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

Exceptional Quantum Efficiency Powers Biomass Production in Halotolerant Algae Picochlorum sp.. Photosynth Res 2024:10.1007/s11120-024-01075-9. [PMID: 38329705 DOI: 10.1007/s11120-024-01075-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The green algal genus Picochlorum is of biotechnological interest because of its robust response to multiple environmental stresses. We compared the metabolic performance of P. SE3 and P. oklahomense to diverse microbial phototrophs and observed exceptional performance of photosystem II (PSII) in light energy conversion in both Picochlorum species. The quantum yield (QY) for O2 evolution is the highest of any phototroph yet observed, 32% (20%) by P. SE3 (P. okl) when normalized to total PSII subunit PsbA (D1) protein, and 80% (75%) normalized per active PSII, respectively. Three factors contribute: (1) an efficient water oxidizing complex (WOC) with the fewest photochemical misses of any organism; (2) faster reoxidation of reduced (PQH2)B in P. SE3 than in P. okl. (period-2 Fourier amplitude); and (3) rapid reoxidation of the plastoquinol pool by downstream electron carriers (Cyt b6f/PETC) that regenerates PQ faster in P. SE3. This performance gain is achieved without significant residue changes around the QB site and thus points to a pull mechanism involving faster PQH2 reoxidation by Cyt b6f/PETC that offsets charge recombination. This high flux in P. SE3 may be explained by genomically encoded plastoquinol terminal oxidases 1 and 2, whereas P. oklahomense has neither. Our results suggest two distinct types of PSII centers exist, one specializing in linear electron flow and the other in PSII-cyclic electron flow. Several amino acids within D1 differ from those in the low-light-descended D1 sequences conserved in Viridiplantae, and more closely match those in cyanobacterial high-light D1 isoforms, including changes near tyrosine Yz and a water/proton channel near the WOC. These residue changes may contribute to the exceptional performance of Picochlorum at high-light intensities by increasing the water oxidation efficiency and the electron/proton flux through the PSII acceptors (QAQB).

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.

Manipulating the Expression of Glycogen Phosphorylase in Synechococcus elongatus PCC 7942 to Mobilize Glycogen Storage for Sucrose Synthesis

Cyanobacteria are a promising photosynthetic chassis to produce biofuels, biochemicals, and pharmaceuticals at the expense of CO₂ and light energy. Glycogen accumulation represents a universal carbon sink mechanism among cyanobacteria, storing excess carbon and energy from photosynthesis and may compete with product synthesis. Therefore, the glycogen synthesis pathway is often targeted to increase cyanobacterial production of desired carbon-based products. However, these manipulations caused severe physiological and metabolic impairments and often failed to optimize the overall performance of photosynthetic production. Here, in this work, we explored to mobilize the glycogen storage by strengthening glycogen degradation activities. In Synechococcus elongatus PCC 7942, we manipulated the abundances of glycogen phosphorylase (GlgP) with a theophylline dose-responsive riboswitch approach, which holds control over the cyanobacterial glycogen degradation process and successfully regulated the glycogen contents in the recombinant strain. Taking sucrose synthesis as a model, we explored the effects of enhanced glycogen degradation on sucrose production and glycogen storage. It is confirmed that under non-hypersaline conditions, the overexpressed glgP facilitated the effective mobilization of glycogen storage and resulted in increased secretory sucrose production. The findings in this work provided fresh insights into the area of cyanobacteria glycogen metabolism engineering and would inspire the development of novel metabolic engineering approaches for efficient photosynthetic biosynthesis.

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.

Modifying the Cyanobacterial Metabolism as a Key to Efficient Biopolymer Production in Photosynthetic Microorganisms

Cyanobacteria are photoautotrophic bacteria commonly found in the natural environment. Due to the ecological benefits associated with the assimilation of carbon dioxide from the atmosphere and utilization of light energy, they are attractive hosts in a growing number of biotechnological processes. Biopolymer production is arguably one of the most critical areas where the transition from fossil-derived chemistry to renewable chemistry is needed. Cyanobacteria can produce several polymeric compounds with high applicability such as glycogen, polyhydroxyalkanoates, or extracellular polymeric substances. These important biopolymers are synthesized using precursors derived from central carbon metabolism, including the tricarboxylic acid cycle. Due to their unique metabolic properties, i.e., light harvesting and carbon fixation, the molecular and genetic aspects of polymer biosynthesis and their relationship with central carbon metabolism are somehow different from those found in heterotrophic microorganisms. A greater understanding of the processes involved in cyanobacterial metabolism is still required to produce these molecules more efficiently. This review presents the current state of the art in the engineering of cyanobacterial metabolism for the efficient production of these biopolymers.

Enhancing photosynthetic production of glycogen-rich biomass for use as a fermentation feedstock

Current sources of fermentation feedstocks, i.e. corn, sugar cane, or plant biomass, fall short of demand for liquid transportation fuels and commodity chemicals in the United States. Aquatic phototrophs including cyanobacteria have the potential to supplement the supply of current fermentable feedstocks. In this strategy, cells are engineered to accumulate storage molecules including glycogen, cellulose, and/or lipid oils that can be extracted from harvested biomass and fed to heterotrophic organisms engineered to produce desired chemical products. In this manuscript, we examine the production of glycogen in the model cyanobacteria, Synechococcus sp. strain PCC 7002, and subsequent conversion of cyanobacterial biomass by an engineered Escherichia coli to octanoic acid as a model product. In effort to maximize glycogen production, we explored the deletion of catabolic enzymes and overexpression of GlgC, an enzyme that catalyzes the first committed step towards glycogen synthesis. We found that deletion of glgP increased final glycogen titers when cells were grown in diurnal light. Overexpression of GlgC led to a temporal increase in glycogen content but not in an overall increase in final titer or content. The best strains were grown, harvested, and used to formulate media for growth of E. coli. The cyanobacterial media was able to support the growth of an engineered E. coli and produce octanoic acid at the same titer as common laboratory media.

Lethality caused by ADP-glucose accumulation is suppressed by salt-induced carbon flux redirection in cyanobacteria

Cyanobacteria are widely distributed photosynthetic organisms. During the day they store carbon, mainly as glycogen, to provide the energy and carbon source they require for maintenance during the night. Here, we generate a mutant strain of the freshwater cyanobacterium Synechocystis sp. PCC 6803 lacking both glycogen synthases. This mutant has a lethal phenotype due to massive accumulation of ADP-glucose, the substrate of glycogen synthases. This accumulation leads to alterations in its photosynthetic capacity and a dramatic decrease in the adenylate energy charge of the cell to values as low as 0.1. Lack of ADP-glucose pyrophosphorylase, the enzyme responsible for ADP-glucose synthesis, or reintroduction of any of the glycogen synthases abolishes the lethal phenotype. Viability of the glycogen synthase mutant is also fully recovered in NaCl-supplemented medium, which redirects the surplus of ADP-glucose to synthesize the osmolite glucosylglycerol. This alternative metabolic sink also suppresses phenotypes associated with the defective response to nitrogen deprivation characteristic of glycogen-less mutants, restoring the capacity to degrade phycobiliproteins. Thus, our system is an excellent example of how inadequate management of the adenine nucleotide pools results in a lethal phenotype, and the influence of metabolic carbon flux in cell viability and fitness.

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.

Adopting a Theophylline-Responsive Riboswitch for Flexible Regulation and Understanding of Glycogen Metabolism in Synechococcus elongatus PCC7942

Cyanobacteria are supposed to be promising photosynthetic microbial platforms that recycle carbon dioxide driven into biomass and bioproducts by solar energy. Glycogen synthesis serves as an essential natural carbon sink mechanism, storing a large portion of energy and organic carbon source of photosynthesis. Engineering glycogen metabolism to harness and rewire carbon flow is an important strategy to optimize efficacy of cyanobacteria platforms. ADP-glucose pyrophosphorylase (GlgC) catalyzes the rate-limiting step for glycogen synthesis. However, knockout of glgC fails to promote cell growth or photosynthetic production in cyanobacteria, on the contrary, glgC deficiency impairs cellular fitness and robustness. In this work, we adopted a theophylline-responsive riboswitch to engineer and control glgC expression in Synechococcus elongatus PCC7942 and achieved flexible regulation of intracellular GlgC abundance and glycogen storage. With this approach, glycogen synthesis and glycogen contents in PCC7942 cells could be regulated in a range from about 40 to 300% of wild type levels. In addition, the results supported a positive role of glycogen metabolism in cyanobacteria cellular robustness. When glycogen storage was reduced, cellular physiology and growth under standard conditions was not impaired, while cellular tolerance toward environmental stresses was weakened. While when glycogen synthesis was enhanced, cells of PCC7942 displayed optimized cellular robustness. Our findings emphasize the significance of glycogen metabolism for cyanobacterial physiology and the importance of flexible approaches for engineering and understanding cellular physiology and metabolism.

Rerouting of Metabolism into Desired Cellular Products by Nutrient Stress: Fluxes Reveal the Selected Pathways in Cyanobacterial Photosynthesis

Boosting cellular growth rates while redirecting metabolism to make desired products are the preeminent goals of gene engineering of photoautotrophs, yet so far these goals have been hardly achieved owing to lack of understanding of the functional pathways and their choke points. Here we apply a ¹³C mass isotopic method (INST-MFA) to quantify instantaneous fluxes of metabolites during photoautotrophic growth. INST-MFA determines the globally most accurate set of absolute fluxes for each metabolite from a finite set of measured ¹³C-isotopomer fluxes by minimizing the sum of squared residuals between experimental and predicted mass isotopomers. We show that the widely observed shift in biomass composition in cyanobacteria, demonstrated here with Synechococcus sp. PCC 7002, favoring glycogen synthesis during nitrogen starvation is caused by (1) increased flux through a bottleneck step in gluconeogenesis (3PG → GAP/DHAP), and (2) flux overflow through a previously unrecognized hybrid gluconeogenesis-pentose phosphate (hGPP) pathway. Our data suggest the slower growth rate and biomass accumulation under N starvation is due to a reduced carbon fixation rate and a reduced flux of carbon into amino acid precursors. Additionally, ¹³C flux from α-ketoglutarate to succinate is demonstrated to occur via succinic semialdehyde, an alternative to the conventional TCA cycle, in Synechococcus 7002 under photoautotrophic conditions. We found that pyruvate and oxaloacetate are synthesized mainly by malate dehydrogenase with minimal flux into acetyl coenzyme-A via pyruvate dehydrogenase. Nutrient stress induces major shifts in fluxes into new pathways that deviate from historical metabolic pathways derived from model bacteria.

Effects of Reduced and Enhanced Glycogen Pools on Salt-Induced Sucrose Production in a Sucrose-Secreting Strain of Synechococcus elongatus PCC 7942

Sucrose and glycogen syntheses in cyanobacteria share the common precursor glucose-1-phosphate. It is generally assumed that lowering glycogen synthesis could drive more carbon toward sucrose synthesis that can be induced by salt stress among cyanobacteria. By using a theophylline-dependent riboswitch system, the expression of glgC, a key gene in glycogen synthesis, was downregulated in a quantitative manner in a sucrose-secreting strain of Synechococcus elongatus PCC 7942. We observed that the stepwise suppression of glycogen synthesis limited rather than stimulated sucrose production in the salt-stressed cells, suggesting that glycogen could serve as a carbon pool for the synthesis of sucrose. Accordingly, we generated glycogen-overproducing strains, but the increased glycogen pool alone did not stimulate sucrose production, indicating that alternative steps limit the carbon flux toward the synthesis of sucrose. Consistent with previous studies that showed that sucrose-phosphate synthase (SPS) catalyzes the rate-limiting step in sucrose synthesis, the combination of glycogen overproduction and sps overexpression resulted in increased sucrose production. Our results indicate that the glycogen and sucrose pools are closely linked in Synechococcus elongatus PCC 7942, and we propose that enhancing the glycogen pool could be a promising strategy for the improvement of sucrose production by cyanobacteria in the presence of a strong sucrose synthesis sink.IMPORTANCE Many cyanobacteria naturally synthesize and accumulate sucrose when stressed by NaCl, which provides novel possibilities for obtaining sugar feedstock by engineering of cyanobacteria. It has been assumed that glycogen synthesis competes with sucrose synthesis for the carbon flux. However, our results showed that the suppression of glycogen synthesis decreased rather than stimulated sucrose production in a sucrose-secreting strain of Synechococcus elongatus PCC 7942. This result suggests that glycogen could serve as a supportive rather than a competitive carbon pool for the synthesis of sucrose, providing new insights about the relation between glycogen synthesis and sucrose synthesis in cyanobacteria. This finding is also useful to guide metabolic engineering work to optimize the production of sucrose and possibly other products by cyanobacteria.

Rewiring of Cyanobacterial Metabolism for Hydrogen Production: Synthetic Biology Approaches and Challenges

With the demand for renewable energy growing, hydrogen (H₂) is becoming an attractive energy carrier. Developing H₂ production technologies with near-net zero carbon emissions is a major challenge for the "H₂ economy." Certain cyanobacteria inherently possess enzymes, nitrogenases, and bidirectional hydrogenases that are capable of H₂ evolution using sunlight, making them ideal cell factories for photocatalytic conversion of water to H₂. With the advances in synthetic biology, cyanobacteria are currently being developed as a "plug and play" chassis to produce H₂. This chapter describes the metabolic pathways involved and the theoretical limits to cyanobacterial H₂ production and summarizes the metabolic engineering technologies pursued.

Production of Industrial Chemicals from CO2 by Engineering Cyanobacteria

As photosynthetic prokaryotes, cyanobacteria can directly convert CO₂ to organic compounds and grow rapidly using sunlight as the sole source of energy. The direct biosynthesis of chemicals from CO₂ and sunlight in cyanobacteria is therefore theoretically more attractive than using glucose as carbon source in heterotrophic bacteria. To date, more than 20 different target chemicals have been synthesized from CO₂ in cyanobacteria. However, the yield and productivity of the constructed strains is about 100-fold lower than what can be obtained using heterotrophic bacteria, and only a few products reached the gram level. The main bottleneck in optimizing cyanobacterial cell factories is the relative complexity of the metabolism of photoautotrophic bacteria. In heterotrophic bacteria, energy metabolism is integrated with the carbon metabolism, so that glucose can provide both energy and carbon for the synthesis of target chemicals. By contrast, the energy and carbon metabolism of cyanobacteria are separated. First, solar energy is converted into chemical energy and reducing power via the light reactions of photosynthesis. Subsequently, CO₂ is reduced to organic compounds using this chemical energy and reducing power. Finally, the reduced CO₂ provides the carbon source and chemical energy for the synthesis of target chemicals and cell growth. Consequently, the unique nature of the cyanobacterial energy and carbon metabolism determines the specific metabolic engineering strategies required for these organisms. In this chapter, we will describe the specific characteristics of cyanobacteria regarding their metabolism of carbon and energy, summarize and analyze the specific strategies for the production of chemicals in cyanobacteria, and propose metabolic engineering strategies which may be most suitable for cyanobacteria.

Extensive Turnover of Compatible Solutes in Cyanobacteria Revealed by Deuterium Oxide (D2O) Stable Isotope Probing

Cyanobacteria are important primary producers of organic matter in diverse environments on a global scale. While mechanisms of CO₂ fixation are well understood, the distribution of the flow of fixed organic carbon within individual cells and complex microbial communities is less well characterized. To obtain a general overview of metabolism, we describe the use of deuterium oxide (D₂O) to measure deuterium incorporation into the intracellular metabolites of two physiologically diverse cyanobacteria: a terrestrial filamentous strain (Microcoleus vaginatus PCC 9802) and a euryhaline unicellular strain (Synechococcus sp. PCC 7002). D₂O was added to the growth medium during different phases of the diel cycle. Incorporation of deuterium into metabolites at nonlabile positions, an indicator of metabolite turnover, was assessed using liquid chromatography mass spectrometry. Expectedly, large differences in turnover among metabolites were observed. Some metabolites, such as fatty acids, did not show significant turnover over 12-24 h time periods but did turn over during longer time periods. Unexpectedly, metabolites commonly regarded to act as compatible solutes, including glutamate, glucosylglycerol, and a dihexose, showed extensive turnover compared to most other metabolites already after 12 h, but only during the light phase in the cycle. The observed extensive turnover is surprising considering the conventional view on compatible solutes as biosynthetic end points given the relatively slow growth and constant osmotic conditions. This suggests the possibility of a metabolic sink for some compatible solutes (e.g., into glycogen) that allows for rapid modulation of intracellular osmolarity. To investigate this, uniformly ¹³C-labeled Synechococcus sp. PCC 7002 were exposed to ¹²C glucosylglycerol. Following metabolite extraction, amylase treatment of methanol-insoluble polymers revealed ¹²C labeling of glycogen. Overall, our work shows that D₂O probing is a powerful method for analysis of cyanobacterial metabolism including discovery of novel metabolic processes.

Flux balance analysis of photoautotrophic metabolism: Uncovering new biological details of subsystems involved in cyanobacterial photosynthesis

We have constructed and experimentally tested a comprehensive genome-scale model of photoautotrophic growth, denoted iSyp821, for the cyanobacterium Synechococcus sp. PCC 7002. iSyp821 incorporates a variable biomass objective function (vBOF), in which stoichiometries of the major biomass components vary according to light intensity. The vBOF was constrained to fit the measured cellular carbohydrate/protein content under different light intensities. iSyp821 provides rigorous agreement with experimentally measured cell growth rates and inorganic carbon uptake rates as a function of light intensity. iSyp821 predicts two observed metabolic transitions that occur as light intensity increases: 1) from PSI-cyclic to linear electron flow (greater redox energy), and 2) from carbon allocation as proteins (growth) to carbohydrates (energy storage) mode. iSyp821 predicts photoautotrophic carbon flux into 1) a hybrid gluconeogenesis-pentose phosphate (PP) pathway that produces glycogen by an alternative pathway than conventional gluconeogenesis, and 2) the photorespiration pathway to synthesize the essential amino acid, glycine. Quantitative fluxes through both pathways were verified experimentally by following the kinetics of formation of ¹³C metabolites from ¹³CO₂ fixation. iSyp821 was modified to include changes in gene products (enzymes) from experimentally measured transcriptomic data and applied to estimate changes in concentrations of metabolites arising from nutrient stress. Using this strategy, we found that iSyp821 correctly predicts the observed redistribution pattern of carbon products under nitrogen depletion, including decreased rates of CO₂ uptake, amino acid synthesis, and increased rates of glycogen and lipid synthesis.

High-level intra- and extra-cellular production of d-psicose 3-epimerase via a modified xylose-inducible expression system in Bacillus subtilis

d-Psicose 3-epimerase (DPEase) converts d-fructose into d-psicose which exists in nature in limited quantities and has key physiological functions. In this study, RDPE (DPEase from Ruminococcus sp. 5_1_39BFAA) was successfully constitutively expressed in Bacillus subtilis, which is the first report of its kind. Three sugar-inducible promoters were compared, and the xylose-inducible promoter PxylA was proved to be the most efficient for RDPE production. Based on the analysis of the inducer concentration and RDPE expression, we surmised that there was an extremely close correlation between the intracellular RDPE expression and xylose accumulation level. Subsequently, after the metabolic pathway of xylose was blocked by deletion of xylAB, the intra- and extra-cellular RDPE expression was significantly enhanced. Meanwhile, the optimal xylose induction concentration was reduced from 4.0 to 0.5 %. Eventually, the secretion level of RDPE reached 95 U/mL and 2.6 g/L in a 7.5-L fermentor with the fed-batch fermentation, which is the highest production of DPEase by a microbe to date.

Inactivation of nitrate reductase alters metabolic branching of carbohydrate fermentation in the cyanobacterium Synechococcus sp. strain PCC 7002

To produce cellular energy, cyanobacteria reduce nitrate as the preferred pathway over proton reduction (H2 evolution) by catabolizing glycogen under dark anaerobic conditions. This competition lowers H2 production by consuming a large fraction of the reducing equivalents (NADPH and NADH). To eliminate this competition, we constructed a knockout mutant of nitrate reductase, encoded by narB, in Synechococcus sp. PCC 7002. As expected, ΔnarB was able to take up intracellular nitrate but was unable to reduce it to nitrite or ammonia, and was unable to grow photoautotrophically on nitrate. During photoautotrophic growth on urea, ΔnarB significantly redirects biomass accumulation into glycogen at the expense of protein accumulation. During subsequent dark fermentation, metabolite concentrations--both the adenylate cellular energy charge (∼ATP) and the redox poise (NAD(P)H/NAD(P))--were independent of nitrate availability in ΔnarB, in contrast to the wild type (WT) control. The ΔnarB strain diverted more reducing equivalents from glycogen catabolism into reduced products, mainly H2 and d-lactate, by 6-fold (2.8% yield) and 2-fold (82.3% yield), respectively, than WT. Continuous removal of H2 from the fermentation medium (milking) further boosted net H2 production by 7-fold in ΔnarB, at the expense of less excreted lactate, resulting in a 49-fold combined increase in the net H2 evolution rate during 2 days of fermentation compared to the WT. The absence of nitrate reductase eliminated the inductive effect of nitrate addition on rerouting carbohydrate catabolism from glycolysis to the oxidative pentose phosphate (OPP) pathway, indicating that intracellular redox poise and not nitrate itself acts as the control switch for carbon flux branching between pathways.

Genetic manipulation of a metabolic enzyme and a transcriptional regulator increasing succinate excretion from unicellular cyanobacterium

Succinate is a building block compound that the U.S. Department of Energy (DOE) has declared as important in biorefineries, and it is widely used as a commodity chemical. Here, we identified the two genes increasing succinate production of the unicellular cyanobacterium Synechocystis sp. PCC 6803. Succinate was excreted under dark, anaerobic conditions, and its production level increased by knocking out ackA, which encodes an acetate kinase, and by overexpressing sigE, which encodes an RNA polymerase sigma factor. Glycogen catabolism and organic acid biosynthesis were enhanced in the mutant lacking ackA and overexpressing sigE, leading to an increase in succinate production reaching five times of the wild-type levels. Our genetic and metabolomic analyses thus demonstrated the effect of genetic manipulation of a metabolic enzyme and a transcriptional regulator on succinate excretion from this cyanobacterium with the data based on metabolomic technique.

Dynamics of Photosynthesis in a Glycogen-Deficient glgC Mutant of Synechococcus sp. Strain PCC 7002

Cyanobacterial glycogen-deficient mutants display impaired degradation of light-harvesting phycobilisomes under nitrogen-limiting growth conditions and secrete a suite of organic acids as a putative reductant-spilling mechanism. This genetic background, therefore, represents an important platform to better understand the complex relationships between light harvesting, photosynthetic electron transport, carbon fixation, and carbon/nitrogen metabolisms. In this study, we conducted a comprehensive analysis of the dynamics of photosynthesis as a function of reductant sink manipulation in a glycogen-deficient glgC mutant of Synechococcus sp. strain PCC 7002. The glgC mutant showed increased susceptibility to photoinhibition during the initial phase of nitrogen deprivation. However, after extended periods of nitrogen deprivation, glgC mutant cells maintained higher levels of photosynthetic activity than the wild type, supporting continuous organic acid secretion in the absence of biomass accumulation. In contrast to the wild type, the glgC mutant maintained efficient energy transfer from phycobilisomes to photosystem II (PSII) reaction centers, had an elevated PSII/PSI ratio as a result of reduced PSII degradation, and retained a nitrogen-replete-type ultrastructure, including an extensive thylakoid membrane network, after prolonged nitrogen deprivation. Together, these results suggest that multiple global signals for nitrogen deprivation are not activated in the glgC mutant, allowing the maintenance of active photosynthetic complexes under conditions where photosynthesis would normally be abolished.

High-yield production of extracellular type-I cellulose by the cyanobacterium Synechococcus sp. PCC 7002

Cellulose synthase, encoded by the cesA gene, is responsible for the synthesis of cellulose in nature. We show that the cell wall of the cyanobacterium Synechococcus sp. PCC 7002 naturally contains cellulose. Cellulose occurs as a possibly laminated layer between the inner and outer membrane, as well as being an important component of the extracellular glycocalyx in this cyanobacterium. Overexpression of six genes, cmc–ccp–cesAB–cesC–cesD–bgl, from Gluconacetobacter xylinus in Synechococcus sp. PCC 7002 resulted in very high-yield production of extracellular type-I cellulose. High-level cellulose production only occurred when the native cesA gene was inactivated and when cells were grown at low salinity. This system provides a method for the production of lignin-free cellulose from sunlight and CO₂ for biofuel production and other biotechnological applications.

Lauric Acid Production in a Glycogen-Less Strain of Synechococcus sp. PCC 7002

The cyanobacterium Synechococcus sp. Pasteur culture collection 7002 was genetically engineered to synthesize biofuel-compatible medium-chain fatty acids (FAs) during photoautotrophic growth. Expression of a heterologous lauroyl-acyl carrier protein (C12:0-ACP) thioesterase with concurrent deletion of the endogenous putative acyl-ACP synthetase led to secretion of transesterifiable C12:0 FA in CO2-supplemented batch cultures. When grown at steady state over a range of light intensities in a light-emitting diode turbidostat photobioreactor, the C12-secreting mutant exhibited a modest reduction in growth rate and increased O2 evolution relative to the wild-type (WT). Inhibition of (i) glycogen synthesis by deletion of the glgC-encoded ADP-glucose pyrophosphorylase (AGPase) and (ii) protein synthesis by nitrogen deprivation were investigated as potential mechanisms for metabolite redistribution to increase FA synthesis. Deletion of AGPase led to a 10-fold decrease in reducing carbohydrates and secretion of organic acids during nitrogen deprivation consistent with an energy spilling phenotype. When the carbohydrate-deficient background (ΔglgC) was modified for C12 secretion, no increase in C12 was achieved during nutrient replete growth, and no C12 was recovered from any strain upon nitrogen deprivation under the conditions used. At steady state, the growth rate of the ΔglgC strain saturated at a lower light intensity than the WT, but O2 evolution was not compromised and became increasingly decoupled from growth rate with rising irradiance. Photophysiological properties of the ΔglgC strain suggest energy dissipation from photosystem II and reconfiguration of electron flow at the level of the plastoquinone pool.

Engineering cyanobacteria as photosynthetic feedstock factories

Carbohydrate feedstocks are at the root of bioindustrial production and are needed in greater quantities than ever due to increased prioritization of renewable fuels with reduced carbon footprints. Cyanobacteria possess a number of features that make them well suited as an alternative feedstock crop in comparison to traditional terrestrial plant species. Recent advances in genetic engineering, as well as promising preliminary investigations of cyanobacteria in a number of distinct production regimes have illustrated the potential of these aquatic phototrophs as biosynthetic chassis. Further improvements in strain productivities and design, along with enhanced understanding of photosynthetic metabolism in cyanobacteria may pave the way to translate cyanobacterial theoretical potential into realized application.

Comparative genomic and phylogenetic analyses of Gammaproteobacterial glg genes traced the origin of the Escherichia coli glycogen glgBXCAP operon to the last common ancestor of the sister orders Enterobacteriales and Pasteurellales

Production of branched α-glucan, glycogen-like polymers is widely spread in the Bacteria domain. The glycogen pathway of synthesis and degradation has been fairly well characterized in the model enterobacterial species Escherichia coli (order Enterobacteriales, class Gammaproteobacteria), in which the cognate genes (branching enzyme glgB, debranching enzyme glgX, ADP-glucose pyrophosphorylase glgC, glycogen synthase glgA, and glycogen phosphorylase glgP) are clustered in a glgBXCAP operon arrangement. However, the evolutionary origin of this particular arrangement and of its constituent genes is unknown. Here, by using 265 complete gammaproteobacterial genomes we have carried out a comparative analysis of the presence, copy number and arrangement of glg genes in all lineages of the Gammaproteobacteria. These analyses revealed large variations in glg gene presence, copy number and arrangements among different gammaproteobacterial lineages. However, the glgBXCAP arrangement was remarkably conserved in all glg-possessing species of the orders Enterobacteriales and Pasteurellales (the E/P group). Subsequent phylogenetic analyses of glg genes present in the Gammaproteobacteria and in other main bacterial groups indicated that glg genes have undergone a complex evolutionary history in which horizontal gene transfer may have played an important role. These analyses also revealed that the E/P glgBXCAP genes (a) share a common evolutionary origin, (b) were vertically transmitted within the E/P group, and (c) are closely related to glg genes of some phylogenetically distant betaproteobacterial species. The overall data allowed tracing the origin of the E. coli glgBXCAP operon to the last common ancestor of the E/P group, and also to uncover a likely glgBXCAP transfer event from the E/P group to particular lineages of the Betaproteobacteria.

Salt acclimation of cyanobacteria and their application in biotechnology

The long evolutionary history and photo-autotrophic lifestyle of cyanobacteria has allowed them to colonize almost all photic habitats on Earth, including environments with high or fluctuating salinity. Their basal salt acclimation strategy includes two principal reactions, the active export of ions and the accumulation of compatible solutes. Cyanobacterial salt acclimation has been characterized in much detail using selected model cyanobacteria, but their salt sensing and regulatory mechanisms are less well understood. Here, we briefly review recent advances in the identification of salt acclimation processes and the essential genes/proteins involved in acclimation to high salt. This knowledge is of increasing importance because the necessary mass cultivation of cyanobacteria for future use in biotechnology will be performed in sea water. In addition, cyanobacterial salt resistance genes also can be applied to improve the salt tolerance of salt sensitive organisms, such as crop plants.

HPLC-MS/MS analyses show that the near-Starchless aps1 and pgm leaves accumulate wild type levels of ADPglucose: further evidence for the occurrence of important ADPglucose biosynthetic pathway(s) alternative to the pPGI-pPGM-AGP pathway

In leaves, it is widely assumed that starch is the end-product of a metabolic pathway exclusively taking place in the chloroplast that (a) involves plastidic phosphoglucomutase (pPGM), ADPglucose (ADPG) pyrophosphorylase (AGP) and starch synthase (SS), and (b) is linked to the Calvin-Benson cycle by means of the plastidic phosphoglucose isomerase (pPGI). This view also implies that AGP is the sole enzyme producing the starch precursor molecule, ADPG. However, mounting evidence has been compiled pointing to the occurrence of important sources, other than the pPGI-pPGM-AGP pathway, of ADPG. To further explore this possibility, in this work two independent laboratories have carried out HPLC-MS/MS analyses of ADPG content in leaves of the near-starchless pgm and aps1 mutants impaired in pPGM and AGP, respectively, and in leaves of double aps1/pgm mutants grown under two different culture conditions. We also measured the ADPG content in wild type (WT) and aps1 leaves expressing in the plastid two different ADPG cleaving enzymes, and in aps1 leaves expressing in the plastid GlgC, a bacterial AGP. Furthermore, we measured the ADPG content in ss3/ss4/aps1 mutants impaired in starch granule initiation and chloroplastic ADPG synthesis. We found that, irrespective of their starch contents, pgm and aps1 leaves, WT and aps1 leaves expressing in the plastid ADPG cleaving enzymes, and aps1 leaves expressing in the plastid GlgC accumulate WT ADPG content. In clear contrast, ss3/ss4/aps1 leaves accumulated ca. 300 fold-more ADPG than WT leaves. The overall data showed that, in Arabidopsis leaves, (a) there are important ADPG biosynthetic pathways, other than the pPGI-pPGM-AGP pathway, (b) pPGM and AGP are not major determinants of intracellular ADPG content, and (c) the contribution of the chloroplastic ADPG pool to the total ADPG pool is low.

Engineering Limonene and Bisabolene Production in Wild Type and a Glycogen-Deficient Mutant of Synechococcus sp. PCC 7002

The plant terpenoids limonene (C₁₀H₁₆) and α-bisabolene (C₁₅H₂₄) are hydrocarbon precursors to a range of industrially relevant chemicals. High-titer microbial synthesis of limonene and α-bisabolene could pave the way for advances in in vivo engineering of tailor-made hydrocarbons, and production at commercial scale. We have engineered the fast-growing unicellular euryhaline cyanobacterium Synechococcus sp. PCC 7002 to produce yields of 4 mg L⁻¹ limonene and 0.6 mg L⁻¹ α-bisabolene through heterologous expression of the Mentha spicatal-limonene synthase or the Abies grandis (E)-α-bisabolene synthase genes, respectively. Titers were significantly higher when a dodecane overlay was applied during culturing, suggesting either that dodecane traps large quantities of volatile limonene or α-bisabolene that would otherwise be lost to evaporation, and/or that continuous product removal in dodecane alleviates product feedback inhibition to promote higher rates of synthesis. We also investigate limonene and bisabolene production in the ΔglgC genetic background, where carbon partitioning is redirected at the expense of glycogen biosynthesis. The Synechococcus sp. PCC 7002 ΔglgC mutant excreted a suite of overflow metabolites (α-ketoisocaproate, pyruvate, α-ketoglutarate, succinate, and acetate) during nitrogen-deprivation, and also at the onset of stationary growth in nutrient-replete media. None of the excreted metabolites, however, appeared to be effectively utilized for terpenoid metabolism. Interestingly, we observed a 1.6- to 2.5-fold increase in the extracellular concentration of most excreted organic acids when the ΔglgC mutant was conferred with the ability to produce limonene. Overall, Synechococcus sp. PCC 7002 provides a highly promising platform for terpenoid biosynthetic and metabolic engineering efforts.

Cyanobacterial biomass as carbohydrate and nutrient feedstock for bioethanol production by yeast fermentation

BACKGROUND

Microbial bioconversion of photosynthetic biomass is a promising approach to the generation of biofuels and other bioproducts. However, rapid, high-yield, and simple processes are essential for successful applications. Here, biomass from the rapidly growing photosynthetic marine cyanobacterium Synechococcus sp. PCC 7002 was fermented using yeast into bioethanol.

RESULTS

The cyanobacterium accumulated a total carbohydrate content of about 60% of cell dry weight when cultivated under nitrate limitation. The cyanobacterial cells were harvested by centrifugation and subjected to enzymatic hydrolysis using lysozyme and two alpha-glucanases. This enzymatic hydrolysate was fermented into ethanol by Saccharomyces cerevisiae without further treatment. All enzyme treatments and fermentations were carried out in the residual growth medium of the cyanobacteria with the only modification being that pH was adjusted to the optimal value. The highest ethanol yield and concentration obtained was 0.27 g ethanol per g cell dry weight and 30 g ethanol L(-1), respectively. About 90% of the glucose in the biomass was converted to ethanol. The cyanobacterial hydrolysate was rapidly fermented (up to 20 g ethanol L(-1) day(-1)) even in the absence of any other nutrient additions to the fermentation medium.

CONCLUSIONS

Cyanobacterial biomass was hydrolyzed using a simple enzymatic treatment and fermented into ethanol more rapidly and to higher concentrations than previously reported for similar approaches using cyanobacteria or microalgae. Importantly, as well as fermentable carbohydrates, the cyanobacterial hydrolysate contained additional nutrients that promoted fermentation. This hydrolysate is therefore a promising substitute for the relatively expensive nutrient additives (such as yeast extract) commonly used for Saccharomyces fermentations.

Glycogen production for biofuels by the euryhaline cyanobacteria Synechococcus sp. strain PCC 7002 from an oceanic environment

BACKGROUND

Oxygenic photosynthetic microorganisms such as cyanobacteria and microalgae have attracted attention as an alternative carbon source for the next generation of biofuels. Glycogen abundantly accumulated in cyanobacteria is a promising feedstock which can be converted to ethanol through saccharification and fermentation processes. In addition, the utilization of marine cyanobacteria as a glycogen producer can eliminate the need for a freshwater supply. Synechococcus sp. strain PCC 7002 is a fast-growing marine coastal euryhaline cyanobacteria, however, the glycogen yield has not yet been determined. In the present study, the effects of light intensity, CO2 concentration, and salinity on the cell growth and glycogen content were investigated in order to maximize glycogen production in Synechococcus sp. strain PCC 7002.

RESULTS

The optimal culture conditions for glycogen production in Synechococcus sp. strain PCC 7002 were investigated. The maximum glycogen production of 3.5 g L(-1) for 7 days (a glycogen productivity of 0.5 g L(-1) d(-1)) was obtained under a high light intensity, a high CO2 level, and a nitrogen-depleted condition in brackish water. The glycogen production performance in Synechococcus sp. strain PCC 7002 was the best ever reported in the α-polyglucan (glycogen or starch) production of cyanobacteria and microalgae. In addition, the robustness of glycogen production in Synechococcus sp. strain PCC 7002 to salinity was evaluated in seawater and freshwater. The peak of glycogen production of Synechococcus sp. strain PCC 7002 in seawater and freshwater were 3.0 and 1.8 g L(-1) in 7 days, respectively. Glycogen production in Synechococcus sp. strain PCC 7002 maintained the same level in seawater and half of the level in freshwater compared with the optimal result obtained in brackish water.

CONCLUSIONS

We conclude that Synechococcus sp. strain PCC 7002 has high glycogen production activity and glycogen can be provided from coastal water accompanied by a fluctuation of salinity. This work supports Synechococcus sp. strain PCC 7002 as a promising carbohydrate source for biofuel production.