Hydrogen formation in nearly stoichiometric amounts from glucose by a Rhodopseudomonas sphaeroides mutant

Optimization of the hydrogen yield from single-stage photofermentation of glucose by Rhodobacter capsulatus JP91 using response surface methodology

Hydrogen production from glucose via single-stage photofermentation was examined with the photosynthetic bacterium Rhodobacter capsulatus JP91 (hup-). Response surface methodology with Box-Behnken design was used to optimize the independent experimental variables of glucose concentration, glutamate concentration and light intensity, as well as examining their interactive effects for maximization of molar hydrogen yield. Under optimal condition with a light intensity of 175W/m(2), 35mM glucose, and 4.5mM glutamate, a maximum hydrogen yield of 5.5 (±0.15)molH(2)/molglucose, and a maximum nitrogenase activity of 246 (±3.5)nmolC(2)H(4)/ml/min were obtained. Densitometric analysis of nitrogenase Fe-protein expression under different conditions showed significant variation in Fe-protein expression with a maximum at the optimized central point. Even under optimum conditions for hydrogen production, a significant fraction of the Fe-protein was found in the ADP-ribosylated state, suggesting that further improvement in yields might be possible.

Kinetic model of hydrogen generation by Rhodobacter sphaeroides in the presence of NH ions

AIMS

To examine the effects of ammonium ion concentration and illumination intensity on the effectivness of the hydrogen generation process of Rhodobacter sphaeroides.

METHODS AND RESULTS

In all experiments the amount of evolved hydrogen, biomass growth, concentration of ammonium ions, pH and chemical oxygen demand were measured. A nonstructural kinetic model was applied in description of biomass growth, the amount of evolved hydrogen and consumption of organic compounds and ammonium ions. An increase of ammonium ions concentration resulted in a decrease of maximal specific hydrogen potential production. At higher ammonium ion concentrations, no hydrogen evolution was observed. The efficiency of malic acid conversion into hydrogen and the PHB content in the biomass were the highest with lower concentrations of nitrogen compounds.

CONCLUSION

The presence of ammonium ions inhibits hydrogen photogeneration. A good agreement between the experimental data and model simulations were obtained. In all cases, hydrogen evolution started after an exhaustion of ammonium ions and the increase was proportional to the C/N ratio in the medium. The accumulation of PHB competes with the hydrogen evolution process while the conversion of acids into biomass was limited at higher levels of hydrogen generation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Confirmation of the suitability of the selected model for kinetic studies of hydrogen photoevolution.

Perspectives and advances of biological H2 production in microorganisms

The rapid development of clean fuels for the future is a critically important global challenge for two main reasons. First, new fuels are needed to supplement and ultimately replace depleting oil reserves. Second, fuels capable of zero CO2 emissions are needed to slow the impact of global warming. This review summarizes the development of solar powered bio-H2 production processes based on the conversion of photosynthetic products by fermentative bacteria, as well as using photoheterotrophic and photoautrophic organisms. The use of advanced bioreactor systems and their potential and limitations in terms of process design, efficiency, and cost are also briefly reviewed.

Effects of Ethanolamine as a nitrogen source on hydrogen production by Rhodobacter capsulatus

Ethanolamine was examined as a nitrogen source in the production of hydrogen by Rhodobacter capsulatus ST-410, a hydrogenase-deficient mutant of the strain B-100. It was found that ethanolamine supports cell growth as the sole nitrogen source and permits a large amount of hydrogen evolution, detected at 138 micromol/ml-culture from 3.5 mM ethanolamine and 30 mM DL-malate. The amount corresponded to a stoichiometric yield of 77% and was close to that obtained from 7.0 mM L-glutamate and 30 mM DL-malate. The hydrogen evolution rate per unit biomass (cells) was higher than that with L-glutamate, and the cells grown with ethanolamine had higher nitrogenase activity than the cells grown with L-glutamate. In terms of bioconversion of cellulosic and hemicellulosic biomass to hydrogen, D-glucose, D-xylose, and D-cellobiose were tested as substrates. The results indicated that those sugars permit a large evolution of hydrogen through cultivation with ethanolamine as a nitrogen source. For instance, the cells grown with 3.5 mM ethanolamine evolved hydrogen of 289 micromol/ml-culture (80% yield) from 30 mM D-glucose under a controlled pH of 6.4 to 6.9.

Microbial production of hydrogen: an overview

Production of hydrogen by anaerobes, facultative anaerobes, aerobes, methylotrophs, and photosynthetic bacteria is possible. Anaerobic Clostridia are potential producers and immobilized C. butyricum produces 2 mol H2/mol glucose at 50% efficiency. Spontaneous production of H2 from formate and glucose by immobilized Escherichia coli showed 100% and 60% efficiencies, respectively. Enterobactericiae produces H2 at similar efficiency from different monosaccharides during growth. Among methylotrophs, methanogenes, rumen bacteria, and thermophilic archae, Ruminococcus albus, is promising (2.37 mol/mol glucose). Immobilized aerobic Bacillus licheniformis optimally produces 0.7 mol H2/mol glucose. Photosynthetic Rhodospirillum rubrum produces 4, 7, and 6 mol of H2 from acetate, succinate, and malate, respectively. Excellent productivity (6.2 mol H2/mol glucose) by co-cultures of Cellulomonas with a hydrogenase uptake (Hup) mutant of R. capsulata on cellulose was found. Cyanobacteria, viz., Anabaena, Synechococcus, and Oscillatoria sp., have been studied for photoproduction of H2. Immobilized A. cylindrica produces H2 (20 ml/g dry wt/h) continually for 1 year. Increased H2 productivity was found for Hup mutant of A. variabilis. Synechococcus sp. has a high potential for H2 production in fermentors and outdoor cultures. Simultaneous productions of oxychemicals and H2 by Klebseilla sp. and by enzymatic methods were also attempted. The fate of H2 biotechnology is presumed to be dictated by the stock of fossil fuel and state of pollution in future.

Biochemical genetics revisited: the use of mutants to study carbon and nitrogen metabolism in the photosynthetic bacteria

The biochemical genetics approach is defined as the use of mutants, in comparative studies with the wild-type, to obtain information about biochemical and physiological processes in complex metabolic systems. This approach has been used extensively, for example in studies on the bioenergetics of the photosynthetic bacteria, but has been applied less frequently to studies of intermediary carbon and nitrogen metabolism in phototrophic organisms. Several important processes in photosynthetic bacteria--the regulation of nitrogenase synthesis and activity, the control of intracellular redox balance during photoheterotrophic growth, and chemotaxis--have been shown to involve metabolism. However, current understanding of carbon and nitrogen metabolism in these organisms is insufficient to allow a complete understanding of these phenomena. The purpose of the present review is to give an overview of carbon and nitrogen metabolism in the photosynthetic bacteria, with particular emphasis on work carried out with mutants, and to indicate areas in which the biochemical genetics approach could be applied successfully. In particular, it will be argued that, in the case of Rhodobacter capsulatus and Rb. sphaeroides, two species which are fast-growing, possess a versatile metabolism, and have been extensively studied genetically, it should be possible to obtain a complete, integrated description of carbon and nitrogen metabolism, and to undertake a qualitative and quantitative analysis of the flow of carbon and reducing equivalents during photoheterotrophic growth. This would require a systematic biochemical genetic study employing techniques such as HPLC, NMR, and mass spectrometry, which are briefly discussed. The review is concerned mainly with Rb. capsulatus and Rb. sphaeroides, since most studies with mutants have been carried out with these organisms. However, where possible, a comparison is made with other species of purple non-sulphur bacteria and with purple and green sulphur bacteria, and recent literature relevant to these organisms has been cited.

Hydrogen Gas Production by an Ectothiorhodospira vacuolata Strain

A hydrogen gas (H 2 )-producing strain of Ectothiorhodospira vacuolata isolated from Soap Lake, Washington, possessed nitrogenase activity. Increasing evolution of H 2 with decreasing ammonium chloride concentrations provided evidence that nitrogenase was the catalyst in gas production. Cells were grown in a mineral medium plus 0.2% acetate with sodium sulfide as an electron donor. Factors increasing H 2 production included addition of reduced carbon compounds such as propionate and succinate, increased reducing power by increasing sodium sulfide concentrations, and increased energy charge (ATP) by increasing light intensity.

Carbon allocation in wild-type and Glc+ Rhodobacter sphaeroides under photoheterotrophic conditions

The photosynthetic bacterium Rhodobacter sphaeroides is capable of producing H2 via nitrogenase when grown photoheterotrophically in the absence of N2. By using 14C-labeled malate, it was found that greater than 95% of this substrate was catabolized completely to CO2 during H2 production. About 60% of this catabolism was associated with H2 biosynthesis, while almost 40% provided reductant for other cellular purposes. Thus, only a small fraction of malate provided carbon skeletons. The addition of ammonium, which inhibited nitrogenase activity, increased substrate conversion into carbon skeletons threefold. Catabolism of malate occurred primarily via the tricarboxylic acid cycle, but gluconeogenesis was also observed. The wild-type organism grew poorly on glucose, accumulated gluconate and 2-keto-3-deoxygluconate, and did not produce H2. More than 50% of metabolized glucose appeared in carbon skeletons or in storage compounds. A glucose-utilizing mutant was five times more effective in utilizing this substrate. This mutant produced H2 from glucose, using 74% of metabolized substrate for this purpose. Glucose converted to storage products or to other carbon skeletons was reduced to 8%. Fixation of CO2 competed directly with H2 production for reducing equivalents and ATP. Refixation of CO2 released from these substrates under H2-producing conditions was, at most, 10 to 12%. Addition of ammonium increased refixation of respired CO2 to 83%. Patterns of carbon flow of fixation products were associated with the particular strains and culture conditions.

Expression of cellulase genes in Rhodobacter capsulatus by use of plasmid expression vectors

Broad-host-range plasmid vectors were constructed for expression of heterologous genes in the photosynthetic bacterium Rhodobacter capsulatus. These plasmids utilize an RK2-derived replicon for maintenance and conjugative transfer and the R. capsulatus rxcA promoter to obtain transcription of genes within appropriately positioned DNA fragments. The expression vectors were used to obtain synthesis of endoglucanase and exoglucanase in R. capsulatus from cellulase genes present on exogenously derived DNA fragments. The cellulase genes were expressed either by use of their native translation initiation signals or by in-frame fusion with the rxcA B870 beta gene translation initiation signals to form a hybrid protein. The level of cellulase gene expression was found to be modulated in response to the extent of aeration of plasmid host cultures.

Increased photoproduction of hydrogen by non-autotrophic mutants of Rhodopseudomonas capsulata

Non-autotrophic ( Aut -) mutants of Rhodopseudomonas capsulata B10 were tested for their efficiency of nitrogenase-mediated H2 production. Three of these mutants ( IR3 , IR4 and IR5 ) showed an increase stoichiometry of H2 production, mediated by nitrogenase, from certain organic substrates. For example, in a medium containing 7 mM-L-glutamate as nitrogen source, strain IR4 produced 10-20% more H2 than did the wild type with DL-lactate or L-malate as major carbon source, 20-50% more H2 with DL-malate, and up to 70% more with D-malate. Strain IR4 was deficient in 'uptake' hydrogenase activity as measured by H2-dependent reduction of Methylene Blue or Benzyl Viologen. However, this observation did not explain the increased efficiency of H2 production, since H2 uptake (H2 recycling) was undetectable in cells of the wild type. Instead, increased H2 production by the mutant appeared to be due to an improved conversion of organic substrates to H2 and CO2, presumably due to an altered carbon metabolism. The metabolism of D-malate by different strains was studied. An NAD+-dependent D-malic enzyme was synthesized constitutively by the wild type, and showed a Km for D-malate of 3 mM. The activity of this enzyme was approx. 50% higher in strain IR4 than in the wild type, and the mutant also grew twice as fast as the wild type with D-malate as sole carbon source.