Effects of growth condition on the structure of glycogen produced in cyanobacterium Synechocystis sp. PCC6803

Analysis of Cell Glycogen with Quantitation and Determination of Branching Using Liquid Chromatography-Mass Spectrometry

Glycogen is a highly branched biomacromolecule that functions as a glucose buffer. It is involved in multiple diseases such as glycogen storage disorders, diabetes, and even liver cancer, where the imbalance between biosynthetic and catabolic enzymes results in structural alterations and abnormal accumulation of glycogen that can be toxic to cells. Accurate and sensitive glycogen quantification and structural determination are prerequisites for understanding the phenotypes and biological functions of glycogen under these conditions. In this research, we furthered cell glycogen characterization by presenting a highly sensitive method to measure the glycogen content and degree of branching. The method employed a novel fructose density gradient as an alternative to the traditional sucrose gradient to fractionate glycogen from cell mixtures using ultracentrifugation. Fructose was used to avoid the large glucose background, allowing the method to be highly quantitative. The glycogen content was determined by quantifying 1-phenyl-3-methyl-5-pyrazolone (PMP)-derivatized glucose residues obtained from acid-hydrolyzed glycogen using ultra-high-performance liquid chromatography/triple quadrupole mass spectrometry (UHPLC/QqQ-MS). The degree of branching was determined through linkage analysis where the glycogen underwent permethylation, hydrolysis, PMP derivatization, and UHPLC/QqQ-MS analysis. The new approach was used to study the effect of insulin on the glycogen phenotypes of human hepatocellular carcinoma (Hep G2) cells. We observed that cells produced greater amounts of glycogen with less branching under increasing insulin levels before reaching the cell's insulin-resistant state, where the trend reversed and the cells produced less but higher-branched glycogen. The advantage of this method lies in its high sensitivity in characterizing both the glycogen level and the structure of biological samples.

Alpha-1,4-transglycosylation Activity of GH57 Glycogen Branching Enzymes Is Higher in the Absence of a Flexible Loop with a Conserved Tyrosine Residue

Starch-like polymers can be created through the use of enzymatic modification with glycogen branching enzymes (GBEs). GBEs are categorized in the glycoside hydrolase (GH) family 13 and 57. Both GH13 and GH57 GBEs exhibit branching and hydrolytic activity. While GH13 GBEs are also capable of α-1,4-transglycosylation, it is yet unknown whether GH57 share this capability. Among the four crystal structures of GH57 GBEs that have been solved, a flexible loop with a conserved tyrosine was identified to play a role in the branching activity. However, it remains unclear whether this flexible loop is also involved in α-1,4-transglycosylation activity. We hypothesize that GH57 GBEs with the flexible loop and tyrosine are also capable of α-1,4-transglycosylation, similar to GH13 GBEs. The aim of the present study was to characterize the activity of GH57 GBEs to investigate a possible α-1,4-transglycosylation activity. Three GH57 GBEs were selected, one from Thermococcus kodakarensis with the flexible loop and two beta-strands; one from Thermotoga maritima, missing the flexible loop and beta-strands; and one from Meiothermus sp., missing the flexible loop but with the two beta-strands. The analysis of chain length distribution over time of modified maltooctadecaose, revealed, for the first time, that all three GH57 GBEs can generate chains longer than the substrate itself, showing that α-1,4-transglycosylation activity is generally present in GH57 GBEs.

The Structure of Maltooctaose-Bound Escherichia coli Branching Enzyme Suggests a Mechanism for Donor Chain Specificity

Glycogen is the primary storage polysaccharide in bacteria and animals. It is a glucose polymer linked by α-1,4 glucose linkages and branched via α-1,6-linkages, with the latter reaction catalyzed by branching enzymes. Both the length and dispensation of these branches are critical in defining the structure, density, and relative bioavailability of the storage polysaccharide. Key to this is the specificity of branching enzymes because they define branch length. Herein, we report the crystal structure of the maltooctaose-bound branching enzyme from the enterobacteria E. coli. The structure identifies three new malto-oligosaccharide binding sites and confirms oligosaccharide binding in seven others, bringing the total number of oligosaccharide binding sites to twelve. In addition, the structure shows distinctly different binding in previously identified site I, with a substantially longer glucan chain ordered in the binding site. Using the donor oligosaccharide chain-bound Cyanothece branching enzyme structure as a guide, binding site I was identified as the likely binding surface for the extended donor chains that the E. coli branching enzyme is known to transfer. Furthermore, the structure suggests that analogous loops in branching enzymes from a diversity of organisms are responsible for branch chain length specificity. Together, these results suggest a possible mechanism for transfer chain specificity involving some of these surface binding sites.

Nutrient-Limited Operational Strategies for the Microbial Production of Biochemicals

Limiting an essential nutrient has a profound impact on microbial growth. The notion of growth under limited conditions was first described using simple Monod kinetics proposed in the 1940s. Different operational modes (chemostat, fed-batch processes) were soon developed to address questions related to microbial physiology and cell maintenance and to enhance product formation. With more recent developments of metabolic engineering and systems biology, as well as high-throughput approaches, the focus of current engineers and applied microbiologists has shifted from these fundamental biochemical processes. This review draws attention again to nutrient-limited processes. Indeed, the sophisticated gene editing tools not available to pioneers offer the prospect of metabolic engineering strategies which leverage nutrient limited processes. Thus, nutrient- limited processes continue to be very relevant to generate microbially derived biochemicals.

Impact of glgA1, glgA2 or glgC overexpression on growth and glycogen production in Synechocystis sp. PCC 6803

Low production rates are still one limiting factor for the industrial climate-neutral production of biovaluable compounds in cyanobacteria. Next to optimized cultivation conditions, new production strategies are required. Hence, the use of established molecular tools could lead to increased product yields in the cyanobacterial model organism Synechocystis sp. PCC6803. Its main storage compound glycogen was chosen to be increased by the use of these tools. In this study, the three genes glgC, glgA1 and glgA2, which are part of the glycogen synthesis pathway, were combined with the P_cpc560 promoter and the neutral cloning site NSC1. The complete genome integration, protein formation, biomass production and glycogen accumulation were determined to select the most productive transformants. The overexpression of glgA2 did not increase the biomass or glycogen production in short-term trials compared to the other two genes but caused transformants death in long-term trials. The transformants glgA1_11 and glgC_2 showed significantly increased biomass (1.6-fold - 1.7-fold) and glycogen production (3.5-fold - 4-fold) compared to the wild type after 96 h making them a promising energy source for further applications. Those could include for example a two-stage production process, with first energy production (glycogen) and second increased product formation (e.g. ethanol).

Photoautotrophic production of renewable ethylene by engineered cyanobacteria: Steering the cell metabolism towards biotechnological use

Ethylene is a volatile hydrocarbon with a massive global market in the plastic industry. The ethylene now used for commercial applications is produced exclusively from nonrenewable petroleum sources, while competitive biotechnological production systems do not yet exist. This review focuses on the currently developed photoautotrophic bioproduction strategies that enable direct solar-driven conversion of CO₂ into ethylene, based on the use of genetically engineered photosynthetic cyanobacteria expressing heterologous ethylene forming enzyme (EFE) from Pseudomonas syringae. The emphasis is on the different engineering strategies to express EFE and to direct the cellular carbon flux towards the primary metabolite 2-oxoglutarate, highlighting associated metabolic constraints, and technical considerations on cultivation strategies and conditional parameters. While the research field has progressed towards more robust strains with better production profiles, and deeper understanding of the associated metabolic limitations, it is clear that there is room for significant improvement to reach industrial relevance. At the same time, existing information and the development of synthetic biology tools for engineering cyanobacteria open new possibilities for improving the prospects for the sustainable production of renewable ethylene.

Enhancing carbohydrate repartitioning into lipid and carotenoid by disruption of microalgae starch debranching enzyme

Light/dark cycling is an inherent condition of outdoor microalgae cultivation, but is often unfavorable for lipid accumulation. This study aims to identify promising targets for metabolic engineering of improved lipid accumulation under outdoor conditions. Consequently, the lipid-rich mutant Chlamydomonas sp. KOR1 was developed through light/dark-conditioned screening. During dark periods with depressed CO₂ fixation, KOR1 shows rapid carbohydrate degradation together with increased lipid and carotenoid contents. KOR1 was subsequently characterized with extensive mutation of the ISA1 gene encoding a starch debranching enzyme (DBE). Dynamic time-course profiling and metabolomics reveal dramatic changes in KOR1 metabolism throughout light/dark cycles. During light periods, increased flux from CO₂ through glycolytic intermediates is directly observed to accompany enhanced formation of small starch-like particles, which are then efficiently repartitioned in the next dark cycle. This study demonstrates that disruption of DBE can improve biofuel production under light/dark conditions, through accelerated carbohydrate repartitioning into lipid and carotenoid.

Characterizing glucose, illumination, and nitrogen-deprivation phenotypes of Synechocystis PCC6803 with Raman spectroscopy

Background

Synechocystis sp. PCC6803 is a model cyanobacterium that has been studied widely and is considered for metabolic engineering applications. Here, Raman spectroscopy and Raman chemometrics (Rametrix™) were used to (i) study broad phenotypic changes in response to growth conditions, (ii) identify phenotypic changes associated with its circadian rhythm, and (iii) correlate individual Raman bands with biomolecules and verify these with more accepted analytical methods.

Methods

Synechocystis cultures were grown under various conditions, exploring dependencies on light and/or external carbon and nitrogen sources. The Rametrix™ LITE Toolbox for MATLAB® was used to process Raman spectra and perform principal component analysis (PCA) and discriminant analysis of principal components (DAPC). The Rametrix™ PRO Toolbox was used to validate these models through leave-one-out routines that classified a Raman spectrum when growth conditions were withheld from the model. Performance was measured by classification accuracy, sensitivity, and specificity. Raman spectra were also subjected to statistical tests (ANOVA and pairwise comparisons) to identify statistically relevant changes in Synechocystis phenotypes. Finally, experimental methods, including widely used analytical and spectroscopic assays were used to quantify the levels of glycogen, fatty acids, amino acids, and chlorophyll a for correlations with Raman data.

Results

PCA and DAPC models produced distinct clustering of Raman spectra, representing multiple Synechocystis phenotypes, based on (i) growth in the presence of 5 mM glucose, (ii) illumination (dark, light/dark [12 h/12 h], and continuous light at 20 µE), (iii) nitrogen deprivation (0–100% NaNO₃ of native BG-11 medium in continuous light), and (iv) throughout a 24 h light/dark (12 h/12 h) circadian rhythm growth cycle. Rametrix™ PRO was successful in identifying glucose-induced phenotypes with 95.3% accuracy, 93.4% sensitivity, and 96.9% specificity. Prediction accuracy was above random chance values for all other studies. Circadian rhythm analysis showed a return to the initial phenotype after 24 hours for cultures grown in light/dark (12 h/12 h) cycles; this did not occur for cultures grown in the dark. Finally, correlation coefficients (R > 0.7) were found for glycogen, all amino acids, and chlorophyll a when comparing specific Raman bands to other experimental results.

Multiscale Multiobjective Systems Analysis (MiMoSA): an advanced metabolic modeling framework for complex systems

In natural environments, cells live in complex communities and experience a high degree of heterogeneity internally and in the environment. Even in 'ideal' laboratory environments, cells can experience a high degree of heterogeneity in their environments. Unfortunately, most of the metabolic modeling approaches that are currently used assume ideal conditions and that each cell is identical, limiting their application to pure cultures in well-mixed vessels. Here we describe our development of Multiscale Multiobjective Systems Analysis (MiMoSA), a metabolic modeling approach that can track individual cells in both space and time, track the diffusion of nutrients and light and the interaction of cells with each other and the environment. As a proof-of concept study, we used MiMoSA to model the growth of Trichodesmium erythraeum, a filamentous diazotrophic cyanobacterium which has cells with two distinct metabolic modes. The use of MiMoSA significantly improves our ability to predictively model metabolic changes and phenotype in more complex cell cultures.

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.

Engineering cyanobacteria for production of terpenoids

This review summarizes recent advances in cyanobacterial terpenoid production. The challenges and opportunities of improving terpenoid production by cyanobacteria are discussed. Terpenoids are a diverse group of natural products with a variety of commercial applications. With recent advances in synthetic biology and metabolic engineering, microbial terpenoid synthesis is being viewed as a feasible approach for industrial production. Among different microbial hosts, cyanobacteria have the potential of sustainable production of terpenoids using light and CO₂. Terpene synthases and the precursor pathways have been expressed in cyanobacteria for enhanced production of various terpene hydrocarbons, including isoprene, limonene, β-phellandrene, and farnesene. However, the productivities need to be further improved for commercial production. Many barriers remain to be overcome in order to efficiently convert CO₂ to terpenoids. In this review, we will summarize recent efforts on photosynthetic production of terpenoids and discuss the challenges and opportunities of engineering cyanobacteria for terpenoid bioproduction.

Cyanobacterial PHA Production-Review of Recent Advances and a Summary of Three Years' Working Experience Running a Pilot Plant

Cyanobacteria, as photoautotrophic organisms, provide the opportunity to convert CO2 to biomass with light as the sole energy source. Like many other prokaryotes, especially under nutrient deprivation, most cyanobacteria are able to produce polyhydroxyalkanoates (PHAs) as intracellular energy and carbon storage compounds. In contrast to heterotrophic PHA producers, photoautotrophic cyanobacteria do not consume sugars and, therefore, do not depend on agricultural crops, which makes them a green alternative production system. This review summarizes the recent advances in cyanobacterial PHA production. Furthermore, this study reports the working experience with different strains and cultivating conditions in a 200 L pilot plant. The tubular photobioreactor was built at the coal power plant in Dürnrohr, Austria in 2013 for direct utilization of flue gases. The main challenges were the selection of robust production strains, process optimization, and automation, as well as the CO2 availability.

High-Speed Scanning for the Quantitative Evaluation of Glycogen Concentration in Bioethanol Feedstock Synechocystis sp. PCC6803 Using a Near-Infrared Hyperspectral Imaging System with a New Near-Infrared Spectral Camera

In the present study, the high-speed quantitative evaluation of glycogen concentration accumulated in bioethanol feedstock Synechocystis sp. PCC6803 was performed using a near-infrared (NIR) imaging system with a hyperspectral NIR spectral camera named Compovision. The NIR imaging system has a feature for high-speed and wide area monitoring and the two-dimensional scanning speed is almost 100 times faster than the general NIR imaging systems for the same pixel size. For the quantitative analysis of glycogen concentration, partial least squares regression (PLSR) and moving window PLSR (MWPLSR) were performed with the information of glycogen concentration measured by high performance liquid chromatography (HPLC) and the calibration curves for the concentration within the Synechocystis sp. PCC6803 cell were constructed. The results had high accuracy for the quantitative estimation of glycogen concentration as the best squared correlation coefficient R² was bigger than 0.99 and a root mean square error (RMSE) was less than 2.9%. The present results proved not only the potential for the applicability of NIR spectroscopy to the high-speed quantitative evaluation of glycogen concentration in the bioethanol feedstock but also the expansivity of the NIR imaging instrument to in-line or on-line product evaluation on a factory production line of bioethanol in the future.

The effect of enhanced acetate influx on Synechocystis sp. PCC 6803 metabolism

Background

Acetate is a common microbial fermentative end-product, which can potentially be used as a supplementary carbon source to enhance the output of biotechnological production systems. This study focuses on the acetate metabolism of the photosynthetic cyanobacterium Synechocystis sp. PCC 6803 which is unable to grow on acetate as a sole carbon source but still can assimilate it via acetyl-CoA—derived metabolic intermediates. In order to gain insight into the acetate uptake, associated limitations and metabolic effects, a heterologous acetate transporter ActP from Escherichia coli was introduced into Synechocystis to facilitate the transport of supplemented acetate from the medium into the cell.

Results

The results show that enhanced acetate intake can efficiently promote the growth of the cyanobacterial host. The effect is apparent specifically under low-light conditions when the photosynthetic activity is low, and expected to result from increased availability of acetyl-CoA precursors, accompanied by changes induced in cellular glycogen metabolism which may include allocation of resources towards enhanced growth instead of glycogen accumulation. Despite the stimulated growth of the mutant, acetate is shown to suppress the activity of the photosynthetic apparatus, further emphasizing the contribution of glycolytic metabolism in the acetate-induced effect.

Conclusions

The use of acetate by the cyanobacterium Synechocystis sp. PCC 6803 is at least partially restricted by the import into the cell. This can be improved by the introduction of a heterologous acetate transporter into the system, thereby providing a potential advantage by expanding the scope of acetate utilization for various biosynthetic processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0640-x) contains supplementary material, which is available to authorized users.

Altering the Structure of Carbohydrate Storage Granules in the Cyanobacterium Synechocystis sp. Strain PCC 6803 through Branching-Enzyme Truncations

Carbohydrate storage is an important element of metabolism in cyanobacteria and in the chloroplasts of plants. Understanding how to manipulate the metabolism and storage of carbohydrate is also an important factor toward harnessing cyanobacteria for energy production. While most cyanobacteria produce glycogen, some have been found to accumulate polysaccharides in the form of water-insoluble α-glucan similar to amylopectin. Notably, this alternative form, termed "semi-amylopectin," forms in cyanobacterial species harboring three branching-enzyme (BE) homologs, designated BE1, BE2, and BE3. In this study, mutagenesis of the branching genes found in Synechocystis sp. strain PCC 6803 was performed in order to characterize their possible impact on polysaccharide storage granule morphology. N-terminal truncations were made to the native BE gene of Synechocystis sp. PCC 6803. In addition, one of the two native debranching enzyme genes was replaced with a heterologous debranching enzyme gene from a semi-amylopectin-forming strain. Growth and glycogen content of mutant strains did not significantly differ from those of the wild type, and ultrastructure analysis revealed only slight changes to granule morphology. However, analysis of chain length distribution by anion-exchange chromatography revealed modest changes to the branched-chain length profile. The resulting glycogen shared structure characteristics similar to that of granules isolated from semi-amylopectin-producing strains.

IMPORTANCE

This study is the first to investigate the impact of branching-enzyme truncations on the structure of storage carbohydrates in cyanobacteria. The results of this study are an important contribution toward understanding the relationship between the enzymatic repertoire of a cyanobacterial species and the morphology of its storage carbohydrates.

Proteomic and cellular views of Arthrospira sp. PCC 8005 adaptation to nitrogen depletion

Cyanobacteria are photosynthetic prokaryotes that play a crucial role in the Earth's nitrogen and carbon cycles. Nitrogen availability is one of the most important factors in cyanobacterial growth. Interestingly, filamentous non-diazotrophic cyanobacteria, such as Arthrospira sp. PCC 8005, have developed survival strategies that enable them to adapt to nitrogen deprivation. Metabolic studies recently demonstrated a substantial synthesis and accumulation of glycogen derived from amino acids during nitrogen starvation. Nevertheless, the regulatory mechanism of this adaptation is poorly understood. To the best of our knowledge, this study is the first proteomic and cellular analysis of Arthrospira sp. PCC 8005 under nitrogen depletion. Label-free differential proteomic analysis indicated the global carbon and nitrogen reprogramming of the cells during nitrogen depletion as characterized by an upregulation of glycogen synthesis and the use of endogenous nitrogen sources. The degradation of proteins and cyanophycin provided endogenous nitrogen when exogenous nitrogen was limited. Moreover, formamides, cyanates and urea were also potential endogenous nitrogen sources. The transporters of some amino acids and alternative nitrogen sources such as ammonium permease 1 were induced under nitrogen depletion. Intriguingly, although Arthrospira is a non-diazotrophic cyanobacterium, we observed the upregulation of HetR and HglK proteins, which are involved in heterocyst differentiation. Moreover, after a long period without nitrate, only a few highly fluorescent cells in each trichome were observed, and they might be involved in the long-term survival mechanism of this non-diazotrophic cyanobacterium under nitrogen deprivation.

Glycogen synthase isoforms in Synechocystis sp. PCC6803: identification of different roles to produce glycogen by targeted mutagenesis

Synechocystis sp. PCC6803 belongs to cyanobacteria which carry out photosynthesis and has recently become of interest due to the evolutionary link between bacteria and plant species. Similar to other bacteria, the primary carbohydrate storage source of Synechocystis sp. PCC6803 is glycogen. While most bacteria are not known to have any isoforms of glycogen synthase, analysis of the genomic DNA sequence of Synechocystis sp. PCC6803 predicts that this strain encodes two isoforms of glycogen synthase (GS) for synthesizing glycogen structure. To examine the functions of the putative GS genes, each gene (sll1393 or sll0945) was disrupted by double cross-over homologous recombination. Zymogram analysis of the two GS disruption mutants allowed the identification of a protein band corresponding to each GS isoform. Results showed that two GS isoforms (GSI and GSII) are present in Synechocystis sp. PCC6803, and both are involved in glycogen biosynthesis with different elongation properties: GSI is processive and GSII is distributive. Total GS activities in the mutant strains were not affected and were compensated by the remaining isoform. Analysis of the branch-structure of glycogen revealed that the sll1393⁻ mutant (GSI⁻) produced glycogen containing more intermediate-length chains (DP 8–18) at the expense of shorter and longer chains compared with the wild-type strain. The sll0945⁻ mutant (GSII⁻) produced glycogen similar to the wild-type, with only a slightly higher proportion of short chains (DP 4–11). The current study suggests that GS isoforms in Synechocystis sp. PCC6803 have different elongation specificities in the biosynthesis of glycogen, combined with ADP-glucose pyrophosphorylase and glycogen branching enzyme.

The nitrogen-regulated response regulator NrrA controls cyanophycin synthesis and glycogen catabolism in the cyanobacterium Synechocystis sp. PCC 6803

The cellular metabolism in cyanobacteria is extensively regulated in response to changes of environmental nitrogen availability. Multiple regulators are involved in this process, including a nitrogen-regulated response regulator NrrA. However, the regulatory role of NrrA in most cyanobacteria remains to be elucidated. In this study, we combined a comparative genomic reconstruction of NrrA regulons in 15 diverse cyanobacterial species with detailed experimental characterization of NrrA-mediated regulation in Synechocystis sp. PCC 6803. The reconstructed NrrA regulons in most species included the genes involved in glycogen catabolism, central carbon metabolism, amino acid biosynthesis, and protein degradation. A predicted NrrA-binding motif consisting of two direct repeats of TG(T/A)CA separated by an 8-bp A/T-rich spacer was verified by in vitro binding assays with purified NrrA protein. The predicted target genes of NrrA in Synechocystis sp. PCC 6803 were experimentally validated by comparing the transcript levels and enzyme activities between the wild-type and nrrA-inactivated mutant strains. The effect of NrrA deficiency on intracellular contents of arginine, cyanophycin, and glycogen was studied. Severe impairments in arginine synthesis and cyanophycin accumulation were observed in the nrrA-inactivated mutant. The nrrA inactivation also resulted in a significantly decreased rate of glycogen degradation. Our results indicate that by directly up-regulating expression of the genes involved in arginine synthesis, glycogen degradation, and glycolysis, NrrA controls cyanophycin accumulation and glycogen catabolism in Synechocystis sp. PCC 6803. It is suggested that NrrA plays a role in coordinating the synthesis and degradation of nitrogen and carbon reserves in cyanobacteria.

Enhanced accumulation of glycogen, lipids and polyhydroxybutyrate under optimal nutrients and light intensities in the cyanobacterium Synechocystis sp. PCC 6803

AIMS

Glycogen (GL) and lipids (LP) are efficient biofuel substrates, whereas polyhydroxybutyrate (PHB) is a potent biodegradable plastic. This study aimed to increase accumulation of these three compounds in Synechocystis sp. PCC 6803.

METHODS AND RESULTS

Under autophototrophic growth, co-accumulation of the three compounds reached maximum level in a mid-stationary phase at 39·2% of dry weight (22·7% GL, 14·1% LP and 2·4% PHB). Nitrogen deprivation increased this to 61·5% (36·8% GL, 11·2% LP and 13·5% PHB), higher than that achieved by phosphorus, sulfur, iron or calcium deprivation. Combining nitrogen deprivation with 0·4% (w/v) glucose addition for heterophototrophic growth and optimizing the light intensity (200 μmol photons m(-2) s(-1) ) synergistically enhanced combined accumulation to 71·1% of biomass (41·3% GL, 16·7% LP and 13·1% PHB), a higher level than previously reported in Synechocystis. However, the maximum coproductivity of GL, LP and PHB (at 0·72 g l(-1) ) was obtained in the 12-day heterophototrophic culture without nitrogen deprivation.

CONCLUSION

Accumulation of GL, LP and PHB was enhanced under both autophototrophic and heterophototrophic conditions by optimizations of nutrient and light supplies.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides a means for increasing co-production of potent bioenergy substrates and useful biomaterials in Synechocystis which may also be applicable to other cyanobacteria.

Dynamic metabolic profiling of cyanobacterial glycogen biosynthesis under conditions of nitrate depletion

Cyanobacteria represent a globally important biomass because they are responsible for a substantial proportion of primary production in the hydrosphere. Arthrospira platensis is a fast-growing halophilic cyanobacterium capable of accumulating glycogen and has the potential to serve as a feedstock in the fermentative production of third-generation biofuels. Accordingly, enhancing cyanobacterial glycogen production is a promising biofuel production strategy. However, the regulatory mechanism of glycogen metabolism in cyanobacteria is poorly understood. The aim of the present study was to determine the metabolic flux of glycogen biosynthesis using a dynamic metabolomic approach. Time-course profiling of widely targeted cyanobacterial metabolic intermediates demonstrated a global metabolic reprogramming that involves transient increases in the levels of some amino acids during the glycogen production phase induced by nitrate depletion. Also, in vivo labelling with NaH(13)CO3 enabled direct measurement of metabolic intermediate turnover in A. platensis, revealing that under conditions of nitrate depletion glycogen is biosynthesized with carbon derived from amino acids released from proteins via gluconeogenesis. This dynamic metabolic profiling approach provided conclusive evidence of temporal alterations in the metabolic profile in cyanobacterial cells.

Natural osmolytes are much less effective substrates than glycogen for catabolic energy production in the marine cyanobacterium Synechococcus sp. strain PCC 7002

ADP-glucose pyrophosphorylase, encoded by glgC, catalyzes the first step of glycogen and glucosylglycer(ol/ate) biosynthesis. Here we report the construction of the first glgC null mutant of a marine cyanobacterium (Synechococcus sp. PCC 7002) and investigate its impact on dark anoxic metabolism (autofermentation). The glgC mutant had 98% lower ADP-glucose, synthesized no glycogen and produced appreciably more soluble sugars (mainly sucrose) than wild type (WT). Some glucosylglycerol was still observed, which suggests that the mutant has another, inefficient ADP-glucose synthesis pathway. In contrast, hypersaline conditions (1M NaCl) were lethal to the mutant strain, indicating that, unlike other strains, the elevated sucrose does not compensate for the reduced GG as osmolyte. In contrast to WT, nitrate limitation did not cause bleaching of N-containing pigments or carbohydrate accumulation in the glgC mutant, indicating impaired recycling of nitrogen stores. Despite the 2-fold increase in osmolytes, both the respiration and autofermentation rates of the glgC mutant were appreciably slower (2-4-fold) and correlated quantitatively with the lower fraction of insoluble carbohydrates relative to WT (85% vs. 12%). However, the remaining insoluble carbohydrates still accounted for a high fraction of the carbohydrate catabolized (38%), indicating that insoluble carbohydrates rather than osmolytes were the preferred substrate for autofermentation.

Altered carbohydrate metabolism in glycogen synthase mutants of Synechococcus sp. strain PCC 7002: Cell factories for soluble sugars

Glycogen and compatible solutes are the major polymeric and soluble carbohydrates in cyanobacteria and function as energy reserves and osmoprotectants, respectively. Glycogen synthase null mutants (glgA-I glgA-II) were constructed in the cyanobacterium Synechococcus sp. strain PCC 7002. Under standard conditions the double mutant produced no glycogen and more soluble sugars. When grown under hypersaline conditions, the glgA-I glgA-II mutant accumulated 1.8-fold more soluble sugars (sucrose and glucosylglycer-(ol/ate)) than WT, and these cells spontaneously excreted soluble sugars into the medium at high levels without the need for additional transporters. An average of 27% more soluble sugars was released from the glgA-I glgA-II mutant than WT by hypo-osmotic shock. Extracellular vesicles budding from the outer membrane were observed by transmission electron microscopy in glgA-I glgA-II cells grown under hypersaline conditions. The glgA-I glgA-II mutant serves as a starting point for developing cell factories for photosynthetic production and excretion of sugars.

Experimental and modeling analysis of Synechocystis sp. PCC 6803 growth

BACKGROUND/AIMS

The influence of different parameters such as temperature, irradiance, nitrate concentration, pH, and an external carbon source on Synechocystis PCC 6803 growth was evaluated.

METHODS

4.5-ml cuvettes containing 2 ml of culture, a high-throughput system equivalent to batch cultures, were used with gas exchange ensured by the use of a Parafilm™ cover. The effect of the different variables on maximum growth was assessed by a multi-way statistical analysis.

RESULTS

Temperature and pH were identified as the key factors. It was observed that Synechocystis cells have a strong influence on the external pH. The optimal growth temperature was 33°C while light-saturating conditions were reached at 40 µE·m⁻²·s⁻¹.

CONCLUSION

It was demonstrated that Synechocystis exhibits a marked difference in behavior between autotrophic and glucose-based mixotrophic conditions, and that nitrate concentrations did not have a significant influence, probably due to endogenous nitrogen reserves. Furthermore, a dynamic metabolic model of Synechocystis photosynthesis was developed to gain insights on the underlying mechanism enabling this cyanobacterium to control the levels of external pH. The model showed a coupled effect between the increase of the pH and ATP production which in turn allows a higher carbon fixation rate.

Photosynthetic accumulation of carbon storage compounds under CO₂ enrichment by the thermophilic cyanobacterium Thermosynechococcus elongatus

The growth characteristics of Thermosynechococcus elongatus on elevated CO₂ were studied in a photobioreactor. Cultures were able to grow on up to 20% CO₂. The maximum productivity and CO₂ fixation rates were 0.09 ± 0.01 and 0.17 ± 0.01 mg ml⁻¹ day⁻¹, respectively, for cultures grown on 20% CO₂. Three major carbon pools--lipids, polyhydroxybutyrates (PHBs), and glycogen--were measured. These carbon stores accounted for 50% of the total biomass carbon in cultures grown on atmospheric CO₂ (no supplemental CO₂), but only accounted for 30% of the total biomass carbon in cultures grown on 5-20% CO₂. Lipid content was approximately 20% (w/w) under all experimental conditions, while PHB content reached 14.5% (w/w) in cultures grown on atmospheric CO₂ and decreased to approximately 2.0% (w/w) at 5-20% CO₂. Glycogen levels did not vary significantly and remained about 1.4% (w/w) under all test conditions. The maximum amount of CO₂ sequestered over the course of the nine-day chemostat experiment was 1.15 g l⁻¹ in cultures grown on 20% CO₂.

Renewable energy from Cyanobacteria: energy production optimization by metabolic pathway engineering

The need to develop and improve sustainable energy resources is of eminent importance due to the finite nature of our fossil fuels. This review paper deals with a third generation renewable energy resource which does not compete with our food resources, cyanobacteria. We discuss the current state of the art in developing different types of bioenergy (ethanol, biodiesel, hydrogen, etc.) from cyanobacteria. The major important biochemical pathways in cyanobacteria are highlighted, and the possibility to influence these pathways to improve the production of specific types of energy forms the major part of this review.

Characterization of 4-alpha-glucanotransferase from Synechocystis sp. PCC 6803 and its application to various corn starches

A putative 4-alpha-glucanotransferase (alphaGTase) gene from Synechocystis sp. PCC 6803 was identified being composed of 1505 nucleotides, and the overexpressed protein was purified with an affinity chromatography. The recombinant alphaGTase had about 57kDa of molecular mass when judged by SDS-PAGE analysis. The optimum reaction condition of the alphaGTase was shown to be pH 7 at 45 degrees C in 50mm phosphate buffer. This enzyme displayed transglycosylating activity on various maltooligosaccharides, of which the smallest donor and acceptor molecules were determined to be maltose and glucose, respectively. Various corn starches consisting of different proportions of amylopectin and amylose were incubated with the recombinant alphaGTase. The change in molecular weight distribution of alphaGTase-modified starch was analyzed by HPSEC. The reaction pattern of alphaGTase showed substantial decrease in amylopectin and increase in the peak corresponding to cycloamylose (CA). The production yield of CA tended to increase from 5 to 30% along with the increase in the apparent amylose content in corn starch, which suggested that linear amylose chain would be preferred to produce CA in the alphaGTase treatment. The detectable minimum degree of polymerization (DP) of CA was shown to be 22 by MALDI-TOF-MS analysis. As another action mode of alphaGTase, the rearrangement of amylopectin branch-chain distribution occurred without hydrolysis to small oligosaccharides. After isoamylolysis, alphaGTase-treated starch displayed the increase in DP 4-9 and longer than DP 21 when the relative proportion of branch chains in amylopectin was determined by HPAEC.