Genetic engineering of cyanobacteria to enhance biohydrogen production from sunlight and water

To mitigate global warming caused by burning fossil fuels, a renewable energy source available in large quantity is urgently required. We are proposing large-scale photobiological H(2) production by mariculture-raised cyanobacteria where the microbes capture part of the huge amount of solar energy received on earth's surface and use water as the source of electrons to reduce protons. The H(2) production system is based on photosynthetic and nitrogenase activities of cyanobacteria, using uptake hydrogenase mutants that can accumulate H(2) for extended periods even in the presence of evolved O(2). This review summarizes our efforts to improve the rate of photobiological H(2) production through genetic engineering. The challenges yet to be overcome to further increase the conversion efficiency of solar energy to H(2) also are discussed.     

Design,Engineering, and Construction of Photosynthetic Microbial Cell Factories for Renewable Solar Fuel Production

There is an urgent need to develop sustainable solutions to convert solar energy into energy carriers used in the society. In addition to solar cells generating electricity, there are several options to generate solar fuels. This paper outlines and discusses the design and engineering of photosynthetic microbial systems for the generation of renewable solar fuels, with a focus on cyanobacteria. Cyanobacteria are prokaryotic microorganisms with the same type of photosynthesis as higher plants. Native and engineered cyanobacteria have been used by us and others as model systems to examine, demonstrate, and develop photobiological H₂ production. More recently, the production of carbon-containing solar fuels like ethanol, butanol, and isoprene have been demonstrated. We are using a synthetic biology approach to develop efficient photosynthetic microbial cell factories for direct generation of biofuels from solar energy. Present progress and advances in the design, engineering, and construction of such cyanobacterial cells for the generation of a portfolio of solar fuels, e.g., hydrogen, alcohols, and isoprene, are presented and discussed. Possibilities and challenges when introducing and using synthetic biology are highlighted.     

Design, engineering, and construction of photosynthetic microbial cell factories for renewable solar fuel production

There is an urgent need to develop sustainable solutions to convert solar energy into energy carriers used in the society. In addition to solar cells generating electricity, there are several options to generate solar fuels. This paper outlines and discusses the design and engineering of photosynthetic microbial systems for the generation of renewable solar fuels, with a focus on cyanobacteria. Cyanobacteria are prokaryotic microorganisms with the same type of photosynthesis as higher plants. Native and engineered cyanobacteria have been used by us and others as model systems to examine, demonstrate, and develop photobiological H(2) production. More recently, the production of carbon-containing solar fuels like ethanol, butanol, and isoprene have been demonstrated. We are using a synthetic biology approach to develop efficient photosynthetic microbial cell factories for direct generation of biofuels from solar energy. Present progress and advances in the design, engineering, and construction of such cyanobacterial cells for the generation of a portfolio of solar fuels, e.g., hydrogen, alcohols, and isoprene, are presented and discussed. Possibilities and challenges when introducing and using synthetic biology are highlighted.     

Biohythane production from organic wastes: present state of art

The economy of an industrialized country is greatly dependent on fossil fuels. However, these nonrenewable sources of energy are nearing the brink of extinction. Moreover, the reliance on these fuels has led to increased levels of pollution which have caused serious adverse impacts on the environment. Hydrogen has emerged as a promising alternative since it does not produce CO₂ during combustion and also has the highest calorific value. The biohythane process comprises of biohydrogen production followed by biomethanation. Biological H₂ production has an edge over its chemical counterpart mainly because it is environmentally benign. Maximization of gaseous energy recovery could be achieved by integrating dark fermentative hydrogen production followed by biomethanation. Intensive research work has already been carried out on the advancement of biohydrogen production processes, such as the development of suitable microbial consortium (mesophiles or thermophiles), genetically modified microorganism, improvement of the reactor designs, use of different solid matrices for the immobilization of whole cells, and development of two-stage process for higher rate of H₂ production. Scale-up studies of the dark fermentation process was successfully carried out in 20- and 800-L reactors. However, the total gaseous energy recovery for two stage process was found to be 53.6 %. From single-stage H₂ production, gaseous energy recovery was only 28 %. Thus, two-stage systems not only help in improving gaseous energy recovery but also can make biohythane (mixture of H₂ and CH₄) concept commercially feasible.     

South Pole Antarctica observations and modeling results: New insights on HOx radical and sulfur chemistry

Measurements of OH, H₂SO₄, and MSA at South Pole (SP) Antarctica were recorded as a part of the 2003 Antarctic Chemistry Investigation (ANTCI 2003). The time period 22 November, 2003–2 January, 2004 provided a unique opportunity to observe atmospheric chemistry at SP under both natural conditions as well as those uniquely defined by a solar eclipse event. Results under natural solar conditions generally confirmed those reported previously in the year 2000. In both years the major chemical driver leading to large scale fluctuations in OH was shifts in the concentration levels of NO. Like in 2000, however, the 2003 observational data were systematically lower than model predictions. This can be interpreted as indicating that the model mechanism is still missing a significant HO_x sink reaction(s); or, alternatively, that the OH calibration source may have problems. Still a final possibility could involve the integrity of the OH sampling scheme which involved a fixed building site. As expected, during the peak in the solar eclipse both NO and OH showed large decreases in their respective concentrations. Interestingly, the observational OH profile could only be approximated by the model mechanism upon adding an additional HO_x radical source in the form of snow emissions of CH₂O and/or H₂O₂. This would lead one to think that either CH₂O and/or H₂O₂ snow emissions represent a significant HO_x radical source under summertime conditions at SP. Observations of H₂SO₄ and MSA revealed both species to be present at very low concentrations (e.g., 5 × 10⁵ and 1 × 10⁵ molec cm^?3, respectively), but similar to those reported in 2000. The first measurements of SO₂ at SP demonstrated a close coupling with the oxidation product H₂SO₄. The observed low concentrations of MSA appear to be counter to the most recent thinking by glacio-chemists who have suggested that the plateau's lower atmosphere should have elevated levels of MSA. We speculate here that the absence of MSA may reflect efficient atmospheric removal mechanisms for this species involving either dynamical and/or chemical processes.     

Analysis of experiments on ion-induced nucleation and aerosol formation in the presence of UV light and ionizing radiation

Under typical atmospheric conditions, sulfuric acid and water vapors are likely most important species in the nucleation of new aerosol particles. The main source of H₂SO₄ in the atmosphere is oxidation of SO₂. Hence, an understanding of the subsequent chemical reactions followed by aerosol-particles formation is of fundamental importance. Here we analyze the results of laboratory experiments in Svensmark et al. (2007) in which (i) the formation of neutral aerosol particles was observed at reported sulfuric acid concentrations well below the range where the binary homogeneous nucleation in a mixture of H₂SO₄–H₂O vapors could be important and (ii) an electron catalytic effect on particle nucleation was suggested as an explanation of the experimental results and as a potential source of aerosol-particles formation in the Earth's atmosphere. In the article we give an interpretation of these experimental data based on a known mechanism of the neutral particles formation via ion-induced nucleation followed by recombination of charged clusters. The main results of our investigation are the following: (i) the observed neutral particles were likely formed via the recombination of ion clusters; (ii) the phenomena of electron photodetachment from ion clusters under UV radiation was improbable in conditions of this experiment and likely unrealized for typical negative ion clusters found in the Earth's atmosphere. In total, these experiments and model investigations show that far more and specially directed laboratory experiments are needed to clarify the ways by which cosmic rays and solar radiation may link to the Earth's climate.     

Factors affecting the levels of hydrogen peroxide in rainwater

Measurements of hydrogen peroxide (H₂O₂) and several meteorological and chemical parameters were made for 34 rain events which occurred in Miami, Florida between April, 1995 and October, 1996. The measured H₂O₂ concentrations ranged from 0.3 to 38.6 μM with an average concentration of 6.9 μM. A strong seasonal dependence for H₂O₂ concentrations was observed during this period, with highest concentrations in the summer and lower levels in the winter, which corresponds to the stronger solar radiation and higher vaporization of volatile organic compounds (VOCs) in the summer and fall, and the weaker sunlight and lower vaporization in the winter and spring. Measurements also showed a significant increase trend of H₂O₂ with increasing ambient rainwater temperature. Rains that were out from lower latitude were exposed to higher solar irradiation and contained relatively higher levels of H₂O₂ than those from the north. All these observations indicate that photochemical reactions that involved volatile organic compounds are the predominant source of H₂O₂ observed in rainwater. During several individual rainstorms, H₂O₂ concentration was found to increase as a function of time due to electrical storm activities. This finding suggests that lightning could be an important factor that determines the level of H₂O₂ during thunderstorms. Statistical data showed that the highest concentrations of H₂O₂ were observed only in rains containing low levels of nonsea-salt sulfate (NSS), nitrate and hydrogen ion. H₂O₂ concentrations in continental originated rains were much lower than marine originated ones, indicating that air pollutants in continental rains could significantly deplete the H₂O₂ concentration in atmospheric gas-phase, clouds and rainwater.     

Atmospheric hydrogen peroxide: Evidence for aqueous-phase formation from a historic perspective and a one-year measurement campaign

H₂O₂ is produced in the atmospheric gas phase only through a single pathway, the HO₂ radical recombination. Its main role has been identified in oxidizing SO₂ dissolved in hydrometeors to sulphate. Thus aqueous-phase chemistry has been considered to be a main sink (apart from dry deposition and scavenging) but rarely a source of H₂O₂ despite early findings of its heterogeneous and aqueous-phase production. The aim of this paper is to discuss the atmospheric budget of H₂O₂ from the multiphase chemistry approach with special emphasis on new sources other than gas-phase HO₂ recombination. After providing a brief historic view on H₂O₂ chemistry, often unknown to young atmospheric chemists but important for a complete understanding, the results of a one-year study of simultaneous measurements of H₂O₂ in rain and air are presented that show strong evidence for aqueous-phase H₂O₂ formation. Implications for future changes in atmospheric chemistry are discussed from the viewpoint of an “interfacial chemistry”.     

From first generation biofuels to advanced solar biofuels

Roadmaps towards sustainable bioeconomy, including the production of biofuels, in many EU countries mostly rely on biomass use. However, although biomass is renewable, the efficiency of biomass production is too low to be able to fully replace the fossil fuels. The use of land for fuel production also introduces ethical problems in increasing the food price. Harvesting solar energy by the photosynthetic machinery of plants and autotrophic microorganisms is the basis for all biomass production. This paper describes current challenges and possibilities to sustainably increase the biomass production and highlights future technologies to further enhance biofuel production directly from sunlight. The biggest scientific breakthroughs are expected to rely on a new technology called “synthetic biology”, which makes engineering of biological systems possible. It will enable direct conversion of solar energy to a fuel from inexhaustible raw materials: sun light, water and CO₂. In the future, such solar biofuels are expected to be produced in engineered photosynthetic microorganisms or in completely synthetic living factories.     

Kinetic degradation of the pollutant guaiacol by dark Fenton and solar photo-Fenton processes

This work is first intended to optimize the experimental conditions for the maximum degradation of guaiacol (2-methoxyphenol) by Fenton’s reagent, and second, to improve the process efficiency through the use of solar radiation. Guaiacol is considered as a model compound of pulp and paper mill effluent. The experiments were carried out in a laboratory-scale reactor subjected or not to solar radiation. Hydrogen peroxide solution was continuously introduced into the reactor at a constant flow rate. The kinetics of organic matter decay was evaluated by means of the chemical oxygen demand (COD) and the absorbance measurements. The experimental results showed that the Fenton and solar photo-Fenton systems lead successfully to 90% elimination of COD and absorbance at 604 nm from a guaiacol solution under particular experimental conditions. The COD removal always obeyed a pseudo-first-order kinetics. The effect of pH, temperature, H₂O₂ dosing rate, initial concentration of Fe²⁺, and initial COD was investigated using the Fenton process. The solar photo-Fenton system needed less time and consequently less quantity of H₂O₂. Under the optimum experimental conditions, the solar photo-Fenton process needs a dose of H₂O₂ 40% lower than that used in the Fenton process to remove 90% of COD.     

Hydroxyl (OH) radical production rates in snowpacks from photolysis of hydrogen peroxide (H2O2) and nitrate (NO3−)

Atmospheric chemistry directly above snowpacks is strongly influenced by ultraviolet (UV) radiation initiated emissions of chemicals from the snowpack. The emission of gases from the snowpack to the atmosphere is in part due to chemical reactions between hydroxyl radical, OH (produced from photolysis of hydrogen peroxide (H₂O₂) or nitrate (NO₃⁻)) and impurities in the snowpack. The work presented here is a radiative-transfer modelling study to calculate the depth-integrated production rates of hydroxyl radical from the photolysis of hydrogen peroxide and nitrate anion in snow for four different snowpacks and for solar zenith angles 30°–90°. This work also demonstrates the importance of hydrogen peroxide photolysis to produce hydroxyl radical relative to nitrate photolysis with (a) different snowpacks, (b) different ozone column depths, and (c) snowpack depths. The importance of hydrogen peroxide photolysis over nitrate photolysis for hydroxyl radical production increases with increasing depth in snowpack, column ozone depth, and solar zenith angle. With a solar zenith angle of 60° the production of hydroxyl radical from hydrogen peroxide photolysis accounts for 91–99% of all hydroxyl radical production from hydrogen peroxide and nitrate photolysis.     

Solar-driven photocatalytic treatment of diclofenac using immobilized TiO<Subscript>2</Subscript>-based zeolite composites

The study is aimed at evaluating the potential of immobilized TiO₂-based zeolite composite for solar-driven photocatalytic water treatment. In that purpose, TiO₂-iron-exchanged zeolite (FeZ) composite was prepared using commercial Aeroxide TiO₂ P25 and iron-exchanged zeolite of ZSM5 type, FeZ. The activity of TiO₂-FeZ, immobilized on glass support, was evaluated under solar irradiation for removal of diclofenac (DCF) in water. TiO₂-FeZ immobilized in a form of thin film was characterized for its morphology, structure, and composition using scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM/EDX). Diffuse reflectance spectroscopy (DRS) was used to determine potential changes in band gaps of prepared TiO₂-FeZ in comparison to pure TiO₂. The influence of pH, concentration of hydrogen peroxide, FeZ wt% within the composite, and photocatalyst dosage on DCF removal and conversion efficiency by solar/TiO₂-FeZ/H₂O₂ process was investigated. TiO₂-FeZ demonstrated higher photocatalytic activity than pure TiO₂ under solar irradiation in acidic conditions and presence of H₂O₂.     

Analysis of a solar still with photovoltaic modules and electrical heater - Energy and exergy approach

The availability of drinkable water, along with food and air, is a fundamental human necessity. Because of the presence of higher amounts of salt and pollution, direct use of water from sources such as lakes, sea, rivers, and subsurface water reservoirs is not normally suggested. Solar is still a basic technology that can use solar energy to transform accessible waste or brackish water into drinkable water. Exergy analysis is a strong inferential technique for evaluating the performance of thermal systems. Exergy is becoming more popular as a predictive tool for analysis, and there is a rising interest in using it. In this paper, performance analysis on the aspect of energy and exergy from the proposed solar still (PSS) (conventional solar still with the photovoltaic modules-AC heater) was analyzed on three different water depths (W_d) conditions (1, 2, and 3 cm). Using a solar still with an electric heater, the daily potable water production was found as 8.54, 6.37, and 4.43 kg, for the variations in water depth (W_d) of 1, 2, and 3 cm respectively. The energy and exergy efficiency of the PSS at the W_d of 1, 2, and 3 cm were 75.67, 51.45, and 37.21% and 5.08, 2.29, and 1.03%, respectively. At 1 cm W_d, PSS produced the maximum freshwater yield as compared to the other two water depths. When the W_d is increased from 1 to 2 cm and from 1 to 3 cm, the yield is decreased up to 27.3 and 52.7%, respectively. Similarly, the energy and exergy efficiency is decreased up to 36.8 and 53.2% and 50.4 and 80.6%, respectively. The water cost of the modified solar still is calculated as 0.028 $/kg for the least water thickness.

     

Nitrogen fixation by cyanobacteria stimulates production in Baltic food webs

Filamentous, nitrogen-fixing cyanobacteria form extensive summer blooms in the Baltic Sea. Their ability to fix dissolved N₂ allows cyanobacteria to circumvent the general summer nitrogen limitation, while also generating a supply of novel bioavailable nitrogen for the food web. However, the fate of the nitrogen fixed by cyanobacteria remains unresolved, as does its importance for secondary production in the Baltic Sea. Here, we synthesize recent experimental and field studies providing strong empirical evidence that cyanobacterial nitrogen is efficiently assimilated and transferred in Baltic food webs via two major pathways: directly by grazing on fresh or decaying cyanobacteria and indirectly through the uptake by other phytoplankton and microbes of bioavailable nitrogen exuded from cyanobacterial cells. This information is an essential step toward guiding nutrient management to minimize noxious blooms without overly reducing secondary production, and ultimately most probably fish production in the Baltic Sea.     

An assessment of the polar HOx photochemical budget based on 2003 Summit Greenland field observations

An interpretative modeling analysis is conducted to simulate the diurnal variations in OH and HO₂+RO₂ observed at Summit, Greenland in 2003. The main goal is to assess the HO_x budget and to quantify the impact of snow emissions on ambient HO_x as well as on CH₂O and H₂O₂. This analysis is based on composite diurnal profiles of HO_x precursors recorded during a 3-day period (July 7–9), which were generally compatible with values reported in earlier studies. The model simulations can reproduce the observed diurnal variation in HO₂+RO₂ when they are constrained by observations of H₂O₂ and CH₂O. By contrast, model predictions of OH were about factor of 2 higher than the observed values. Modeling analysis of H₂O₂ suggests that its distinct diurnal variation is likely controlled by snow emissions and loss by deposition and/or scavenging. Similarly, deposition and/or scavenging sinks are needed to reproduce the observed diel profile in CH₂O. This study suggests that for the Summit 2003 period snow emissions contribute ∼25% of the total CH₂O production, while photochemical oxidation of hydrocarbon appears to be the dominant source. A budget assessment of HO_x radicals shows that primary production from O(¹D)+H₂O and photolysis of snow emitted precursors (i.e., H₂O₂ and CH₂O) are the largest primary HO_x sources at Summit, contributing 41% and 40%, respectively. The snow contribution to the HO_x budget is mostly in the form of emissions of H₂O₂. The dominant HO_x sink involves the HO₂+HO₂ reaction forming H₂O₂, followed by its deposition to snow. These results differ from those previously reported for the South Pole (SP), in that primary production of HO_x was shown to be largely driven by both the photolysis of CH₂O and H₂O₂ emissions (46%) with smaller contributions coming from the oxidation of CH₄ and the O(¹D)+H₂O reaction (i.e., 27% each). In sharp contrast to the findings at Summit in 2003, due to the much higher levels of NO_x, the SP HO_x sinks are dominated by HO_x–NO_x reactions, leading to the formation and deposition of HNO₃ and HO₂NO₂. Thus, a comparison between SP and Summit studies suggests that snow emissions appear to play a prominent role in controlling primary HO_x production in both environments. However, as regards to maintaining highly elevated levels of OH, the two environments differ substantially. At Summit the elevated rate for primary production of HO_x is most important; whereas, at SP it is the rapid recycling of the more prevalent HO₂ radical, through reaction with NO, back to OH that is primarily responsible.     

Utilization of waste bittern from saltern as a source for magnesium and an absorbent for carbon dioxide capture

During solar salt production, large quantities of bittern, a liquid by-product containing high inorganic substance concentrations, are produced. The purpose of this research was to examine the utilization of waste bittern generated from salterns as a source for Mg production and as an absorbent for carbon dioxide (CO₂) capture. The study was conducted in a sequential two-step process. At NaOH/Mg molar ratios of 2.70–2.75 and pH 9.5–10.0, > 99% Mg precipitation from the bittern was achieved. After washing with water, 100–120 g/L of precipitate containing 94% Mg(OH)₂ was recovered from the bittern. At the optimum NH₄OH concentration of 5%, 120 g of sodium bicarbonate precipitate per liter of bittern were recovered, which was equivalent to 63 g CO₂ captured per liter of bittern. These results can be used to support the use of bittern as a resource and reduce economic losses during solar salt production.     

A Simple Technique for Measuring Mean H2S Concentration

Lead acetate impregnated ceramic tiles are useful devices for determining H₂S levels in the outdoor air. The exposure of tiles in simple shelters and for an overnight period is effective in an areawide sampling program to determine: (1) whether a significant H₂S source exists, (2) the source location, (3) the area affected, and (4) the relative intensity pattern.

On the basis of an overnight exposure, tiles can “see” a mean H₂S concentration range of 0.003 to 0.3 ppm. The lower level of sensitivity is near 0.03 ppm X hr. Hence, tiles offer a way to verify whether hourly air quality standards are being exceeded.

Tiles can be qualitatively evaluated against: experience, known effects, or by ranking against each other. Tiles can be semi-quantitatively evaluated by visual grading against painted standards developed by exposing tiles of particular manufacture to known H₂S dosages.     

Photo-sensitized oxidation in natural water via OH radicals

A method is presented for determining production and consumption rates of ^.OH radicals produced photochemically in natural surface waters. It is based on the determination of the kinetics by which the concentration of a specified trace compound decreases during irradiation. In samples from Lake Greifensee (Switzerland) low production rates for ^.OH limit its possible effects. In addition, fast consumptions by the natural dissolved organic solutes and by the bicarbonate protect organic micropollutants from oxidation by ^.OH. Neither direct nor indirect H₂O₂ photolysis was a significant source of ^.OH in the lakewater studied lacking iron, whereas nitrate photolysis could have been a source. Comparison with reaction kinetic formulations allows generalizations for other types of waters.     

Kinetics study of fermentative hydrogen production from liquid swine manure supplemented with glucose under controlled pH

Kinetics of H₂ production from liquid swine manure supplemented with glucose by mixed anaerobic cultures was investigated using batch experiments under four different pH conditions (4.4, 5.0, 5.6, and uncontrolled). The temperature for the experiments was controlled at 37 ± 1°C and the length of experiments varied between 50 and 120 hours, depending upon the time needed for completion of each individual experiment. The modified Gompertz model was evaluated for its suitability for describing the H₂ production potential, H₂ production rate, and substrate consumption rate for all the experiments. The results showed that the Gompertz model could adequately fit the experimental results. The effect of pH was significant on all kinetic parameters for H₂ production including yield, production rate and lag time, and the substrate utilization rate. The optimal pH was found to be 5.0, at which a maximum H₂ production rate (0.64 L H₂/h) was obtained, and deviation from the optimal pH could result in substantial reductions in H₂ production rate (0.32 L H₂/h for pH 4.0 and 0.43 L H₂/h for pH 5.6). The results also showed that if pH was not controlled for the batch fermentation process, the substrate utilization efficiency could steeply decrease from 98.8% to 33.7%.     

Gas reforming and tar decomposition performance of nickel oxide (NiO)/SBA-15 catalyst in gasification of woody biomass

In the gasification of biomass, it is necessary to limit the amount of by-product tar and increase the yields of hydrogen (H₂) and carbon monoxide (CO) (syngas). Therefore, we conducted gasification and reforming experiments on woody biomass using an electric tubular furnace, to evaluate the gas reforming and tar decomposition performance of a NiO/SBA-15 catalyst. As a result, we found that this catalyst is effective for H₂ production. It is believed that the increase in H₂ volume due to the catalyst occurs through a steam reforming reaction involving hydrocarbons, including methane (CH₄), and the water-gas shift reaction. With respect to the influence of the gasifying agent on the reforming effect of the catalyst, the amount of generated carbon dioxide (CO₂) and hydrogen (H₂) increases because the shift reaction is promoted by supplying steam. On the other hand, it was inferred that the shift reaction rarely occurred because it approaches equilibrium by supplying O₂. Furthermore, it is suggested that light aromatic hydrocarbons are decomposed by the catalyst.

Implications: The mesoporous silica catalyst NiO/SBA-15 was highly effective for H₂ production and decomposition of light aromatic compounds in the gasification of woody biomass. In the catalyst reaction, supplying steam promoted H₂ production. From thermodynamic analysis and discussion, it was also inferred that supplying O₂ might prevent the water gas shift reaction. The results are useful for designing a process needed for rich H₂ production and gas refining process for further use of syngas.