Glycogen content and nitrogenase activity in Anabaena variabilis

Two-Stage System for Hydrogen Production by Immobilized CyanobacteriumGloeocapsa alpicolaCALU 743

Previous studies showed that cell suspensions of unicellular nondiazotrophic cyanobacterium G. alpicola grown under nitrate-limiting conditions intensively produces H2 via fermentation of endogenous glycogen with hydrogen yield more then 90% of theoretical maximum (3.8 mol H2 per mol glucose). H2 production is realized by a Hox hydrogenase on the stages of NAD(P)H generation. Exploiting this property, the two-stage cyclic system for sustained hydrogen production was developed using a photobioreactor (PhBR) with G. alpicola immobilized on glass fiber TR-0.3. Immobilization of the cells on the matrix occurred during growth directly in PhBR operated in continuous mode; the density of culture immobilized achieved 37 g Chl alpha cm(-2). The first stage of the cycle was the photosynthetic incubations of G. alpicola in the flow of the culture medium, which contained limiting concentrations of nitrate for efficient glycogen accumulation and activation of hydrogenase. The second stage was the fermentation of glycogen, with H2 production realized in darkness with continuous Ar sparging and without medium flow. Standardization of optimal parameters for both stages provided a stable cyclic regime of the system: photosynthesis (24 hours)-fermentation (24 hours). The total amount of H2 evolved in one cycle was 957.6 mL L(-1)(matrix), and the overage rate of H2 production during the cycle (48 hours) was about 20 mL h(-1) L(-1)(matrix). Ten consequent cycles was carried out in this regime with reproducible H2 production, although PhBR with the same sample of immobilized culture was operated over a period of more then three months.

High photobiological hydrogen production activity of a Nostoc sp. PCC 7422 uptake hydrogenase-deficient mutant with high nitrogenase activity

We describe a strategy to establish cyanobacterial strains with high levels of H(2) production that involves the identification of promising wild-type strains followed by optimization of the selected strains using genetic engineering. Nostoc sp. PCC 7422 was chosen from 12 other heterocystous strains, because it has the highest nitrogenase activity. We sequenced the uptake hydrogenase (Hup) gene cluster as well as the bidirectional hydrogenase gene cluster from the strain, and constructed a mutant (Delta hupL) by insertional disruption of the hupL gene. The Delta hupL mutant produced H(2) at 100 mumoles mg chlorophyll a (-1) h(-1), a rate three times that of the wild-type. The Delta hupL cells could accumulate H(2) to about 29% (v/v) accompanied by O(2) evolution in 6 days, under a starting gas phase of Ar + 5% CO(2). The presence of 20% O(2) in the initial gas phase inhibited H(2) accumulation of the Delta hupL cells by less than 20% until day 7.

Production of H2 by sulphur-deprived cells of the unicellular cyanobacteria Gloeocapsa alpicola and Synechocystis sp. PCC 6803 during dark incubation with methane or at various extracellular pH

AIMS

To examine sulphur (S) deprivation in combination with the presence of methane (CH4) and changes in extracellular pH as a method to enhance in situ hydrogen (H2) generation during fermentation in the unicellular non-diazotrophic cyanobacteria Gloeocapsa alpicola and Synechocystis PCC 6803.

METHODS AND RESULTS

The level of H2 production, measured using a gas chromatography, was determined in S-deprived cells of G. alpicola and Synechocystis PCC 6803 during fermentation. Starvation on S enhanced the rate of H2 production by more than fourfold in both strains. S-deprived cyanobacteria were able to maintain maximum rate of H2 production during at least 8 h of fermentation representing the entire dark period of a day. Increased H2 production was observed during dark anoxic incubation with a gas phase of 100% CH4 (up to four times) at lower pH of the medium (5.0-5.5).

CONCLUSIONS

S-deprivation in combination with CH4, added or maybe produced by another micro-organisms, and changes in the pH of the media can be used to further increase the specific capacity of unicellular non-N2-fixing cyanobacteria to produce H2 during fermentation with the overall aim of applying it for outdoor photobiological H2 production.

SIGNIFICANCE AND IMPACT OF THE STUDY

S-deprivation with respect to H2 production is well studied in the green algae Chlamydomonas reinhardtii while its application for H2 production in cyanobacteria is novel. Similarly, the stimulation of H2 generation in the presence of CH4 opens up new possibilities to increase the H2 production. Natural gas enriched with H2 seems to be a perspective fuel and may be an intermediate step on the pathway to the exploitation of pure biohydrogen.

FesM, a membrane iron-sulfur protein, is required for cyclic electron flow around photosystem I and photoheterotrophic growth of the cyanobacterium Synechococcus sp. PCC 7002

While it is known that cyclic electron flow around photosystem I is an important pathway of photosynthetic electron transfer for converting light energy to chemical energy, some components of cyclic electron flow remain to be revealed. Here, we show that fesM, encoding a novel membrane iron-sulfur protein, is essential to cyclic electron flow in the cyanobacterium Synechococcus sp. PCC 7002. The FesM protein is predicted to have a cAMP-binding domain, an NtrC-like domain, a redox sensor motif, and an iron-sulfur (4Fe-4S) motif. Deletion of fesM (fesM-D) led to an inability for Synechococcus cells to grow in the presences of glycerol and 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Photoheterotrophic growth was restored by a complete fesM gene present on a replicable plasmid. A mutant fesM gene encoding a truncated FesM protein lacking the cAMP domain failed to restore the phenotype, suggesting this domain is important to the function of FesM. Measurements of reduction of P700(+) and the redox state of interphotosystem electron carriers showed that cells had a slower rate of respiratory electron donation to the interphotosystem electron transport chain, and cyclic electron flow around photosystem I in fesM-D was impaired, suggesting that FesM is a critical protein for respiratory and cyclic electron flow. Phosphatase fusion analysis showed that FesM contains nine membrane-spanning helices, and all functional domains of FesM are located on the cytoplasmic face of the thylakoid membranes.

Role of polyhydroxybutyrate and glycogen as carbon storage compounds in pea and bean bacteroids

Rhizobium leguminosarum synthesizes polyhydroxybutyrate and glycogen as its main carbon storage compounds. To examine the role of these compounds in bacteroid development and in symbiotic efficiency, single and double mutants of R. leguminosarum bv. viciae were made which lack polyhydroxybutyrate synthase (phaC), glycogen synthase (glgA), or both. For comparison, a single phaC mutant also was isolated in a bean-nodulating strain of R. leguminosarum bv. phaseoli. In one large glasshouse trial, the growth of pea plants inoculated with the R. leguminosarum bv. viciae phaC mutant were significantly reduced compared with wild-type-inoculated plants. However, in subsequent glasshouse and growth-room studies, the growth of pea plants inoculated with the mutant were similar to wildtype-inoculated plants. Bean plants were unaffected by the loss of polyhydroxybutyrate biosynthesis in bacteroids. Pea plants nodulated by a glycogen synthase mutant, or the glgA/phaC double mutant, grew as well as the wild type in growth-room experiments. Light and electron micrographs revealed that pea nodules infected with the glgA mutant accumulated large amounts of starch in the II/III interzone. This suggests that glycogen may be the dominant carbon storage compound in pea bacteroids. Polyhydroxybutyrate was present in bacteria in the infection thread of pea plants but was broken down during bacteroid formation. In nodules infected with a phaC mutant of R. leguminosarum bv. viciae, there was a drop in the amount of starch in the II/III interzone, where bacteroids form. Therefore, we propose a carbon burst hypothesis for bacteroid formation, where polyhydroxybutyrate accumulated by bacteria is degraded to fuel bacteroid differentiation.

Structure and functional analysis of the microbial community in an aerobic: anaerobic sequencing batch reactor (SBR) with no phosphorus removal

The bacterial community of an aerobic:anaerobic non-P removing SBR biomass fed a mixture of acetate and glucose was analysed using several 16S rRNA based methods. Populations responsible for anaerobic glucose and acetate assimilation were determined with fluorescent in situ hybridization (FISH) in combination with microautoradiography (FISH/MAR). At 'steady state' this community consisted of alpha-Proteobacteria (26%) and gamma-Proteobacteria (14%), mainly appearing as large cocci in tetrads (i.e. typical 'G-Bacteria'). Large numbers of low G+C bacteria (22%), and high G+C Gram-positive bacteria (29%) seen as small cocci in clusters or in sheets were also detected after FISH. DGGE fingerprinting of PCR amplified 16S rDNA fragments and subsequent cloning and sequencing of several of the major bands led to the identification of some of these populations. They included an organism 98% similar in its 16S rRNA sequence to Micropruina glycogenica, and ca. 76% of the high G+C bacteria responded to a probe MIC 184, designed against it. The rest responded to the KSB 531 probe designed against a high G+C clone sequence, sbr-gs28 reported in other similar systems. FISH analyses showed that both these high G+C populations were almost totally dominated by small clustered cocci. Only ca. 2% of cells were beta-Proteobacteria. None of the alpha- and gamma-Proteobacterial 'G-bacteria' responded to FISH probes designed for the 'G-Bacteria' Amaricoccus spp. or Defluvicoccus vanus. FISH/MAR revealed that not all the alpha-Proteobacterial 'G-Bacteria' could take up acetate or glucose anaerobically. Almost all of the gamma-Proteobacterial 'G-Bacteria' assimilated acetate anaerobically but not glucose, the low G+C clustered cocci only took up glucose, whereas the high G+C bacteria including M. glycogenica and the sbr-gs28 clone assimilated both acetate and glucose. All bacteria other than the low G+C small cocci and a few of the alpha-Proteobacteria accumulated PHB. The low G+C bacteria showing anaerobic glucose assimilation ability were considered responsible for the lactic acid produced anaerobically by this SBR biomass, and M. glycogenica for its high glycogen content.

Nitrogenase activity in cyanobacteria measured by the acetylene reduction assay: a comparison between batch incubation and on-line monitoring

A new on-line method for measuring acetylene reduction is described. It consists of a gas-flow cell connected to an electronic gas-mixing system and an automatic sample loop in the gas chromatograph. Alternatively, ethylene can be determined by using laser-based trace gas detection. The laser-based trace gas detection technique achieves a detection limit that is three orders of magnitude better than gas chromatography. We have applied the on-line method to the measurement of nitrogen fixation in a culture of the heterocystous cyanobacterium Nodularia spumigena and compared it with conventional batch-type incubations. Incubation of N. spumigena in the gas-flow cell resulted in very short response times with a steady-state flux of ethylene obtained within 2 min. Nitrogenase was shown to respond immediately to changes in light and oxygen. Monitoring of nitrogenase activity could be continued for several hours without having a negative impact on nitrogen fixation rates in N. spumigena. This was not the case in batch incubations, in which changes in nitrogenase activities were recorded during incubations, probably as a result of varying oxygen concentrations. It was therefore concluded that the on-line method is superior to batch incubations when rates of nitrogenase activity are to be measured. The method is suitable for natural samples (water or sediment).

H(2) photoproduction by batch culture of Anabaena variabilis ATCC 29413 and its mutant PK84 in a photobioreactor

Hydrogen production by Anabaena variabilis ATCC 29413 and of its mutant PK84, grown in batch cultures, was studied in a photobioreactor. The highest volumetric H(2) production rates of native and mutant strains were found in cultures grown at gradually increased irradiation. The native strain evolved H(2) only under an argon atmosphere with the actual rate as high as the potential rate (measured in small vials under optimal conditions). In this case 61% of oxygenic photosynthesis was used for H(2) production. In contrast the mutant PK84 produced H(2) during growth under CO(2)-enriched air. Under these conditions at the maximum rate of H(2) production (10 mL h(-1) L(-1)), 13% of oxygenic photosynthesis was used for H(2) production and the actual H(2) production was only 33% of the potential. Under an atmosphere of 98% argon + 2% CO(2) actual H(2) production by mutant PK84 was 85% of the potential rate and 66% of oxygenic photosynthesis was used for H(2) production. Hydrogen production under argon + CO(2) by the mutant was strictly light-dependent with saturation at about 300 microE m(-2) s(-1). However, the rate of photosynthesis was not saturated at this irradiation. At limiting light intensities (below 250 microE m(-2) s(-1)) 33-58% of photosynthesis was used for H(2) production. Hydrogen evolution by PK84 under air + 2% CO(2) was also stimulated by light; but was not saturated at 332 microE m(-2) s(-1) and did not cease completely in darkness. The rate of oxygen photoevolution was also not saturated. A mechanism for increasing cyanobacterial hydrogen production is proposed.

The DevBCA exporter is essential for envelope formation in heterocysts of the cyanobacterium Anabaena sp. strain PCC 7120

The gene devA of the filamentous heterocyst-form-ing cyanobacterium Anabaena sp. strain PCC 7120 encodes a protein with high similarity to ATP-binding cassettes of ABC transporters. Mutant M7 defective in the devA gene is arrested in the development of heterocysts at an early stage and is not able to fix N2 under aerobic conditions. The devA gene is differentially expressed in heterocysts. To gain a better understanding of the structural components of this putative ABC transporter, we determined the complete nucleotide sequence of the entire gene cluster. The two additional genes, named devB and devC, encode proteins with similarities to membrane fusion proteins (DevB) of several ABC exporters and to membrane-spanning proteins (DevC) of ABC transporters in general. Site-directed mutations in each of the three genes resulted in identical phenotypes. Heterocyst-specific glycolipids forming the laminated layer of the envelope were identified in lipid extracts of M7 and in the site-directed mutants. However, transmission electron microscopy revealed unequivocally that the glycolipid layer is missing in mutant M7. Ultrastructural analysis also confirmed a developmental block at an early stage of differentiation. The results of this study suggest that the devBCA operon encodes an exporter of glycolipids or of an enzyme that is necessary for the formation of the laminated layer. The hypothesis is proposed that an intact envelope could be required for further heterocyst differentiation.

Isolation and characterization of multiple adenylate cyclase genes from the cyanobacterium Anabaena sp. strain PCC 7120

Adenylate cyclase genes, designated cyaA, cyaB1, cyaB2, cyaC, and cyaD, were isolated from the filamentous cyanobacterium Anabaena sp. strain PCC 7120 by complementation of a strain of Escherichia coli defective for the presence of cya. These genes encoded polypeptides consisting of 735, 859, 860, 1,155, and 546 amino acid residues, respectively. Deduced amino acid sequences of the regions near the C-terminal ends of these cya genes were similar to those of catalytic domains of eukaryotic adenylate cyclases. The remaining part of each cya gene towards its N-terminal end showed a characteristic structure. CyaA had two putative membrane-spanning regions. Both CyaB1 and CyaB2 had regions that were very similar to the cyclic GMP (cGMP)-binding domain of cGMP-stimulated cGMP phosphodiesterase. CyaC consisted of four distinct domains forming sequentially from the N terminus: a response regulator-like domain, a histidine kinase-like domain, a response regulator-like domain, and the catalytic domain of adenylate cyclase. CyaD contained the forkhead-associated domain in its N-terminal region. Expression of these genes was examined by reverse transcription-PCR. The transcript of cyaC was shown to be predominant in this cyanobacterium. The cellular cyclic AMP level in the disruptant of the cyaC mutant was much lower than that in the wild type.

Photodissimilation of Fructose to H(inf2) and CO(inf2) by a Dinitrogen-Fixing Cyanobacterium, Anabaena variabilis

The ability of cyanobacteria to serve as biocatalysts in the production of H(inf2) as a fuel and chemical feedstock was investigated with Anabaena variabilis. The results show that A. variabilis, when incubated under argon, dissimilated fructose to H(inf2) and CO(inf2) in a light-dependent reaction. The H(inf2) production had an obligate requirement for fructose and was heterocyst dependent, since NH(inf4)(sup+)-grown cultures lacking heterocysts failed to produce H(inf2). Differential inhibition studies with CO showed that nitrogenase is the main enzyme catalyzing the H(inf2) production. Net H(inf2) yield increased with increasing concentrations of fructose up to 10 mM in the medium. The average apparent conversion efficiency of fructose to H(inf2) (net H(inf2) produced/fructose removed from the medium) was about 10, although higher conversion efficiencies of 15 to 17 could be obtained during shorter periods and at optimum fructose concentrations. Under the same conditions, the ratio of CO(inf2) released to fructose removed from the medium was about 3.5, suggesting that only a fraction of the fructose carbon was completely oxidized to CO(inf2). Under conditions of carbon excess, which prevents H(inf2) uptake, the maximum ratio of H(inf2) to CO(inf2) was found to be 3.0. This is higher than the expected value of 2.0, indicating that water was also a source of reductant in this fructose-mediated H(inf2) production. Inhibition of H(inf2) evolution by 3-(3,4-dichlorophenyl)-1,1-dimethylurea confirmed a role for photosystem II in this process. The rate of H(inf2) production by A. variabilis SA1 was 46 ml h(sup-1) g (dry weight)(sup-1). This high rate was maintained for over 15 days. About 30% of this H(inf2) was derived from water (10 ml of H(inf2) h(sup-1) g [dry weight](sup-1)). These results show that filamentous, heterocystous cyanobacteria can serve as biocatalysts in the high-efficiency conversion of biomass-derived sugars to H(inf2) as a fuel source while simultaneously dissimilating water to H(inf2).

Genetic and physiological characterization of a Rhizobium etli mutant strain unable to synthesize poly-beta-hydroxybutyrate

Rhizobium etli accumulates poly-beta-hydroxybutyrate (PHB) in symbiosis and in free life. PHB is a reserve material that serves as a carbon and/or electron sink when optimal growth conditions are not met. It has been suggested that in symbiosis PHB can prolong nitrogen fixation until the last stages of seed development, but experiments to test this proposition have not been done until now. To address these questions in a direct way, we constructed an R. etli PHB-negative mutant by the insertion of an Omega-Km interposon within the PHB synthase structural gene (phaC). The identification and sequence of the R. etli phaC gene are also reported here. Physiological studies showed that the PHB-negative mutant strain was unable to synthesize PHB and excreted more lactate, acetate, pyruvate, beta-hydroxybutyrate, fumarate, and malate than the wild-type strain. The NAD+/NADH ratio in the mutant strain was lower than that in the parent strain. The oxidative capacity of the PHB-negative mutant was reduced. Accordingly, the ability to grow in minimal medium supplemented with glucose or pyruvate was severely diminished in the mutant strain. We propose that in free life PHB synthesis sequesters reductive power, allowing the tricarboxylic acid cycle to proceed under conditions in which oxygen is a limiting factor. In symbiosis with Phaseolus vulgaris, the PHB-negative mutant induced nodules that prolonged the capacity to fix nitrogen.

Effect of the nitrogen source on phycobiliprotein synthesis and cell reserves in a chromatically adapting filamentous cyanobacterium

Cyanobacteria can utilize nitrate or ammonium as a source of fixed nitrogen for cell growth. In the filamentous Calothrix sp. strain PCC 7601, these two sources of nitrogen differently influenced the phycobiliprotein composition of the phycobilisomes, the major light-harvesting antennae. When compared to nitrate, growth in the presence of ammonium resulted in intracellular steady-state levels 35% lower for phycoerythrin and 46% higher for phycocyanin. Besides these differences in cell pigmentation, a rapid but transient accumulation of cyanophycin granule polypeptide occurred in ammonium-grown cells, while these macromolecules were not detected in cells grown with nitrate. In contrast, glycogen reserves displayed a dynamic pattern of accumulation and disappearance during cell growth which varied only slightly with the nitrogen source. The observed changes in cell pigmentation are reminiscent of the phenomenon of complementary chromatic adaptation, in which green and red wavelengths promote the syntheses of phycoerythrin and phycocyanin-2, respectively. As in complementary chromatic adaptation, the regulation of synthesis of phycoerythrin and phycocyanin-2 by the nitrogen source occurred mainly at the mRNA level. Moreover, the transcriptional start sites for the expression of the cpeBA and the cpc2 operons, which respectively encode the two subunits of phycoerythrin and phycocyanin-2, were the same in cells grown in nitrate or ammonium, and identical to those in green- and red-light-grown cells. The results of this study suggest that acclimation to the spectral light quality and to the nitrogen source share some common regulatory elements.

Synthesis of nitrogenase in mutants of the cyanobacterium Anabaena sp. strain PCC 7120 affected in heterocyst development or metabolism

Mutants of Anabaena sp. strain PCC 7120 that are incapable of sustained growth with air as the sole source of nitrogen were generated by using Tn5-derived transposons. Nitrogenase was expressed only in mutants that showed obvious morphological signs of heterocyst differentiation. Even under rigorously anaerobic conditions, nitrogenase was not synthesized in filaments that were unable to develop heterocysts. These results suggest that competence to synthesize nitrogenase requires a process that leads to an early stage of visible heterocyst development and are consistent with the idea that synthesis of nitrogenase is under developmental control (J. Elhai and C. P. Wolk, EMBO J. 9:3379-3388, 1990). We isolated mutants in which differentiation was arrested at an intermediate stage of heterocyst formation, suggesting that differentiation proceeds in stages; those mutants, as well as mutants with aberrant heterocyst envelopes and a mutant with defective respiration, expressed active nitrogenase under anaerobic conditions only. These results support the idea that the heterocyst envelope and heterocyst respiration are required for protection of nitrogenase from inactivation by oxygen. In the presence of air, such mutants contained less nitrogenase than under anaerobic conditions, and the Fe-protein was present in a posttranslationally modified inactive form. We conclude that internal partial oxygen pressure sufficient to inactivate nitrogenase is insufficient to repress synthesis of the enzyme completely. Among mutants with an apparently intact heterocyst envelope and normal respiration, three had virtually undetectable levels of dinitrogenase reductase under all conditions employed. However, three others expressed oxygen-sensitive nitrogenase activity, suggesting that respiration and barrier to diffusion of gases may not suffice for oxygen protection of nitrogenase in these mutants; two of these mutants reduced acetylene to ethylene and ethane.

Modification of dinitrogenase reductase in the cyanobacterium Anabaena variabilis due to C starvation and ammonia

In the heterocystous cyanobacterium Anabaena variabilis, a change in nitrogenase activity and concomitant modification of dinitrogenase reductase (the Fe protein of nitrogenase) was induced either by NH4Cl at pH 10 (S. Reich and P. Böger, FEMS Microbiol. Lett. 58:81-86, 1989) or by cessation of C supply resulting from darkness, CO2 limitation, or inhibition of photosystem II activity. Modification induced by both C limitation and NH4Cl was efficiently prevented by anaerobic conditions. Under air, endogenously stored glycogen and added fructose protected against modification triggered by C limitation but not by NH4Cl. With stored glycogen present, dark modification took place after inhibition of respiration by KCN. Reactivation of inactivated nitrogenase and concomitant demodification of dinitrogenase reductase occurred after restoration of diazotrophic growth conditions. In previously C-limited cultures, reactivation was also observed in the dark after addition of fructose (heterotrophic growth) and under anaerobiosis upon reillumination in the presence of a photosynthesis inhibitor. The results indicate that modification of dinitrogenase reductase develops as a result of decreased carbohydrate-supported reductant supply of the heterocysts caused by C limitation or by increased diversion of carbohydrates towards ammonia assimilation. Apparently, a product of N assimilation such as glutamine is not necessary for modification. The increase of oxygen concentration in the heterocysts is a plausible consequence of all treatments causing Fe protein modification.

Morphological and ultrastructural changes in vegetative cells and heterocysts of Anabaena variabilis grown with fructose

The morphology and ultrastructure of Anabaena variabilis grown in medium with and without 40 mM fructose were compared. Vegetative cells and young heterocysts in fructose-supplemented medium were significantly larger, were filled with glycogen granules, and had fewer thylakoids. Developing heterocysts contained large numbers of glycogen granules well into mature stages, and envelope formation was precocious. As heterocysts enlarged in fructose medium, their shape became more broadly oblong compared with the more rectangular heterocysts in fructose-free medium.