Synechocystis sp. PCC 6803 overexpressing genes involved in CBB cycle and free fatty acid cycling enhances the significant levels of intracellular lipids and secreted free fatty acids

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.

Tailoring regulatory components for metabolic engineering in cyanobacteria

The looming climate crisis has prompted an ever-growing interest in cyanobacteria due to their potential as sustainable production platforms for the synthesis of energy carriers and value-added chemicals from CO2 and sunlight. Nonetheless, cyanobacteria are yet to compete with heterotrophic systems in terms of space-time yields and consequently production costs. One major drawback leading to the low production performance observed in cyanobacteria is the limited ability to utilize the full capacity of the photosynthetic apparatus and its associated systems, i.e. CO2 fixation and the directly connected metabolism. In this review, novel insights into various levels of metabolic regulation of cyanobacteria are discussed, including the potential of targeting these regulatory mechanisms to create a chassis with a phenotype favorable for photoautotrophic production. Compared to conventional metabolic engineering approaches, minor perturbations of regulatory mechanisms can have wide-ranging effects.

Improved lipid production and component of mycosporine-like amino acids by co-overexpression of amt1 and aroB genes in Synechocystis sp. PCC6803

Implementing homologous overexpression of the amt1 (A) and aroB (B) genes involved in ammonium transporter and the synthesis of mycosporine-like amino acids (MAAs) and aromatic amino acids, respectively, we created three engineered Synechocystis sp. PCC6803 strains, including Ox-A, Ox-B, and Ox-AB, to study the utilization of carbon and nitrogen in cyanobacteria for the production of valuable products. With respect to amt1 overexpression, the Ox-A and Ox-AB strains had a greater growth rate under (NH4)2SO4 supplemented condition. Both the higher level of intracellular accumulation of lipids in Ox-A and Ox-AB as well as the increased secretion of free fatty acids from the Ox-A strain were impacted by the late-log phase of cell growth. It is noteworthy that among all strains, the Ox-B strain undoubtedly spotted a substantial accumulation of glycogen as a consequence of aroB overexpression. Additionally, the ammonium condition drove the potent antioxidant activity in Ox strains with a late-log phase, particularly in the Ox-B and Ox-AB strains. This was probably related to the altered MAA component inside the cells. The higher proportion of P4-fraction was induced by the ammonium condition in both Ox-B and Ox-AB, while the noted increase of the P1 component was found in the Ox-A strain.

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.

Advances in Genetic Engineering in Improving Photosynthesis and Microalgal Productivity

Even though sunlight energy far outweighs the energy required by human activities, its utilization is a key goal in the field of renewable energies. Microalgae have emerged as a promising new and sustainable feedstock for meeting rising food and feed demand. Because traditional methods of microalgal improvement are likely to have reached their limits, genetic engineering is expected to allow for further increases in the photosynthesis and productivity of microalgae. Understanding the mechanisms that control photosynthesis will enable researchers to identify targets for genetic engineering and, in the end, increase biomass yield, offsetting the costs of cultivation systems and downstream biomass processing. This review describes the molecular events that happen during photosynthesis and microalgal productivity through genetic engineering and discusses future strategies and the limitations of genetic engineering in microalgal productivity. We highlight the major achievements in manipulating the fundamental mechanisms of microalgal photosynthesis and biomass production, as well as promising approaches for making significant contributions to upcoming microalgal-based biotechnology.

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.

Native SodB Overexpression of Synechocystis sp. PCC 6803 Improves Cell Growth Under Alcohol Stresses Whereas Its Gpx2 Overexpression Impacts on Growth Recovery from Alcohol Stressors

To overcome the limited resistance to alcohol stress, genetically engineered Synechocystis sp. PCC 6803 strains with overexpressions of genes related with the ROS detoxification system (sodB and gpx2, which encode superoxide dismutase and glutathione peroxidase, respectively) were developed. Three engineered strains including a sodB-overexpressing strain (OE + S), a gpx2-overexpressing strain (OE + G), and a sodB/gpx2-overexpressing strain (OE + SG) grew similarly as wild-type control under normal condition. When compared to wild-type control, OE + S and OE + SG strains grew faster for 4 days under 2.0% (v/v) ethanol and 0.3% (v/v) n-butanol conditions, as well as having higher chlorophyll a levels. On the other hand, the prominent growth recovery of OE + G and OE + SG was noted within 4 days in normal BG₁₁ medium after treating cells with high alcohol stresses for 1 h, in particular 15% ethanol and 2.5% n-butanol. Under 4 days of recovery from butanol stress, specific levels of intracellular pigments including chlorophyll a and carotenoids were dramatically increased in all modified strains. The overexpression of antioxidant genes then revealed a significant improvement of alcohol tolerance in Synechocystis sp. PCC 6803.

Metabolic transformation of cyanobacteria for biofuel production

World-wide, an emerging demand is moving towards the biofuels to replace the fossil fuels. In alternative biofuel production strategies, cyanobacteria have unique characteristic of accumulating glycogen, lipid, and fuel molecules through natural mechanisms. Moreover, the cyanobacteria can be easily engineered to synthesis a plenty of fuel molecules from CO₂. To obtain the fuel molecule from cyanobacteria, various techniques were invented in which the metabolic engineering is found to be a prerequisite to develop an economically feasible process. The expression of indigenous or heterologous pathways plays an important role in developing successful production process. In addition, the engineering of photosynthetic apparatus, destruction of competitive pathways and improvement of tolerance were also proven to improve the product specific synthesis. Although various metabolic engineering approaches have been developed, there are certain obstacles when it comes to implementation for the production. In this review, the important biosynthetic pathways for biofuels, alteration of other genes to improve the actual pathway and possibilities of developing cyanobacterial fuel production have been elaborated.

Identification of the key functional genes in salt-stress tolerance of Cyanobacterium Phormidium tenue using in silico analysis

The development of artificial biocrust using cyanobacterium Phormidium tenue has been suggested as an effective strategy to prevent soil degradation. Here, a combination of in silico approaches with growth rate, photosynthetic pigment, morphology, and transcript analysis was used to identify specific genes and their protein products in response to 500 mM NaCl in P. tenue. The results show that 500 mM NaCl induces the expression of genes encoding glycerol-3-phosphate dehydrogenase (glpD) as a Flavoprotein, ribosomal protein S12 methylthiotransferase (rimO), and a hypothetical protein (sll0939). The constructed co-expression network revealed a group of abiotic stress-responsive genes. Using the Basic Local Alignment Search Tool (BLAST), the homologous proteins of rimO, glpD, and sll0939 were identified in the P. tenue genome. Encoded proteins of glpD, rimO, and DUF1622 genes, respectively, contain (DAO and DAO C), (UPF0004, Radical SAM and TRAM 2), and (DUF1622) domains. The predicted ligand included 22B and MG for DUF1622, FS5 for rimO, and FAD for glpD protein. There was no direct disruption in ligand-binding sites of these proteins by Na+, Cl-, or NaCl. The growth rate, photosynthetic pigment, and morphology of P. tenue were investigated, and the result showed an acceptable tolerance rate of this microorganism under salt stress. The quantitative real-time polymerase chain reaction (qRT-PCR) results revealed the up-regulation of glpD, rimO, and DUF1622 genes under salt stress. This is the first report on computational and experimental analyses of the glpD, rimO, and DUF1622 genes in P. tenue under salt stress to the best of our knowledge.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03050-w.

Using different cultivation strategies and methods for the production of microalgal biomass as a raw material for the generation of bioproducts

Microalgal biomass and its fine chemical production from microalgae have pioneered algal bioprocess technology with few limitations such as lab-to-industry. However, laboratory-scale transitions and industrial applications are hindered by a plethora of limitations comprising expensive in culturing methods. Therefore, to emphasize the profitable benefits, the algal culturing techniques appropriately employed for large-scale microalgal biomass yield necessitates intricate assessment to emphasize the profitable benefits. The present review holistically compiles the culturing strategies for improving microalgal biomass production based on appropriate factors like designing better bioreactor designs. On the other hand, synthetic biology approaches for abridging the effective industrial transition success explored recently. Prospects in synthetic biology for enhanced microalgal biomass production based on cultivation strategies and various mechanistic modes approach to enrich cost-effective and viable output are discussed. The State-of-the-art culturing techniques encompassing enhancement of photosynthetic activity, designing bioreactor design, and potential augmenting protocols for biomass yield employing indoor cultivation in both (Open and or/closed) methods are enumerated. Further, limitations hindering the microalgal bioproducts development are critically evaluated for improving culturing techniques for microalgal cell factories, subsequently escalating the cost-benefit ratio in bioproducts synthesis from microalgae. The comprehensive analysis could provide a rational and deeper detailed insight for microalgal entrepreneurs through alternative culturing technology viz., synthetic biology and genome engineering in an Industrial perspective arena.

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.

Current advances in engineering cyanobacteria and their applications for photosynthetic butanol production

Cyanobacteria are natural photosynthetic microbes which can be engineered for sustainable conversion of solar energy and carbon dioxide into chemical products. Attempts to improve target production often require an improved understanding of the native cyanobacterial host system. Valuable insights into cyanobacterial metabolism, biochemistry and physiology have been steadily increasing in recent years, stimulating key advancements of cyanobacteria as cell factories for biochemical, including biofuel, production. In the present review, we summarize the current progress in engineering cyanobacteria and discuss the achieved and potential utilization of these advances in cyanobacteria for the production of the bulk chemical butanol, specifically isobutanol and 1-butanol.

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.

Improvement of Free Fatty Acid Secretory Productivity in Aspergillus oryzae by Comprehensive Analysis on Time-Series Gene Expression

Aspergillus oryzae is a filamentous fungus that has historically been utilized in the fermentation of food products. In recent times, it has also been introduced as a component in the industrial biosynthesis of consumable compounds, including free fatty acids (FFAs), which are valuable and versatile products that can be utilized as feedstocks in the production of other commodities, such as pharmaceuticals and dietary supplements. To improve the FFA secretory productivity of A. oryzae in the presence of Triton X-100, we analyzed the gene expression of a wild-type control strain and a disruptant strain of an acyl-CoA synthetase gene, faaA, in a time-series experiment. We employed a comprehensive analysis strategy using the baySeq, DESeq2, and edgeR algorithms to clarify the vital pathways for FFA secretory productivity and select genes for gene modification. We found that the transport and metabolism of inorganic ions are crucial in the initial stages of FFA production and revealed 16 candidate genes to be modified in conjunction with the faaA disruption. These genes were verified through the construction of overexpression strains, and showed that the manipulation of reactions closer to the FFA biosynthesis step led to a higher increase in FFA secretory productivity. This resulted in the most successful overexpression strains to have an FFA secretory productivity more than two folds higher than that of the original faaA disruptant. Our study provides guidance for further gene modification for FFA biosynthesis in A. oryzae and for enhancing the productivity of other metabolites in other microorganisms through metabolic engineering.

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.

Untargeted Lipidomics Analysis of the Cyanobacterium Synechocystis sp. PCC 6803: Lipid Composition Variation in Response to Alternative Cultivation Setups and to Gene Deletion

Cyanobacteria play an important role in several ecological environments, and they are widely accepted to be the ancestors of chloroplasts in modern plants and green algae. Cyanobacteria have become attractive models for metabolic engineering, with the goal of exploring them as microbial cell factories. However, the study of cyanobacterial lipids’ composition and variation, and the assessment of the lipids’ functional and structural roles have been largely overlooked. Here, we aimed at expanding the cyanobacterial lipidomic analytical pipeline by using an untargeted lipidomics approach. Thus, the lipid composition variation of the model cyanobacterium Synechocystis sp. PCC 6803 was investigated in response to both alternative cultivation setups and gene deletion. This approach allowed for detecting differences in total lipid content, alterations in fatty-acid unsaturation level, and adjustments of specific lipid species among the identified lipid classes. The employed method also revealed that the cultivation setup tested in this work induced a deeper alteration of the cyanobacterial cell lipidome than the deletion of a gene that results in a dramatic increase in the release of lipid-rich outer membrane vesicles. This study further highlights how growth conditions must be carefully selected when cyanobacteria are to be engineered and/or scaled-up for lipid or fatty acids production.