Optimization of cultural and nutritional conditions for accumulation of poly-beta-hydroxybutyrate in Synechocystis sp. PCC 6803

Cost-effective production of bioplastic polyhydroxybutyrate via introducing heterogeneous constitutive promoter and elevating acetyl-Coenzyme A pool of rapidly growing cyanobacteria

Bioplastic production using cyanobacteria can be an effective strategy to cope with environmental problems caused by using petroleum-based plastics. Synechococcus elongatus UTEX 2973 with heterogeneous phaCAB can produce bioplastic polyhydroxybutyrate (PHB) with a high CO2 uptake rate. For cost-effective production of PHB in S. elongatus UTEX 2973, phaCAB was expressed by the constitutive Pcpc560, resulting in the production of 226 mg/L of PHB by only photoautotrophic cultivation without the addition of inducer. Several culture conditions were applied to increase PHB productivity, and when acetate was supplied at a concentration of 1 g/L as an organic carbon source, productivity significantly increased resulting in 607.2 mg/L of PHB and additive cost reduction of more than 300 times was achieved compared to IPTG. Consequently, these results suggest the possibility of cyanobacteria as an agent that can economically produce PHB and as a solution to the problem of petroleum-based plastics.

Polyhydroxybutyrate (PHB) accumulation by a mangrove isolated cyanobacteria Limnothrix planktonica using fruit waste

Cyanobacterial polyhydroxybutyrate (PHB) is preferred over bacteria for low-cost production due to its photoautotrophic nature and lower carbon requirement. Considering its impact on the environment and circular economy, the valorization of fruit waste is the need of the hour. In the present study, fruit peels of banana, orange, pea, jackfruit, watermelon and waste flowers were tried as carbon sources for mangrove-isolated cyanobacteria Limnothrix planktonica to accumulate PHB. Alterations in the ASN-III culture medium and the introduction of untreated and pre-treated (acid/alkali-treated) peels as carbon sources are tried to enhance PHB. Banana peel showed the maximum PHB accumulation potential of 25.73 mg/L on the 12th day of incubation, followed by jackfruit (22.46 mg/L) and watermelon peels (20.72 mg/L); whereas, commercial carbon sources showed lower PHB accumulation up to 19.26 mg/L and 18.21 mg/L with fructose and glucose respectively. PHB accumulation was boosted to 5-fold higher (39.39 mg/L) in NP deficiency medium along with banana peel supplement, as compared to photoautotrophic conditions (8.49 mg/L) after the 9th day of incubation. Additionally, the PHB obtained by using the fruit wastes has a higher molecular weight than the PHB accumulated during photoautotrophic conditions. Optimization of parameters using fruit wastes and characterization of PHB would lead to its potential use.

Acylated plastoquinone is a novel neutral lipid accumulated in cyanobacteria

Although cyanobacteria do not possess bacterial triacylglycerol (TAG)-synthesizing enzymes, the accumulation of TAGs and/or lipid droplets has been repeatedly reported in a wide range of species. In most cases, the identification of TAG has been based on the detection of the spot showing the mobility similar to the TAG standard in thin-layer chromatography (TLC) of neutral lipids. In this study, we identified monoacyl plastoquinol (acyl PQH) as the predominant molecular species in the TAG-like spot from the unicellular Synechocystis sp. PCC 6803 (S.6803) as well as the filamentous Nostocales sp., Nostoc punctiforme PCC 73102, and Anabaena sp. PCC 7120. In S.6803, the accumulation level of acyl PQH but not TAG was affected by deletion or overexpression of slr2103, indicating that acyl PQH is the physiological product of Slr2103 having homology with the eukaryotic diacylglycerol acyltransferase-2 (DGAT2). Electron microscopy revealed that cyanobacterial strains used in this study do not accumulate lipid droplet structures such as those observed in oleaginous microorganisms. Instead, they accumulate polyhydroxybutyrate (PHB) granules and/or aggregates of alkane, free C16 and C18 saturated fatty acids, and low amounts of TAG in the cytoplasmic area, which can be detected by staining with a fluorescent dye specific to neutral lipids. Unlike these lipophilic materials, acyl PQH is exclusively localized in the membrane fraction. There must be DGAT2-like enzymatic activity esterifying de novo-synthesized C16 and C18 fatty acids to PQH₂ in the thylakoid membranes.

Fermentation development using fruit waste derived mixed sugars for poly(3-hydroxybutyrate) production and property evaluation

Free sugars from fruit wastes were evaluated for the production of poly(3-hydroxybutyrate) (PHB) in Paraburkholderia sacchari fed-batch bioreactor fermentations. Different initial sugar concentration, carbon to inorganic phosphorus (C/IP) ratio, IP addition during feeding and volumetric oxygen transfer coefficient (k_La) were evaluated to promote PHB production. The highest intracellular PHB accumulation (66.6%), PHB concentration (108.3 g/L), productivity (3.28 g/L/h) and yield (0.33 g/g) were achieved at 40 g/L initial sugars, C/IP 26.5, 202.6 h^-1k_La value and 20% IP supplementation in the feeding solution. The effect of different cell's harvesting time on PHB properties showed no influence in weight average molecular weight and thermal properties. The harvest time influenced the tensile strength that was reduced from 28.7 MPa at 22 h to 13.3 MPa at 36 h. The elongation at break and Young's modulus were in the range 3.6-14.8% and 830-2000 MPa, respectively.

Highlighting the potential of Synechococcus elongatus PCC 7942 as platform to produce α-linolenic acid through an updated genome-scale metabolic modeling

Cyanobacteria are prokaryotic organisms that capture energy from sunlight using oxygenic photosynthesis and transform CO2 into products of interest such as fatty acids. Synechococcus elongatus PCC 7942 is a model cyanobacterium efficiently engineered to accumulate high levels of omega-3 fatty acids. However, its exploitation as a microbial cell factory requires a better knowledge of its metabolism, which can be approached by using systems biology tools. To fulfill this objective, we worked out an updated, more comprehensive, and functional genome-scale model of this freshwater cyanobacterium, which was termed iMS837. The model includes 837 genes, 887 reactions, and 801 metabolites. When compared with previous models of S. elongatus PCC 7942, iMS837 is more complete in key physiological and biotechnologically relevant metabolic hubs, such as fatty acid biosynthesis, oxidative phosphorylation, photosynthesis, and transport, among others. iMS837 shows high accuracy when predicting growth performance and gene essentiality. The validated model was further used as a test-bed for the assessment of suitable metabolic engineering strategies, yielding superior production of non-native omega-3 fatty acids such as α-linolenic acid (ALA). As previously reported, the computational analysis demonstrated that fabF overexpression is a feasible metabolic target to increase ALA production, whereas deletion and overexpression of fabH cannot be used for this purpose. Flux scanning based on enforced objective flux, a strain-design algorithm, allowed us to identify not only previously known gene overexpression targets that improve fatty acid synthesis, such as Acetyl-CoA carboxylase and β-ketoacyl-ACP synthase I, but also novel potential targets that might lead to higher ALA yields. Systematic sampling of the metabolic space contained in iMS837 identified a set of ten additional knockout metabolic targets that resulted in higher ALA productions. In silico simulations under photomixotrophic conditions with acetate or glucose as a carbon source boosted ALA production levels, indicating that photomixotrophic nutritional regimens could be potentially exploited in vivo to improve fatty acid production in cyanobacteria. Overall, we show that iMS837 is a powerful computational platform that proposes new metabolic engineering strategies to produce biotechnologically relevant compounds, using S. elongatus PCC 7942 as non-conventional microbial cell factory.

Process Technologies of Cyanobacteria

Although the handling and exploitation of cyanobacteria is associated with some challenges, these phototrophic bacteria offer great opportunities for innovative biotechnological processes. This chapter covers versatile aspects of working with cyanobacteria, starting with up-to-date in silico and in vitro screening methods for bioactive substances. Subsequently, common conservation techniques and vitality/viability estimation methods are compared and supplemented by own data regarding the non-invasive vitality evaluation via pulse amplitude modulated fluorometry. Moreover, novel findings about the influence the state of the pre-cultures have on main cultures are presented. The following sub-chapters deal with different photobioreactor-designs, with special regard to biofilm photobioreactors, as well as with heterotrophic and mixotrophic cultivation modes. The latter topic provides information from literature on successfully enhanced cyanobacterial production processes, augmented by own data.

Optimization of Production of Polyhydroxyalkanoates (PHAs) from Newly Isolated Ensifer sp. Strain HD34 by Response Surface Methodology

Petroleum-based plastics have become a big problem in many countries because of their non-degradability and that they become microplastics in the environment. This study focused on the optimization of production medium and conditions of polyhydroxyalkanoates (PHAs), which are biodegradable bioplastics and are accumulated in microbial cells. Among 341 isolates from 40 composted soil samples, the best isolate was the HD34 strain, which was identified using morphological, molecular, and biochemical methods. The results showed that the strain was most closely related to Ensifer adhaerens LMG20216T, with 99.6% similarity. For optimization of production medium and conditions using response surface methodology, it exhibited an optimal medium containing 3.99% (w/v) of potato dextrose broth (PDB) and 1.54% (w/v) of D-glucose with an adjusted initial pH of 9.0. The optimum production was achieved under culture conditions of a temperature of 28 °C, inoculum size of 2.5% (v/v), and a shaking speed of 130 rpm for 5 days. The results showed the highest PHA content, total cell dry weight, and PHA yield as 72.96% (w/w) of cell dry weight, 9.30 g/L, and 6.78 g/L, respectively. The extracted PHA characterization was studied using gas chromatography, 1H NMR, FTIR, and XRD. The results found that the polymer was a polyhydroxybutyrate (PHB) with a melting temperature (Tm) and degradation temperature (Td) of 173.5 °C and 260.8 °C, respectively.

Cyanobacteria as a Promising Alternative for Sustainable Environment: Synthesis of Biofuel and Biodegradable Plastics

With the aim to alleviate the increasing plastic burden and carbon footprint on Earth, the role of certain microbes that are capable of capturing and sequestering excess carbon dioxide (CO2) generated by various anthropogenic means was studied. Cyanobacteria, which are photosynthetic prokaryotes, are promising alternative for carbon sequestration as well as biofuel and bioplastic production because of their minimal growth requirements, higher efficiency of photosynthesis and growth rates, presence of considerable amounts of lipids in thylakoid membranes, and cosmopolitan nature. These microbes could prove beneficial to future generations in achieving sustainable environmental goals. Their role in the production of polyhydroxyalkanoates (PHAs) as a source of intracellular energy and carbon sink is being utilized for bioplastic production. PHAs have emerged as well-suited alternatives for conventional plastics and are a parallel competitor to petrochemical-based plastics. Although a lot of studies have been conducted where plants and crops are used as sources of energy and bioplastics, cyanobacteria have been reported to have a more efficient photosynthetic process strongly responsible for increased production with limited land input along with an acceptable cost. The biodiesel production from cyanobacteria is an unconventional choice for a sustainable future as it curtails toxic sulfur release and checks the addition of aromatic hydrocarbons having efficient oxygen content, with promising combustion potential, thus making them a better choice. Here, we aim at reporting the application of cyanobacteria for biofuel production and their competent biotechnological potential, along with achievements and constraints in its pathway toward commercial benefits. This review article also highlights the role of various cyanobacterial species that are a source of green and clean energy along with their high potential in the production of biodegradable plastics.

Polyhydroxybutyrate production by Chlorella sorokiniana SVMIICT8 under Nutrient-deprived mixotrophy

Polyhydroxybutyrates (PHBs) are naturally occurring biopolymeric compounds that accumulate in a variety of microorganisms, including microalgae as energy and carbon storage sources. The present study was designed to evaluate nature-based PHB production using microalgae (Chlorella sorokiniana SVMIICT8) in biphasic (growth (GP) and stress phase (SP)) nutritional mode of cultivation. Microalgal PHB accumulation was driven by nutrient constraint, with a maximal production of 29.5% of PHB from 0.94 gm L^-1 of biomass. Fluorescence microscopy revealed PHB granules in the cell cytoplasm, while NMR (¹H and ¹³C), XRD and TGA analysis confirmed the structure. The biopolymer obtained was homopolymer of PHB with carbonyl (C=O) stretch of the aliphatic ester moiety. In GC-MS analysis, major peak representing butyric acid methyl ester also confirmed the PHB. Chlorophyll a fluorescence transients inferred through OJIP, exhibited significant variation in photosynthetic process during growth and nutrient limiting conditions. Mining of bio-based products from microalgae cultivation embrace nature-based approach addressing climate change and sustainability inclusively.

Patterning of the Autotrophic, Mixotrophic, and Heterotrophic Proteomes of Oxygen-Evolving Cyanobacterium Synechocystis sp. PCC 6803

Proteomes of an oxygenic photosynthetic cyanobacterium, Synechocystis sp. PCC 6803, were analyzed under photoautotrophic (low and high CO₂, assigned as ATLC and ATHC), photomixotrophic (MT), and light-activated heterotrophic (LAH) conditions. Allocation of proteome mass fraction to seven sub-proteomes and differential expression of individual proteins were analyzed, paying particular attention to photosynthesis and carbon metabolism–centered sub-proteomes affected by the quality and quantity of the carbon source and light regime upon growth. A distinct common feature of the ATHC, MT, and LAH cultures was low abundance of inducible carbon-concentrating mechanisms and photorespiration-related enzymes, independent of the inorganic or organic carbon source. On the other hand, these cells accumulated a respiratory NAD(P)H dehydrogenase I (NDH-1₁) complex in the thylakoid membrane (TM). Additionally, in glucose-supplemented cultures, a distinct NDH-2 protein, NdbA, accumulated in the TM, while the plasma membrane-localized NdbC and terminal oxidase decreased in abundance in comparison to both AT conditions. Photosynthetic complexes were uniquely depleted under the LAH condition but accumulated under the ATHC condition. The MT proteome displayed several heterotrophic features typical of the LAH proteome, particularly including the high abundance of ribosome as well as amino acid and protein biosynthesis machinery-related components. It is also noteworthy that the two equally light-exposed ATHC and MT cultures allocated similar mass fractions of the total proteome to the seven distinct sub-proteomes. Unique trophic condition-specific expression patterns were likewise observed among individual proteins, including the accumulation of phosphate transporters and polyphosphate polymers storing energy surplus in highly energetic bonds under the MT condition and accumulation under the LAH condition of an enzyme catalyzing cyanophycin biosynthesis. It is concluded that the rigor of cell growth in the MT condition results, to a great extent, by combining photosynthetic activity with high intracellular inorganic carbon conditions created upon glucose breakdown and release of CO₂, besides the direct utilization of glucose-derived carbon skeletons for growth. This combination provides the MT cultures with excellent conditions for growth that often exceeds that of mere ATHC.

Development of a New Method for Obtaining the Bioplastics Based on Microbial Biopolymers and Lignin

Background. The ever-increasing demand for plastic polymer products with simultaneous depleting fossil fuels such as oil and natural gas, as well as the growing problem of waste disposal, creates a need to find alternative technologies that meet current trends in both environmental and economic development. Bioplastic materials that are synthesized from renewable sources and have the ability to biodegrade are considered as such an alternative. The main obstacle of modern bioplastics which makes it impossible to completely replace traditional plastics is the high cost of production. In order to reduce the cost of existing biopolymers, production waste is added to the polymer matrix. One such waste is lignin – the second most common biopolymer. An additional way to reduce the cost of production is to find more cost-effective producers. Thus, although the classical microbial synthesis has fairly high productivity, the source of carbon for the cultivation of microorganisms are sugars obtained from agricultural raw materials which could cause a threat for food industry. The new producer for production of polyhydroxyalkanoates (PHA) is cyanobacteria, the carbon source of which is carbon (IV) oxide or gas emissions from enterprises, which reduces the cost of the target product. Objective. Development of a method for obtaining bioplastics using products of microbial synthesis and lignin. Methods. Cyanobacteria Nostoc commune was grown using a nutrient medium BG-11 with subsequent limitation of Nitrogen for the synthesis of PHA. Hydrolyzed lignin from hardwoods was combined with polylactic acid (PLA) or cyanobacteria-synthesized PHA in different ratios with further casting of the solution to determine the ability of lignin and polymer matrix to form polymer films. Results. The content of PHA in the cells of cyanobacteria Nostoc commune, when grown in a nutrient medium limited to Nitrogen, reached 7.8%. The synthesized polymer films based on PLA and lignin were not homogeneous, and films based on PHA and lignin were fragile. Conclusions. The possibility of obtaining PHA by using cyanobacteria of the Nostoc commune species under environmental conditions that differ from the optimal ones for both cultivation and PHA production is shown. The possibility of obtaining a biopolymer based on lignin and PLA is shown. To form homogeneous films, it is necessary to change the standard conditions for obtaining a mixture of components. The interaction of lignin with PHA forms a homogeneous polymer mixture, which is fragile and requires the addition of plasticizers to obtain the necessary properties.

Recovery of value-added products by mining microalgae

Microalgae blooms are always blamed for the interruption of the aquatic environment and pose a risk to the source of drinking water. Meanwhile, microalgae as primary producers are a kind of resource pool and could benefit the environment and contribute to building a circular economy. The lipid and polyhydroxybutyrate (PHB) in the cells of microalgae could be alternatives to fossil fuels and plastics, respectively, which are the culprits of global warming and plastic pollution. Besides, some microalgae are rich in nutrients, such as proteins and astaxanthin, which make themselves suitable for feed additives. As wastewater is rich in nutrients necessary for microalgae, thus, value-added product recovery via microalgae could be an approach to valorizing wastewater. However, a one-size-fits-all approach deploying various wastewater for the above products cannot be summarized. On the contrary, specific technical protocols should be tailored regarding each product in microalgae biomass with various wastewater. Thus, this review is to summarize the research effort by far on wastewater-cultivated microalgae for value-added products. Wastewater type, regulation methods, and targeted product yields are compiled and discussed and are expected to guide future extrapolation into a commercial scale.

Leptolyngbya sp. NIVA-CYA 255, a Promising Candidate for Poly(3-hydroxybutyrate) Production under Mixotrophic Deficiency Conditions

Cyanobacteria are a promising source for the sustainable production of biodegradable bioplastics such as poly(3-hydroxybutyrate) (PHB). The auto-phototrophic biomass formation is based on light and CO₂, which is an advantage compared to heterotrophic PHB-producing systems. So far, only a handful of cyanobacterial species suitable for the high-yield synthesis of PHB have been reported. In the present study, the PHB formation, biomass, and elemental composition of Leptolyngbya sp. NIVA-CYA 255 were investigated. Therefore, a three-stage cultivation process was applied, consisting of a growth stage; an N-, P-, and NP-depleted phototrophic stage; and a subsequent mixotrophic deficiency stage, initiated by sodium acetate supplementation. The extracted cyanobacterial PHB was confirmed by FTIR- and GC-MS analyses. Furthermore, the fluorescent dyes LipidGreen2 and Nile red were used for fluorescence-based monitoring and the visualization of PHB. LipidGreen2 was well suited for PHB quantification, while the application of Nile red was limited by fluorescence emission crosstalk with phycocyanin. The highest PHB yields were detected in NP- (325 mg g⁻¹) and N-deficiency (213 mg g⁻¹). The glycogen pool was reduced in all cultures during mixotrophy, while lipid composition was not affected. The highest glycogen yield was formed under N-deficiency (217 mg g⁻¹). Due to the high carbon storage capacity and PHB formation, Leptolyngbya sp. NIVA-CYA 255 is a promising candidate for PHB production. Further work will focus on upscaling to a technical scale and monitoring the formation by LipidGreen2-based fluorometry.

Microalgal Biorefinery Concepts’ Developments for Biofuel and Bioproducts: Current Perspective and Bottlenecks

Microalgae have received much interest as a biofuel feedstock. However, the economic feasibility of biofuel production from microalgae does not satisfy capital investors. Apart from the biofuels, it is necessary to produce high-value co-products from microalgae fraction to satisfy the economic aspects of microalgae biorefinery. In addition, microalgae-based wastewater treatment is considered as an alternative for the conventional wastewater treatment in terms of energy consumption, which is suitable for microalgae biorefinery approaches. The energy consumption of a microalgae wastewater treatment system (0.2 kW/h/m³) was reduced 10 times when compared to the conventional wastewater treatment system (to 2 kW/h/m³). Microalgae are rich in various biomolecules such as carbohydrates, proteins, lipids, pigments, vitamins, and antioxidants; all these valuable products can be utilized by nutritional, pharmaceutical, and cosmetic industries. There are several bottlenecks associated with microalgae biorefinery. Hence, it is essential to promote the sustainability of microalgal biorefinery with innovative ideas to produce biofuel with high-value products. This review attempted to bring out the trends and promising solutions to realize microalgal production of multiple products at an industrial scale. New perspectives and current challenges are discussed for the development of algal biorefinery concepts.

Optimization of Propagation Medium for Enhanced Polyhydroxyalkanoate Production by Pseudomonas oleovorans

Polyhydroxyalkanoates (PHAs) represent a promising alternative to commercially used petroleum-based plastics. Pseudomonas oleovorans is a natural producer of medium-chain-length PHA (mcl-PHA) under cultivation conditions with nitrogen limitation and carbon excess. Two-step cultivation appears to be an efficient but more expensive method of PHA production. Therefore, the aim of this work was to prepare a minimal synthetic medium for maximum biomass yield and to optimize selected independent variables by response surface methodology (RSM). The highest biomass yield (1.71 ± 0.04 g/L) was achieved in the optimized medium containing 8.4 g/L glucose, 5.7 g/L sodium ammonium phosphate and 35.4 mM phosphate buffer. Under these conditions, both carbon and nitrogen sources were completely consumed after 48 h of the cultivation and the biomass yield was 1.7-fold higher than in the conventional medium recommended by the literature. This approach demonstrates the possibility of using two-stage PHA cultivation to obtain the maximum amount of biomass and PHA.

Enhanced biomass production of Synechocystis sp. PCC 6803 by two associated bacteria Paenibacillus camelliae and Curtobacterium ammoniigenes

Synechocystis sp. PCC 6803 grown continuously in a 5-L photo-bioreactor for 20 months was found to have associated consortia with heterotrophic microorganisms. Two strains of bacteria were isolated from the long-term cultures of cyanobacteria with the aim to test whether their presence affects cyanobacterial growth and metabolism. The two strains were phylogenetically identified as Paenibacillus camelliae and Curtobacterium ammoniigenes, respectively. Co-culturing the Synechocystis sp. with either of the isolates under photoautotrophic and photoheterotrophic conditions exerted a statistically significant growth enhancement effect on cyanobacteria. Under co-culture experiments, the addition of P. camelliae resulted in a four-fold higher biomass yield with a considerable decrease in the stationary period. The growth was more pronounced on the addition of acetate to the culture media. Growth-enhancing factors like indole acetic acid (IAA) and siderophores were detected in the co-culture conditions which proved to be the main driving force in boosting cyanobacterial growth. Thus, the cyanobacteria-bacteria consortia can be very useful for augmenting biomass production by circumventing the time factor which can be further exploited for various biotechnological applications.

Sustainable production of polyhydroxybutyrate from autotrophs using CO2 as feedstock: Challenges and opportunities

Due to industrialization and rapid increase in world population, the global energy consumption has increased dramatically. As a consequence, there is increased consumption of fossil fuels, leading to a rapid increase in CO₂ concentration in the atmosphere. This accumulated CO₂ can be efficiently used by autotrophs as a carbon source to produce chemicals and biopolymers. There has been increasing attention on the production of polyhydroxybutyrate (PHB), a biopolymer, with focus on reducing the production cost. For this, cheaper renewable feedstocks, molecular tools, including metabolic and genetic engineering have been explored to improve microbial strains along with process engineering aspects for scale-up of PHB production. This review discusses the recent advents on the utilization of CO₂ as feedstock especially by engineered autotrophs, for sustainable production of PHB. The review also discusses the innovations in cultivation technology and process monitoring while understanding the underlying mechanisms for CO₂ to biopolymer conversion.

Photoautotrophic poly(3-hydroxybutyrate) production by a wild-type Synechococcus elongatus isolated from an extreme environment

The photoautotrophic poly(3-hydroxybutyrate) (PHB) production by cyanobacteria is an attractive option as it only requires CO₂ and light. In this work, a new wild-type strain producing PHB, Synechococcus elongatus UAM-C/S03, was identified using a polyphasic approach. The strain was cultured in a photobioreactor operated under N-sufficiency conditions at different pH values (7 to 11) and fed with CO₂ on demand. We also evaluated the production of PHB under N-starving conditions. Highest biomass productivity, 324 mg L^-1 d^-1, and CO₂ capture, 674 mg L^-1 d^-1, were obtained at pH 7 and under N-sufficiency conditions. The strain accumulated 29.42% of PHB in dry cell weight (DCW) under N-starvation conditions without pH control, and highest PHB productivity was 58.10 mg L^-1 d^-1. The highest carbohydrate content registered at pH 8, 50.84% in DCW, along with a release of carbon-based organic compounds, suggested the presence of exopolysaccharides in the culture medium.

Cyanobacterium Synechocystis sp. PCC 6803 lacking adc1 gene produces higher polyhydroxybutyrate accumulation under modified nutrients of acetate supplementation and nitrogen-phosphorus starvation

Increased polyhydroxybutyrate production in cyanobacterium Synechocystis sp. PCC 6803 lacking adc1 gene (Δadc1) is first-timely reported in this study. We constructed the mutant by disrupting adc1 gene encoding arginine decarboxylase, thereby exhibiting a partial blockade of polyamine synthesis. This Δadc1 mutant had a proliferative growth and certain contents of intracellular pigments including chlorophyll a and carotenoids as similar as those of wild type (WT). Highest PHB production was certainly induced by BG₁₁-N-P+A condition in both WT and Δadc1 mutant of about 24.9 %w/DCW at day 9 and 36.1 %w/DCW at day 7 of adaptation time, respectively. Abundant PHB granules were also visualized under both BG₁₁-N-P and BG₁₁-N-P+A conditions. All pha transcript amounts of Δadc1 mutant grown at 7 days-adaptation time were clearly upregulated corresponding to its PHB content under BG₁₁-N-P+A condition. Our finding indicated that this adc1 perturbation is alternatively achieved for PHB production in Synechocystis sp. PCC 6803.

Polyhydroxybutyrate: A Useful Product of Chlorotic Cyanobacteria

Polyhydroxybutyrate (PHB) is a carbon polymer with diverse functions, varying greatly on the organism producing it. This microreview describes the current knowledge about PHB metabolism, structure, and different physiological roles with a special focus on cyanobacteria. Despite the physiological function of PHB in the cyanobacterial phylum still being unknown, these organisms provide the unique opportunity to directly convert atmospheric CO2 into bioplastic using a solar-based process. Recent research on PHB metabolism in the cyanobacterial model organism Synechocystis revealed a sophisticated control of PHB granule formation. Novel insights about the metabolic background of PHB synthesis resulted in the engineering of the first cyanobacterial superproducer strain.

Challenges and Perspectives of Polyhydroxyalkanoate Production From Microalgae/Cyanobacteria and Bacteria as Microbial Factories: An Assessment of Hybrid Biological System

Polyhydroxyalkanoates (PHAs) are the biopolymer of choice if we look for a substitute of petroleum-based non-biodegradable plastics. Microbial production of PHAs as carbon reserves has been studied for decades and PHAs are gaining attention for a wide range of applications in various fields. Still, their uneconomical production is the major concern largely attributed to high cost of organic substrates for PHA producing heterotrophic bacteria. Therefore, microalgae/cyanobacteria, being photoautotrophic, prove to have an edge over heterotrophic bacteria. They have minimal metabolic requirements, such as inorganic nutrients (CO2, N, P, etc.) and light, and they can survive under adverse environmental conditions. PHA production under photoautotrophic conditions has been reported from cyanobacteria, the only candidate among prokaryotes, and few of the eukaryotic microalgae. However, an efficient cultivation system is still required for photoautotrophic PHA production to overcome the limitations associated with (1) stringent management of closed photobioreactors and (2) optimization of monoculture in open pond culture. Thus, a hybrid system is a necessity, involving the participation of microalgae/cyanobacteria and bacteria, i.e., both photoautotrophic and heterotrophic components having mutual interactive benefits for each other under different cultivation regime, e.g., mixotrophic, successive two modules, consortium based, etc. Along with this, further strategies like optimization of culture conditions (N, P, light exposure, CO2 dynamics, etc.), bioengineering, efficient downstream processes, and the application of mathematical/network modeling of metabolic pathways to improve PHA production are the key areas discussed here. Conclusively, this review aims to critically analyze cyanobacteria as PHA producers and proposes economically sustainable production of PHA from microbial autotrophs and heterotrophs in "hybrid biological system."

Homologs of Circadian Clock Proteins Impact the Metabolic Switch Between Light and Dark Growth in the Cyanobacterium Synechocystis sp. PCC 6803

The putative circadian clock system of the facultative heterotrophic cyanobacterial strain Synechocystis sp. PCC 6803 comprises the following three Kai-based systems: a KaiABC-based potential oscillator that is linked to the SasA-RpaA two-component output pathway and two additional KaiBC systems without a cognate KaiA component. Mutants lacking the genes encoding the KaiAB1C1 components or the response regulator RpaA show reduced growth in light/dark cycles and do not show heterotrophic growth in the dark. In the present study, the effect of these mutations on central metabolism was analyzed by targeted and non-targeted metabolite profiling. The strongest metabolic changes were observed in the dark in ΔrpaA and, to a lesser extent, in the ΔkaiAB1C1 mutant. These observations included the overaccumulation of 2-phosphoglycolate, which correlated with the overaccumulation of the RbcL subunit in the mutants, and taken together, these data suggest enhanced RubisCO activity in the dark. The imbalanced carbon metabolism in the ΔrpaA mutant extended to the pyruvate family of amino acids, which showed increased accumulation in the dark. Hence, the deletion of the response regulator rpaA had a more pronounced effect on metabolism than the deletion of the kai genes. The larger impact of the rpaA mutation is in agreement with previous transcriptomic analyses and likely relates to a KaiAB1C1-independent function as a transcription factor. Collectively, our data demonstrate an important role of homologs of clock proteins in Synechocystis for balanced carbon and nitrogen metabolism during light-to-dark transitions.

Light excess stimulates Poly-beta-hydroxybutyrate yield in a mangrove-isolated strain of Synechocystis sp

Poly-β-hydroxybutyrate (PHB) is a biodegradable biopolymer that may replace fossil-based plastics reducing its negative environmental impact. One highly sustainable strategy to produce these biopolymers is the exploitation of photosynthetic microorganisms that use sunlight and CO₂ to produce biomass and subsequently, PHB. Exploring environmental biological diversity is a powerful tool to find resilient microorganisms potentially exploitable to produce bioproducts. In this work, a cyanobacterium (Synechocystis sp.) isolated from a contaminated area close to an important industrial complex was shown to produce PHB under different culture conditions. Carbon, nutrients supply and light intensity impact on biomass and PHB productivity were assessed, showing that the highest yield of PHB achieved was 241 mg L^-1 (31%_dcw) under high light intensity. Remarkably this condition not only stimulated PHB accumulation by 70% compared to other conditions tested but also high cellular duplication rate, maximizing the potential of this strain for PHB production.

Accumulation of PHA in the Microalgae Scenedesmus sp. under Nutrient-Deficient Conditions

Traditional plastics have undoubted utility and convenience for everyday life; but when they are derived from petroleum and are non-biodegradable, they contribute to two major crises today's world is facing: fossil resources depletion and environmental degradation. Polyhydroxyalkanoates are a promising alternative to replace them, being biodegradable and suitable for a wide variety of applications. This biopolymer accumulates as energy and carbon storage material in various microorganisms, including microalgae. This study investigated the influence of glucose, N, P, Fe, and salinity over the production of polyhydroxyalkanoate (PHA) by Scenedesmus sp., a freshwater microalga strain not previously explored for this purpose. To assess the effect of the variables, a fractional Taguchi experimental design involving 16 experimental runs was planned and executed. Biopolymer was obtained in all the experiments in a wide range of concentrations (0.83-29.92%, w/w DW), and identified as polyhydroxybutyrate (PHB) by FTIR analysis. The statistical analysis of the response was carried out using Minitab 16, where phosphorus, glucose, and iron were identified as significant factors, together with the P-Fe and glucose-N interactions. The presence of other relevant macromolecules was also quantified. Doing this, this work contributes to the understanding of the critical factors that control PHA production and present Scenedesmus sp. as a promising species to produce bio-resources in commercial systems.

Maximizing PHB content in Synechocystis sp. PCC 6803: a new metabolic engineering strategy based on the regulator PirC

BACKGROUND

PHB (poly-hydroxy-butyrate) represents a promising bioplastic alternative with good biodegradation properties. Furthermore, PHB can be produced in a completely carbon-neutral fashion in the natural producer cyanobacterium Synechocystis sp. PCC 6803. This strain has been used as model system in past attempts to boost the intracellular production of PHB above ~ 15% per cell-dry-weight (CDW).

RESULTS

We have created a new strain that lacks the regulatory protein PirC (product of sll0944), which exhibits a higher activity of the phosphoglycerate mutase resulting in increased PHB pools under nutrient limiting conditions. To further improve the intracellular PHB content, two genes involved in PHB metabolism, phaA and phaB, from the known producer strain Cupriavidus necator, were introduced under the control of the strong promotor PpsbA2. The resulting strain, termed PPT1 (ΔpirC-REphaAB), produced high amounts of PHB under continuous light as well under a day-night regime. When grown in nitrogen and phosphorus depleted medium, the cells produced up to 63% per CDW. Upon the addition of acetate, the content was further increased to 81% per CDW. The produced polymer consists of pure PHB, which is highly isotactic.

CONCLUSION

The amounts of PHB achieved with PPT1 are the highest ever reported in any known cyanobacterium and demonstrate the potential of cyanobacteria for a sustainable, industrial production of PHB.

Cyanobacterial Polyhydroxyalkanoates: A Sustainable Alternative in Circular Economy

Conventional petrochemical plastics have become a serious environmental problem. Its unbridled use, especially in non-durable goods, has generated an accumulation of waste that is difficult to measure, threatening aquatic and terrestrial ecosystems. The replacement of these plastics with cleaner alternatives, such as polyhydroxyalkanoates (PHA), can only be achieved by cost reductions in the production of microbial bioplastics, in order to compete with the very low costs of fossil fuel plastics. The biggest costs are carbon sources and nutrients, which can be appeased with the use of photosynthetic organisms, such as cyanobacteria, that have a minimum requirement for nutrients, and also using agro-industrial waste, such as the livestock industry, which in turn benefits from the by-products of PHA biotechnological production, for example pigments and nutrients. Circular economy can help solve the current problems in the search for a sustainable production of bioplastic: reducing production costs, reusing waste, mitigating CO2, promoting bioremediation and making better use of cyanobacteria metabolites in different industries.

Industrial Production of Poly-β-hydroxybutyrate from CO2: Can Cyanobacteria Meet this Challenge?

The increasing impact of plastic materials on the environment is a growing global concern. In regards to this circumstance, it is a major challenge to find new sources for the production of bioplastics. Poly-β-hydroxybutyrate (PHB) is characterized by interesting features that draw attention for research and commercial ventures. Indeed, PHB is eco-friendly, biodegradable, and biocompatible. Bacterial fermentation processes are a known route to produce PHB. However, the production of PHB through the chemoheterotrophic bacterial system is very expensive due to the high costs of the carbon source for the growth of the organism. On the contrary, the production of PHB through the photoautotrophic cyanobacterium system is considered an attractive alternative for a low-cost PHB production because of the inexpensive feedstock (CO2 and light). This paper regards the evaluation of four independent strategies to improve the PHB production by cyanobacteria: (i) the design of the medium; (ii) the genetic engineering to improve the PHB accumulation; (iii) the development of robust models as a tool to identify the bottleneck(s) of the PHB production to maximize the production; and (iv) the continuous operation mode in a photobioreactor for PHB production. The synergic effect of these strategies could address the design of the optimal PHB production process by cyanobacteria. A further limitation for the commercial production of PHB via the biotechnological route are the high costs related to the recovery of PHB granules. Therefore, a further challenge is to select a low-cost and environmentally friendly process to recover PHB from cyanobacteria.

Contribution of protein synthesis depression to poly-β-hydroxybutyrate accumulation in Synechocystis sp. PCC 6803 under nutrient-starved conditions

Poly-β-hydroxybutyrate (PHB) in cyanobacteria, which accumulates as energy and carbon sources through the action of photosynthesis, is expected to substitute for petroleum-based plastics. This study first demonstrated that PHB accumulation was induced, with the appearance of lipid droplets, in sulfur (S)-starved cells of a cyanobacterium, Synechocystis sp. PCC 6803, however, to a lower level than in nitrogen (N)- or phosphorus (P)-starved cells. Concomitantly found was repression of the accumulation of total cellular proteins in the S-starved cells to a similar level to that in N-starved cells, and a severer level than in P-starved cells. Intriguingly, PHB accumulation was induced in Synechocystis even under nutrient-replete conditions, upon repression of the accumulation of total cellular proteins through treatment of the wild type cells with a protein synthesis inhibitor, chloramphenicol, or through disruption of the argD gene for Arg synthesis. Meanwhile, the expression of the genes for PHB synthesis was hardly induced in S-starved cells, in contrast to their definite up-regulation in N- or P-starved cells. It therefore seemed that PHB accumulation in S-starved cells is achieved through severe repression of protein synthesis, but is smaller than in N- or P-starved cells, owing to little induction of the expression of PHB synthesis genes.

Exploitation of Wheat Straw Biorefinery Side Streams as Sustainable Substrates for Microorganisms: A Feasibility Study

Lignocellulosic agricultural side products, like wheat straw, are widely seen as an important contribution to a future sustainable economy. However, optimization of biorefinery processes and exploitation of all side streams are crucial for an economically viable biorefinery. Pretreatment of lignocellulosic raw material, which is necessary for further processing steps, can generate low-value side streams. In this feasibility study, side streams from a liquid hot water (LHW) pretreatment of wheat straw were utilized for the production of polyhydroxybutyrate (PHB) and highly valuable tetraether lipids (TELs). Additional value created by these products can benefit the biorefinery’s economic operation. The utilized wheat straw was pretreated at 120 °C and 170 °C for up to two hours in laboratory and lab scale. The resulting side stream consists mainly of carbohydrates from hemicelluloses and fermentation inhibitors such as acetic acid. In order to achieve a successful production of both products, an acetic acid separation via distillation was necessary. Subsequently, the acetic acid fraction was utilized for the PHB production using cyanobacteria. The carbohydrate-rich fraction was applied in the cultivation of Sulfolobus acidocaldarius and resulted in the successful production of TELs. Both fractions achieved better fermentation yields compared to their corresponding reference media.

Polyhydroxyalkanoates (PHA) production from fermented thermal-hydrolyzed sludge by mixed microbial cultures: The link between phosphorus and PHA yields

Production of polyhydroxyalkanoates (PHA) from wastes has gained increasing attention for the related low costs and high environmental benefits. Phosphorus limitation is a potential strategy used to facilitate PHA production, yet excessive limitation was previously reported to cause negative effects. This study was the first to investigate the optimum phosphorus limitation for PHA accumulation from thermal-hydrolyzed sludge. The results showed that the maximum PHA content increased from 23 wt% to 51 wt% when phosphorus concentration was limited from 127.60 to 1.35 mg/L, indicating that a lower phosphorus concentration would promote maximum PHA accumulation. Batch tests performed with synthetic substrates (containing one specific VFA for each batch) confirmed that the effect of phosphorus content on PHA production was mainly devoted by the efficiency of the conversion of acetate to PHA. The PHA yields on acetate (Y_PHA/ac) were 0.68 and 0.05 Cmol/Cmol under phosphorus-limited (1 mg/L) and -excess (100 mg/L) conditions, respectively. A mathematical model was developed to describe the correlation between phosphorus concentration and Y_PHA/ac, which can fit the experimental data and predict the results properly. Finally, further (ammonium-) nitrogen restriction did not efficiently cause the additional improvement of PHA production under the conditions of phosphorus limitation.

Disruption of cyanobacterial γ-aminobutyric acid shunt pathway reduces metabolites levels in tricarboxylic acid cycle, but enhances pyruvate and poly(3-hydroxybutyrate) accumulation

The photoautotrophic cyanobacterium Synechocystis sp. PCC 6803 assimilates carbon dioxide as the sole carbon source, and a major portion of the assimilated carbon is metabolically consumed by the tricarboxylic acid (TCA) cycle. Effects of partial interference of TCA cycle metabolic activity on other carbon metabolism have yet to be examined. Here, the γ-aminobutyric acid (GABA) shunt, one of the metabolic pathways for completing TCA cycle in Synechocystis, was disrupted via inactivating the glutamate decarboxylase gene (gdc). Under normal photoautotrophic condition, cell growth and the level of the TCA cycle metabolites succinate, malate and citrate were decreased by 25%, 35%, 19% and 28%, respectively, in Δgdc mutant relative to those in the wild type (WT). The cellular levels of glycogen and total lipids of the Δgdc mutant were comparable to those of the WT, but the intracellular levels of pyruvate and bioplastic poly(3-hydroxybutyrate) (PHB) were 1.23- and 2.50-fold higher, respectively, in Δgdc mutant. Thus, disruption of the GABA shunt pathway reduced the TCA cycle metabolites levels, but positively enhanced the bioaccumulation of pyruvate and PHB. The PHB production rate in Δgdc mutant was 2.0-fold higher than in the WT under normal photoautotrophy.

Poly-β-Hydroxybutyrate Production by the Cyanobacterium Scytonema geitleri Bharadwaja under Varying Environmental Conditions

The production of poly-β-hydroxybutyrate (PHB) under varying environmental conditions (pH, temperature and carbon sources) was examined in the cyanobacterium Scytonema geitleri Bharadwaja isolated from the roof-top of a building. The S. geitleri produced PHB and the production of PHB was linear with the growth of cyanobacterium. The maximum PHB production (7.12% of dry cell weight) was recorded when the cells of S. geitleri were at their stationary growth phase. The production of PHB was optimum at pH 8.5 and 30 °C, and acetate (30 mM) was the preferred carbon source.

Microalgae as source of polyhydroxyalkanoates (PHAs) - A review

Polyhydroxyalkanoates (PHA) are biopolymers synthesized by different microorganisms and considered substitute powers for petroleum-based plastics because they have similar mechanical properties as synthetic polymers, can be processed in a similar way and are fully biodegradable. Currently commercial PHAs are produced in fermenters using bacteria and large amounts of organic carbon sources and salts in the culture media, accounting for approximately 50% of the total production costs. A greater commercial application of the PHA is limited to a decrease in the cost of production. Several studies suggest that microalgae are a type of microorganisms that can be used to obtain PHAs at a lower cost because they have minimum nutrient requirements for growth and are photoautotrophic in nature, i.e. they use light and CO₂ as their main sources of energy. Thus, this work aims to provide a review on the production of PHAs of different microalgae, focusing on the properties and composition of biopolymers, verifying the potential of using these bioplastics instead of petroleum based plastics. Studies of stimulation PHA synthesis by microalgae are still considered incipient. Still, it is clear that microalgae have the potential to produce biopolymers with lower cost and can play a vital role in the environment.

Bioprocess Engineering Aspects of Sustainable Polyhydroxyalkanoate Production in Cyanobacteria

Polyhydroxyalkanoates (PHAs) are a group of biopolymers produced in various microorganisms as carbon and energy reserve when the main nutrient, necessary for growth, is limited. PHAs are attractive substitutes for conventional petrochemical plastics, as they possess similar material properties, along with biocompatibility and complete biodegradability. The use of PHAs is restricted, mainly due to the high production costs associated with the carbon source used for bacterial fermentation. Cyanobacteria can accumulate PHAs under photoautotrophic growth conditions using CO₂ and sunlight. However, the productivity of photoautotrophic PHA production from cyanobacteria is much lower than in the case of heterotrophic bacteria. Great effort has been focused to reduce the cost of PHA production, mainly by the development of optimized strains and more efficient cultivation and recovery processes. Minimization of the PHA production cost can only be achieved by considering the design and a complete analysis of the whole process. With the aim on commercializing PHA, this review will discuss the advances and the challenges associated with the upstream processing of cyanobacterial PHA production, in order to help the design of the most efficient method on the industrial scale.

Generation of a functional, semisynthetic [FeFe]-hydrogenase in a photosynthetic microorganism

[FeFe]-Hydrogenases are hydrogen producing metalloenzymes with excellent catalytic capacities, highly relevant in the context of a future hydrogen economy. Here we demonstrate the synthetic activation of a heterologously expressed [FeFe]-hydrogenase in living cells of Synechocystis PCC 6803, a photoautotrophic microbial chassis with high potential for biotechnological energy applications. H2-Evolution assays clearly show that the non-native, semi-synthetic enzyme links to the native metabolism in living cells.

Analysis of the light intensity dependence of the growth of Synechocystis and of the light distribution in a photobioreactor energized by 635 nm light

Synechocystis gathered momentum in modelling studies and biotechnological applications owing to multiple factors like fast growth, ability to fix carbon dioxide into valuable products, and the relative ease of genetic manipulation. Synechocystis physiology and metabolism, and consequently, the productivity of Synechocystis-based photobioreactors (PBRs), are heavily light modulated. Here, we set up a turbidostat-controlled lab-scale cultivation system in order to study the influence of varying orange–red light intensities on Synechocystis growth characteristics and photosynthetic activity. Synechocystis growth and photosynthetic activity were found to raise as supplied light intensity increased up to 500 μmol photons m⁻² s⁻¹ and to enter the photoinhibition state only at 800 μmol photons m⁻² s⁻¹. Interestingly, reverting the light to a non-photo-inhibiting intensity unveiled Synechocystis to be able to promptly recover. Furthermore, our characterization displayed a clear correlation between variations in growth rate and cell size, extending a phenomenon previously observed in other cyanobacteria. Further, we applied a modelling approach to simulate the effects produced by varying the incident light intensity on its local distribution within the PBR vessel. Our model simulations suggested that the photosynthetic activity of Synechocystis could be enhanced by finely regulating the intensity of the light incident on the PBR in order to prevent cells from experiencing light-induced stress and induce their exploitation of areas of different local light intensity formed in the vessel. In the latter case, the heterogeneous distribution of the local light intensity would allow Synechocystis for an optimized usage of light.

Novel quantitative insights into carbon sources for synthesis of poly hydroxybutyrate in Synechocystis PCC 6803

Many freshwater cyanobacteria accumulate polyhydroxybutyrate (PHB) under nitrogen or phosphorus deprivation. While prior literature has shed lights on transcriptomic and metabolomic changes in the model cyanobacterium Synechocystis PCC 6803 cells, the quantitative contributions of the newly fixed carbon following nitrogen deprivation or the externally added acetate to PHB synthesis are not clear. Similarly, it is not clear how photomixotrophy affects precursor contributions. In this study, we show that (i) the pre-growth mode (photoautotrophic or photomixotrophic), while significantly impacting glycogen levels, does not have any significant effect on PHB levels, (ii) the carbon fixed following nitrogen deprivation contributes 26% of C for PHB synthesis in photoautotrophically pre-grown cells and its contribution to the PHB synthesis goes down with the addition of acetate at the resuspension phase or with photomixotrophic pre-growth, (iii) the acetate added at the start of nitrogen deprivation, doubles the intracellular PHB levels and contributes 44-48% to PHB synthesis and this value is not greatly affected by how the cells were pre-grown. Indirectly, the labeling studies also show that the intracellular C recycling is the most important source of precursors for PHB synthesis, contributing about 74-87% of the C for PHB synthesis in the absence of acetate. The addition of acetate significantly reduces its contribution. In photoautotrophic pre-growth followed by acetate addition under nitrogen starvation, the contribution of intracellular C reduces to about 34%. Thus, our study provides several novel quantitative insights on how prior nutritional status affects the precursor contributions for PHB synthesis.

A balanced ATP driving force module for enhancing photosynthetic biosynthesis of 3-hydroxybutyrate from CO2

Using engineered photoautotrophic microorganisms for the direct chemical synthesis from CO₂ is an attractive direction for both sustainability and CO₂ mitigation. However, the behaviors of non-native metabolic pathways may be difficult to control due to the different intracellular contexts between natural and heterologous hosts. While most metabolic engineering efforts focus on strengthening driving forces in pathway design to favor biochemical production in these organisms, excessive driving force may be detrimental to product biosynthesis due to imbalanced cellular intermediate distribution. In this study, an ATP-hydrolysis based driving force module was engineered into cyanobacterium Synechococcus elongatus PCC 7942 to produce 3-hydroxybutyrate (3HB), a valuable chemical feedstock for the synthesis of biodegradable plastics and antibiotics. However, while the ATP driving force module is effective for increasing product formation, uncontrolled accumulation of intermediate metabolites likely led to metabolic imbalance and thus to cell growth inhibition. Therefore, the ATP driving force module was reengineered by providing a reversible outlet for excessive carbon flux. Upon expression of this balanced ATP driving force module with 3HB biosynthesis, engineered strain produced 3HB with a cumulative titer of 1.2 g/L, a significant increase over the initial strain. This result highlighted the importance of pathway reversibility as an effective design strategy for balancing driving force and intermediate accumulation, thereby achieving a self-regulated control for increased net flux towards product biosynthesis.

Production of polyhydroxybutyrates and carbohydrates in a mixed cyanobacterial culture: Effect of nutrients limitation and photoperiods

In the present study, different photoperiods and nutritional conditions were applied to a mixed wastewater-borne cyanobacterial culture in order to enhance the intracellular accumulation of polyhydroxybutyrates (PHBs) and carbohydrates. Two different experimental set-ups were used. In the first, the culture was permanently exposed to illumination, while in the second it was submitted to light/dark alternation (12 h cycles). In both cases, two different nutritional regimes were also evaluated, N-limitation and P-limitation. Results showed that the highest PHB concentration (104 mg L^-1) was achieved under P limited conditions and permanent illumination, whereas the highest carbohydrate concentration (838 mg L^-1) was obtained under N limited condition and light/dark alternation. With regard to bioplastics and biofuel generation, this study demonstrates that the accumulation of PHBs (bioplastics) and carbohydrates (potential biofuel substrate) is favored in wastewater-borne cyanobacteria under conditions where nutrients are limited.

Photosynthetic poly-β-hydroxybutyrate accumulation in unicellular cyanobacterium Synechocystis sp. PCC 6714

Poly-β-hydroxybutyrate (PHB) production from CO₂ has the potential to reduce the production cost of this biodegradable polyesters, and also to make the material more sustainable compared to utilization of sugar feedstocks. In this study the unicellular cyanobacterium, Synechocystis sp. PCC 6714 has been identified as an unexplored potential organism for production of PHB. Synechocystis sp. PCC 6714 was studied under various cultivation conditions and nutritional limitations. Combined effects of nitrogen and phosphorus deficiency led to highest PHB accumulation under photoautotrophic conditions. Multivariate experimental design and quantitative bioprocess development methodologies were used to identify the key cultivation parameters for PHB accumulation. Biomass growth and PHB accumulation were studied under controlled defined conditions in a lab-scale photobioreactor. Specific growth rates were fourfold higher in photobioreactor experiments when cultivation conditions were controlled. After 14 days of cultivation in nitrogen and phosphorus, limited media intracellular PHB levels reached up to 16.4% from CO₂. The highest volumetric production rate of PHB was 59 ± 6 mg L⁻¹ day⁻¹. Scanning electron microscopy of isolated PHB granules of Synechocystis sp. PCC 6714 cultivated under nitrogen and phosphorus limitations showed an average diameter of 0.7 µm. The results of this study might contribute towards a better understanding of photoautotrophic PHB production from cyanobacteria.

FabG can function as PhaB for poly-3-hydroxybutyrate biosynthesis in photosynthetic cyanobacteria Synechocystis sp. PCC 6803

The production of poly-3-hydroxybutyrate (PHB) by photosynthetic cyanobacteria is a potentially sustainable production method for the biodegradable plastics industry. β-Ketoacyl-ACP reductase (FabG), from the photosynthetic cyanobacterium Synechocystis sp. PCC 6803 (SpFabG), is the first NADPH-dependent reductase in the fatty acid biosynthesis pathway. Its structure is similar to that of acetoacetyl-CoA reductase (SpPhaB), which is critical for PHB synthesis and can replace SpPhaB for acetoacetyl-CoA reduction in vitro. However, the specific function of SpFabG in fatty acid synthesis and whether SpFabG could participate in PHB synthesis in vivo were not yet clear. In this study, the role of SpFabG in fatty acid synthesis was first verified in vivo by knocking down and overexpressing of fabG. It was shown that SpFabG was essential yet not rate-limiting for fatty acid biosynthesis. The biochemical characterization of SpFabG using acetoacetyl-CoA as the substrate showed that the optimum temperature, optimum pH, K_m and k_cat were 30°C, 7, 2.30 mM, and 19.85 s^-1, respectively, which exemplified the ability of SpFabG to reduce acetoacetyl-CoA with a relatively low affinity and weak catalytic efficiency. Functional analysis of SpFabG in vivo indicated that SpFabG was able to partially complement SpPhaB under nitrogen-deprived conditions, and overexpression of fabG led to the diversion of partial carbon flux from fatty acid toward PHB synthesis.