Acetylene reduction and hydrogen photoproduction by wild-type and mutant strains ofAnabaenaat different CO2and O2concentrations

Influence of Mo and Fe on Photosynthetic and Nitrogenase Activities of Nitrogen-Fixing Cyanobacteria under Nitrogen Starvation

The potential of cyanobacteria to perform a variety of distinct roles vital for the biosphere, including nutrient cycling and environmental detoxification, drives interest in studying their biodiversity. Increasing soil erosion and the overuse of chemical fertilizers are global problems in developed countries. The option might be to switch to organic farming, which entails largely the use of biofertilisers. Cyanobacteria are prokaryotic, photosynthetic organisms with considerable potential, within agrobiotechnology, to produce biofertilisers. They contribute significantly to plant drought resistance and nitrogen enrichment in the soil. This study sought, isolated, and investigated nitrogen-fixing cyanobacterial strains in rice fields, and evaluated the effect of Mo and Fe on photosynthetic and nitrogenase activities under nitrogen starvation. Cyanobacterial isolates, isolated from rice paddies in Kazakhstan, were identified as Trichormus variabilis K-31 (MZ079356), Cylindrospermum badium J-8 (MZ079357), Nostoc sp. J-14 (MZ079360), Oscillatoria brevis SH-12 (MZ090011), and Tolypothrix tenuis J-1 (MZ079361). The study of the influence of various concentrations of Mo and Fe on photosynthetic and nitrogenase activities under conditions of nitrogen starvation revealed the optimal concentrations of metals that have a stimulating effect on the studied parameters.

Microalgal hydrogen production - A review

Bio-hydrogen from microalgae including cyanobacteria has attracted commercial awareness due to its potential as an alternative, reliable and renewable energy source. Photosynthetic hydrogen production from microalgae can be interesting and promising options for clean energy. Advances in hydrogen-fuel-cell technology may attest an eco-friendly way of biofuel production, since, the use of H₂ to generate electricity releases only water as a by-product. Progress in genetic/metabolic engineering may significantly enhance the photobiological hydrogen production from microalgae. Manipulation of competing metabolic pathways by modulating the certain key enzymes such as hydrogenase and nitrogenase may enhance the evolution of H₂ from photoautotrophic cells. Moreover, biological H₂ production at low operating costs is requisite for economic viability. Several photobioreactors have been developed for large-scale biomass and hydrogen production. This review highlights the recent technological progress, enzymes involved and genetic as well as metabolic engineering approaches towards sustainable hydrogen production from microalgae.

Cyanobacterial nitrogenases: phylogenetic diversity, regulation and functional predictions

Cyanobacteria is a remarkable group of prokaryotic photosynthetic microorganisms, with several genera capable of fixing atmospheric nitrogen (N2) and presenting a wide range of morphologies. Although the nitrogenase complex is not present in all cyanobacterial taxa, it is spread across several cyanobacterial strains. The nitrogenase complex has also a high theoretical potential for biofuel production, since H2 is a by-product produced during N2 fixation. In this review we discuss the significance of a relatively wide variety of cell morphologies and metabolic strategies that allow spatial and temporal separation of N2 fixation from photosynthesis in cyanobacteria. Phylogenetic reconstructions based on 16S rRNA and nifD gene sequences shed light on the evolutionary history of the two genes. Our results demonstrated that (i) sequences of genes involved in nitrogen fixation (nifD) from several morphologically distinct strains of cyanobacteria are grouped in similarity with their morphology classification and phylogeny, and (ii) nifD genes from heterocytous strains share a common ancestor. By using this data we also discuss the evolutionary importance of processes such as horizontal gene transfer and genetic duplication for nitrogenase evolution and diversification. Finally, we discuss the importance of H2 synthesis in cyanobacteria, as well as strategies and challenges to improve cyanobacterial H2 production.

Investigation of the links between heterocyst and biohydrogen production by diazotrophic cyanobacterium A. variabilis ATCC 29413

This work investigates the effect of heterocyst toward biohydrogen production by A. variabilis. The heterocyst frequency was artificially promoted by adding an amino acid analog, in this case DL-7-azatryptophan into the growth medium. The frequency of heterocyst differentiation was found to be proportional to the concentration of azatryptophan (0-25 µM) in the medium. Conversely, the growth and nitrogenase activity were gradually suppressed. In addition, there was also a distinct shortening of the cells filaments and detachment of heterocyst from the vegetative cells. Analysis on the hydrogen production performance revealed that both the frequency and distribution of heterocyst in the filaments affected the rate of hydrogen production. The highest hydrogen production rate and yield (41 µmol H2 mg chl a(-1) h(-1) and 97 mL H2 mg chl a(-1), respectively) were achieved by cells previously grown in 15 µM of azatryptophan with 14.5 % of heterocyst frequency. The existence of more isolated heterocyst has been shown to cause a relative loss in nitrogenase activity thus lowering the hydrogen production rate.

Hydrogen photoproduction by immobilized n2-fixing cyanobacteria: understanding the role of the uptake hydrogenase in the long-term process

We have investigated two approaches to enhance and extend H2 photoproduction yields in heterocystous, N2-fixing cyanobacteria entrapped in thin alginate films. In the first approach, periodic CO2 supplementation was provided to alginate-entrapped, N-deprived cells. N deprivation led to the inhibition of photosynthetic activity in vegetative cells and the attenuation of H2 production over time. Our results demonstrated that alginate-entrapped ΔhupL cells were considerably more sensitive to high light intensity, N deficiency, and imbalances in C/N ratios than wild-type cells. In the second approach, Anabaena strain PCC 7120, its ΔhupL mutant, and Calothrix strain 336/3 films were supplemented with N2 by periodic treatments of air, or air plus CO2. These treatments restored the photosynthetic activity of the cells and led to a high level of H2 production in Calothrix 336/3 and ΔhupL cells (except for the treatment air plus CO2) but not in the Anabaena PCC 7120 strain (for which H2 yields did not change after air treatments). The highest H2 yield was obtained by the air treatment of ΔhupL cells. Notably, the supplementation of CO2 under an air atmosphere led to prominent symptoms of N deficiency in the ΔhupL strain but not in the wild-type strain. We propose that uptake hydrogenase activity in heterocystous cyanobacteria not only supports nitrogenase activity by removing excess O2 from heterocysts but also indirectly protects the photosynthetic apparatus of vegetative cells from photoinhibition, especially under stressful conditions that cause an imbalance in the C/N ratio in cells.

Auto-flotation of heterocyst enables the efficient production of renewable energy in cyanobacteria

Utilizing cyanobacteria as a bioenergy resource is difficult due to the cost and energy consuming harvests of microalgal biomass. In this study, an auto-floating system was developed by increasing the photobiological H₂ production in the heterocysts of filamentous cyanobacteria. An amount of 1.0 μM of diuron, which inhibited O₂ production in cyanobacteria, resulted in a high rate of H₂ production in heterocysts. The auto-floating process recovered 91.71% ± 1.22 of the accumulated microalgal biomass from the liquid media. Quantification analysis revealed that 0.72–1.10 μmol H₂ per mg dry weight microalgal biomass was necessary to create this auto-floating system. Further bio-conversion by using anaerobic digestion converted the harvested microalgal biomass into biogas. Through this novel coupled system of photobiological H₂ production and anaerobic digestion, a high level of light energy conversion efficiency from solar energy to bioenergy was attained with the values of 3.79% ± 0.76.

The uptake hydrogenase in the unicellular diazotrophic cyanobacterium Cyanothece sp. strain PCC 7822 protects nitrogenase from oxygen toxicity

Cyanothece sp. strain PCC 7822 is a unicellular, diazotrophic cyanobacterium that can produce large quantities of H2 when grown diazotrophically. This strain is also capable of genetic manipulations and can represent a good model for improving H2 production from cyanobacteria. To this end, a knockout mutation was made in the hupL gene (ΔhupL), and we determined how this would affect the amount of H2 produced. The ΔhupL mutant demonstrated virtually no nitrogenase activity or H2 production when grown under N2-fixing conditions. To ensure that this mutation only affected the hupL gene, a complementation strain was constructed readily with wild-type properties; this indicated that the original insertion was only in hupL. The mutant had no uptake hydrogenase activity but had increased bidirectional hydrogenase (Hox) activity. Western blotting and immunocytochemistry under the electron microscope indicated that the mutant had neither HupL nor NifHDK, although the nif genes were transcribed. Interestingly, biochemical analysis demonstrated that both HupL and NifH could be membrane associated. The results indicated that the nif genes were transcribed but that NifHDK was either not translated or was translated but rapidly degraded. We hypothesized that the Nif proteins were made but were unusually susceptible to O2 damage. Thus, we grew the mutant cells under anaerobic conditions and found that they grew well under N2-fixing conditions. We conclude that in unicellular diazotrophs, like Cyanothece sp. strain PCC 7822, the HupLS complex helps remove oxygen from the nitrogenase, and that this is a more important function than merely oxidizing the H2 produced by the nitrogenase.

High rates of photobiological H2 production by a cyanobacterium under aerobic conditions

Among the emerging renewable and green energy sources, biohydrogen stands out as an appealing choice. Hydrogen can be produced by certain groups of microorganisms that possess functional nitrogenase and/or bidirectional hydrogenases. In particular, the potential of photobiological hydrogen production by oxygenic photosynthetic microbes has attracted significant interest. However, nitrogenase and hydrogenase are generally oxygen sensitive, and require protective mechanisms to function in an aerobic extracellular environment. Here, we describe Cyanothece sp. ATCC 51142, a unicellular, diazotrophic cyanobacterium with the capacity to generate high levels of hydrogen under aerobic conditions. Wild-type Cyanothece 51142 can produce hydrogen at rates as high as 465 μmol per mg of chlorophyll per hour in the presence of glycerol. Hydrogen production in this strain is mediated by an efficient nitrogenase system, which can be manipulated to convert solar energy into hydrogen at rates that are several fold higher, compared with any previously described wild-type hydrogen-producing photosynthetic microbe.

Growth, CO2 consumption and H2 production of Anabaena variabilis ATCC 29413-U under different irradiances and CO2 concentrations

AIMS

The objective of this study is to develop kinetic models based on batch experiments describing the growth, CO(2) consumption, and H(2) production of Anabaena variabilis ATCC 29413-U(TM) as functions of irradiance and CO(2) concentration.

METHODS AND RESULTS

A parametric experimental study is performed for irradiances from 1120 to 16100 lux and for initial CO(2) mole fractions from 0.03 to 0.20 in argon at pH 7.0 +/- 0.4 with nitrate in the medium. Kinetic models are successfully developed based on the Monod model and on a novel scaling analysis employing the CO(2) consumption half-time as the time scale.

CONCLUSIONS

Monod models predict the growth, CO(2) consumption and O(2) production within 30%. Moreover, the CO(2) consumption half-time is an appropriate time scale for analysing all experimental data. In addition, the optimum initial CO(2) mole fraction is 0.05 for maximum growth and CO(2) consumption rates. Finally, the saturation irradiance is determined to be 5170 lux for CO(2) consumption and growth whereas, the maximum H(2) production rate occurs around 10,000 lux.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study presents kinetic models predicting the growth, CO(2) consumption and H(2) production of A. variabilis. The experimental and scaling analysis methods can be generalized to other micro-organisms.

Hydrogen production by Cyanobacteria

The limited fossil fuel prompts the prospecting of various unconventional energy sources to take over the traditional fossil fuel energy source. In this respect the use of hydrogen gas is an attractive alternate source. Attributed by its numerous advantages including those of environmentally clean, efficiency and renew ability, hydrogen gas is considered to be one of the most desired alternate. Cyanobacteria are highly promising microorganism for hydrogen production. In comparison to the traditional ways of hydrogen production (chemical, photoelectrical), Cyanobacterial hydrogen production is commercially viable. This review highlights the basic biology of cynobacterial hydrogen production, strains involved, large-scale hydrogen production and its future prospects. While integrating the existing knowledge and technology, much future improvement and progress is to be done before hydrogen is accepted as a commercial primary energy source.

Hydrogen production by cyanobacteria in an automated outdoor photobioreactor under aerobic conditions

The possibility of hydrogen production by a hydrogenase impaired mutant strain of Anabaena variabilis in outdoor culture was studied. A computer-controlled rooftop (outdoor) tubular photobioreactor (4.35 L) was assembled. H(2) production rates by A. variabilis PK84 grown in CO(2) + air in the photobioreactor were measured together with other parameters such as temperature, irradiance, pH, dry biomass weight, and pO(2), and Chl a concentrations during summer months of 1998 and 1999. Efficiencies of light energy bioconversion to H(2) energy and energy accumulated in biomass were calculated. The influence of irradiance, temperature, and mode of cultivation on H(2) production and efficiency of light energy bioconversion were evaluated. The culture produced up to 1.1 L H(2) day(-1) PhBR(-1). The efficiency of light energy to H(2) energy bioconversion on some days was 0.094%. However, the conditions for maximum H(2) photoproduction and for maximum efficiency of light energy to H(2) energy bioconversion were not the same. A. variabilis PK84 could produce hydrogen for prolonged periods (up to 40 days) without injection of fresh inoculum. During this period photobioreactor produced 24.5 L of H(2). Possibilities for increasing the efficiency of light energy conversion are discussed.

Hydrogenases and hydrogen metabolism of cyanobacteria

Cyanobacteria may possess several enzymes that are directly involved in dihydrogen metabolism: nitrogenase(s) catalyzing the production of hydrogen concomitantly with the reduction of dinitrogen to ammonia, an uptake hydrogenase (encoded by hupSL) catalyzing the consumption of hydrogen produced by the nitrogenase, and a bidirectional hydrogenase (encoded by hoxFUYH) which has the capacity to both take up and produce hydrogen. This review summarizes our knowledge about cyanobacterial hydrogenases, focusing on recent progress since the first molecular information was published in 1995. It presents the molecular knowledge about cyanobacterial hupSL and hoxFUYH, their corresponding gene products, and their accessory genes before finishing with an applied aspect--the use of cyanobacteria in a biological, renewable production of the future energy carrier molecular hydrogen. In addition to scientific publications, information from three cyanobacterial genomes, the unicellular Synechocystis strain PCC 6803 and the filamentous heterocystous Anabaena strain PCC 7120 and Nostoc punctiforme (PCC 73102/ATCC 29133) is included.

Ultrastructure of the fresh water cyanobacterium Anabaena variabilis SPU 003 and its application for oxygen-free hydrogen production

Photoproduction of hydrogen has been studied as one of the ways to produce a clean, renewable energy source. Ultrastructure of the selected strain Anabaena variabilis SPU 003, a heterocystous cyanobacterium, has been done to understand the cell structure. The organism was found to be essentially a dark hydrogen producer. While pH had no significant effect on hydrogen production, optimum temperature was found to be 30 degrees C. Various sugars increased the production of hydrogen while the presence of various nitrogen sources inhibits the production. The production of hydrogen is highly sensitive to salinity and micronutrients.

Hydrogen production by Anabaena variabilis PK84 under simulated outdoor conditions

Hydrogen production by autotrophic, vanadium-grown cells of Anabaena variabilis PK84, a cyanobacterial mutant impaired in the utilization of molecular hydrogen, has been studied under simulated outdoor conditions. The cyanobacterium was cultivated in an automated helical tubular photobioreactor (4.35 L) under air containing 2% CO(2), with alternating 12-h light (36 degrees C) and 12-h dark (14 degrees to 30 degrees C) periods. A. variabilis steadily produced H(2) directly in the photobioreactor during continuous cultivation for 2.5 months. The maximum H(2) production by the continuously aerated culture under light of 332 microE. s(-1). m(-2) was 230 mL per 12-h light period per photobioreactor and was observed at a growth density corresponding to 3.6 to 4.6 microgram Chl a. mL(-1) (1.2 to 1.6 mg dry weight. mL(-1)). Replacement of air with an argon atmosphere enhanced H(2) evolution by a factor of 2. This stimulatory effect was caused mainly by N(2) deprivation in the cell suspension. A short-term decrease of the CO(2) concentration in the air suppressed H(2) evolution. Anoxygenic conditions over the dark periods had a negative effect on H(2) production. The peculiarity of hydrogen production and some physiological characteristics of A. variabilis PK84 during cultivation in the photobioreactor under a light-dark regime are investigated.

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.