Increased carbohydrate production from carbon dioxide in randomly mutated cells of cyanobacterial strain Synechocystis sp. PCC 6714: Bioprocess understanding and evaluation of productivities

One-Pot Biosynthesis of Acetone from Waste Poly(hydroxybutyrate)

The plastic waste crisis is catalyzing change across the plastics life cycle. Central to this is increased production and application of bioplastics and biodegradable plastics. In particular, poly(hydroxybutyrate) (PHB) is a biodegradable bioplastic that can be produced from various renewable and waste feedstocks and is a promising alternative to some petrochemical-derived and non-biodegradable plastics. Despite its advantages, PHB biodegradation depends on environmental conditions, and the effects of degradation into microplastics, oligomers, and the 3-hydroxybutyrate (3-HB) monomer on soil microbiomes are unknown. We hypothesized that the ease of PHB biodegradation renders this next-generation plastic an ideal feedstock for microbial recycling into platform chemicals currently produced from fossil fuels. To demonstrate this, we report the one-pot degradation and recycling of PHB into acetone using a single strain of engineered Escherichia coli. Following strain development and initial bioprocess optimization, we report maximum titers of 123 mM acetone (7 g/L) from commercial PHB granules after 24 h fermentation at 30 °C. We further report biorecycling of an authentic sample of post-consumer PHB waste at a preparative scale. This is the first demonstration of biological recycling of PHB into a second-generation chemical, and it demonstrates next-generation plastic waste as a novel feedstock for the circular bioeconomy.

PHB Processability and Property Improvement with Linear-Chain Polyester Oligomers Used as Plasticizers

In 2021, global petroleum-based plastic production reached over 400 million metric tons (Mt), and the accumulation of these non-biodegradable plastics in the environment is a worldwide concern. Polyhydroxybutyrate (PHB) offers many advantages over traditional petroleum-based plastics, being biobased, completely biodegradable, and non-toxic. However, its production and use are still challenging due to its low deformation capacity and narrow processing window. In this work, two linear-chain polyester oligomers were used as plasticizers to improve the processability and properties of PHB. Thermal analyses, XRD, and polarized optical microscopy were performed to evaluate the plasticizing effect on the PHB and the reflection on the mechanical behavior. Both oligomers acted as PHB plasticizers, with a reduction in T_g and T_m as a function of the plasticizer concentration, which can make it easier to handle the material in thermal processing and reduce the probability of thermal degradation. Plasticizer 2 proved to be the most promising between the two with an optimized condition of 20%, in which there was a decrease in elastic modulus of up to 72% and an increase in the maximum elongation of 467%.

Inorganic carbon stimulates the metabolic routes related to the polyhdroxybutyrate production in a Synechocystis sp. strain (cyanobacteria) isolated from wastewater

Cyanobacteria are capable of transforming CO₂ into polyhydroxybutyrate (PHB). In this study, different inorganic carbon concentrations (0-2 gC L^-1) were evaluated for a Synechocystis sp. strain isolated from wastewater. Quantitative RT-qPCR was also performed to decipher the links between inorganic carbon and PHB and glycogen metabolism. 2 gC L^-1 of bicarbonate stimulated cell growth, nutrients consumption and production of PHB. Using this concentration, a 14%_dcw of PHB and an average productivity of 2.45 mgPHB L^-1 d^-1 were obtained. Gene expression analysis revelated that these conditions caused the overexpression of genes related to glycogen and PHB synthesis. Moreover, a positive correlation between the genes codifying for the glycogen phosphorylase, the acetyl-CoA reductase and the poly(3-hydroxyalkanoate) polymerase was found, meaning that PHB synthesis and glycogen catabolism are strongly related. These results provide an exhaustive evaluation of the effect of carbon on the PHB production and cyanobacterial metabolism.

Process optimization of the polyhydroxybutyrate production in the cyanobacteria Synechocystis sp. and Synechococcus sp

The effect of four parameters (acetate, NaCl, inorganic carbon and days in darkness) affecting the polyhydroxybutyrate (PHB) production were tested and optimized for Synechococcus sp. and Synechocystis sp. using a Box-Behnken design. The optimal conditions for Synechocystis sp. were found to be 1.2 g L^-1 of acetate, 4 gC L^-1 of NaHCO₃, 18 g L^-1 of NaCl and 0 days in darkness. For Synechococcus sp., equal acetate concentration and days in darkness, and lower inorganic carbon and NaCl concentrations than those for Synechocystis sp. were needed (0.05 g L^-1 inorganic carbon and 9 g L^-1 NaCl). Optimal conditions were scaled up to 3 L photobioreactors. Using Synechocystis sp., 5.6 %_dcw of PHB was obtained whether adding or not acetate. In opposition, a maximum of 26.1 %_dcw by using acetate was reached with Synechococcus sp. These results provide an easy method to optimize the cultivation conditions to enhance PHB production with cyanobacteria.

Bio-Based Plastics Production, Impact and End of Life: A Literature Review and Content Analysis

The accumulation of plastic wastes is one of the most widely spread problems affecting the environment. The reality that plastics can be made from renewable resources and degrade naturally has prompted academics to think outside the box to develop “better for the environment” items. In this paper, a bibliometric analysis of the scholarly publications related to bio-based plastics within the last 20 years is presented. Annual progression, geographic and research area distribution, and keyword co-occurrence were all examined. Six distinct clusters emerged from keyword analysis, which were further categorized into three directions: production to marketing; impact on the environment, economy, and society; and end-of-life (EoL) options. The major focus was on how to counter the weaknesses and challenges of bio-based plastics and take opportunities using the inherent advantages of bio-based plastics. Comprehensive studies regarding the impact of bio-based plastics on the environment, economy and social sustainability are still deficient. Although there are many promising innovations in this area, most of them are at the research stage. The benefits of bio-based plastics and better EoL options can be enjoyed only after increased production.

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.

Optimization of the phototrophic Cyanobacteria polyhydroxybutyrate (PHB) production by kinetic model simulation

Cyanobacteria can grow using inorganic substrates, such as CO₂ from industrial sources and nutrients from wastewaters, and therefore are promising microorganisms to produce polyhydroxybutyrate in a cleaner circular context. However, this biotechnological production is highly challenging because it involves different interlinked reactions that are affected by environmental conditions, which hinders process optimization. In this study a new biokinetic mechanistic model using novel experimental approaches was developed to optimize polyhydroxybutyrate (PHB) and glycogen production. The model includes, for the first time, the production of glycogen and its conversion into PHB, which has been found as the main pathway to produce PHB. Model was successfully (r²: 0.6-0.99) calibrated and validated with experimental data from photobioreactors inoculated with Synechocystis sp. The developed model was used to determine suitable initial conditions for a lab scale batch reactor (6.4 mgN·L^-1 and 2 mgP·L^-1) and a new configuration for the continuous industrial production of PHB was proposed and optimized using this tool. The maximum productivity (5.1 mgPHB·L^-1·d^-1) and the optimal configuration and operation of the serial reactors to produce PHB in an industrial scale was achieved using a hydraulic retention time of 4 days in the growth reactor. Then, this reactor daily fed 20 batch accumulation reactors, which were discharged after 20 days. The optimal influent nutrients concentrations for this configuration was found to be 50 mgN·L^-1 and 10 mgP·L^-1. Results found in this study show the necessity to optimize biopolymers production with Cyanobacteria considering environmental conditions, and demonstrated the potential of this model as a tool to increase PHB productivity.

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.

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.

Recent progress and challenges in microbial polyhydroxybutyrate (PHB) production from CO2 as a sustainable feedstock: A state-of-the-art review

The recalcitrance of petroleum-based plastics causes severe environmental problems and has accelerated research into production of biodegradable polymers from inexpensive and sustainable feedstocks. Various microorganisms are capable of producing Polyhydroxybutyrate (PHB), a representative biodegradable polymer, under nutrient-limited conditions, among which CO₂-utilizing microorganisms are of primary interest. Herein, we discuss recent progress on bacterial strains including proteobacteria, purple non-sulfur bacteria, and cyanobacteria in terms of CO₂-containing carbon sources, PHB-production capability, and genetic modification. In addition, this review introduces recent technical approaches used to improve PHB production from CO₂ such as two-stage bioprocesses and bioelectrochemical systems. Challenges and future perspectives for the development of economically feasible PHB production are also discussed. Finally, this review might provide insights into the construction of a closed-carbon-loop to cope with climate change.

Genome-wide identification and characterization of Fur-binding sites in the cyanobacteria Synechocystis sp. PCC 6803 and PCC 6714

The Ferric uptake regulator (Fur) is crucial to both pathogenic and non-pathogenic bacteria for the maintenance of iron homeostasis as well as the defence against reactive oxygen species. Based on datasets from the genome-wide mapping of transcriptional start sites and transcriptome data, we identified a high confidence regulon controlled by Fur for the model cyanobacterium Synechocystis sp. PCC 6803 and its close relative, strain 6714, based on the conserved strong iron starvation response and Fur-binding site occurrence. This regulon comprises 33 protein-coding genes and the sRNA IsaR1 that are under the control of 16 or 14 individual promoters in strains 6803 and 6714, respectively. The associated gene functions are mostly restricted to transporters and enzymes involved in the uptake and storage of iron ions, with few exceptions or unknown functional relevance. Within the isiABC operon, we identified a previously neglected gene encoding a small cysteine-rich protein, which we suggest calling, IsiE. The regulation of iron uptake, storage, and utilization ultimately results from the interplay between the Fur regulon, several other transcription factors, the FtsH3 protease, and the sRNA IsaR1.

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.

Biogas-based production of glycogen by Nostoc muscorum: Assessing the potential of transforming CO2 into value added products

The potential of the filamentous N₂-fixing cyanobacterium Nostoc muscorum for CO₂ capture from high-loaded streams (i.e. flue gas or biogas) combined with the accumulation of glycogen (GL) and polyhydroxybutyrate (PHB), was evaluated under nutrient-sufficient and nutrient-limited conditions. N. muscorum was able to grow under CO₂ contents from 0.03 up to 30% v/v, thus tolerating CO₂ concentrations similar to those found in raw biogas or flue-gas, with maximum CO₂-fixation rates of 191.9 ± 46 g m^-3 d^-1 at a biomass concentration of 733.3 ± 207.4 mg TSS L^-1. Despite N. muscorum was inhibited by the presence of H₂S, the co-inoculation with activated sludge resulted in both CO₂ and H₂S depletion. Moreover, N. muscorum accumulated GL up to ∼54% dcw under N and P-deprivation, almost 36 times higher than that recorded under nutrients sufficient condition. The addition of 10% extra carbon in the form of valeric acid not only did not hamper the growth of N. muscorum (336.0 ± 113.1 mg TSS L^-1) but also increased the GL content to ∼58% dcw. On the contrary, a negligible PHB accumulation was found under the tested conditions, likely due to the high CO₂ concentration of 30% v/v in the headspace and therefore the high availability of inorganic carbon for the cultures. N. muscorum cultures achieved VFAs degradations up to ∼78% under controlled pH. These results supported N. muscorum as a sustainable alternative for CO₂-capture and greenhouse gas mitigation or for photosynthetic biogas upgrading coupled with value added biomass production.

Improved CO2-derived polyhydroxybutyrate (PHB) production by engineering fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 for potential utilization of flue gas

Industrial application of cyanobacterial poly-β-hydroxybutyrate (PHB) production from CO₂ is currently challenged by slow growth rate and low photoautotrophic PHB productivity of existing cyanobacteria species. Herein, a novel PHB-producing cyanobacterial strain was developed by harnessing fast-growing cyanobacteria Synechococcus elongatus UTEX 2973 with introduction of heterologous phaCAB genes. Under photoautotrophic condition, the engineered strain produced 420 mg L^-1 (16.7% of dry cell weight) with the highest specific productivity of 75.2 mg L^-1 d^-1. When compared with a native PHB producer Synechocystis PCC 6803 under nitrogen deprivation, the engineered strain exhibited 2.4-fold higher PHB productivity. The performance of the engineered strain was further demonstrated in large scale cultivation using photobioreactor and outdoor cultivation employing industrial flue gas as the sole carbon source. This study can provide a promising solution to address petroleum-based plastic waste and contribute to CO₂ mitigation.

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.

Polyhydroxybutyrate and glycogen production in photobioreactors inoculated with wastewater borne cyanobacteria monocultures

The aim of this study was to investigate the PHB and glycogen accumulation dynamics in two photobioreactors inoculated with different monocultures of wastewater-borne cyanobacteria, using a three-stage feeding strategy (growth phase, feast-famine phase and feast phase). Two cyanobacterial monocultures containing members of Synechocystis sp. or Synechococcus sp. were collected from treated wastewater and inoculated in lab-scale photobioreactors to evaluate the PHB and glycogen accumulation. A third photobioreactor with a complex microbial community grown in real wastewater was also set up. During each experimental phase different concentrations of inorganic carbon were applied to the cultures, these shifts allowed to discern the accumulation mechanism of carbon storage polymers (PHB and glycogen) in cyanobacteria. Conversion of one into the other was directly related to the carbon content. The highest PHB and glycogen contents (5.04%_dcw and 69%_dcw, respectively) were achieved for Synechocystis sp.

Simple, fast and accurate method for the determination of glycogen in the model unicellular cyanobacterium Synechocystis sp. PCC 6803

Glycogen is a highly soluble branched polymer composed of glucose monomers linked by glycosidic bonds that represents, together with starch, one of the main energy storage compounds in living organisms. While starch is present in plant cells, glycogen is present in bacteria, protozoa, fungi and animal cells. Due to its essential function, it has been the subject of intense research for almost two centuries. Different procedures for the isolation and quantification of glycogen, according to the origin of the sample and/or the purpose of the study, have been reported in the literature. The objective of this study is to optimize the methodology for the determination of glycogen in cyanobacteria, as the interest in cyanobacterial glycogen has increased in recent years due to the biotechnological application of these microorganisms. In the present work, the methodology reported for the quantification of glycogen in cyanobacteria has been reviewed and an extensive empirical analysis has been performed showing how this methodology can be optimized significantly to reduce time and improve reliability and reproducibility. Based on these results, a simple and fast protocol for quantification of glycogen in the model unicellular cyanobacterium Synechocystis sp. PCC 6803 is presented, which could also be successfully adapted to other cyanobacteria.

PHB is Produced from Glycogen Turn-over during Nitrogen Starvation in Synechocystis sp. PCC 6803

Polyhydroxybutyrate (PHB) is a polymer of great interest as a substitute for conventional plastics, which are becoming an enormous environmental problem. PHB can be produced directly from CO₂ in photoautotrophic cyanobacteria. The model cyanobacterium Synechocystis sp. PCC 6803 produces PHB under conditions of nitrogen starvation. However, it is so far unclear which metabolic pathways provide the precursor molecules for PHB synthesis during nitrogen starvation. In this study, we investigated if PHB could be derived from the main intracellular carbon pool, glycogen. A mutant of the major glycogen phosphorylase, GlgP2 (slr1367 product), was almost completely impaired in PHB synthesis. Conversely, in the absence of glycogen synthase GlgA1 (sll0945 product), cells not only produced less PHB, but were also impaired in acclimation to nitrogen depletion. To analyze the role of the various carbon catabolic pathways (EMP, ED and OPP pathways) for PHB production, mutants of key enzymes of these pathways were analyzed, showing different impact on PHB synthesis. Together, this study clearly indicates that PHB in glycogen-producing Synechocystis sp. PCC 6803 cells is produced from this carbon-pool during nitrogen starvation periods. This knowledge can be used for metabolic engineering to get closer to the overall goal of a sustainable, carbon-neutral bioplastic production.

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.