-based production is feasible. However, there are regulations and guidelines in place for the use of genetically modified organisms (GMOs) and some further developments are still needed before commercialization can be reached.

Characterization of a cyanobacterial rep protein with broad-host range and its utilization for expression vectors

Owing to their photosynthetic capabilities, cyanobacteria are regarded as ecologically friendly hosts for production of biomaterials. However, compared to other bacteria, tools for genetic engineering, especially expression vector systems, are limited. In this study, we characterized a Rep protein, exhibiting replication activity in multiple cyanobacteria and established an expression vector using this protein. Our comprehensive screening using a genomic library of Synechocystis sp. PCC 6803 revealed that a certain region encoding a Rep-related protein (here named Cyanobacterial Rep protein A2: CyRepA2) exhibits high autonomous replication activity in a heterologous host cyanobacterium, Synechococcus elongatus PCC 7942. A reporter assay using GFP showed that the expression vector pYS carrying CyRepA2 can be maintained in not only S. 6803 and S. 7942, but also Synechococcus sp. PCC 7002 and Anabaena sp. PCC 7120. In S. 7942, GFP expression in the pYS-based system was tightly regulated by IPTG, achieving 10-fold higher levels than in the chromosome-based system. Furthermore, pYS could be used together with the conventional vector pEX, which was constructed from an endogenous plasmid in S. 7942. The combination of pYS with other vectors is useful for genetic engineering, such as modifying metabolic pathways, and is expected to improve the performance of cyanobacteria as bioproduction chassis.

Mutualistic microbial community of Bacillus amyloliquefaciens and recombinant Yarrowia lipolytica co-produced lipopeptides and fatty acids from food waste

Bioconversion is an important method for transforming food waste (FW) into high value-added products, rendering it harmless, and recycling resources. An artificial microbial consortium (AMC) was constructed to produce FW-based lipopeptides in order to investigate the strategy of FW bioconversion into value-added products. Exogenous fatty acids as a precursor significantly improved the lipopeptide production of Bacillus amyloliquefaciens HM618. To enhance fatty acid synthesis and efflux in AMC, the recombinant Yarrowia lipolytica YL21 (strain YL21) was constructed by screening 12 target genes related to fatty acids to replace exogenous fatty acids in order to improve lipopeptide production. The levels of fengycin, surfactin, and iturin A in the AMC of strains HM618 and YL21 reached 76.19, 192.80, and 31.32 mg L^-1, increasing 7.24-, 12.13-, and 3.23-fold compared to the results from the pure culture of strain HM618 in flask with Landy medium, respectively. Furthermore, free fatty acids were almost undetectable in the co-culture of strains HM618 and YL21, although its level was around 1.25 g L^-1 in the pure culture of strain YL21 with Landy medium. Interestingly, 470.24 mg L^-1 of lipopeptides and 18.11 g L^-1 of fatty acids were co-produced in this AMC in a bioreactor with FW medium. To our knowledge, it is the first report of FW biotransformation into co-produce of lipopeptides and fatty acids in the AMC of B. amyloliquefaciens and Y. lipolytica. These results provide new insights into the biotransformation potential of FW for value-added co-products by AMC.

Metabolic Engineering Design Strategies for Increasing Carbon Fluxes Relevant for Biosynthesis in Cyanobacteria

Cyanobacteria are promising microbial cell factories for the direct production of biochemicals and biofuels from CO2. Through genetic and metabolic engineering, they can be modified to produce a variety of both natural and non-natural compounds. To enhance the yield of these products, various design strategies have been developed. In this chapter, strategies used to enhance metabolic fluxes towards common precursors used in biosynthesis, including pyruvate, acetyl-CoA, malonyl-CoA, TCA cycle intermediates, and aromatics, are discussed. Additionally, strategies related to cofactor availability and mixotrophic conditions for bioproduction are also summarize.

Role of regulatory pathways and multi-omics approaches for carbon capture and mitigation in cyanobacteria

Cyanobacteria are known for their metabolic potential and carbon capture and sequestration capabilities. These cyanobacteria are not only an effective source for carbon minimization and resource mobilization into value-added products for biotechnological gains. The present review focuses on the detailed description of carbon capture mechanisms exerted by the various cyanobacterial strains, the role of important regulatory pathways, and their subsequent genes responsible for such mechanisms. Moreover, this review will also describe effectual mechanisms of central carbon metabolism like isoprene synthesis, ethylene production, MEP pathway, and the role of Glyoxylate shunt in the carbon sequestration mechanisms. This review also describes some interesting facets of using carbon assimilation mechanisms for valuable bio-products. The role of regulatory pathways and multi-omics approaches in cyanobacteria will not only be crucial towards improving carbon utilization but also will give new insights into utilizing cyanobacterial bioresource for carbon neutrality.

Enhanced productivity of extracellular free fatty acids by gene disruptions of acyl-ACP synthetase and S-layer protein in Synechocystis sp. PCC 6803

Background

Based on known metabolic response to excess free fatty acid (FFA) products, cyanobacterium Synechocystis sp. PCC 6803 preferentially both recycles via FFA recycling process and secrets them into medium. Engineered cyanobacteria with well growth and highly secreted FFA capability are considered best resources for biofuel production and sustainable biotechnology. In this study, to achieve the higher FFA secretion goal, we successfully constructs Synechocystis sp. PCC 6803 mutants disrupting genes related to FFA recycling reaction (aas gene encoding acyl–acyl carrier protein synthetase), and surface layer protein (encoded by sll1951).

Results

Three Synechocystis sp. PCC 6803 engineered strains, including two single mutants lacking aas (KA) and sll1951 (KS), and one double mutant lacking both aas and sll1951 (KAS), significantly secreted FFAs higher than that of wild type (WT). Certain increase of secreted FFAs was noted when cells were exposed to nitrogen-deficient conditions, BG₁₁-half N and BG₁₁-N conditions, with the exception of strain KS. Under BG₁₁-N condition at day 10, strain KAS strikingly secreted FFAs products up to 40%w/DCW or 238.1 mg/L, with trace amounts of PHB. Unexpectedly, strain KS, with S-layer disruption, appeared to have endured longer in BG₁₁-N growth medium. This strain KS significantly acclimated to the BG₁₁-N environment by accumulating a greater glycogen pool with lower FFA production, whereas strain KA favored higher PHB and intracellular lipid accumulations with moderate FFA secretion.

Conclusions

Mutations of both aas and sll1951 genes in Synechocystis sp. PCC 6803 significantly improved the productivity of secreted FFAs, especially under nitrogen deprivation.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Rhythm of the Night (and Day): Predictive Metabolic Modeling of Diurnal Growth in Chlamydomonas

Economical production of photosynthetic organisms requires the use of natural day/night cycles. These induce strong circadian rhythms that lead to transient changes in the cells, requiring complex modeling to capture. In this study, we coupled times series transcriptomic data from the model green alga Chlamydomonas reinhardtii to a metabolic model of the same organism in order to develop the first transient metabolic model for diurnal growth of algae capable of predicting phenotype from genotype. We first transformed a set of discrete transcriptomic measurements (D. Strenkert, S. Schmollinger, S. D. Gallaher, P. A. Salomé, et al., Proc Natl Acad Sci U S A 116:2374–2383, 2019, https://doi.org/10.1073/pnas.1815238116) into continuous curves, producing a complete database of the cell’s transcriptome that can be interrogated at any time point. We also decoupled the standard biomass formation equation to allow different components of biomass to be synthesized at different times of the day. The resulting model was able to predict qualitative phenotypical outcomes of a starchless mutant. We also extended this approach to simulate all single-knockout mutants and identified potential targets for rational engineering efforts to increase productivity. This model enables us to evaluate the impact of genetic and environmental changes on the growth, biomass composition, and intracellular fluxes for diurnal growth.

IMPORTANCE We have developed the first transient metabolic model for diurnal growth of algae based on experimental data and capable of predicting phenotype from genotype. This model enables us to evaluate the impact of genetic and environmental changes on the growth, biomass composition and intracellular fluxes of the model green alga, Chlamydomonas reinhardtii. The availability of this model will enable faster and more efficient design of cells for production of fuels, chemicals, and pharmaceuticals.

Heterologous Lactate Synthesis in Synechocystis sp. Strain PCC 6803 Causes a Growth Condition-Dependent Carbon Sink Effect

Cyanobacteria are considered promising hosts for product synthesis directly from CO₂ via photosynthetic carbon assimilation. The introduction of heterologous carbon sinks in terms of product synthesis has been reported to induce the so-called “carbon sink effect,” described as the release of unused photosynthetic capacity by the introduction of additional carbon. This effect is thought to arise from a limitation of carbon metabolism that represents a bottleneck in carbon and electron flow, thus enforcing a downregulation of photosynthetic efficiency. It is not known so far how the cellular source/sink balance under different growth conditions influences the extent of the carbon sink effect and in turn product formation from CO₂, constituting a heterologous carbon sink. We compared the Synechocystis sp. strain PCC 6803 wild type (WT) with an engineered lactate-producing strain (SAA023) in defined metabolic states. Unexpectedly, high-light conditions combined with carbon limitation enabled additional carbon assimilation for lactate production without affecting biomass formation. Thus, a strong carbon sink effect only was observed under carbon and thus sink limitation, but not under high-sink conditions. We show that the carbon sink effect was accompanied by an increased rate of alternative electron flow (AEF). Thus, AEF plays a crucial role in the equilibration of source/sink imbalances, presumably via ATP/NADPH balancing. This study emphasizes that the evaluation of the biotechnological potential of cyanobacteria profits from cultivation approaches enabling the establishment of defined metabolic states and respective quantitative analytics. Factors stimulating photosynthesis and carbon fixation are discussed.

IMPORTANCE Previous studies reported various and differing effects of the heterologous production of carbon-based molecules on photosynthetic and growth efficiency of cyanobacteria. The typically applied cultivation in batch mode, with continuously changing growth conditions, however, precludes a clear differentiation between the impact of cultivation conditions on cell physiology and effects related to the specific nature of the product and its synthesis pathway. In this study, we employed a continuous cultivation system to maintain defined source/sink conditions and thus metabolic states. This allowed a systematic and quantitative analysis of the effect of NADPH-consuming lactate production on photosynthetic and growth efficiency. This approach enables a realistic evaluation of the biotechnological potential of engineered cyanobacterial strains. For example, the quantum requirement for carbon production was found to constitute an excellent indicator of the source/sink balance and thus a key parameter for photobioprocess optimization. Such knowledge is fundamental for rational and efficient strain and process development.

Cytochrome P450s in algae: Bioactive natural product biosynthesis and light-driven bioproduction

Algae are a large group of photosynthetic organisms responsible for approximately half of the earth's total photosynthesis. In addition to their fundamental ecological roles as oxygen producers and as the food base for almost all aquatic life, algae are also a rich source of bioactive natural products, including several clinical drugs. Cytochrome P450 enzymes (P450s) are a superfamily of biocatalysts that are extensively involved in natural product biosynthesis by mediating various types of reactions. In the post-genome era, a growing number of P450 genes have been discovered from algae, indicating their important roles in algal life-cycle. However, the functional studies of algal P450s remain limited. Benefitting from the recent technical advances in algae cultivation and genetic manipulation, the researches on P450s in algal natural product biosynthesis have been approaching to a new stage. Moreover, some photoautotrophic algae have been developed into “photo-bioreactors” for heterologous P450s to produce high-value added pharmaceuticals and chemicals in a carbon-neutral or carbon-negative manner. Here, we comprehensively review these advances of P450 studies in algae from 2000 to 2021.

Photosynthetic symbiotic therapeutics - An innovative, effective treatment for ischemic cardiovascular diseases

Ischemic heart disease is a major cause of global morbidity and mortality, affecting over 15 million patients in the United States. Recent advances in research and innovation have greatly broadened clinicians' ability to treatment ischemic heart disease and associated heart failure using various preventive, pharmacologic, and surgical strategies. Specifically, innovative photosynthetic symbiotic systems using Synechococcus elongatus has gained significant attention. S. elongatus is a unicellular cyanobacterium that can carry out oxygenic photosynthesis. Photosynthetic therapies have been developed to rescue ischemic tissue by taking up tissue-derived carbon dioxide and in turn releasing oxygen for sustained aerobic metabolism during ischemia. In this article, we review the application of cyanobacteria, specifically S. elongatus, in the field of biotechnology, ischemic heart disease, and other clinical applications in ischemic diseases. We also address the motivation for innovation and current limitations in the field of S. elongatus photosynthetic therapeutics for ischemic cardiovascular disease.

Potential of Producing Flavonoids Using Cyanobacteria As a Sustainable Chassis

Numerous plant secondary metabolites have remarkable impacts on both food supplements and pharmaceuticals for human health improvement. However, higher plants can only generate small amounts of these chemicals with specific temporal and spatial arrangements, which are unable to satisfy the expanding market demands. Cyanobacteria can directly utilize CO₂, light energy, and inorganic nutrients to synthesize versatile plant-specific photosynthetic intermediates and organic compounds in large-scale photobioreactors with outstanding economic merit. Thus, they have been rapidly developed as a "green" chassis for the synthesis of bioproducts. Flavonoids, chemical compounds based on aromatic amino acids, are considered to be indispensable components in a variety of nutraceutical, pharmaceutical, and cosmetic applications. In contrast to heterotrophic metabolic engineering pioneers, such as yeast and Escherichia coli, information about the biosynthesis flavonoids and their derivatives is less comprehensive than that of their photosynthetic counterparts. Here, we review both benefits and challenges to promote cyanobacterial cell factories for flavonoid biosynthesis. With increasing concerns about global environmental issues and food security, we are confident that energy self-supporting cyanobacteria will attract increasing attention for the generation of different kinds of bioproducts. We hope that the work presented here will serve as an index and encourage more scientists to join in the relevant research area.

MoS2 Nanosheets-Cyanobacteria Interaction: Reprogrammed Carbon and Nitrogen Metabolism

Fully understanding the environmental implications of engineered nanomaterials is crucial for their safe and sustainable use. Cyanobacteria, as the pioneers of the planet earth, play important roles in global carbon and nitrogen cycling. Here, we evaluated the biological effects of molybdenum disulfide (MoS₂) nanosheets on a N₂-fixation cyanobacteria (Nostoc sphaeroides) by monitoring growth and metabolome changes. MoS₂ nanosheets did not exert overt toxicity to Nostoc at the tested doses (0.1 and 1 mg/L). On the contrary, the intrinsic enzyme-like activities and semiconducting properties of MoS₂ nanosheets promoted the metabolic processes of Nostoc, including enhancing CO₂-fixation-related Calvin cycle metabolic pathway. Meanwhile, MoS₂ boosted the production of a range of biochemicals, including sugars, fatty acids, amino acids, and other valuable end products. The altered carbon metabolism subsequently drove proportional changes in nitrogen metabolism in Nostoc. These intracellular metabolic changes could potentially alter global C and N cycles. The findings of this study shed light on the nature and underlying mechanisms of bio-nanoparticle interactions, and offer the prospect of utilization bio-nanomaterials for efficient CO₂ sequestration and sustainable biochemical production.

Overexpression of lipA or glpD_RuBisCO in the Synechocystis sp. PCC 6803 Mutant Lacking the Aas Gene Enhances Free Fatty-Acid Secretion and Intracellular Lipid Accumulation

Although engineered cyanobacteria for the production of lipids and fatty acids (FAs) are intelligently used as sustainable biofuel resources, intracellularly overproduced FAs disturb cellular homeostasis and eventually generate lethal toxicity. In order to improve their production by enhancing FFAs secretion into a medium, we constructed three engineered Synechocystis 6803 strains including KA (a mutant lacking the aas gene), KAOL (KA overexpressing lipA, encoding lipase A in membrane lipid hydrolysis), and KAOGR (KA overexpressing quadruple glpD/rbcLXS, related to the CBB cycle). Certain contents of intracellular lipids and secreted FFAs of all engineered strains were higher than those of the wild type. Remarkably, the KAOL strain attained the highest level of secreted FFAs by about 21.9%w/DCW at day 5 of normal BG₁₁ cultivation, with a higher growth rate and shorter doubling time. TEM images provided crucial evidence on the morphological changes of the KAOL strain, which accumulated abundant droplets on regions of thylakoid membranes throughout the cell when compared with wild type. On the other hand, BG₁₁-N condition significantly induced contents of both intracellular lipids and secreted FFAs of the KAOL strain up to 37.2 and 24.5%w/DCW, respectively, within 5 days. Then, for the first time, we shone a spotlight onto the overexpression of lipA in the aas mutant of Synechocystis as another potential strategy to achieve higher FFAs secretion with sustainable growth.

Approaches in the photosynthetic production of sustainable fuels by cyanobacteria using tools of synthetic biology

Cyanobacteria, photosynthetic prokaryotic microorganisms having a simple genetic composition are the prospective photoautotrophic cell factories for the production of a wide range of biofuel molecules. The simple genetic composition of cyanobacteria allows effortless genetic manipulation which leads to increased research endeavors from the synthetic biology approach. Various unicellular model cyanobacterial strains like Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 have been successfully engineered for biofuels generation. Improved development of synthetic biology tools, genetic modification methods and advancement in transformation techniques to construct a strain that can contain multiple foreign genes in a single operon have vastly expanded the functions that can be used for engineering photosynthetic cyanobacteria for the generation of various biofuel molecules. In this review, recent advancements and approaches in synthetic biology tools used for cyanobacterial genome editing have been discussed. Apart from this, cyanobacterial productions of various fuel molecules like isoprene, limonene, α-farnesene, squalene, alkanes, butanol, and fatty acids, which can be a substitute for petroleum and fossil fuels in the future, have been elaborated.

Development of CRISPR-Cas9 knock-in tools for free fatty acid production using the fast-growing cyanobacterial strain Synechococcus elongatus UTEX 2973

Synechococcus elongatus UTEX 2973 has one of the fastest measured doubling time of cyanobacteria making it an important candidate for metabolic engineering. Traditional genetic engineering methods, which rely on homologous recombination, however, are inefficient, labor-intensive, and time-consuming due to the oligoploidy or polyploidy nature of cyanobacteria and the reliance on unique antibiotic resistance markers. CRISPR-Cas9 has emerged as an effective and versatile editing platform in a wide variety of organisms, but its application for cyanobacterial engineering is limited by the inherent toxicity of Cas9 resulting in poor transformation efficiencies. Here, we demonstrated that a single-plasmid CRISPR-Cas9 system, pCRISPOmyces-2, can effectively knock-in a truncated thioesterase gene from Escherichia coli to generate free fatty acid (FFA) producing mutants of Syn2973. To do so, three parameters were evaluated on the effect of generating recipient colonies after conjugation with pCRISPOmyces-2-based plasmids: 1) a modified conjugation protocol termed streaked conjugation, 2) the deletion of the gene encoding RecJ exonuclease, and 3) single guide RNA (sgRNA) sequence. With the use of the streaked conjugation protocol and a ΔrecJ mutant strain of Syn2973, the conjugation efficiency for the pCRISPomyces-2 plasmid could be improved by 750-fold over the wildtype (WT) for a conjugation efficiency of 2.0 × 10^-6 transconjugants/recipient cell. While deletion of the RecJ exonuclease alone increased the conjugation efficiency by 150-fold over the WT, FFA generation was impaired in FFA-producing mutants with the ΔrecJ background, and the large number of poor FFA-producing isolates indicated the potential increase in spontaneous mutation rates. The sgRNA sequence was found to be critical in achieving the desired CRISPR-Cas9-mediated knock-in mutation as the sgRNA impacts conjugation efficiency, likelihood of homogenous recombinants, and free fatty acid production in engineered strains.

Emerging technologies for conversion of sustainable algal biomass into value-added products: A state-of-the-art review

Concerns regarding high energy demand and gradual depletion of fossil fuels have attracted the desire of seeking renewable and sustainable alternatives. Similar to but better than the first- and second-generation biomass, algae derived third-generation biorefinery aims to generate value-added products by microbial cell factories and has a great potential due to its abundant, carbohydrate-rich and lignin-lacking properties. However, it is crucial to establish an efficient process with higher competitiveness over the current petroleum industry to effectively utilize algal resources. In this review, we summarize the recent technological advances in maximizing the bioavailability of different algal resources. Following an overview of approaches to enhancing the hydrolytic efficiency, we review prominent opportunities involved in microbial conversion into various value-added products including alcohols, organic acids, biogas and other potential industrial products, and also provide key challenges and trends for future insights into developing biorefineries of marine biomass.

In Silico Study of the Structure and Ligand Preference of Pyruvate Kinases from Cyanobacterium Synechocystis sp. PCC 6803

Finding reliable cheap sources for producing chemicals and materials is always challenging. During recent decades, photosynthetic organisms such as cyanobacteria, which used CO₂ as a carbon source for making products, have attracted a great deal of attention. Among cyanobacteria, Synechocystis sp. PCC 6803 has been considered as a model strain and has some desirable features that make it suitable for use as an industrial strain. Pyruvate kinase (PK) catalyzes the transformation of phosphoenolpyruvate (PEP) to pyruvate in the last step of glycolysis that is an essential enzyme to produce adenosine triphosphate (ATP) in all organisms. Therefore, it plays a critical role in regulating cell metabolism. However, active and allosteric sites of PK and allosteric mechanisms governing PK activity are poorly understood in many bacteria. This study was aimed to provide more insight into PKs of Synechocystis sp. PCC 6803, using in silico methods. The results indicated that predicted structures of PKs from Synechocystis sp. PCC 6803 are reliable and can be considered for further studies. Molecular docking studies suggested that for predicted structures of sll0587 and sll1275, respectively, there are three and two possible active or allosteric sites. Furthermore, molecular interaction analysis of modeled structures proposes that sll0587 is strongly inhibited by ATP and when ATP concentration is low, this isoenzyme is active.

Harnessing Solar Energy using Phototrophic Microorganisms: A Sustainable Pathway to Bioenergy, Biomaterials, and Environmental Solutions

Phototrophic microorganisms (microbial phototrophs) use light as an energy source to carry out various metabolic processes producing biomaterials and bioenergy and supporting their own growth. Among them, microalgae and cyanobacteria have been utilized extensively for bioenergy, biomaterials, and environmental applications. Their superior photosynthetic efficiency, lipid content, and shorter cultivation time compared to terrestrial biomass make them more suitable for efficient production of bioenergy and biomaterials. Other phototrophic microorganisms, especially anoxygenic phototrophs, demonstrated the ability to survive and flourish while producing renewable energy and high-value products under harsh environmental conditions. This review presents a comprehensive overview of microbial phototrophs on their (i) production of bioenergy and biomaterials, (ii) emerging and innovative applications for environmental conservation, mitigation, and remediation, and (iii) physical, genetic, and metabolic pathways to improve light harvesting and biomass/biofuel/biomaterial production. Both physical (e.g., incremental irradiation) and genetic approaches (e.g., truncated antenna) are implemented to increase the light-harvesting efficiency. Increases in biomass yield and metabolic products are possible through the manipulation of metabolic pathways and selection of a proper strain under optimal cultivation conditions and downstream processing, including harvesting, extraction, and purification. Finally, the current barriers in harnessing solar energy using phototrophic microorganisms are presented, and future research perspectives are discussed, such as integrating phototrophic microorganisms with emerging technologies.

Potential of cyanobacteria in the conversion of wastewater to biofuels

Environmental and energy security has now become a serious global problem, requiring a lot of research to find and implement its cost-effective and environmentally friendly alternatives. The development and use of renewable energy sources is necessary and important in order to avoid the emergence of a global economic crisis. One of the solution to prevent a future crisis caused by energy shortages is to introduce biofuels into the fuel market. Despite the fact that various forms of renewable energy are currently used, the prospects for the production of biofuels from cyanobacteria are quite high due to their unique properties, such as a high lipid content and a suitable fatty acid (FA) composition for the production of biofuels, their suitability for growing open water and the ability to grow on wastewater. The purpose of this article is to provide a comprehensive overview of the potential of cyanobacteria in the conversion of wastewater into biofuels. The article covers comparative data on the accumulation of lipids and the content of fatty acids in various representatives of cyanobacteria and their possibilities in the remediation of wastewater. Various approaches to the extraction of lipids from phototrophic microorganisms that are currently available, their advantages and disadvantages, and the results of the monitoring of the main key points of the development of the technology for converting cyanobacterial biomass into biofuels, with an emphasis on the existing barriers, effects and solutions, are also considered. Further research in this field is required for the successful implementation of this technology on an industrial scale.

Improved Bioproduction of 1-Octanol Using Engineered Synechocystis sp. PCC 6803

1-Octanol has gained interest as a chemical precursor for both high and low value commodities including fuel, solvents, surfactants, and fragrances. By harnessing the power from sunlight and CO₂ as carbon source, cyanobacteria has recently been engineered for renewable production of 1-octanol. The productivity, however, remained low. In the present work, we report efforts to further improve the 1-octanol productivity. Different N-terminal truncations were evaluated on three thioesterases from different plant species, resulting in several candidate thioesterases with improved activity and selectivity toward octanoyl-ACP. The structure/function trials suggest that current knowledge and/or state-of-the art computational tools are insufficient to determine the most appropriate cleavage site for thioesterases in Synechocystis. Additionally, by tuning the inducer concentration and light intensity, we further improved the 1-octanol productivity, reaching up to 35% (w/w) carbon partitioning and a titer of 526 ± 5 mg/L 1-octanol in 12 days. Long-term cultivation experiments demonstrated that the improved strain can be stably maintained for at least 30 days and/or over ten times serial dilution. Surprisingly, the improved strain was genetically stable in contrast to earlier strains having lower productivity (and hence a reduced chance of reaching toxic product concentrations). Altogether, improved enzymes and environmental conditions (e.g., inducer concentration and light intensity) substantially increased the 1-octanol productivity. When cultured under continuous conditions, the bioproduction system reached an accumulative titer of >3.5 g/L 1-octanol over close to 180 days.

Synthetic counter-selection markers and their application in genetic modification of Synechococcus elongatus UTEX2973

Due to its robustness to environmental stresses and fast growth, Synechococcus elongatus UTEX2973 is developed as a new model for researches on cyanobacterial molecular biology and biotechnology. However, systematic genetic modifications of S. elongatus UTEX2973 were hindered by the lack of effective genetic manipulation tools, especially available counter-selection markers. Here, six synthetic counter-selection markers (SCOMs) were assembled by fusing six toxin genes from either Escherichia coli or cyanobacteria with a theophylline-inducible promoter. The SCOMs containing SYNPCC7002_G0085 from Synechococcus sp. PCC7002 or mazF from E. coli were proved to be inducible by theophylline in S. elongatus UTEX2973. By using the mazF-based SCOM, the neutral locus 1 and 23 small regulatory RNAs were completely deleted from the genome of S. elongatus UTEX2973 after one round of selection with both kanamycin and theophylline. The genetic tools developed in this work will facilitate future researches on molecular genetics and synthetic biology in S. elongatus UTEX2973. KEY POINTS: • Two inducible counter-selection markers are lethal to S. elongatus UTEX2973. • The counter-selection marker benefits the gene targeting in S. elongatus UTEX2973. • Twentry-three small regulatory RNAs were fully deleted via the novel gene targeting method.

Screening and characterization of polyhydroxyalkanoate granules, and phylogenetic analysis of polyhydroxyalkanoate synthase gene PhaC in cyanobacteria

Using Nile Red and BODIPY 493/503 dye-staining and fluorescence microscopy, twenty cyanobacterial strains, including ten commercially available strains and ten environmental isolates from estuaries, freshwater ponds, and lagoons, were screened for the accumulation of ecologically important and potentially biotechnologically significant carbon storage granules such as polyhydroxyalkanoates (PHA). Dye-staining granules were observed in six strains. Three Synechocystis, spp. strains WHSYN, LSNM, and CGF-1, and a Phormidium-like sp. CGFILA were isolated from environmental sources and found to produce granules of polyhydroxyalkanoate (PHA) according to PHA synthase gene (phaC) PCR screening and ¹ H NMR analyses. The environmental isolate, Nodularia sp. Las Olas and commercially available Phormidium cf. iriguum CCALA 759 displayed granules but screened negative for PHA according to phaC PCR and ¹ H NMR analyses. Partial polyhydroxyalkanoate synthase subunit C (phaC) and 16S rRNA gene sequences obtained from the PHA-accumulating strains and analyzed alongside publicly available phaC, phaE, 16S rRNA, and 23S rRNA data help in understanding the distribution and evolutionary history of PHA biosynthesis within the phylum Cyanobacteria. The data show that the presence of phaC is highly conserved within the genus Synechocystis, and present in at least one isolate of Phormidium. Maximum likelihood analyses and cophylogenetic modeling of PHA synthase gene sequences provide evidence of a recent horizontal gene transfer event between distant genera of cyanobacteria related to Pleurocapsa sp. PCC 7327 and Phormidium-like sp. CGFILA. These findings will help guide additional screening for PHA producers, and may explain why some Phormidium species produce PHAs, while others do not.

Engineering a Central Carbon Metabolism Pathway to Increase the Intracellular Acetyl-CoA Pool in Synechocystis sp. PCC 6803 Grown under Photomixotrophic Conditions

In cyanobacteria, photomixotrophic growth is considered as a promising strategy to achieve both high cell density and product accumulation. However, the conversion of glucose to acetyl coenzyme A (acetyl-CoA) in the native glycolytic pathway is insufficient, which decreases the carbon utilization and productivity of engineered cyanobacteria under photomixotrophic conditions. To increase the carbon flux from glucose to key intracellular precursor acetyl-CoA in Synechocystis sp. PCC 6803 (hereafter, Synechocystis 6803) under photomixotrophic conditions, a synthetic nonoxidative cyclic glycolysis (NOG) pathway was introduced into the wild type strain, which successfully increased the intracellular pool of acetyl-CoA by approximately 1-fold. To minimize the competition for glucose, the native Embden-Meyerhof-Parnas (EMP) and Entner-Doudoroff (ED) pathways were knocked out, respectively. Notably, eliminating the native ED pathway in the engineered strain carrying the NOG pathway further increased the intracellular pool of acetyl-CoA up to 2.8-fold. Another carbon consuming pathway in Synechocystis 6803, the glycogen biosynthesis pathway, was additionally knocked out in the above-mentioned engineered strain, which enabled an increase of the intracellular acetyl-CoA pool by up to 3.5-fold when compared with the wild type strain. Finally, the content of intracellular lipids was analyzed as an index of the productive capacity of the engineered Synechocystis 6803 cell factory under photomixotrophic conditions. The results showed the total lipids yield increased about 26% compared to the wild type (from 15.71% to 34.12%, g/g glucose), demonstrating that this integrated approach could represent a general strategy not only for the improvement of the intracellular concentration of acetyl-CoA, but also for the production of value-added chemicals that require acetyl-CoA as a key precursor in cyanobacteria.

Understanding Omics Driven Plant Improvement and de novo Crop Domestication: Some Examples

In the current era, one of biggest challenges is to shorten the breeding cycle for rapid generation of a new crop variety having high yield capacity, disease resistance, high nutrient content, etc. Advances in the "-omics" technology have revolutionized the discovery of genes and bio-molecules with remarkable precision, resulting in significant development of plant-focused metabolic databases and resources. Metabolomics has been widely used in several model plants and crop species to examine metabolic drift and changes in metabolic composition during various developmental stages and in response to stimuli. Over the last few decades, these efforts have resulted in a significantly improved understanding of the metabolic pathways of plants through identification of several unknown intermediates. This has assisted in developing several new metabolically engineered important crops with desirable agronomic traits, and has facilitated the de novo domestication of new crops for sustainable agriculture and food security. In this review, we discuss how "omics" technologies, particularly metabolomics, has enhanced our understanding of important traits and allowed speedy domestication of novel crop plants.

Catalytic Hydroprocessing of Single-Cell Oils to Hydrocarbon Fuels : Converting microbial lipids to fuels is a promising approach to replace fossil fuels

Microbial lipids hold great promise as biofuel precursors, and research efforts to convert such lipids to renewable diesel fuels have been increasing in recent years. In contrast to the numerous literature reviews on growing, characterising and extracting lipids from oleaginous microbes, and on converting vegetable oils to hydrocarbon fuels, this review aims to provide insight into aspects that are specific to hydroprocessing microbial lipids. While standard hydrotreating catalysts generally perform well with terrestrial oils, differences in lipid speciation and the presence of co-extracted compounds, such as chlorophyll and sterols, introduce additional complexities into the process for microbial lipids. Lipid cleanup steps can be introduced to produce suitable feedstocks for catalytic upgrading.

Microbial fatty acid transport proteins and their biotechnological potential

Fatty acid metabolism has been widely studied in various organisms. However, fatty acid transport has received less attention, even though it plays vital physiological roles, such as export of toxic free fatty acids or uptake of exogenous fatty acids. Hence, there are important knowledge gaps in how fatty acids cross biological membranes, and many mechanisms and proteins involved in these processes still need to be determined. The lack of information is more predominant in microorganisms, even though the identification of fatty acids transporters in these cells could lead to establishing new drug targets or improvements in microbial cell factories. This review provides a thorough analysis of the current information on fatty acid transporters in microorganisms, including bacteria, yeasts and microalgae species. Most available information relates to the model organisms Escherichia coli and Saccharomyces cerevisiae, but transport systems of other species are also discussed. Intracellular trafficking of fatty acids and their transport through organelle membranes in eukaryotic organisms is described as well. Finally, applied studies and engineering efforts using fatty acids transporters are presented to show the applied potential of these transporters and to stress the need for further identification of new transporters and their engineering.

Photosynthetic production of biodiesel in Synechocystis sp. PCC6803 transformed with insect or plant fatty acid methyltransferase

Biodiesel contains methyl or ethyl esters of long-chain fatty acids and has recently attracted increasing attention. Microalgae have emerged as a sustainable biodiesel production system owing to their photosynthetic potential. However, the conversion of microalgal biomass to biodiesel requires high energy and is costly. This study aimed to overcome the high cost of the pretreatment process by generating cyanobacteria converting fatty acids to fatty acids methyl ester (FAME) in vivo by introducing the fatty acid methyl ester transferase (FAMT) gene. Two FAMT genes from Drosophila melanogaster and Arabidopsis thaliana were selected and their codons were optimized for insertion in the Synechocystis sp. PCC6803 genome through homologous recombination, respectively. FAMT mRNA and protein expression levels were confirmed through reverse-transcription PCR and western blot analysis, respectively. Furthermore, heterologous expression of the FAMT genes yielded FAME, which was analyzed by gas chromatography. We found that FAMT transformants can be further metabolically optimized and applied for commercial production of biodiesel.

Biotechnological Production of the Sunscreen Pigment Scytonemin in Cyanobacteria: Progress and Strategy

Scytonemin is a promising UV-screen and antioxidant small molecule with commercial value in cosmetics and medicine. It is solely biosynthesized in some cyanobacteria. Recently, its biosynthesis mechanism has been elucidated in the model cyanobacterium Nostoc punctiforme PCC 73102. The direct precursors for scytonemin biosynthesis are tryptophan and p-hydroxyphenylpyruvate, which are generated through the shikimate and aromatic amino acid biosynthesis pathway. More upstream substrates are the central carbon metabolism intermediates phosphoenolpyruvate and erythrose-4-phosphate. Thus, it is a long route to synthesize scytonemin from the fixed atmospheric CO2 in cyanobacteria. Metabolic engineering has risen as an important biotechnological means for achieving sustainable high-efficiency and high-yield target metabolites. In this review, we summarized the biochemical properties of this molecule, its biosynthetic gene clusters and transcriptional regulations, the associated carbon flux-driving progresses, and the host selection and biosynthetic strategies, with the aim to expand our understanding on engineering suitable cyanobacteria for cost-effective production of scytonemin in future practices.

Recombinant overexpression of the Escherichia coli acetyl-CoA carboxylase gene in Synechocystis sp. boosts lipid production

Microalgae have received continued attention as a potential source for biofuel production. However, the lack of suitable strains that provide a lipid-rich biomass and tolerate harsh condition inhibits their industrial application. This report describes an effort to transform Synechocystis sp. with genes encoding acetyl-CoA carboxylase (ACC), a key regulatory enzyme in the lipogenesis pathway, from the white mustard plant (Sinapis alba) and the bacterium Escherichia coli DH5α using chitosan nanoparticles. Although a recombinant plasmid encoding S. alba ACC failed to express, successful transformation was achieved with a recombinant plasmid encoding E. coli DH5α ACC. The successful transformant, Synechocystis sp. PAK13, exhibited increased ACC expression compared with its wild-type parent (11.8 vs. 7.2 ng), which significantly increased its lipid content (by 3.6-fold). Synechocystis sp. PAK13 also exhibited a significant (20%) reduction in photosynthetic pigments, a 1.52-fold higher glucose content and a 3.5-fold lower sucrose content than the wild-type. In conclusion, this report introduces a useful strategy to overexpress the ACC gene in microalgae, creating strains with improved lipid production that are suited to industrial applications.

Engineering Photosynthetic Bioprocesses for Sustainable Chemical Production: A Review

Microbial production of chemicals using renewable feedstocks such as glucose has emerged as a green alternative to conventional chemical production processes that rely primarily on petroleum-based feedstocks. The carbon footprint of such processes can further be reduced by using engineered cells that harness solar energy to consume feedstocks traditionally considered to be wastes as their carbon sources. Photosynthetic bacteria utilize sophisticated photosystems to capture the energy from photons to generate reduction potential with such rapidity and abundance that cells often cannot use it fast enough and much of it is lost as heat and light. Engineering photosynthetic organisms could enable us to take advantage of this energy surplus by redirecting it toward the synthesis of commercially important products such as biofuels, bioplastics, commodity chemicals, and terpenoids. In this work, we review photosynthetic pathways in aerobic and anaerobic bacteria to better understand how these organisms have naturally evolved to harness solar energy. We also discuss more recent attempts at engineering both the photosystems and downstream reactions that transfer reducing power to improve target chemical production. Further, we discuss different methods for the optimization of photosynthetic bioprocess including the immobilization of cells and the optimization of light delivery. We anticipate this review will serve as an important resource for future efforts to engineer and harness photosynthetic bacteria for chemical production.

Opportunities and Challenges for Microbial Synthesis of Fatty Acid-Derived Chemicals (FACs)

Global warming and uneven distribution of fossil fuels worldwide concerns have spurred the development of alternative, renewable, sustainable, and environmentally friendly resources. From an engineering perspective, biosynthesis of fatty acid-derived chemicals (FACs) is an attractive and promising solution to produce chemicals from abundant renewable feedstocks and carbon dioxide in microbial chassis. However, several factors limit the viability of this process. This review first summarizes the types of FACs and their widely applications. Next, we take a deep look into the microbial platform to produce FACs, give an outlook for the platform development. Then we discuss the bottlenecks in metabolic pathways and supply possible solutions correspondingly. Finally, we highlight the most recent advances in the fast-growing model-based strain design for FACs biosynthesis.

Biocatalysis in Green and Blue: Cyanobacteria

Recently, several studies have proven the potential of cyanobacteria as whole-cell biocatalysts for biotransformation. Compared to heterotrophic hosts, cyanobacteria show unique advantages thanks to their photoautotrophic metabolism. Their ability to use light as energy and CO₂ as carbon source promises a truly sustainable production platform. Their photoautotrophic metabolism offers an encouraging source of reducing power, which makes them attractive for redox-based biotechnological purposes. To exploit the full potential of these whole-cell biocatalysts, cyanobacterial cells must be considered in their entirety. With this emphasis, this review summarizes the latest developments in cyanobacteria research with a strong focus on the benefits associated with their unique metabolism. Remaining bottlenecks and recent strategies to overcome them are evaluated for their potential in future applications.