Cyanobacterial production of plant essential oils

Toward combined photobiological-photochemical formation of kerosene-type biofuels: which small 1,3-diene photodimerizes most efficiently?

A transition from fossil- to bio-based hydrocarbon fuels is required to reduce greenhouse gas emissions; yet, traditional biomass cultivation for biofuel production competes with food production and impacts negatively on biodiversity. Recently, we reported a proof-of-principle study of a two-step photobiological-photochemical approach to kerosene biofuels in which a volatile hydrocarbon (isoprene) is produced by photosynthetic cyanobacteria, followed by its photochemical dimerization into C₁₀ hydrocarbons. Both steps can utilize solar irradiation. Here, we report the triplet state (T₁)-sensitized photodimerization of a broader set of small 1,3-dienes to identify which structural features lead to rapid photodimerization. Neat 1,3-cyclohexadiene gave the highest yield (93%) after 24 h of irradiation at 365 nm, followed by isoprene (66%). The long triplet lifetime of 1,3-cyclohexadiene, which is two orders of magnitude longer than those of acyclic dienes, is key to its high photoreactivity and stem from its planar T₁ state structure. In contrast, while isoprene is conformationally flexible, it has both photochemical and photobiological advantages, as it is the most reactive among the volatile 1,3-dienes and it can be produced by cyanobacteria. Finally, we explored the influence of solvent viscosity, diene concentration, and triplet sensitizer loading on the photodimerization, with a focus on conditions that are amenable when the dienes are produced photobiologically. Our findings should be useful for the further development of the two-step photobiological-photochemical approach to kerosene biofuels.

Versatile Applications of Cyanobacteria in Biotechnology

Cyanobacteria are blue-green Gram-negative and photosynthetic bacteria which are seen as one of the most morphologically numerous groups of prokaryotes. Because of their ability to fix gaseous nitrogen and carbon dioxide to organic materials, they are known to play important roles in the universal nutrient cycle. Cyanobacteria has emerged as one of the promising resources to combat the issues of global warming, disease outbreaks, nutrition insecurity, energy crises as well as persistent daily human population increases. Cyanobacteria possess significant levels of macro and micronutrient substances which facilitate the versatile popularity to be utilized as human food and protein supplements in many countries such as Asia. Cyanobacteria has been employed as a complementary dietary constituent of feed for poultry and as vitamin and protein supplement in aquatic lives. They are effectively used to deal with numerous tasks in various fields of biotechnology, such as agricultural (including aquaculture), industrial (food and dairy products), environmental (pollution control), biofuel (bioenergy) and pharmaceutical biotechnology (such as antimicrobial, anti-inflammatory, immunosuppressant, anticoagulant and antitumor); recently, the growing interest of applying them as biocatalysts has been observed as well. Cyanobacteria are known to generate a numerous variety of bioactive compounds. However, the versatile potential applications of cyanobacteria in biotechnology could be their significant growth rate and survival in severe environmental conditions due to their distinct and unique metabolic pathways as well as active defensive mechanisms. In this review, we elaborated on the versatile cyanobacteria applications in different areas of biotechnology. We also emphasized the factors that could impede the implementation to cyanobacteria applications in biotechnology and the execution of strategies to enhance their effective applications.

Phycocyanin Fusion Constructs for Heterologous Protein Expression Accumulate as Functional Heterohexameric Complexes in Cyanobacteria

Overexpression of heterologous proteins from plants, bacteria, and human as fusion constructs in cyanobacteria has been documented in the literature. Typically, the heterologous protein "P" of interest is expressed as a fusion with the abundant CpcB β-subunit of phycocyanin (PC), which was placed in the leader sequence position. The working hypothesis for such overexpressions is that CpcB*P fusion proteins somehow accumulate in a soluble and stable form in the cytosol of the cyanobacteria, retaining the activity of the trailing heterologous "P" protein of interest. The present work revealed a substantially different and previously unobvious picture, comprising the following properties of the above-mentioned CpcB*P fusion constructs: (i) the CpcB*P proteins assemble as functional (α,β*P)₃CpcG heterohexameric discs, where α is the CpcA α-subunit of PC, β*P is the CpcB*P fusion protein, the asterisk denotes fusion, and CpcG is the 28.9 kDa PC disc linker polypeptide CpcG1. (ii) The (α,β*P)₃CpcG1 complexes covalently bind one open tetrapyrrole bilin co-factor per α-subunit and two bilins per β-subunit. (iii) The (α,β*P)₃CpcG1 heterohexameric discs are functionally attached to the Synechocystis allophycocyanin (AP) core cylinders and efficiently transfer excitation energy from the assembled (α,β*P)₃CpcG1 heterohexamer to the PSII reaction center, enhancing the rate of photochemical charge separation and electron transfer activity in this photosystem. (iv) In addition to the human interferon α-2 and tetanus toxin fragment C tested in this work, we have shown that enzymes such as the plant-origin isoprene synthase, β-phellandrene synthase, geranyl diphosphate synthase, and geranyl linalool synthase are also overexpressed, while retaining their catalytic activity in the respective fusion construct configuration. (v) Folding models for the (α,β*P)₃CpcG1 heterohexameric discs showed the recombinant proteins P to be radially oriented with respect to the (α,β)₃ compact disc. Elucidation of the fusion construct configuration and function will pave the way for the rational design of fusion constructs harboring and overexpressing multiple proteins of scientific and commercial interest.

Synechococcus elongatus PCC7942: a cyanobacterium cell factory for producing useful chemicals and fuels under abiotic stress conditions

Sucrose, a compatible osmolyte in cyanobacteria, functions both as an energy reserve and as osmoprotectant. Sugars are the most common substrates used by microorganisms to produce hydrogen (H₂) by means of anaerobic dark fermentation. Cells of the unicellular, non-nitrogen fixing, freshwater cyanobacterium Synechococcus elongatus PCC7942 accumulate sucrose under salt stress. In the present work, we used this cyanobacterium and a genetically engineered strain of it (known as PAMCOD) to investigate the optimal conditions for (a) photosynthetic activity, (b) cell proliferation and (c) sucrose accumulation, which are necessary for H₂ production via anaerobic dark fermentation of the accumulated sucrose. PAMCOD (Deshnium et al. in Plant Mol Biol 29:897-902, 1995) contains the gene codA that codes for choline oxidase, the enzyme which converts choline to the zwitterion glycine betaine. Glycine betaine is a compatible osmolyte which increases the salt tolerance of Synechococcus elongatus PCC7942. Furthermore, glycine betaine maintains cell proliferation under salt stress and results in increased sucrose accumulation. In the present study, we examine the environmental factors, such as the NaCl concentration, the culture medium pH, and the carbon dioxide content of the air bubbled through it. At optimal conditions, sucrose accumulated in the cyanobacteria cells up to 13.5 mol per mole Chl a. Overall, genetically engineered Synechococcus elongatus PCC7942 produces sucrose in sufficient quantities such that it may be a viable alternative (a) to sucrose synthesis, and (b) to H₂ formation via anaerobic dark fermentation.

Ten years of algal biofuel and bioproducts: gains and pains

It has been proposed that future efforts should focus on basic studies, biotechnology studies and synthetic biology studies related to algal biofuels and various high-value bioproducts for the economically viable production of algal biof uels. In recognition of diminishing fossil fuel reserves and the worsening environment, microalgal biofuel has been proposed as a renewable energy source with great potential. Algal biofuel thus became one of the hottest topics in renewable energy research in the new century, especially over the past decade. Between 2007 and 2017, research related to microalgal biofuels experienced a dramatic, three-stage development, rising, growing exponentially, and then declining rapidly due to overheating of the subject. However, biofuel-driven algal biotechnology and bioproducts research has been thriving since 2010. To clarify the gains (and pains) of the past decade and detail prospects for the future, this review summarizes the extensive scientific progress and substantial technical advances in algal biofuel over the past decade, covering basic biology, applied research, as well as the production of value-added natural products. Even after 10 years of hard work and billions of dollars in investments, its unacceptably high cost remains the ultimate bottleneck for the industrialization of algal biofuel. To maximize the total research benefits, both economically and socially, it has been proposed that future efforts should focus on basic studies to characterize oilgae, on biotechnology studies into various high-value bioproducts. Moreover, the development of synthetic biology provides new possibilities for the economically viable production of biofuels via the directional manufacture of microalgal bioproducts in algal cell factories.

Terpenes and isoprenoids: a wealth of compounds for global use

Role of terpenes and isoprenoids has been pivotal in the survival and evolution of higher plants in various ecoregions. These products find application in the pharmaceutical, flavor fragrance, and biofuel industries. Fitness of plants in a wide range of environmental conditions entailed (i) evolution of secondary metabolic pathways enabling utilization of photosynthate for the synthesis of a variety of biomolecules, thereby facilitating diverse eco-interactive functions, and (ii) evolution of structural features for the sequestration of such compounds away from the mainstream primary metabolism to prevent autotoxicity. This review summarizes features and applications of terpene and isoprenoid compounds, comprising the largest class of secondary metabolites. Many of these terpene and isoprenoid biomolecules happen to be high-value bioproducts. They are essential components of all living organisms that are chemically highly variant. They are constituents of primary (quinones, chlorophylls, carotenoids, steroids) as well as secondary metabolism compounds with roles in signal transduction, reproduction, communication, climatic acclimation, defense mechanisms and more. They comprise single to several hundreds of repetitive five-carbon units of isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In plants, there are two pathways that lead to the synthesis of terpene and isoprenoid precursors, the cytosolic mevalonic acid (MVA) pathway and the plastidic methylerythritol phosphate (MEP) pathway. The diversity of terpenoids can be attributed to differential enzyme and substrate specificities and to secondary modifications acquired by terpene synthases. The biological role of secondary metabolites has been recognized as pivotal in the survival and evolution of higher plants. Terpenes and isoprenoids find application in pharmaceutical, nutraceutical, synthetic chemistry, flavor fragrance, and possibly biofuel industries.