The effect of Ni,Fe and Mg concentration on photo-hydrogen production by Rhodopseudomonas faecalis RLD-53

In the present study, the effect of Ni²⁺ (0–10 μmol/l), Fe²⁺ (0–200 μmol/l) and Mg²⁺ (0–15 mmol/l) concentration on photo-hydrogen production from acetate was investigated by batch culture. Results showed that under a proper concentration range, Ni²⁺ was able to enhance the hydrogen production rate and the hydrogen yield; Fe²⁺ was able to increase the hydrogen yield, and hydrogen production rate was enhanced only when the culturing time was 24–72 h. Ni²⁺ and Fe²⁺ at a higher concentration inhibited cell growth. When Ni²⁺ and Fe²⁺ concentrations were 4 μmol/l and 80 μmol/l, respectively, maximal hydrogen yield of 2.87 and 2.78 mol H₂/mol acetate was obtained when batch culturing at 35 °C with initial pH 7.0. Mg²⁺ did not significantly affect hydrogen production and hydrogen yield which maintained at about 2.45 mol H₂/mol acetate, but it was favorable to cell growth.     

Production of biohydrogen from sugars and lignocellulosic biomass using Thermoanaerobacter GHL15

The effect of culture parameters on hydrogen production using strain GHL₁₅ in batch culture was investigated. The strain belongs to the genus Thermoanaerobacter with 98.9% similarity to Thermoanaerobacter yonseiensis and 98.5% to Thermoanaerobacter keratinophilus with a temperature optimum of 65–70 °C and a pH optimum of 6–7. The strain metabolizes various pentoses, hexoses, and disaccharides to acetate, ethanol, hydrogen, and carbon dioxide. However substrate inhibition was observed above 10 mM glucose concentration. Maximum hydrogen yields on glucose were 3.1 mol H₂ mol⁻¹ glucose at very low partial pressure of hydrogen. Hydrogen production from various lignocellulosic biomass hydrolysates was investigated in batch culture. Various pretreatment methods were examined including acid, base, and enzymatic (Celluclast^® and Novozyme 188) hydrolysis. Maximum hydrogen production (5.8–6.0 mmol H₂ g⁻¹ dw) was observed from Whatman paper (cellulose) hydrolysates although less hydrogen was produced by hydrolysates from other examined lignocellulosic materials (maximally 4.83 mmol H₂ g⁻¹ dw of grass hydrolysate). The hydrogen yields from all lignocellulosic hydrolysates were improved by acid and alkaline pretreatments, with maximum yields on grass, 7.6 mmol H₂ g⁻¹ dw.     

Microbial diversity of hydrogen-producing bacteria in batch reactors fed with cellulose using leachate as inoculum

Hydrogen production using cellulosic residues offers the possibility of waste minimization with renewable energy recovery. In the present study, heat-treated biomass purified from leachate was used as inoculum in batch reactors for hydrogen production fed with different concentrations of cellulose (2.5, 5.0 and 10 g/L), in the presence and absence of exogenous cellulase. The heat-treated biomass did not degrade cellulose and hydrogen production was not detected in the absence of cellulase. In reactors with cellulase, the hydrogen yields were 1.2, 0.6 and 2.3 mol H₂/mol of hydrolyzed cellulose with substrate degradation of 41.4, 28.4 and 44.7% for 2.5, 5.0 and 10 g/L cellulose, respectively. Hydrogen production potentials (P) varied from 19.9 to 125.9 mmol H₂ and maximum hydrogen production rates (R_m) were among 0.8–2.3 mmol H₂/h. The reactor containing 10 g/L of cellulose presented the highest P and R_m among the conditions tested. The main acid produced in reactors were butyric acid, followed by acetic, isobutyric and propionic acids. Bacteria similar to Clostridium sp. (98–99%) were identified in the reactors with cellulase. The heat-treated leachate can be used as an inoculum source for hydrogen production from hydrolyzed cellulose.     

Glycerol fermentation to hydrogen by Thermotoga maritima: Proposed pathway and bioenergetic considerations

The production of biohydrogen from glycerol, by the hyperthermophilic bacterium Thermotoga maritima DSM 3109, was investigated in batch and chemostat systems. T. maritima converted glycerol to mainly acetate, CO₂ and H₂. Maximal hydrogen yields of 2.84 and 2.41 hydrogen per glycerol were observed for batch and chemostat cultivations, respectively. For batch cultivations: i) hydrogen production rates decreased with increasing initial glycerol concentration, ii) growth and hydrogen production was optimal in the pH range of 7–7.5, and iii) a yeast extract concentration of 2 g/l led to optimal hydrogen production. Stable growth could be maintained in a chemostat, however, when dilution rates exceeded 0.025 h⁻¹ glycerol conversion was incomplete. A detailed overview of the catabolic pathway involved in glycerol fermentation to hydrogen by T. maritima is given. Based on comparative genomics the ability to grow on glycerol can be considered as a general trait of Thermotoga species. The exceptional bioenergetics of hydrogen formation from glycerol is discussed.     

Effect of pH on glucose and starch fermentation in batch and sequenced-batch mode with a recently isolated strain of hydrogen-producing Clostridium butyricum CWBI1009

This paper reports investigations carried out to determine the optimum culture conditions for the production of hydrogen with a recently isolated strain Clostridium butyricum CWBI1009. The production rates and yields were investigated at 30 °C in a 2.3 L bioreactor operated in batch and sequenced-batch mode using glucose and starch as substrates. In order to study the precise effect of a stable pH on hydrogen production, and the metabolite pathway involved, cultures were conducted with pH controlled at different levels ranging from 4.7 to 7.3 (maximum range of 0.15 pH unit around the pH level). For glucose the maximum yield (1.7 mol H₂ mol⁻¹ glucose) was measured when the pH was maintained at 5.2. The acetate and butyrate yields were 0.35 mol acetate mol⁻¹ glucose and 0.6 mol butyrate mol⁻¹ glucose. For starch a maximum yield of 2.0 mol H₂ mol⁻¹ hexose, and a maximum production rate of 15 mol H₂ mol⁻¹ hexose h⁻¹ were obtained at pH 5.6 when the acetate and butyrate yields were 0.47 mol acetate mol⁻¹ hexose and 0.67 mol butyrate mol⁻¹ hexose.     

Hydrogen production performance by Enterobacter cloacae KBH3 isolated from termite guts

Two out of six bacterial isolates obtained from the guts of Globitermes sp. termites were identified as hydrogen-producing bacteria. One isolate, Enterobacter cloacae KBH3, was characterised using the BIOLOG identification system and 16S rRNA gene analysis. In a batch fermentation study to evaluate its growth in defined medium, E. cloacae KBH3 produced 154 ml H₂ per litre medium with approximately 50% hydrogen content. The carbon utilisation results suggest that E. cloacae KBH3 have the potential to be a good hydrogen producer. This strain is also able to produce hydrogen within a wide range of temperatures (28–40 °C) and pH (4.5–8). In several fermentation runs, the pH of the culture dropped from 6.5 to 5.36 within the first 3 h, which was mostly due to the biosynthesis of formate. An increase of cumulative hydrogen production was recorded as well as a decrease in the concentration of formate, indicating the importance of the formate pathway for hydrogen production. The highest rate of hydrogen production of 180.74 ml H₂/l/h was achieved when lactate and acetate were at their highest concentrations. Most of the hydrogen gas was produced during the exponential growth phase, and the biogas continued to be produced during the stationary phase. The specific growth rate was calculated to be 0.224 per hour while the hydrogen yield was 1.8 mol of hydrogen per mol of glucose. At the end of the batch study, the highest cumulative hydrogen production was 2404 ml H₂ per litre of fermentation medium.     

The kinetic characterization of photofermentative bacterium Rhodopseudomonas faecalis RLD-53 and its application for enhancing continuous hydrogen production

In this work, the kinetic characterization of hydrogen production by the photofermentative bacteria Rhodopseudomonas faecalis RLD-53 was investigated at different growth phase. During entire fermentation, 89.30% of total biomass was accumulated in exponential growth phase, while hydrogen yield was only 1.82 mol H₂/mol acetate at the expense of 51.25% substrate. In the stationary phase, biomass synthesis was minimal (7.51%), and 38.17% of the substrate was directly converted into hydrogen. As a result, hydrogen (59.19%) was mainly produced in stationary phase with highest hydrogen yield of 3.67 mol H₂/mol acetate. Consequently, bacteria in stationary phase were most effective for hydrogen production. Based on these findings, a novel membrane photobioreactor was developed to retain bacteria during stationary phase in reactor through membrane separation. Maximum rate (32.82 ml/l/h) and yield (3.27 mol H₂/mol acetate) of hydrogen production were achieved using membrane photobioreactor under the continuous operation. Therefore, using bacteria in stationary phase as hydrogen producer can offer considerable benefits for enhancing photo-hydrogen production.     

Enhanced bio-hydrogen production from sugarcane juice by immobilized Clostridium butyricum on sugarcane bagasse

Immobilized Clostridium butyricum TISTR 1032 on sugarcane bagasse improved hydrogen production rate (HPR) approximately 1.2 times in comparison to free cells. The optimum conditions for hydrogen production by immobilized C. butyricum were initial pH 6.5 and initial sucrose concentration of 25 g COD/L. The maximum HPR and hydrogen yield (HY) of 3.11 L H₂/L substrate·d and 1.34 mol H₂/mol hexose consumed, respectively, were obtained. Results from repeated batch fermentation indicated that the highest HPR of 3.5 L H₂/L substrate·d and the highest HY of 1.52 mol H₂/mol hexose consumed were obtained at the medium replacement ratio of 75% and 50% respectively. The major soluble metabolites in both batch and repeated batch fermentation were butyric and acetic acids.     

Enhancement of phototrophic hydrogen production by Rhodobacter sphaeroides ZX-5 using fed-batch operation based on ORP level

In this study, a new outer-cycle flat-panel photobioreactor was designed for an anaerobic, photo-fermentation process by Rhodobacter sphaeroides ZX-5. In order to obtain the high hydrogen yield, photo-hydrogen production by fed-batch culture with on-line oxidation-reduction potential (ORP) feedback control was investigated. Meanwhile, the effects of feeding malic acid concentration and pH adjustment on the growth and hydrogen production of R. sphaeroides ZX-5 were studied. In the entire fed-batch culture, biomass (i.e., OD₆₆₀) rapidly increased up to 1.79 within 18 h, and then OD₆₆₀ value stayed constant within a range of 1.85-2.18 until the end of the photo-fermentation. The cumulative hydrogen volumes in each phase of fed-batch process were 2339, 1439, 1328, and 510 ml H₂/l-culture, respectively. Throughout the entire repeated fed-batch photo-fermentation, the maximum substrate conversion efficiency of 73.03% was observed in the first fed-batch process, obviously higher than that obtained from batch culture process (59.81%). In addition, compared to the batch culture, a much higher maximum hydrogen production rate (102.33 ml H₂/l h) was achieved during fed-batch culture. The results demonstrated that photo-hydrogen production using fed-batch operation based on ORP feedback control is a favorable choice of sustainable and feasible strategy to improve phototrophic hydrogen production efficiency.     

Hydrogen production from soft-drink wastewater in an upflow anaerobic packed-bed reactor

The production of hydrogen from soft-drink wastewater in two upflow anaerobic packed-bed reactors was evaluated. The results show that soft-drink wastewater is a good source for hydrogen generation. Data from both reactors indicate that the reactor without medium containing macro- and micronutrients (R2) provided a higher hydrogen yield (3.5 mol H₂ mol⁻¹ of sucrose) as compared to the reactor (R1) with a nutrient-containing medium (3.3 mol H₂ mol⁻¹ of sucrose). Reactor R2 continuously produced hydrogen, whereas reactor R1 exhibited a short period of production and produced lower amounts of hydrogen. Better hydrogen production rates and percentages of biogas were also observed for reactor R2, which produced 0.4 L h⁻¹ L⁻¹ and 15.8% of H₂, compared to reactor R1, which produced 0.2 L h⁻¹ L⁻¹ and 2.6% of H₂. The difference in performance between the reactors was likely due to changes in the metabolic pathway for hydrogen production and decreases in bed porosity as a result of excessive biomass growth in reactor R1. Molecular biological analyses of samples from reactors R1 and R2 indicated the presence of several microorganisms, including Clostridium (91% similarity), Enterobacter (93% similarity) and Klebsiella (97% similarity).     

Sequential dark–photo fermentation and autotrophic microalgal growth for high-yield and CO2-free biohydrogen production

Dark fermentation, photo fermentation, and autotrophic microalgae cultivation were integrated to establish a high-yield and CO₂-free biohydrogen production system by using different feedstock. Among the four carbon sources examined, sucrose was the most effective for the sequential dark (with Clostridium butyricum CGS5) and photo (with Rhodopseudomonas palutris WP3-5) fermentation process. The sequential dark–photo fermentation was stably operated for nearly 80 days, giving a maximum H₂ yield of 11.61 mol H₂/mol sucrose and a H₂ production rate of 673.93 ml/h/l. The biogas produced from the sequential dark–photo fermentation (containing ca. 40.0% CO₂) was directly fed into a microalga culture (Chlorella vulgaris C–C) cultivated at 30 °C under 60 μmol/m²/s illumination. The CO₂ produced from the fermentation processes was completely consumed during the autotrophic growth of C. vulgaris C–C, resulting in a microalgal biomass concentration of 1999 mg/l composed mainly of 48.0% protein, 23.0% carbohydrate and 12.3% lipid.     

Hydrogen production from sulfate- and ferrous-enriched wastewater

The quantitative relationship between sulfate reducing bacteria (SRB) and hydrogen (H₂) production from sulfate (SO₄²⁻) and ferrous [Fe(II)] enriched wastewater was investigated. Both Fe(II) (0–11,600 mg/L) and SO₄²⁻ (0–20,000 mg/L) improved the H₂ production efficiency from wastewater. The H₂ yields were increased up to 1.9 mol H₂/mol glucose in 580–1750 mg Fe(II)/L and 1000–3000 mg SO₄²⁻/L enriched wastewater at pH 5.8–6.2. Quantitative Fluorescence In Situ Hybridization (FISH) analyses revealed that the specific sulfate reducing activities (SSRA) were increased from 0.08 and 0.06 to 0.16 and 0.21 g TS/g SRB h in response to variations in sulfate concentration from 300–20,000 mg/L at pH 5.8 and 6.2, respectively. H₂ production was not influenced by low SSRA (≤0.1 g TS/g SRB h), which was independent of pH variation. The results demonstrated that the SSRA and Fe(II) concentration can significantly influence on the biological H₂ production from SO₄²⁻ and Fe(II) containing wastewater.     

Optimization of photo-hydrogen production based on cheese whey spent medium

Cheese whey wastewater diluted to 10 g lactose/L was initially subjected to dark-fermentation by Enterobacter aerogenes MTCC 2822, and the VFAs-rich spent medium (acetic acid 1900 mg/L, butyric acid 537 mg/L, and traces of propionic acid) was subjected to photo-fermentation through enrichment by Ni²⁺ (0–8 μmol/L), Fe²⁺ (0–100 μmol/L) or Mg²⁺ (0–15 mmol/L) in batch mode by Rhodopseudomonas BHU 01 strain. The maximum cumulative H₂ production (144 ml) and yield (58 mmol) was obtained at 4 μmol Ni²⁺/L. Likewise, Fe²⁺ (60 μmol/L) resulted in maximum cumulative H₂ production (139 ml) and yield (56 mmol). Nevertheless, 6 mmol of Mg²⁺ did not significantly affect H₂ production (110 ml) or yield (44 mmol); the latter value in close proximity with the control (37 mmol). The concomitant reduction in COD was maximum (15.61%) for 4 μmol Ni²⁺/L, followed by 15.33% for 60 μmol Fe²⁺/L, and the least for 6 mmol Mg²⁺/L (14.5%). The observations suggest the role of Fe²⁺ and Ni²⁺ in regulation of nitrogenase and hydrogenase, while that of Mg²⁺ mainly in the biosynthesis of photopigment bacteriochlorophyll (Bchl).     

Hydrogen production from acid hydrolyzed molasses by the hydrogen overproducing Escherichia coli strain HD701 and subsequent use of the waste bacterial biomass for biosorption of Cd(II) and Zn(II)

This study was devoted to investigate production of hydrogen gas from acid hydrolyzed molasses by Escherichia coli HD701 and to explore the possible use of the waste bacterial biomass in biosorption technology. In variable substrate concentration experiments (1, 2.5, 5, 10 and 15 g L⁻¹), the highest cumulative hydrogen gas (570 ml H₂ L⁻¹) and formation rate (19 ml H₂ h⁻¹ L⁻¹) were obtained from 10 g L⁻¹ reducing sugars. However, the highest yield (132 ml H₂ g⁻¹ reducing sugars) was obtained at a moderate hydrogen formation rate (11 ml H₂ h⁻¹ L⁻¹) from 2.5 g L⁻¹ reducing sugars. Subsequent to H₂ production, the waste E. coli biomass was collected and its biosorption efficiency for Cd²⁺ and Zn²⁺ was investigated. The biosorption kinetics of both heavy metals fitted well with the pseudo second-order kinetic model. Based on the Langmuir biosorption isotherm, the maximum biosorption capacities (q_max) of E. coli waste biomass for Cd²⁺ and Zn²⁺ were 162.1 and 137.9 (mg/g), respectively. These q_max values are higher than those of many other previously studied biosorbents and were around three times more than that of aerobically grown E. coli. The FTIR spectra showed an appearance of strong peaks for the amine groups and an increase in the intensity of many other functional groups in the waste biomass of E. coli after hydrogen production in comparison to that of aerobically grown E. coli which explain the higher biosorption capacity for Cd²⁺ or Zn²⁺ by the waste biomass of E. coli after hydrogen production. These results indicate that E. coli waste biomass after hydrogen production can be efficiently used in biosorption technology. Interlinking such biotechnologies is potentially possible in future applications to reduce the cost of the biosorption technology and duplicate the benefits of biological H₂ production technology.     

Biohydrogen production by Thermoanaerobacterium thermosaccharolyticum TERI S7 from oil reservoir flow pipeline

Thermophilic dark fermentative hydrogen producing bacterial strain, TERI S7, isolated from an oil reservoir flow pipeline located in Mumbai, India, showed 98% identity with Thermoanaerobacterium thermosaccharolyticum by 16S rRNA gene analysis. It produced 1450–1900 ml/L hydrogen under both acidic and alkaline conditions; at a temperature range of 45–60 °C. The maximum hydrogen yield was 2.5 ± 0.2 mol H₂/mol glucose, 2.2 ± 0.2 mol H₂/mol xylose and 5.2 ± 0.2 mol H₂/mol sucrose, when the respective sugars were used as carbon source. The cumulative hydrogen production, hydrogen production rate and specific hydrogen production rate by the strain TERI S7 with sucrose as carbon source was found to be 1704 ± 105 ml/L, 71 ± 6 ml/L/h and 142 ± 13 ml/g/h respectively. Major soluble metabolites produced during fermentation were acetic acid and butyric acid. The strain TERI S7 was also observed to produce hydrogen continuously up to 48 h at pH 3.9.     

Biohydrogen production from beet molasses by sequential dark and photofermentation

Biological hydrogen production using renewable resources is a promising possibility to generate hydrogen in a sustainable way. In this study, a sequential dark and photofermentation has been employed for biohydrogen production using sugar beet molasses as a feedstock. An extreme thermophile Caldicellulosiruptor saccharolyticus was used for the dark fermentation, and several photosynthetic bacteria (Rhodobacter capsulatus wild type, R. capsulatus hup⁻ mutant, and Rhodopseudomonas palustris) were used for the photofermentation. C. saccharolyticus was grown in a pH-controlled bioreactor, in batch mode, on molasses with an initial sucrose concentration of 15 g/L. The influence of additions of NH₄⁺ and yeast extract on sucrose consumption and hydrogen production was determined. The highest hydrogen yield (4.2 mol of H₂/mol sucrose) and maximum volumetric productivity (7.1 mmol H₂/L_c.h) were obtained in the absence of NH₄⁺. The effluent of the dark fermentation containing no NH₄⁺ was fed to a photobioreactor, and hydrogen production was monitored under continuous illumination, in batch mode. Productivity and yield were improved by dilution of the dark fermentor effluent (DFE) and the additions of buffer, iron-citrate and sodium molybdate. The highest hydrogen yield (58% of the theoretical hydrogen yield of the consumed organic acids) and productivity (1.37 mmol H₂/L_c.h) were attained using the hup⁻ mutant of R. capsulatus. The overall hydrogen yield from sucrose increased from the maximum of 4.2 mol H₂/mol sucrose in dark fermentation to 13.7 mol H₂/mol sucrose (corresponding to 57% of the theoretical yield of 24 mol of H₂/mole of sucrose) by sequential dark and photofermentation.     

Sugarcane vinasse as substrate for fermentative hydrogen production: The effects of temperature and substrate concentration

The present study aimed to evaluate the hydrogen production of a microbial consortium using different concentrations of sugarcane vinasse (2–12 g COD L⁻¹) at 37 °C and 55 °C. In mesophilic tests, the increase in vinasse concentration did not significantly impact the hydrogen yield (HY) (from 1.72 to 2.23 mmol H₂ g⁻¹ COD_influent) but had a positive effect on the hydrogen production potential (P) and hydrogen production rate (R_m). On the other hand, the increase in the substrate concentration caused a drop in HY from 2.31 to 0.44 mmol H₂ g⁻¹ COD_influent in the tests performed at 55 °C with vinasse concentrations from 2 to 12 g COD L⁻¹. The mesophilic community was composed of different species within the Clostridium genus, and the thermophilic community was dominated by organisms affiliated with the Thermoanaerobacter genus. Not all isolates affiliated with the Clostridium genus contributed to a high HY, as the homoacetogenic pathway can occur.     

Impact of regulated pH on proto scale hydrogen production from xylose by an alkaline tolerant novel bacterial strain, Enterobacter cloacae DT-1

A hydrogen producing facultative anaerobic alkaline tolerant novel bacterial strain was isolated from crude oil contaminated soil and identified as Enterobacter cloacae DT-1 based on 16S rRNA gene sequence analysis. DT-1 strain could utilize various carbon sources; glycerol, CMCellulose, glucose and xylose, which demonstrates that DT-1 has potential for hydrogen generation from renewable wastes. Batch fermentative studies were carried out for optimization of pH and Fe²⁺ concentration. DT-1 could generate hydrogen at wide range of pH (5–10) at 37 °C. Optimum pH was; 8, at which maximum hydrogen was obtained from glucose (32 mmol/L), when used as substrate in BSH medium containing 5 mg/L Fe²⁺ ion. Decrease in hydrogen partial pressure by lowering the total pressure in the fermenter head space, enhanced the hydrogen production performance of DT-1 from 32 mmol H₂/L to 42 mmol H₂/L from glucose and from 19 mmol H₂/L to 33 mmol H₂/L from xylose. Hydrogen yield efficiency (HY) of DT-1 from glucose and xylose was 1.4 mol H₂/mol glucose and 2.2 mol H₂/mol xylose, respectively. Scale up of batch fermentative hydrogen production in proto scale (20 L working volume) at regulated pH, enhanced the HY efficiency of DT-1 from 2.2 to 2.8 mol H₂/mol xylose (1.27 fold increase in HY from laboratory scale). 84% of maximum theoretical possible HY efficiency from xylose was achieved by DT-1. Acetate and ethanol were the major metabolites generated during hydrogen production.     

Optimization of thermophilic fermentative hydrogen production by the newly isolated Caloranaerobacter azorensis H53214 from deep-sea hydrothermal vent environment

A unique thermophilic fermentative hydrogen-producing strain H53214 was isolated from a deep-sea hydrothermal vent environment, and identified as Caloranaerobacter azorensis based on bacterial 16S rRNA gene analysis. The optimum culture condition for hydrogen production by the bacterium, designated C. azorensis H53214, was investigated by the response surface methodology (RSM). Eight variables including the concentration of NaCl, glucose, yeast, tryptone, FeSO₄ and MgSO₄, initial pH and incubation temperature were screened based on the Plackett–Burman design. The results showed that initial pH, tryptone and yeast were significant variables, which were further optimized using the steepest ascent method and Box–Behnken design. The optimal culture conditions for hydrogen production were an initial pH of 7.7, 8.3 g L⁻¹ tryptone and 7.9 g L⁻¹ yeast. Under these conditions, the maximum cumulative hydrogen volume, hydrogen yield and maximum H₂ production rate were 1.58 L H₂ L⁻¹ medium, 1.46 mol H₂ mol⁻¹ glucose and 25.7 mmol H₂ g⁻¹ cell dry weight (CDW) h⁻¹, respectively. By comparison analysis, strain H53214 was superior to the most thermophilic hydrogen producers because of the high hydrogen production rate. In addition, the isolation of C. azorensis H53214 indicated the deep-sea hydrothermal environment might be a potential source for fermentative hydrogen-producing thermophiles.     

Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen

Increasing awareness of environmental problems caused by the current use of fossil fuel-based energy, has led to the search for alternatives. Hydrogen is a good alternative and the cyanobacterium Anabaena sp. PCC 7120 is naturally able to produce molecular hydrogen, photosynthetically from water and light. However, this H₂ is rapidly consumed by the uptake hydrogenase.This study evaluated the hydrogen production of Anabaena sp. PCC 7120 wild-type and mutants: hupL⁻ (deficient in the uptake hydrogenase), hoxH⁻ (deficient in the bidirectional hydrogenase) and hupL⁻/hoxH⁻ (deficient in both hydrogenases) on several experimental conditions, such as gas atmosphere (argon and propane with or without N₂ and/or CO₂ addition), light intensity (54 and 152 ??Em⁻²s⁻¹), light regime (continuous and light/dark cycles 16 h/8 h) and nickel concentrations in the culture medium.In every assay, the hupL⁻ and hupL⁻/hoxH⁻ mutants stood out over wild-type cells and the hoxH⁻ mutant. Nevertheless, the hupL⁻ mutant showed the best hydrogen production except in an argon atmosphere under 16 h light/8 h dark cycles at 54 ??Em⁻²s⁻¹ in the light period, with 1 ??M of NiCl₂ supplementation in the culture medium, and under a propane atmosphere.In all strains, higher light intensity leads to higher hydrogen production and if there is a daily 1% of CO₂ addition in the gas atmosphere, hydrogen production could increase 5.8 times, related to the great increase in heterocysts differentiation (5 times more, approximately), whereas nickel supplementation in the culture medium was not shown to increase hydrogen production. The daily incorporation of 1% of CO₂ plus 1% of N₂ did not affect positively hydrogen production rate.