Overexpression of hlyB and mdh genes confers halotolerance in Fremyella diplosiphon, a freshwater cyanobacterium

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.

Augmentation of the Photoreactivation Gene in Fremyella diplosiphon Confers UV-B Tolerance

In spite of the enormous potential of cyanobacteria as a renewable energy source, elevated UV exposure is a major impediment to their commercial viability and productivity. Fremyella diplosiphon is a widely explored cyanobacterium with great biofuel capacity due to its high lipid content. To enhance UV stress tolerance in this species, we overexpressed the photoreactivation gene (phr A) that encodes for photolyase DNA repair enzyme in the wild type F. diplosiphon (B481-WT) by genetic transformation. Our efforts resulted in a transformant (B481-ViAnSa) with a 3808-fold increase in the phr A mRNA transcript level and enhanced growth under UV-B stress. Additionally, DNA strand breaks in the transformant were significantly lower after 12 and 16 h of UV radiation, with significantly higher dsDNA recovery in B481-ViAnSa (98.1%) compared to that in B481-WT (81.5%) at 48 h post irradiation. Photosystem II recovery time in the transformant was significantly reduced (48 h) compared to that in the wild type (72 h). Evaluation of high-value fatty acid methyl esters (FAMEs) revealed methyl palmitate, the methyl ester of hexadecenoic acid (C16:0), to be the most dominant component, accounting for 53.43% of the identified FAMEs in the transformant. Results of the study offer a promising approach to enhance UV tolerance in cyanobacteria, thus paving the way to large-scale open or closed pond cultivation for commercial biofuel production.

Augmenting Fremyella diplosiphon Cellular Lipid Content and Unsaturated Fatty Acid Methyl Esters Via Sterol Desaturase Gene Overexpression

Cyanobacteria have immense prospective as a platform for renewable energy; however, a major barrier in achieving optimal productivity is the low lipid yield. Fremyella diplosiphon, a model cyanobacterium, is an ideal biofuel agent due to its desirable fatty acid methyl esters (FAMEs). To enhance lipid content, we overexpressed the sterol desaturase (SD) gene in F. diplosiphon B481 wild type by genetic transformation. This effort resulted in a transformant (B481-SD) with a 64-fold increase in the SD gene at the mRNA transcript level, with no loss in growth and pigmentation. The transformant was persistently grown for over 32 generations indicating long-term stability and vitality. We observed 27.3% and 23% increases in total lipid content and unsaturated FAMEs respectively in B481-SD transesterified lipids with methyl octadecadienoate as the most abundant unsaturated component. In addition, we detected an 81% increase in FAME composition in the transformant compared with the wild type. Theoretical physical and chemical properties confirmed a FAME profile with very high cetane number (65.972-67.494) and oxidative stability (50.493-18.66 h) in the engineered strain. Results of the study offer a promising approach to augment F. diplosiphon total lipid content and unsaturated FAMEs, thus paving the way to enhance biofuel capacity of the organism.

Nanoparticle-mediated Impact on Growth and Fatty Acid Methyl Ester Composition in the Cyanobacterium Fremyella diplosiphon

Insufficient light supply is a major limitation in cultivation of cyanobacteria for scaled up biofuel production and other biotechnological applications, which has driven interest in nanoparticle-mediated enhancement of cellular light capture. In the present study, Fremyella diplosiphon wild type (Fd33) and halotolerant (HSF33-2) strains were grown in solution with 20, 100, and 200 nm-diameter gold nanoparticles (AuNPs) to determine their impact on biomass accumulation, pigmentation, and fatty acid methyl ester (FAME) production. Results revealed a significant increase in growth of Fd33 (0.244 ± 0.006) and HSF33-2 (0.112 ± 0.003) when treated with 200 nm AuNPs. In addition, we observed a significant increase in chlorophyll a accumulation in 200 nm AuNP-treated Fd33 (25.7%) and HSF33-2 (36.3%) indicating that NPs enhanced photosynthetic pigmentation. We did not observe any alteration in FAME composition and biodiesel properties of transesterified F. diplosiphon lipids among all AuNP treatments. Interactions between F. diplosiphon and AuNPs were visualized using scanning electron microscopy. Energy dispersive X-ray spectroscopy confirmed the presence of AuNPs outside cells with aggregation in high cell density locales. Our findings indicate that nanotechnological approaches could significantly enhance growth of the organism with no negative effect on FAME-derived biodiesel properties, thus augmenting F. diplosiphon as a potential biofuel agent.

Fremyella diplosiphon as a biodiesel agent: Identification of fatty acid methyl esters via microwave-assisted direct in situ transesterification

Increasing concerns on environmental and economic issues linked to fossil fuel use has driven great interest in cyanobacteria as third generation biofuel agents. In this study, the biodiesel potential of a model photosynthetic cyanobacterium, Fremyella diplosiphon, was identified by fatty acid methyl esters (FAME) via direct transesterification. Total lipids in wild type (Fd33) and halotolerant (HSF33-1 and HSF33-2) strains determined by gravimetric analysis yielded 19% cellular dry weight (CDW) for HSF33-1 and 20% CDW for HSF33-2, which were comparable to Fd33 (18% CDW). Gas chromatography-mass spectrometry detected a high ratio of saturated to unsaturated FAMEs (2.48-2.61) in transesterified lipids, with methyl palmitate being the most abundant (C16:0). While theoretical biodiesel properties revealed high cetane number and oxidative stability, high cloud and pour point values indicated that fuel blending could be a viable approach. Significantly high FAME abundance in total transesterified lipids of HSF33-1 (40.2%) and HSF33-2 (69.9%) relative to Fd33 (25.4%) was identified using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry, indicating that robust salt stress response corresponds to higher levels of extractable FAME. Alkanes, a key component in conventional fuels, were present in F. diplosiphon transesterified lipids across all strains confirming that natural synthesis of these hydrocarbons is not inhibited during biodiesel production. While analysis of photosynthetic pigments and phycobiliproteins did not reveal significant differences, FAME abundance varied significantly in wild type and halotolerant strains indicating that photosynthetic pathways are not the sole factors that determine fatty acid production. We characterize the potential of F. diplosiphon for biofuel production with FAME yields in halotolerant strains higher than the wild type with no loss in photosynthetic pigmentation.