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

Exploring simultaneous production of poly(3-hydroxybutyrate) and exopolysaccharides in cyanobacteria-rich microbiomes

The aim of this study was to explore the viability of the dual production of poly(3-hydroxybutyrate) (PHB) and exopolysaccharides (EPS) by seven microbiomes rich in cyanobacteria. Our initial experiments involved to screen for EPS-producing candidates and examine the impact of salinity and acetate on EPS synthesis. Salinity's known influence on EPS production and acetate's role in enhancing PHB production guided our parameter selection. Surprisingly, neither the introduction of an external carbon source (acetate) nor exposure to an abiotic stressor (salt) significantly altered EPS synthesis rates, which ranged from 25 to 150 mg·L-1, or its composition, with glucose being the dominant sugar component. Scaling up to a 3 L photobioreactor, we achieved simultaneous biopolymer production, reaching up to 205 mg·L-1 EPS and 87 mg·L-1 PHB. Additionally, the presence of uronic acid in the EPS facilitated biomass flocculation, streamlining the separation process, and potentially reducing associated time and costs.

Microplastics enhance the denitrification of glycogen-accumulating organisms by regulating electronic transport in carbon-nitrogen coupling

The increasing presence of microplastics (MPs) in wastewater treatment systems profoundly impacts microbial metabolism and process performance. However, the effects of MPs on the denitrification process of glycogen-accumulating organisms (GAOs) remain unclear. Herein, various types and concentrations of MPs were introduced into the activate sludge of GAOs to assess their impact on denitrification processes and to investigate the underlying mechanisms. Our findings revealed that adding 100 μm PVC increased the denitrification efficiency of GAOs by 14.6 %, whereas adding 100 nm PVC decreased efficiency by 8.4 %. Additionally, 100 nm PVC inhibited polyhydroxybutyrate (PHB) degradation, while 100 μm PVC promoted it. Furthermore, 100 nm and 100 μm PVC differently influenced metabolic functions, including reactive oxygen species (ROS) levels, electron transport chain (ETC) activity, and intracellular nicotinamide adenine dinucleotide (NADH) content. Metatranscriptome analyses revealed differential expression of genes such as phaC, CS, nuoL, CYC1, and nisK, which are involved in carbon-nitrogen metabolism and oxidative phosphorylation. Consequently, 100 μm PVC enhanced the denitrification rate in GAOs by promoting PHB decomposition, increasing NADH electron-donating capacity, and ultimately enhancing the denitrification rate of GAOs. Our findings reveal a novel mechanism on regulating the carbon-nitrogen coupling in activated sludge under the different particle size of MPs.

Structural elements of cyanobacterial co-factor-independent phosphoglycerate mutase that mediate regulation by PirC

The 2,3-bisphosphoglycerate-independent phosphoglycerate mutase (iPGAM) has been identified as a regulating key point in the carbon storage metabolism of cyanobacteria. Upon nitrogen starvation, the iPGAM is inhibited by the PII-interacting regulator PirC, which is released from its interaction partner PII due to elevated 2-oxoglutarate levels. In silico analysis of 338 different iPGAMs revealed a deep-rooted distinctive evolution of iPGAMs in cyanobacteria. Remarkably, cyanobacterial iPGAMs possess a unique loop structure and an extended C-terminus. Our mass photometry analysis suggests that iPGAM forms a complex with three individual PirC monomers. Biolayer interferometry revealed that the PirC-iPGAM complex is affected by the unique loop and the C-terminal structural elements of iPGAM. A C-terminally truncated iPGAM enzyme showed loss of control by PirC and twofold increased enzymatic activity compared to the iPGAM-WT (wild type), as demonstrated by enzyme assays. By contrast, deleting the loop structure significantly reduced the activity of this variant. Physiological experiments were carried out with different iPGAM variant strains of Synechocystis, in which these structural elements were deleted. The strain expressing the C-terminally truncated iPGAM showed a similar overproduction of polyhydroxybutyrate as deletion of the iPGAM regulator PirC. However, in contrast to the latter, these strains showed higher overall biomass accumulation, making them a better chassis for the production of polyhydroxybutyrate or other valuable substances than the PirC-deficient mutant.IMPORTANCEThe primordial cyanobacteria were responsible for developing oxygenic photosynthesis early in evolution. In the pathways of fixed carbon allocation, the co-factor-independent phosphoglycerate mutase (iPGAM) plays a crucial role by directing the first CO2 fixation product, 3-phosphoglycerate, toward central anabolic glycolytic-derived pathways. This work reveals a distinct evolution of iPGAM within oxygenic photosynthetic organisms. We have identified two specific segments in cyanobacterial iPGAMs that affect the control of iPGAM activity through its specific interactor protein PirC. This understanding of iPGAM has allowed us to engineer cyanobacterial strains with altered carbon fluxes. Since cyanobacteria can directly convert CO2 into valuable products, our results demonstrate a novel approach for developing a chassis for biotechnical use.

Greening the Production of Indigo Blue Exploiting Light and a Recombinant Synechocystis sp. PCC6803 Strain Expressing the Enzyme mFMO

Cyanobacteria are emerging as interesting cell factories, offering the significant advantage of their in-built photosynthetic machinery, which generates NADPH to support redox biocatalysis. In this study, we assessed the potential of the cyanobacterium Synechocystis sp. PCC6803 in producing the dye indigo by light-driven whole-cell biotransformation using indole as a starting compound. A stable transgenic strain expressing a flavin-containing monooxygenase from Methylophaga aminisulfidivorans (mFMO) was engineered, enabling light-dependent indigo production. Upon optimising conditions, effective biotransformations could be performed, resulting in 112 mg/L indigo (86% conversion of the furnished indole). Additionally, we present a method for the recovery of the secreted dye directly from the growth medium through solid-phase absorption on polyamide nets. Overall, the effectiveness and sustainability of the biotransformation in Synechocystis sp. PCC6803 performed at the laboratory scale provide a strong basis for further exploring the applicability of the process.

Cyanobacteria microbiomes for bioplastic production: Critical review of key factors and challenges in scaling from laboratory to industry set-ups

Cyanobacteria are photoautotrophic microorganisms capable of accumulating polyhydroxybutyrate (PHB). A novel approach for PHB production involves the exploration of cyanobacterial microbiomes, potentially reducing costs through non-sterile cultivation with non-pure substrates. Although still in its early stages, this approach shows promise for high yields and sustained synthesis. However, managing microbiome population dynamics in non-sterile environments requires effective monitoring and control. This review covers PHB production by cyanobacteria microbiomes, from sample procurement to laboratory-scale production. It highlights recent insights into optimizing cultivation parameters for enhanced biopolymer yield. Strategies to overcome challenges in PHB production are evaluated, emphasizing integrated molecular biology techniques with quantitative and qualitative PHB analysis. Finally, key challenges in scaling up production to industrial-scale scenarios are discussed, along with potential solutions to support the development of sustainable industrial processes. Cyanobacteria microbiomes show promise PHB production but challenges like managing non-sterile conditions and scaling up require optimized strategies and integrated approaches.

Microcystin production is important for toxic Microcystis to survive long-term nitrogen starvation

Toxic cyanobacterial blooms have expanded and intensified on a global scale. Although microcystins are known as the most abundant cyanotoxins released during cyanobacterial blooms, the physiological role of these toxic secondary metabolites has not been fully resolved. Here, we show that microcystin production is important for toxic Microcystis to maintain carbon metabolism under long-term nitrogen starvation and subsequent recovery. Compared to carbon metabolism in the nonmicrocystin-producing strains, toxic Microcystis could accumulate more carbon reserves under nitrogen limitation, which is important for the survival of cells under stressful conditions. Transcriptomic analysis revealed that the genes involved in microcystin synthesis were significantly up-regulated at the initial recovery phase, indicating their essential role in strengthening glycogen catabolism and fueling recovery. Flow cytometry analysis showed that compared to nontoxic strains, microcystin-producing Microcystis exhibited a higher survival and recovery rate after prolonged nitrogen starvation, which is consistent with the dominance of these species at the early stage of cyanobacterial blooms. The close genetic traits between Microcystis strains suggest that the strategies observed here might be highly conserved. Our results imply that toxic Microcystis establishes a competitive advantage over nontoxic species and provides insights into the seasonal succession of natural Microcystis populations.

Light intensity-regulated glycogen synthesis and pollutant removal in microalgal-bacterial granular sludge for wastewater treatment

As light intensity plays a pivotal role in the microalgal-bacterial granular sludge (MBGS) process, understanding its impact on system performance and energy dynamics is essential. This study investigated the effects of varying light intensities (20, 100, 200, and 300 μ mol/m²/s) on the performance of MBGS in urban wastewater treatment, with a particular focus on glycogen accumulation and pollutant removal. The results demonstrated that light intensity significantly influenced microbial community structure, glycogen accumulation, and pollutant removal efficiency. At the lowest light intensity (20 μ mol/m²/s), filamentous Cyanobacteria dominated, leading to poor settling performance but enhancing phosphate removal due to increased polyphosphate metabolism. As light intensity increased, the microbial community shifted from Cyanobacteria to eukaryotic algae (e.g., Chlorella) at higher intensities. The optimal light condition (200 μ mol/m²/s) favored glycogen synthesis and improved synergistic interactions between microalgae and bacteria, ensuring stable pollutant removal. Notably, increased light intensity upregulated the expression of genes encoding glycogen branching enzyme (EC: 2.4.1.18), promoting glycogen accumulation in the MBGS system. These findings provide valuable insights for optimizing MBGS systems to achieve efficient urban wastewater treatment and resource recovery.

Poly-β-hydroxybutyrate production by Synechocystis MT_a24 in a raceway pond using urban wastewater

Poly-β-hydroxybutyrate (PHB) is a potential source of biodegradable plastics that are environmentally friendly due to their complete degradation to water and carbon dioxide. This study aimed to investigate PHB production in the cyanobacterium Synechocystis sp. PCC6714 MT_a24 in an outdoor bioreactor using urban wastewater as a sole nutrient source. The culture was grown in a thin-layer raceway pond with a working volume of 100 L, reaching a biomass density of up to 3.5 g L-1 of cell dry weight (CDW). The maximum PHB content was found under nutrient-limiting conditions in the late stationary phase, reaching 23.7 ± 2.2% PHB per CDW. These data are one of the highest reported for photosynthetic production of PHB by cyanobacteria, moreover using urban wastewater in pilot-scale cultivation which multiplies the potential of sustainable cultivation approaches. Contamination by grazers (Poterioochromonas malhamensis) was managed by culturing Synechocystis in a highly alkaline environment (pH about 10.5) which did not significantly affect the culture growth. Furthermore, the strain MT_a24 showed significant wastewater nutrient remediation removing about 72% of nitrogen and 67% of phosphorus. These trials demonstrate that the photosynthetic production of PHB by Synechocystis sp. PCC6714 MT_a24 in the outdoor thin-layer bioreactor using urban wastewater and ambient carbon dioxide. It shows a promising approach for the cost-effective and sustainable production of biodegradable carbon-negative plastics. KEY POINTS: • High PHB production by cyanobacteria in outdoor raceway pond • Urban wastewater used as a sole source of nutrients for phototrophic growth • Potential for cost-effective and sustainable production of biodegradable plastics.

Bioplastic production by harnessing cyanobacteria-rich microbiomes for long-term synthesis

Departing from the conventional axenic and heterotrophic cultures, our research ventures into unexplored territory by investigating the potential of photosynthetic microbiomes for polyhydroxybutyrate (PHB) synthesis, a biodegradable polyester that presents a sustainable alternative to conventional plastics. Our investigation focused on a cyanobacteria-enriched microbiome, dominated by Synechocystis sp. and Synechococcus sp., cultivated in a 3 L photobioreactor under non-sterile conditions, achieving significant PHB production-up to 28 % dry cell weight (dcw) over a span of 108 days through alternating cycles of biomass growth and PHB accumulation. Nile Blue staining and Transmission Electron Microscope visualization allowed to successfully confirm the presence of PHB granules within cyanobacteria cells. Furthermore, the overexpression of PHA synthase during the accumulation phase directly correlated with the increased PHB production. Also, gene expression changes revealed glycogen as the primary storage compound, but under prolonged macronutrient stress, there was a shift of the carbon flux towards favoring PHB synthesis. Finally, analysis through Raman, Fourier- transform infrared spectroscopy and proton Nuclear Magnetic Resonance further validated the extracted polymer as PHB. Overall, it was demonstrated for the first time the feasibility of using phototrophic microbiomes to continuous production of PHB in a non-sterile system. This study also offers valuable insights into the metabolic pathways involved.

Overexpression of RuBisCO form I and II genes in Rhodopseudomonas palustris TIE-1 augments polyhydroxyalkanoate production heterotrophically and autotrophically

With the rising demand for sustainable renewable resources, microorganisms capable of producing bioproducts such as bioplastics are attractive. While many bioproduction systems are well-studied in model organisms, investigating non-model organisms is essential to expand the field and utilize metabolically versatile strains. This investigation centers on Rhodopseudomonas palustris TIE-1, a purple non-sulfur bacterium capable of producing bioplastics. To increase bioplastic production, genes encoding the putative regulatory protein PhaR and the depolymerase PhaZ of the polyhydroxyalkanoate (PHA) biosynthesis pathway were deleted. Genes associated with pathways that might compete with PHA production, specifically those linked to glycogen production and nitrogen fixation, were deleted. Additionally, RuBisCO form I and II genes were integrated into TIE-1's genome by a phage integration system, developed in this study. Our results show that deletion of phaR increases PHA production when TIE-1 is grown photoheterotrophically with butyrate and ammonium chloride (NH4Cl). Mutants unable to produce glycogen or fix nitrogen show increased PHA production under photoautotrophic growth with hydrogen and NH4Cl. The most significant increase in PHA production was observed when RuBisCO form I and form I & II genes were overexpressed, five times under photoheterotrophy with butyrate, two times with hydrogen and NH4Cl, and two times under photoelectrotrophic growth with N2 . In summary, inserting copies of RuBisCO genes into the TIE-1 genome is a more effective strategy than deleting competing pathways to increase PHA production in TIE-1. The successful use of the phage integration system opens numerous opportunities for synthetic biology in TIE-1.IMPORTANCEOur planet has been burdened by pollution resulting from the extensive use of petroleum-derived plastics for the last few decades. Since the discovery of biodegradable plastic alternatives, concerted efforts have been made to enhance their bioproduction. The versatile microorganism Rhodopseudomonas palustris TIE-1 (TIE-1) stands out as a promising candidate for bioplastic synthesis, owing to its ability to use multiple electron sources, fix the greenhouse gas CO2, and use light as an energy source. Two categories of strains were meticulously designed from the TIE-1 wild-type to augment the production of polyhydroxyalkanoate (PHA), one such bioplastic produced. The first group includes mutants carrying a deletion of the phaR or phaZ genes in the PHA pathway, and those lacking potential competitive carbon and energy sinks to the PHA pathway (namely, glycogen biosynthesis and nitrogen fixation). The second group comprises TIE-1 strains that overexpress RuBisCO form I or form I & II genes inserted via a phage integration system. By studying numerous metabolic mutants and overexpression strains, we conclude that genetic modifications in the environmental microbe TIE-1 can improve PHA production. When combined with other approaches (such as reactor design, use of microbial consortia, and different feedstocks), genetic and metabolic manipulations of purple nonsulfur bacteria like TIE-1 are essential for replacing petroleum-derived plastics with biodegradable plastics like PHA.

A Comparison of the Effects of Continuous Illumination and Day/Night Regimes on PHB Accumulation in Synechocystis Cells

Poly(3-hydroxybutyrate) (PHB) is a biobased and biodegradable polymer with properties comparable to polypropylene and therefore has the potential to replace conventional plastics. PHB is intracellularly accumulated by prokaryotic organisms. For the cells PHB functions manly as carbon and energy source, but all possible functions of PHB are still not known. Synechocystis (cyanobacteria) accumulates PHB using light as energy and CO2 as carbon source. The main trigger for PHB accumulation in cyanobacteria is nitrogen and phosphorous depletion with simultaneous surplus of carbon and energy. For the above reasons, obtaining knowledge about external factors influencing PHB accumulation is of highest interest. This study compares the effect of continuous light exposure and day/night (16/8 h) cycles on selected physiology parameters of three Synechocystis strains. We show that continuous illumination at moderate light intensities leads to an increased PHB accumulation in Synechocystis salina CCALA 192 (max. 14.2% CDW - cell dry weight) compared to day/night cycles (3.7% CDW). In addition to PHB content, glycogen and cell size increased, while cell density and cell viability decreased. The results offer new approaches for further studies to gain deeper insights into the role of PHB in cyanobacteria to obtain bioplastics in a more sustainable and environmentally friendly way.

Disrupted H2 synthesis combined with methyl viologen treatment inhibits photosynthetic electron flow to synergistically enhance glycogen accumulation in the cyanobacterium Synechocystis sp. PCC 6803

Under nitrogen deprivation (-N), cyanobacterium Synechocystis sp. PCC 6803 exhibits growth arrest, reduced protein content, and remarkably increased glycogen accumulation. However, producing glycogen under this condition requires a two-step process with cell transfer from normal to -N medium. Metabolic engineering and chemical treatment for rapid glycogen accumulation can bypass the need for two-step cultivation. For example, recent studies indicate that individually disrupting hydrogen (H2) or poly(3-hydroxybutyrate) (PHB) synthesis, or treatment with methyl viologen (MV), effectively increases glycogen accumulation in Synechocystis. Here we explore the effects of disrupted H2 or poly(3-hydroxybutyrate) synthesis, together with MV treatment to on enhanced glycogen accumulation in Synechocystis grown in normal medium. Wild-type cells without MV treatment exhibited low glycogen content of less than 6% w/w dry weight (DW). Compared with wild type, disrupting PHB synthesis combined with MV treatment did not increase glycogen content. Disrupted H₂ production without MV treatment yielded up to 11% w/w DW glycogen content. Interestingly, when combined, disrupted H2 production with MV treatment synergistically enhanced glycogen accumulation to 51% and 59% w/w DW within 3 and 7 days, respectively. Metabolomic analysis suggests that MV treatment mediated the conversion of proteins into glycogen. Metabolomic and transcriptional-expression analysis suggests that disrupted H2 synthesis under MV treatment positively influenced glycogen synthesis. Disrupted H₂ synthesis under MV treatment significantly increased NADPH levels. This increased NADPH content potentially contributed to the observed enhancements in antioxidant activity against MV-induced oxidants, O2 evolution, and metabolite substrates levels for glycogen synthesis in normal medium, ultimately leading to enhanced glycogen accumulation in Synechocystis. KEY MESSAGE: Combining disrupted hydrogen-gas synthesis and the treatment by photosynthesis electron-transport inhibitor significantly enhance glycogen production in cyanobacteria.

Deciphering the genetic landscape of enhanced poly-3-hydroxybutyrate production in Synechocystis sp. B12

BACKGROUND

Microbial biopolymers such as poly-3-hydroxybutyrate (PHB) are emerging as promising alternatives for sustainable production of biodegradable bioplastics. Their promise is heightened by the potential utilisation of photosynthetic organisms, thus exploiting sunlight and carbon dioxide as source of energy and carbon, respectively. The cyanobacterium Synechocystis sp. B12 is an attractive candidate for its superior ability to accumulate high amounts of PHB as well as for its high-light tolerance, which makes it extremely suitable for large-scale cultivation. Beyond its practical applications, B12 serves as an intriguing model for unravelling the molecular mechanisms behind PHB accumulation.

RESULTS

Through a multifaceted approach, integrating physiological, genomic and transcriptomic analyses, this work identified genes involved in the upregulation of chlorophyll biosynthesis and phycobilisome degradation as the possible candidates providing Synechocystis sp. B12 an advantage in growth under high-light conditions. Gene expression differences in pentose phosphate pathway and acetyl-CoA metabolism were instead recognised as mainly responsible for the increased Synechocystis sp. B12 PHB production during nitrogen starvation. In both response to strong illumination and PHB accumulation, Synechocystis sp. B12 showed a metabolic modulation similar but more pronounced than the reference strain, yielding in better performances.

CONCLUSIONS

Our findings shed light on the molecular mechanisms of PHB biosynthesis, providing valuable insights for optimising the use of Synechocystis in economically viable and sustainable PHB production. In addition, this work supplies crucial knowledge about the metabolic processes involved in production and accumulation of these molecules, which can be seminal for the application to other microorganisms as well.

The primary carbon metabolism in cyanobacteria and its regulation

Cyanobacteria are the only prokaryotes capable of performing oxygenic photosynthesis. Many cyanobacterial strains can live in different trophic modes, ranging from photoautotrophic and heterotrophic to mixotrophic growth. However, the regulatory mechanisms allowing a flexible switch between these lifestyles are poorly understood. As anabolic fixation of CO2 in the Calvin-Benson-Bassham (CBB) cycle and catabolic sugar-degradation pathways share intermediates and enzymatic capacity, a tight regulatory network is required to enable simultaneous opposed metabolic fluxes. The Entner-Doudoroff (ED) pathway was recently predicted as one glycolytic route, which cooperates with other pathways in glycogen breakdown. Despite low carbon flux through the ED pathway, metabolite analyses of mutants deficient in the ED pathway revealed a distinct phenotype pointing at a strong regulatory impact of this route. The small Cp12 protein downregulates the CBB cycle in darkness by inhibiting phosphoribulokinase and glyceraldehyde 3-phosphate dehydrogenase. New results of metabolomic and redox level analyses on strains with Cp12 variants extend the known role of Cp12 regulation towards the acclimation to external glucose supply under diurnal conditions as well as to fluctuations in CO2 levels in the light. Moreover, carbon and nitrogen metabolism are closely linked to maintain an essential C/N homeostasis. The small protein PirC was shown to be an important regulator of phosphoglycerate mutase, which identified this enzyme as central branching point for carbon allocation from CBB cycle towards lower glycolysis. Altered metabolite levels in the mutant ΔpirC during nitrogen starvation experiments confirm this regulatory mechanism. The elucidation of novel mechanisms regulating carbon allocation at crucial metabolic branching points could identify ways for targeted redirection of carbon flow towards desired compounds, and thus help to further establish cyanobacteria as green cell factories for biotechnological applications with concurrent utilization of sunlight and CO2.

Interplay between photosynthetic electron flux and organic carbon sinks in sucrose-excreting Synechocystis sp. PCC 6803 revealed by omics approaches

BACKGROUND

Advancing the engineering of photosynthesis-based prokaryotic cell factories is important for sustainable chemical production and requires a deep understanding of the interplay between bioenergetic and metabolic pathways. Rearrangements in photosynthetic electron flow to increase the efficient use of the light energy for carbon fixation must be balanced with a strong carbon sink to avoid photoinhibition. In the cyanobacterium Synechocystis sp. PCC 6803, the flavodiiron protein Flv3 functions as an alternative electron acceptor of photosystem I and represents an interesting engineering target for reorganizing electron flow in attempts to enhance photosynthetic CO2 fixation and increase production yield.

RESULTS

We have shown that inactivation of Flv3 in engineered sucrose-excreting Synechocystis (S02:Δflv3) induces a transition from photoautotrophic sucrose production to mixotrophic growth sustained by sucrose re-uptake and the formation of intracellular carbon sinks such as glycogen and polyhydroxybutyrate. The growth of S02:Δflv3 exceeds that of the sucrose-producing strain (S02) and demonstrates unforeseen proteomic and metabolomic changes over the course of the nine-day cultivation. In the absence of Flv3, a down-regulation of proteins related to photosynthetic light reactions and CO2 assimilation occurred concomitantly with up-regulation of those related to glycolytic pathways, before any differences in sucrose production between S02 and S02:Δflv3 strains were observed. Over time, increased sucrose degradation in S02:Δflv3 led to the upregulation of respiratory pathway components, such as the plastoquinone reductase complexes NDH-11 and NDH-2 and the terminal respiratory oxidases Cyd and Cox, which transfer electrons to O2. While glycolytic metabolism is significantly up-regulated in S02:Δflv3 to provide energy for the cell, the accumulation of intracellular storage compounds and the increase in respiration serve as indirect sinks for photosynthetic electrons.

CONCLUSIONS

Our results show that the presence of strong carbon sink in the engineered sucrose-producing Synechocystis S02 strain, operating under high light, high CO2 and salt stress, cannot compensate for the lack of Flv3 by directly balancing the light transducing source and carbon fixing sink reactions. Instead, the cells immediately sense the imbalance, leading to extensive reprogramming of cellular bioenergetic, metabolic and ion transport pathways that favor mixotrophic growth rather than enhancing photoautotrophic sucrose production.

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.

The adc1 knockout with proC overexpression in Synechocystis sp. PCC 6803 induces a diversion of acetyl-CoA to produce more polyhydroxybutyrate

BACKGROUND

Lack of nutrients, in particular nitrogen and phosphorus, has been known in the field to sense glutamate production via 2-oxoglutarate and subsequently accelerate carbon storage, including glycogen and polyhydroxybutyrate (PHB), in cyanobacteria, but a few studies have focused on arginine catabolism. In this study, we first time demonstrated that gene manipulation on proC and adc1, related to proline and polyamine syntheses in arginine catabolism, had a significant impact on enhanced PHB production during late growth phase and nutrient-modified conditions. We constructed Synechocystis sp. PCC 6803 with an overexpressing proC gene, encoding Δ1pyrroline-5-carboxylate reductase in proline production, and adc1 disruption resulted in lower polyamine synthesis.

RESULTS

Three engineered Synechocystis sp. PCC 6803 strains, including a ProC-overexpressing strain (OXP), adc1 mutant, and an OXP strain lacking the adc1 gene (OXP/Δadc1), certainly increased the PHB accumulation under nitrogen and phosphorus deficiency. The possible advantages of single proC overexpression include improved PHB and glycogen storage in late phase of growth and long-term stress situations. However, on day 7 of treatment, the synergistic impact created by OXP/Δadc1 increased PHB synthesis by approximately 48.9% of dry cell weight, resulting in a shorter response to nutrient stress than the OXP strain. Notably, changes in proline and glutamate contents in engineered strains, in particular OXP and OXP/Δadc1, not only partially balanced the intracellular C/N metabolism but also helped cells acclimate under nitrogen (N) and phosphorus (P) stress with higher chlorophyll a content in comparison with wild-type control.

CONCLUSIONS

In Synechocystis sp. PCC 6803, overexpression of proC resulted in a striking signal to PHB and glycogen accumulation after prolonged nutrient deprivation. When combined with the adc1 disruption, there was a notable increase in PHB production, particularly in situations where there was a strong C supply and a lack of N and P.

Metabolic flux phenotyping of secondary metabolism in cyanobacteria

Cyanobacteria generate energy from photosynthesis and produce various secondary metabolites with diverse commercial and pharmaceutical applications. Unique metabolic and regulatory pathways in cyanobacteria present new challenges for researchers to enhance their product yields, titers, and rates. Therefore, further advancements are critically needed to establish cyanobacteria as a preferred bioproduction platform. Metabolic flux analysis (MFA) quantitatively determines the intracellular flows of carbon within complex biochemical networks, which elucidate the control of metabolic pathways by transcriptional, translational, and allosteric regulatory mechanisms. The emerging field of systems metabolic engineering (SME) involves the use of MFA and other omics technologies to guide the rational development of microbial production strains. This review highlights the potential of MFA and SME to optimize the production of cyanobacterial secondary metabolites and discusses the technical challenges that lie ahead.

Determination of Poly(3-hydroxybutyrate) Content in Cyanobacterium Synechocystis sp. PCC 6803 Using Acid Hydrolysis Followed by High-performance Liquid Chromatography

Various photoautotrophic cyanobacteria accumulate intracellular poly(3-hydroxybutyrate) (PHB) granules. This protocol can be used for determining the PHB contents of the cells as % PHB weight per dry cell weight using acid hydrolysis followed by high-performance liquid chromatography (HPLC). This HPLC analysis is rapid, with a running time of approximately 5 min per sample. The technique can accurately determine PHB concentrations in the range of 2-1,000 μg/mL PHB. However, this technique is not applicable for determining the contents of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) in cyanobacteria.

Parallel Proteomic Comparison of Mutants With Altered Carbon Metabolism Reveals Hik8 Regulation of PII Phosphorylation and Glycogen Accumulation in a Cyanobacterium

Carbon metabolism is central to photosynthetic organisms and involves the coordinated operation and regulation of numerous proteins. In cyanobacteria, proteins involved in carbon metabolism are regulated by multiple regulators including the RNA polymerase sigma factor SigE, the histidine kinases Hik8, Hik31 and its plasmid-borne paralog Slr6041, and the response regulator Rre37. To understand the specificity and the cross-talk of such regulations, we simultaneously and quantitatively compared the proteomes of the gene knockout mutants for the regulators. A number of proteins showing differential expression in one or more mutants were identified, including four proteins that are unanimously upregulated or downregulated in all five mutants. These represent the important nodes of the intricate and elegant regulatory network for carbon metabolism. Moreover, serine phosphorylation of PII, a key signaling protein sensing and regulating in vivo carbon/nitrogen (C/N) homeostasis through reversible phosphorylation, is massively increased with a concomitant significant decrease in glycogen content only in the hik8-knockout mutant, which also displays impaired dark viability. An unphosphorylatable PII S49A substitution restored the glycogen content and rescued the dark viability of the mutant. Together, our study not only establishes the quantitative relationship between the targets and the corresponding regulators and elucidated their specificity and cross-talk but also unveils that Hik8 regulates glycogen accumulation through negative regulation of PII phosphorylation, providing the first line of evidence that links the two-component system with PII-mediated signal transduction and implicates them in the regulation of carbon metabolism.

Improving bioplastic production by Rhodopseudomonas palustris TIE-1 using synthetic biology and metabolic engineering

With the increasing demand for sustainably produced renewable resources, it is important to look towards microorganisms capable of producing bioproducts such as biofuels and bioplastics. Though many systems for bioproduct production are well documented and tested in model organisms, it is essential to look beyond to non-model organisms to expand the field and take advantage of metabolically versatile strains. This investigation centers on Rhodopseudomonas palustris TIE-1, a purple, non-sulfur autotrophic, and anaerobic bacterium capable of producing bioproducts that are comparable to their petroleum-based counterparts. To induce bioplastic overproduction, genes that might have a potential role in the PHB biosynthesis such as the regulator, phaR, and phaZ known for its ability to degrade PHB granules were deleted using markerless deletion. Mutants in pathways that might compete with polyhydroxybutyrate (PHB) production such as glycogen and nitrogen fixation previously created to increase n -butanol production by TIE-1 were also tested. In addition, a phage integration system was developed to insert RuBisCO (RuBisCO form I and II genes) driven by a constitutive promoter P _aphII into TIE- 1 genome. Our results show that deletion of the phaR gene of the PHB pathway increases PHB productivity when TIE-1 was grown photoheterotrophically with butyrate and ammonium chloride (NH ₄ Cl). Mutants unable to make glycogen or fix dinitrogen gas show an increase in PHB productivity under photoautotrophic growth conditions with hydrogen. In addition, the engineered TIE-1 overexpressing RuBisCO form I and form II produces significantly more polyhydroxybutyrate than the wild type under photoheterotrophy with butyrate and photoautotrophy with hydrogen. Inserting RuBisCO genes into TIE-1 genome is a more effective strategy than deleting competitive pathways to increase PHB production in TIE-1. The phage integration system developed for TIE-1 thus creates numerous opportunities for synthetic biology in TIE-1.

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.

Exploration of the metabolic flexibility of glycogen accumulating organisms through metatranscriptome analysis and metabolic characterization

Glycogen accumulating organisms (GAOs) are closely related to the deterioration of enhanced biological phosphorus removal systems. However, the metabolic mechanisms that drive GAOs remain unclear. Here, the two-thirds supernatant of a reactor were decanted following the anaerobic period to enrich GAOs. Long-term monitoring demonstrated that the system was stable and exhibited typical characteristics of GAOs metabolism. Acetate was completely consumed after 60 min of the anaerobic phase. The level of glycogen decreased from 0.20 to 0.14 g/gSS during the anaerobic phase, whereas the level of glycogen significantly increased to 0.21g/gSS at the end of the aerobic period. Moreover, there was almost no phosphate release and absorption in the complete periods, thus confirming the successful construction of a GAOs enrichment system. Microbial community analysis demonstrated that Ca. Contendobacter was among the core functional genera and showed the highest activity among all of the communities. Furthermore, our study is the first to identify the involvement of the ethyl-malonyl-CoA pathway in the synthesis of polyhydroxyvalerate via croR, ccr, ecm, mcd, mch and mcl genes. The Embden-Meyerhof-Parnas (EMP) pathway was preferentially used via glgP. Furthermore, the glyoxylate cycle was the main source of ATP under anaerobic conditions, whereas the tricarboxylic acid cycle provided ATP under aerobic conditions. aceA and mdh appeared to be major modulators of the glyoxylate pathway for controlling energy flow. Collectively, our findings not only revealed the crucial metabolic mechanisms in a GAOs enrichment system but also provided insights into the potential application of Ca. Contendobacter for wastewater treatment.

An Alternative Exploitation of Synechocystis sp. PCC6803: A Cascade Approach for the Recovery of High Added-Value Products

Microalgal biomass represents a very interesting biological feedstock to be converted into several high-value products in a biorefinery approach. In this study, the cyanobacterium Synechocystis sp. PCC6803 was used to obtain different classes of molecules: proteins, carotenoids and lipids by using a cascade approach. In particular, the protein extract showed a selective cytotoxicity towards cancer cells, whereas carotenoids were found to be active as antioxidants both in vitro and on a cell-based model. Finally, for the first time, lipids were recovered from Synechocystis biomass as the last class of molecules and were successfully used as an alternative substrate for the production of polyhydroxyalkanoate (PHA) by the native PHA producer Pseudomonas resinovorans. Taken together, our results lead to a significant increase in the valorization of Synechocystis sp. PCC6803 biomass, thus allowing a possible offsetting of the process costs.

Increased Biomass and Polyhydroxybutyrate Production by Synechocystis sp. PCC 6803 Overexpressing RuBisCO Genes

The overexpression of the RuBisCO (rbc) gene has recently become an achievable strategy for increasing cyanobacterial biomass and overcoming the biocompound production restriction. We successfully constructed two rbc-overexpressing Synechocystis sp. PCC 6803 strains (OX), including a strain overexpressing a large subunit of RuBisCO (OXrbcL) and another strain overexpressing all large, chaperone, and small subunits of RuBisCO (OXrbcLXS), resulting in higher and faster growth than wild type under sodium bicarbonate supplementation. This increased biomass of OX strains significantly contributed to the higher polyhydroxybutyrate (PHB) production induced by nutrient-deprived conditions, in particular nitrogen (N) and phosphorus (P). As a result of higher PHB contents in OX strains occurring at days 7 and 9 of nutrient deprivation, this enhancement was apparently made possible by cells preferentially maintaining their internal lipids while accumulating less glycogen. The OXrbcLXS strain, with the highest level of PHB at about 39 %w/dry cell weight (DCW) during 7 days of BG11-NP treatment, contained a lower glycogen level (31.9 %w/DCW) than wild type control (40 %w/DCW). In contrast, the wild type control strain exposed to N- and NP-stresses tended to retain lipid levels and store more glycogen than PHB. In this model, we, for the first time, implemented a RuBisCO-overexpressing cyanobacterial factory for overproducing PHB, destined for biofuel and biomaterial biotechnology.

Phosphoglucomutase comes into the spotlight

This article comments on: Ortega-Martínez P, Roldán M, Díaz-Troya S, Florencio FJ. 2023. Stress response requires an efficient glycogen and central carbon metabolism connection by phosphoglucomutases in cyanobacteria. Journal of Experimental Botany 74, 1532–1550

Enhancement of cyanobacterial PHB production using random chemical mutagenesis with detection through FACS

Poly-hydroxy-butyrate (PHB) bioplastic resin can be made directly from atmospheric CO₂ using cyanobacteria. However, higher PHB productivities are required before large-scale production is economically viable. Random mutagenesis offers a way to create new production strains with increased PHB yields and increased biomass densities without complex technical manipulation associated with genetically modified organisms. This study used staining with lipid fluorescent dye (BODIPY 493/593) and fluorescence-activated cell sorting (FACS) to select high lipid content mutants and followed this with a well plate growth screen. Thirteen mutants were selected for flask cultivation and two strains produced significantly higher PHB yields (29% and 26% higher than wild type), biomass accumulation (36% and 33% higher than wild type) and volumetric PHB density (75% and 67% higher than wild type). The maximum PHB yielding strain (% dcw) was 12.0%, which was 43% higher than the wild type (8.3% in this study). The highest volumetric PHB density was 18.8 mg PHB/L compared to 10.7 mg PHB/L by the wild type. To develop cyanobacterial strain with higher PHB productivities, the combination of random chemical mutagenesis and FACS holds great potential to promote cyanobacteria bioplastic production becoming economically viable.

Chemical Triggering Cyanobacterial Glycogen Accumulation: Methyl Viologen Treatment Increases Synechocystis sp. PCC 6803 Glycogen Storage by Enhancing Levels of Gene Transcript and Substrates in Glycogen Synthesis

Two-stage cultivation is effective for glycogen production by cyanobacteria. Cells were first grown under adequate nitrate supply (BG11) to increase biomass and subsequently transferred to nitrogen deprivation (-N) to stimulate glycogen accumulation. However, the two-stage method is time-consuming and requires extensive energy. Thus, one-stage cultivation that enables both cell growth and glycogen accumulation is advantageous. Such one-stage method could be achieved using a chemical triggering glycogen storage. However, there is a limited study on such chemicals. Here, nine compounds previously reported to affect cyanobacterial cellular functions were examined in Synechocystis sp. PCC 6803. 2-Phenylethanol, phenoxyethanol, 3-(3,4-dichlorophenyl)-1,1-dimethylurea and methyl viologen can stimulate glycogen accumulation. The oxidative stress agent, methyl viologen significantly increased glycogen levels up to 57% and 69% [w/w dry weight (DW)] under BG11 and -N cultivation, respectively. One-stage cultivation where methyl viologen was directly added to the pre-grown culture enhanced glycogen storage to 53% (w/w DW), compared to the 10% (w/w DW) glycogen level of the control cells without methyl viologen. Methyl viologen treatment reduced the contents of total proteins (including phycobiliproteins) but caused increased transcript levels of glycogen synthetic genes and elevated levels of metabolite substrates for glycogen synthesis. Metabolomic results suggested that upon methyl viologen treatment, proteins degraded to amino acids, some of which could be used as a carbon source for glycogen synthesis. Results of oxygen evolution and metabolomic analysis suggested that photosynthesis and carbon fixation were not completely inhibited upon methyl viologen treatment, and these two processes may partially generate upstream metabolites required for glycogen synthesis.

Recent trends of biotechnological production of polyhydroxyalkanoates from C1 carbon sources

Growing concerns over the use of limited fossil fuels and their negative impacts on the ecological niches have facilitated the exploration of alternative routes. The use of conventional plastic material also negatively impacts the environment. One such green alternative is polyhydroxyalkanoates, which are biodegradable, biocompatible, and environmentally friendly. Recently, researchers have focused on the utilization of waste gases particularly those belonging to C1 sources derived directly from industries and anthropogenic activities, such as carbon dioxide, methane, and methanol as the substrate for polyhydroxyalkanoates production. Consequently, several microorganisms have been exploited to utilize waste gases for their growth and biopolymer accumulation. Methylotrophs such as Methylobacterium organophilum produced highest amount of PHA up to 88% using CH₄ as the sole carbon source and 52-56% with CH₃OH. On the other hand Cupriavidus necator, produced 71-81% of PHA by utilizing CO and CO₂ as a substrate. The present review shows the potential of waste gas valorization as a promising solution for the sustainable production of polyhydroxyalkanoates. Key bottlenecks towards the usage of gaseous substrates obstructing their realization on a large scale and the possible technological solutions were also highlighted. Several strategies for PHA production using C1 gases through fermentation and metabolic engineering approaches are discussed. Microbes such as autotrophs, acetogens, and methanotrophs can produce PHA from CO₂, CO, and CH₄. Therefore, this article presents a vision of C1 gas into bioplastics are prospective strategies with promising potential application, and aspects related to the sustainability of the system.

Introduction to Cyanobacteria

Cyanobacteria are highly interesting microbes with the capacity for oxygenic photosynthesis. They fulfill an important purpose in nature but are also potent biocatalysts. This chapter gives a brief overview of this diverse phylum and shortly addresses the functions these organisms have in the natural ecosystems. Further, it introduces the main topics covered in this volume, which is dealing with the development and application of cyanobacteria as solar cell factories for the production of chemicals including potential fuels. We discuss cyanobacteria as industrial workhorses, present established chassis strains, and give an overview of the current target products. Genetic engineering strategies aiming at the photosynthetic efficiency as well as approaches to optimize carbon fluxes are summarized. Finally, main cultivation strategies are sketched.

Engineering type I methanotrophic bacteria as novel platform for sustainable production of 3-hydroxybutyrate and biodegradable polyhydroxybutyrate from methane and xylose

Methylotuvimicrobium alcaliphilum20Z recombinant strain co-utilizing methane and xylose from anthropogenic activities and lignocellulose biomassis a promising cell factory platform. In this study, the production of (R)-3-hydroxybutyrate and poly (3-hydroxybutyrate) inM. alcaliphilum20Z was demonstrated. The production of (R)-3-hydroxybutyrate was optimized by introducing additional thioesterase, and a tunable genetic module. The final recombinant strain produced the highest titer of 334.52 ± 2 mg/L (R)-3-hydroxybutyrate (yield of 1,853 ± 429 mg/g dry cell weight). The poly (3-hydroxybutyrate) yielded 1.29 ± 0.08% (w/w) from methane and xylose in one-stage cultivation. Moreover, the study demonstrated the importance of pathway reversibility as an effective design strategy for balancing the driving force and intermediate accumulation. This is the first demonstration of the production ofbiodegradablepoly (3-hydroxybutyrate) from methane in type I methanotrophs, which is a key step toward sustainable biomanufacturing and carbon-neutral society.

Enhanced productivity of extracellular free fatty acids by gene disruptions of acyl-ACP synthetase and S-layer protein in Synechocystis sp. PCC 6803

Background

Based on known metabolic response to excess free fatty acid (FFA) products, cyanobacterium Synechocystis sp. PCC 6803 preferentially both recycles via FFA recycling process and secrets them into medium. Engineered cyanobacteria with well growth and highly secreted FFA capability are considered best resources for biofuel production and sustainable biotechnology. In this study, to achieve the higher FFA secretion goal, we successfully constructs Synechocystis sp. PCC 6803 mutants disrupting genes related to FFA recycling reaction (aas gene encoding acyl–acyl carrier protein synthetase), and surface layer protein (encoded by sll1951).

Results

Three Synechocystis sp. PCC 6803 engineered strains, including two single mutants lacking aas (KA) and sll1951 (KS), and one double mutant lacking both aas and sll1951 (KAS), significantly secreted FFAs higher than that of wild type (WT). Certain increase of secreted FFAs was noted when cells were exposed to nitrogen-deficient conditions, BG₁₁-half N and BG₁₁-N conditions, with the exception of strain KS. Under BG₁₁-N condition at day 10, strain KAS strikingly secreted FFAs products up to 40%w/DCW or 238.1 mg/L, with trace amounts of PHB. Unexpectedly, strain KS, with S-layer disruption, appeared to have endured longer in BG₁₁-N growth medium. This strain KS significantly acclimated to the BG₁₁-N environment by accumulating a greater glycogen pool with lower FFA production, whereas strain KA favored higher PHB and intracellular lipid accumulations with moderate FFA secretion.

Conclusions

Mutations of both aas and sll1951 genes in Synechocystis sp. PCC 6803 significantly improved the productivity of secreted FFAs, especially under nitrogen deprivation.

The survivor strain: isolation and characterization of Phormidium yuhuli AB48, a filamentous phototactic cyanobacterium with biotechnological potential

Despite their recognized potential, current applications of cyanobacteria as microbial cell factories remain in early stages of development. This is partly due to the fact that engineered strains are often difficult to grow at scale. This technical challenge contrasts with the dense and highly productive cyanobacteria populations thriving in many natural environments. It has been proposed that the selection of strains pre-adapted for growth in industrial photobioreactors could enable more productive cultivation outcomes. Here, we described the initial morphological, physiological, and genomic characterization of Phormidium yuhuli AB48 isolated from an industrial photobioreactor environment. P. yuhuli AB48 is a filamentous phototactic cyanobacterium with a growth rate comparable to Synechocystis sp. PCC 6803. The isolate forms dense biofilms under high salinity and alkaline conditions and manifests a similar nutrient profile to Arthrospira platensis (Spirulina). We sequenced, assembled, and analyzed the P. yuhuli AB48 genome, the first closed circular isolate reference genome for a member of the Phormidium genus. We then used cultivation experiments in combination with proteomics and metabolomics to investigate growth characteristics and phenotypes related to industrial scale cultivation, including nitrogen and carbon utilization, salinity, and pH acclimation, as well as antibiotic resistance. These analyses provide insight into the biological mechanisms behind the desirable growth properties manifested by P. yuhuli AB48 and position it as a promising microbial cell factory for industrial-scale bioproduction[221, 1631].

Polyhydroxybutyrate-producing cyanobacteria from lampenflora: The case study of the “Stiffe” caves in Italy

This study aimed to estimate the green formation lampenflora of “Stiffe” caves in order to evaluate their suitability as an isolation source of cyanobacteria useful for the production of polyhydroxyalkanoates (PHAs). The cave system was chosen as the sampling site due to its touristic use and the presence of high-impact illuminations. The biofilms and the mats of the illuminated walls were sampled. Samples were investigated by 16S rRNA gene analysis and culturable cyanobacteria isolation. The isolated strains were then screened for the production of PHAs under typical culturing and nutritional starvation. Cultures were checked for PHA accumulation, poly-β-hydroxybutyrate (PHB) presence (infrared spectroscopy), and pigment production. The 16S rRNA gene metabarcoding. Highlighted a considerable extent of the pressure exerted by anthropogenic activities. However, the isolation yielded eleven cyanobacteria isolates with good PHA (mainly PHB)-producing abilities and interesting pigment production rates (chlorophyll a and carotenoids). Under normal conditions (BG11₀), the accumulation abilities ranged from 266 to 1,152 ng mg dry biomass^–1. The optimization of bioprocesses through nutritional starvation resulted in a 2.5-fold increase. Fourier transform infrared (FTIR) studies established the occurrence of PHB within PHAs extracted by cyanobacteria isolates. The comparison of results with standard strains underlined good production rates. For C2 and C8 strains, PHA accumulation rates under starvation were higher than Azospirillum brasilense and similar to Synechocystis cf. salina 192. This study broadened the knowledge of the microbial communities of mats and biofilms on the lightened walls of the caves. These findings suggested that these structures, which are common in tourist caves, could be used to isolate valuable strains before remediation measures are adopted.

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.

Rational engineering of natural polyhydroxyalkanoates producing microorganisms for improved synthesis and recovery

Microbial production of biopolymers derived from renewable substrates and waste streams reduces our heavy reliance on petrochemical plastics. One of the most important biodegradable polymers is the family of polyhydroxyalkanoates (PHAs), naturally occurring intracellular polyoxoesters produced for decades by bacterial fermentation of sugars and fatty acids at the industrial scale. Despite the advances, PHA production still suffers from heavy costs associated with carbon substrates and downstream processing to recover the intracellular product, thus restricting market positioning. In recent years, model-aided metabolic engineering and novel synthetic biology approaches have spurred our understanding of carbon flux partitioning through competing pathways and cellular resource allocation during PHA synthesis, enabling the rational design of superior biopolymer producers and programmable cellular lytic systems. This review describes these attempts to rationally engineering the cellular operation of several microbes to elevate PHA production on specific substrates and waste products. We also delve into genome reduction, morphology, and redox cofactor engineering to boost PHA biosynthesis. Besides, we critically evaluate engineered bacterial strains in various fermentation modes in terms of PHA productivity and the period required for product recovery.

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.

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.

Novel Poly(butylene adipate-co-terephthalate)-degrading Bacillus sp. JY35 from wastewater sludge and its broad degradation of various bioplastics

Poly(butylene adipate-co-terephthalate) (PBAT), a bioplastic consisting of aliphatic hydrocarbons and aromatic hydrocarbons, was developed to overcome the shortcomings of aliphatic and aromatic polyesters. Many studies report the use of PBAT as a blending material for improving properties of other bioplastics. However, there are few studies on microorganisms that degrade PBAT. We found six kinds of PBAT-degrading microorganisms from various soils. Among these, Bacillus sp. JY35 showed superior PBAT degradability and robustness to temperature. We monitored the degradation of PBAT films by Bacillus sp. JY35 using scanning electron microscopy, field emission scanning electron microscopy, Fourier-transform infrared spectroscopy, and gel permeation chromatography. GC-MS was used to measure the PBAT film degradation rate at different temperatures and with additional NaCl and carbon sources. Certain additional carbon sources improve the growth of Bacillus sp. JY35. However, this did not increase PBAT film degradation. Time-dependent PBAT film degradation rates were measured during three weeks of cultivation, after which the strain achieved almost 50% degradation. Additionally, various bioplastics were applied to solid cultures to confirm the biodegradation range of Bacillus sp. JY35, which can degrade not only PBAT but also PBS, PCL, PLA, PHB, P(3HB-co-4HB), P(3HB-co-3HV), P(3HB-co-3HHx), and P(3HB-co-3HV-co-3HHx), suggesting its usability as a superior bioplastic degrader.

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.

Glycogen, poly(3-hydroxybutyrate) and pigment accumulation in three Synechocystis strains when exposed to a stepwise increasing salt stress

The cyanobacterial genus Synechocystis is of particular interest to science and industry because of its efficient phototrophic metabolism, its accumulation of the polymer poly(3-hydroxybutyrate) (PHB) and its ability to withstand or adapt to adverse growing conditions. One such condition is the increased salinity that can be caused by recycled or brackish water used in cultivation. While overall reduced growth is expected in response to salt stress, other metabolic responses relevant to the efficiency of phototrophic production of biomass or PHB (or both) have been experimentally observed in three Synechocystis strains at stepwise increasing salt concentrations. In response to recent reports on metabolic strategies to increase stress tolerance of heterotrophic and phototrophic bacteria, we focused particularly on the stress-induced response of Synechocystis strains in terms of PHB, glycogen and photoactive pigment dynamics. Of the three strains studied, the strain Synechocystis cf. salina CCALA192 proved to be the most tolerant to salt stress. In addition, this strain showed the highest PHB accumulation. All the three strains accumulated more PHB with increasing salinity, to the point where their photosystems were strongly inhibited and they could no longer produce enough energy to synthesize more PHB.

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.

Carbon nanogels exert multipronged attack on resistant bacteria and strongly constrain resistance evolution

Developing antimicrobial agents that can eradicate drug-resistant (DR) bacteria and provide sustained protection from DR bacteria is a major challenge. Herein, we report a mild pyrolysis approach to prepare carbon nanogels (CNGs) through polymerization and the partial carbonization of l-lysine hydrochloride at 270 °C as a potential broad-spectrum antimicrobial agent that can inhibit biopolymer-producing bacteria and clinical drug-resistant isolates and tackle drug resistance issues. We thoroughly studied the structures of the CNGs, their antibacterial mechanism, and biocompatibility. CNGs possess superior bacteriostatic effects against drug-resistant bacteria compared to some commonly explored antibacterial nanomaterials (silver, copper oxide, and zinc oxide nanoparticles, and graphene oxide) through multiple antimicrobial mechanisms, including reactive oxygen species generation, membrane potential dissipation, and membrane function disruption, due to the positive charge and flexible colloidal structures resulting strong interaction with bacterial membrane. The minimum inhibitory concentration (MIC) values of the CNGs (0.6 µg mL^-1 against E. coli and S. aureus) remained almost the same against the bacteria after 20 passages; however, the MIC values increased significantly after treatment with silver nanoparticles, antibiotics, the bacteriostatic chlorhexidine, and especially gentamicin (approximately 140-fold). Additionally, the CNGs showed a negligible MIC value difference against the obtained resistant bacteria after acclimation to the abovementioned antimicrobial agents. The findings of this study unveil the development of antimicrobial CNGs as a sustainable solution to combat multidrug-resistant bacteria.

Disruption of Hydrogen Gas Synthesis Enhances the Cellular Levels of NAD(P)H, Glycogen, Poly(3-hydroxybutyrate) and Photosynthetic Pigments Under Specific Nutrient Condition(s) in Cyanobacterium Synechocystis sp. PCC 6803

In photoautotrophic Synechocystis sp. PCC 6803, NADPH is generated from photosynthesis and utilized in various metabolism, including the biosynthesis of glyceraldehyde 3-phosphate (the upstream substrate for carbon metabolism), poly(3-hydroxybutyrate) (PHB), photosynthetic pigments, and hydrogen gas (H2). Redirecting NADPH flow from one biosynthesis pathway to another has yet to be studied. Synechocystis's H2 synthesis, one of the pathways consuming NAD(P)H, was disrupted by the inactivation of hoxY and hoxH genes encoding the two catalytic subunits of hydrogenase. Such inactivation with a complete disruption of H2 synthesis led to 1.4-, 1.9-, and 2.1-fold increased cellular NAD(P)H levels when cells were cultured in normal medium (BG11), the medium without nitrate (-N), and the medium without phosphate (-P), respectively. After 49-52 d of cultivation in BG11 (when the nitrogen source in the media was depleted), the cells with disrupted H2 synthesis had 1.3-fold increased glycogen level compared to wild type of 83-85% (w/w dry weight), the highest level reported for cyanobacterial glycogen. The increased glycogen content observed by transmission electron microscopy was correlated with the increased levels of glucose 6-phosphate and glucose 1-phosphate, the two substrates in glycogen synthesis. Disrupted H2 synthesis also enhanced PHB accumulation up to 1.4-fold under -P and 1.6-fold under -N and increased levels of photosynthetic pigments (chlorophyll a, phycocyanin, and allophycocyanin) by 1.3- to 1.5-fold under BG11. Thus, disrupted H2 synthesis increased levels of NAD(P)H, which may be utilized for the biosynthesis of glycogen, PHB, and pigments. This strategy might be applicable for enhancing other biosynthetic pathways that utilize NAD(P)H.

Screening novel genes by a comprehensive strategy to construct multiple stress-tolerant industrial Saccharomyces cerevisiae with prominent bioethanol production

BACKGROUND

Strong multiple stress-tolerance is a desirable characteristic for Saccharomyces cerevisiae when different feedstocks are used for economical industrial ethanol production. Random mutagenesis or genome shuffling has been applied for improving multiple stress-tolerance, however, these techniques are generally time-consuming and labor cost-intensive and their molecular mechanisms are unclear. Genetic engineering, as an efficient technology, is poorly applied to construct multiple stress-tolerant industrial S. cerevisiae due to lack of clear genetic targets. Therefore, constructing multiple stress-tolerant industrial S. cerevisiae is challenging. In this study, some target genes were mined by comparative transcriptomics analysis and applied for the construction of multiple stress-tolerant industrial S. cerevisiae strains with prominent bioethanol production.

RESULTS

Twenty-eight shared differentially expressed genes (DEGs) were identified by comparative analysis of the transcriptomes of a multiple stress-tolerant strain E-158 and its original strain KF-7 under five stress conditions (high ethanol, high temperature, high glucose, high salt, etc.). Six of the shared DEGs which may have strong relationship with multiple stresses, including functional genes (ASP3, ENA5), genes of unknown function (YOL162W, YOR012W), and transcription factors (Crz1p, Tos8p), were selected by a comprehensive strategy from multiple aspects. Through genetic editing based on the CRISPR/Case9 technology, it was demonstrated that expression regulation of each of these six DEGs improved the multiple stress-tolerance and ethanol production of strain KF-7. In particular, the overexpression of ENA5 significantly enhanced the multiple stress-tolerance of not only KF-7 but also E-158. The resulting engineered strain, E-158-ENA5, achieved higher accumulation of ethanol. The ethanol concentrations were 101.67% and 27.31% higher than those of the E-158 when YPD media and industrial feedstocks (straw, molasses, cassava) were fermented, respectively, under stress conditions.

CONCLUSION

Six genes that could be used as the gene targets to improve multiple stress-tolerance and ethanol production capacities of S. cerevisiae were identified for the first time. Compared to the other five DEGs, ENA5 has a more vital function in regulating the multiple stress-tolerance of S. cerevisiae. These findings provide novel insights into the efficient construction of multiple stress-tolerant industrial S. cerevisiae suitable for the fermentation of different raw materials.

Combinatorial use of environmental stresses and genetic engineering to increase ethanol titres in cyanobacteria

Current industrial bioethanol production by yeast through fermentation generates carbon dioxide. Carbon neutral bioethanol production by cyanobacteria uses biological fixation (photosynthesis) of carbon dioxide or other waste inorganic carbon sources, whilst being sustainable and renewable. The first ethanologenic cyanobacterial process was developed over two decades ago using Synechococcus elongatus PCC 7942, by incorporating the recombinant pdc and adh genes from Zymomonas mobilis. Further engineering has increased bioethanol titres 24-fold, yet current levels are far below what is required for industrial application. At the heart of the problem is that the rate of carbon fixation cannot be drastically accelerated and carbon partitioning towards bioethanol production impacts on cell fitness. Key progress has been achieved by increasing the precursor pyruvate levels intracellularly, upregulating synthetic genes and knocking out pathways competing for pyruvate. Studies have shown that cyanobacteria accumulate high proportions of carbon reserves that are mobilised under specific environmental stresses or through pathway engineering to increase ethanol production. When used in conjunction with specific genetic knockouts, they supply significantly more carbon for ethanol production. This review will discuss the progress in generating ethanologenic cyanobacteria through chassis engineering, and exploring the impact of environmental stresses on increasing carbon flux towards ethanol production.

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.

Photoautotrophic and Mixotrophic Cultivation of Polyhydroxyalkanoate-Accumulating Microalgae Consortia Selected under Nitrogen and Phosphate Limitation

Microalgae consortia were photoautotrophically cultivated in sequencing batch photobioreactors (SBPRs) with an alteration of the normal growth and starvation (nutrient limitation) phases to select consortia capable of polyhydroxyalkanoate (PHA) accumulation. At the steady state of SBPR operation, the obtained microalgae consortia, selected under nitrogen and phosphate limitation, accumulated up to 11.38% and 10.24% of PHA in their biomass, which was identified as poly(3-hydroxybutyrate) (P3HB). Photoautotrophic and mixotrophic batch cultivation of the selected microalgae consortia was conducted to investigate the potential of biomass and PHA production. Sugar source supplementation enhanced the biomass and PHA production, with the highest PHA contents of 10.94 and 6.2%, and cumulative PHA productions of 100 and 130 mg/L, with this being achieved with sugarcane juice under nitrogen and phosphate limitation, respectively. The analysis of other macromolecules during batch cultivation indicated a high content of carbohydrates and lipids under nitrogen limitation, while higher protein contents were detected under phosphate limitation. These results recommended the selected microalgae consortia as potential tools for PHA and bioresource production. The mixed-culture non-sterile cultivation system developed in this study provides valuable information for large-scale microalgal PHA production process development following the biorefinery concept.

Polyhydroxyalkanoates (PHAs) degradation by the newly isolated marine Bacillus sp. JY14

Polyhydroxyalkanoates (PHAs) are bioplastic substitutes for petroleum-derived plastics that may help to reduce the increasing environmental impact of plastic pollution. Among them, polyhydroxybutyrate (PHB) is a promising biopolymer, incentivizing many researchers to search for PHB-producing and PHB-degrading bacteria for improved PHB utilization. Many novel PHB-producing microorganisms have been discovered; however, relatively few PHB-degrading bacteria have been identified. Six PHB-degrading bacteria were found in marine soil and investigated their PHB-degrading abilities under various temperature and salinity conditions using solid-media based culture. Finally, thermotolerant and halotolerant PHB-degrader Bacillus sp. JY14 was selected. PHB degradation was confirmed by monitoring changes in the physical and chemical properties of PHB films incubated with Bacillus sp. JY14 using scanning electron microscopy, Fourier-transform infrared spectroscopy, and gel permeation chromatography. Further, PHB degradation ability of Bacillus sp. JY14 was measured in liquid culture by gas chromatography. After 14 days of cultivation with PHB film, Bacillus sp. JY14 achieved approximately 98% PHB degradation. Applying various bioplastics to assess the bacteria's biodegradation capabilities, the result showed that Bacillus sp. JY14 could also degrade P(3HB-co-4HB) and P(3HB-co-3HV). Overall, this study identified a thermotolerant and halotolerant bacteria capable of PHB degradation under solid and liquid conditions. These results suggest that this bacteria could be utilized to degrade various PHAs.

Cycling between growth and production phases increases cyanobacteria bioproduction of lactate

Decoupling growth from product synthesis is a promising strategy to increase carbon partitioning and maximize productivity in cell factories. However, reduction in both substrate uptake rate and metabolic activity in the production phase are an underlying problem for upscaling. Here, we used CRISPR interference to repress growth in lactate-producing Synechocystis sp. PCC 6803. Carbon partitioning to lactate in the production phase exceeded 90%, but CO₂ uptake was severely reduced compared to uptake during the growth phase. We characterized strains during the onset of growth arrest using transcriptomics and proteomics. Multiple genes involved in ATP homeostasis were regulated once growth was inhibited, which suggests an alteration of energy charge that may lead to reduced substrate uptake. In order to overcome the reduced metabolic activity and take advantage of increased carbon partitioning, we tested a novel production strategy that involved alternating growth arrest and recovery by periodic addition of an inducer molecule to activate CRISPRi. Using this strategy, we maintained lactate biosynthesis in Synechocystis for 30 days in a constant light turbidostat cultivation. Cumulative lactate titers were also increased by 100% compared to a constant growth-arrest regime, and reached 1 g/L. Further, the cultivation produced lactate for 30 days, compared to 20 days for the non-growth arrest cultivation. Periodic growth arrest could be applicable for other products, and in cyanobacteria, could be linked to internal circadian rhythms that persist in constant light.