Brief Discussion on Hydrogen Production by Photosynthetic Biology
Release Date: 2022-01-08 16:21:35 Visit: 303
Photosynthetic bacteria are bacteria that can use organic matter as oxygen donor and carbon source to perform photosynthesis under anaerobic light or aerobic dark conditions, and have the characteristics of changing metabolic types with changes in environmental conditions. There are three types of microorganisms that can realize photobiological hydrogen production: aerobic green algae, blue-green algae and anaerobic photosynthetic bacteria. These so-called photo-oxygen organisms use light as an energy source, make full use of solar energy, and carry out activities that only release hydrogen without producing oxygen. Among them, photosynthetic bacteria have higher oxygen production purity and hydrogen production efficiency than cyanobacteria and green algae. At the same time, the conditions for hydrogen production by photosynthetic bacteria are moderate, and they can use a variety of organic wastes as substrates for hydrogen production to achieve dual effects of energy production and waste utilization. Therefore, hydrogen production by photosynthetic bacteria is considered to be an important form and way of future energy supply. .
Gaffron, Rubin and Spruit also reported the use of photobiological processes for hydrogen and oxygen production, which is the use of solar energy and photosynthetic microorganisms for efficient hydrogen production [18,19]. The hydrogen production process of photosynthetic bacteria can be realized in two ways: removing the global greenhouse gas CO2 and generating non-polluting renewable energy (through the biological photosynthesis of green algae and cyanobacteria); using environmentally harmful wastes as substrates (Photofermentation by photosynthetic bacteria). Since Gest first proved that photosynthetic bacteria can use organic matter as a hydrogen donor to prevent hydrogen release in photosynthesis, the research on the mechanism of photosynthetic hydrogen release has always been a hot and difficult research topic. Researchers in Japan, the United States, and Europe have done a lot of research on this, but the photosynthetic hydrogen release process is very complex and highly precise. The current research is mainly focused on the selection and optimization of highly active hydrogen-producing strains and the selection of environmental conditions. The purpose is to increase the hydrogen production, and the scale of the research level is still basically in the laboratory stage.
Compared with thermochemical and electrochemical hydrogen production, photosynthetic biological hydrogen production has many limitations and the hydrogen production rate is very low. Theoretically, in the process of direct biological photosynthesis, 2 mol of water can produce 2 mol of hydrogen; in indirect biological photosynthesis, 12 mol of hydrogen can be generated from 1 mol of glucose; in the process of photofermentation, 1 mol of acetic acid can produce 4 mol of hydrogen. However, the actual hydrogen production rate is much lower than the maximum theoretical hydrogen production rate, and the reason is that in algae and cyanobacteria, hydrogen-producing enzymes are involved, and if the enzyme and nitrogenase are involved, the oxygen produced by the system makes the activity decrease rapidly. The advantage of using purple photosynthetic bacteria for hydrogen production is that there is no intervention of oxygen, but the absorption of hydrogenase reduces the hydrogen production rate of the whole process.
1. Principle
The physiological functions and metabolic roles of photosynthetic microorganisms are diverse, so they have different hydrogen production pathways. The photosynthetic biological hydrogen production pathway is shown in Figure 3-4 (P106). Both cyanobacteria and green algae can produce hydrogen through both direct and indirect photosynthesis.
The direct photosynthetic hydrogen production process of cyanobacteria and green algae uses solar energy to directly split water to generate hydrogen and oxygen. It shows aerobic photosynthesis similar to higher plants in capturing solar energy, including two photosynthetic systems (PSI and PSII). Hydrogenases can also use electrons from ferredoxin to reduce protons to produce hydrogen gas when oxygen is insufficient. In a photoreactor, partial inhibition of the cell's photosynthetic system PSII produces anaerobic conditions because only a small amount of water is oxidized to oxygen, and the remaining oxygen is consumed by respiration.
The chemical reaction formula is
2H2O+hν→O2↑+4H++Fd(red)(4e-)→Fd(red)(4e-)+4H+→Fd(ox)+2H2
Indirect biological photosynthesis is the process of efficiently separating oxygen from hydrogen, especially most common in cyanobacteria, where stored carbohydrates are oxidized to produce hydrogen.
chemical reaction
12H2O+6CO2→C6H12O6+6O2;C6H12O6+12H2O→12H2+6CO2
Under anaerobic dark conditions, pyruvate ferredoxin oxidoreductase loses the carbonic acid group of pyruvate, and acetyl-CoA generates hydrogen through the reduction of ferredoxin. Pyruvate dehydrogenase (PDH) produces NADH during the metabolism of pyruvate, and ferredoxin is reduced by NADH in places with less sunlight. Nitrogen-fixing cyanobacteria produce hydrogen gas (fixing N2 to NH3) primarily by nitrogenase, not by hydrogenation, which has a two-way effect. However, in many cyanobacteria that do not fix nitrogen, hydrogen production can also be observed by a bidirectional hydrogenase.
The two main types of photosynthetic bacteria are purple bacteria and green bacteria, which utilize only one photosystem for photosynthesis. Green bacteria have PS I type reaction centers. Inorganic/organic substrates are oxidized, donating electrons, reducing ferredoxin via FeS protein. The reduced ferredoxin acts directly as the electron donor for the dark reaction as well as the hydrogen production reaction. In contrast, purple bacteria contain a reaction center-like PSII system that cannot reduce ferredoxin, but can generate ATP through cyclic electron flow. The electrons required for the nitrogenase-promoted hydrogen evolution process come from inorganic/organic substrates. Bacteriochlorophyll enters the "benzoquinone pool" through the reaction center. The energy barrier of benzoquinone is not sufficiently negative to directly reduce NAD+. Therefore, electrons from the quinone pool are forced to reduce NAD+ to NADH in turn. The electrons required for this process are called reverse electron flow. No oxygen is produced during the entire process, and the net total amount of hydrogen produced is affected by the hydrogenase activity. Studies have shown that the photosynthesis of purple bacteria is considered to be the best in terms of hydrogen production by biological photosynthesis, because purple can utilize industrial wastes and the products of fermentation processes (such as organic acids, etc.).
2. Photobioreactor
In order to improve the hydrogen production rate of biological hydrogen production, various photobioreactors have been designed. In a photobioreactor, light energy is converted into biochemical energy. The most basic factors that distinguish the photobioreactor from other ordinary reactors are: the reactor is transparent, so that the light can pass through to the maximum extent; the energy is instantaneous and cannot be stored in the reactor; the cells are self-shading. Self-shading leads to loss of additional absorbed energy, fluorescence and heat increase temperatures, and bioreactors require additional cooling systems. The thickness of the reactor is usually small, thereby increasing the reactor area to volume ratio and avoiding the effects of cell self-shading.
There are many factors affecting the hydrogen production efficiency of photobioreactors. In order to improve the hydrogen production and biomass of photosynthesis, a variety of bioreactors have been designed. Bioreactor. Various photobioreactors have their own advantages and disadvantages. For example, in the tubular photoreactor, a larger light irradiation area can be obtained, but the dissolved oxygen concentration generated by photosynthesis is higher and the energy input by the pump is also higher. The scale of such a reactor is limited. In the column-type photobioreactor, the irradiation area of light is small, but due to its small size, low price, easy operation, and the ability to stir by bubbling, it is widely used in microalgae and photosynthetic bacteria. When hydrogen is used, column-type photobioreactors are still widely used. In the plate reactor, the photosynthesis efficiency is high and the air pressure is controllable. Compared with other reactors, the cost is low, but it is difficult to maintain a constant culture temperature and proper agitation during the hydrogen production process.
In addition to the effect of the shape of the photobioreactor on the hydrogen production efficiency, the physicochemical parameters of the photobioreactor also affect the hydrogen production, such as the pH of the solution, temperature, light intensity, depth of light penetration, dissolved oxygen , dissolved CO2, agitation, gas exchange, carbon and nitrogen sources and the ratio of the two, etc. Whether the photobioreactor is open or closed will affect the setting of these physicochemical parameters.
The main factor limiting large-scale hydrogen production by biological methods is that there is only limited transmission of light energy deep into the reactor; external light sources are placed into the deeper regions of the reactor through fiber optic cables, and the light sources are uniformly distributed. This dispersed light source plays a key role in hydrogen production.
3. Biological hydrogenase
In the process of biological hydrogen production, there are two main enzymes, namely hydrogenase and nitrogenase, which play a catalytic role in biological hydrogen production, reducing proton H+ to generate molecular H2. Both of these enzymes are metalloproteins.
1) Hydrogenase: A series of hydrogen production behaviors that occur in organisms by decomposing water are based on the continuous arrangement of hydrogenase subunits with catalytic functions; according to research, there are 3 different types of hydrogenases, namely [NiFe] hydrogenase, [ FeFe]hydrogenase and Fe-S hydrogenase, of which Fe-S hydrogenase was later named [Fe]hydrogenase; a nucleus with a specific function is present in each enzyme. The hydrogen production of [FeFe]hydrogenase is higher, while the oxygen tolerance of [NiFe]hydrogenase is stronger, and [Fe]hydrogenase does not participate in hydrogen production. [Fe]hydrogenase catalyzes the reduction of CO2 to methane via H2, and most bacteria that rely on this enzyme are alkane-producing organisms.
2) Nitrogenase: Nitrogenase is very important for agricultural production, because through nitrogen fixation, N2 in the atmosphere can be converted into NH3, which naturally fertilizes the soil. Nitrogenase can also produce hydrogen while mainly fixing nitrogen. The study found that nitrogenase can produce more hydrogen under nitrogen-limited conditions.
Biohydrogenases play an important role in the process of biohydrogen production, but studies have found that most biohydrogenases are very sensitive to oxygen. In the process of splitting water to produce hydrogen, the generation of oxygen is inevitable, so the continuity of hydrogen generation will be affected. So far, only a few hydrogenases have shown tolerance to oxygen, and many approaches have been tried to develop oxygen-tolerant variants. It is observed that algal hydrogenase is the smallest and simplest hydrogenase, which is most sensitive to oxygen inactivation. Since only the H group where the active site is located, algal hydrogenase is more sensitive to oxygen inactivation. The F group code protects the active site from oxygen inactivation. The amino acid sequence of the H group or the amino acid composition of the active site region is very critical, which is directly related to the protection of the active site from the influence of oxygen.
Nitrogenase is also sensitive to oxygen, but it is relatively less sensitive to oxygen than hydrogenase. Nitrogenase is mainly found in anaerobic prokaryotes, but also in cyanobacteria. Through heterokaryotic cells, anaerobic hydrogen production is isolated from aerobic photosynthesis. Under nitrogen-limited conditions, hydrogen production is favored, and hydrogen production is an energy-intensive process.
Gaffron, Rubin and Spruit also reported the use of photobiological processes for hydrogen and oxygen production, which is the use of solar energy and photosynthetic microorganisms for efficient hydrogen production [18,19]. The hydrogen production process of photosynthetic bacteria can be realized in two ways: removing the global greenhouse gas CO2 and generating non-polluting renewable energy (through the biological photosynthesis of green algae and cyanobacteria); using environmentally harmful wastes as substrates (Photofermentation by photosynthetic bacteria). Since Gest first proved that photosynthetic bacteria can use organic matter as a hydrogen donor to prevent hydrogen release in photosynthesis, the research on the mechanism of photosynthetic hydrogen release has always been a hot and difficult research topic. Researchers in Japan, the United States, and Europe have done a lot of research on this, but the photosynthetic hydrogen release process is very complex and highly precise. The current research is mainly focused on the selection and optimization of highly active hydrogen-producing strains and the selection of environmental conditions. The purpose is to increase the hydrogen production, and the scale of the research level is still basically in the laboratory stage.
Compared with thermochemical and electrochemical hydrogen production, photosynthetic biological hydrogen production has many limitations and the hydrogen production rate is very low. Theoretically, in the process of direct biological photosynthesis, 2 mol of water can produce 2 mol of hydrogen; in indirect biological photosynthesis, 12 mol of hydrogen can be generated from 1 mol of glucose; in the process of photofermentation, 1 mol of acetic acid can produce 4 mol of hydrogen. However, the actual hydrogen production rate is much lower than the maximum theoretical hydrogen production rate, and the reason is that in algae and cyanobacteria, hydrogen-producing enzymes are involved, and if the enzyme and nitrogenase are involved, the oxygen produced by the system makes the activity decrease rapidly. The advantage of using purple photosynthetic bacteria for hydrogen production is that there is no intervention of oxygen, but the absorption of hydrogenase reduces the hydrogen production rate of the whole process.
1. Principle
The physiological functions and metabolic roles of photosynthetic microorganisms are diverse, so they have different hydrogen production pathways. The photosynthetic biological hydrogen production pathway is shown in Figure 3-4 (P106). Both cyanobacteria and green algae can produce hydrogen through both direct and indirect photosynthesis.
The direct photosynthetic hydrogen production process of cyanobacteria and green algae uses solar energy to directly split water to generate hydrogen and oxygen. It shows aerobic photosynthesis similar to higher plants in capturing solar energy, including two photosynthetic systems (PSI and PSII). Hydrogenases can also use electrons from ferredoxin to reduce protons to produce hydrogen gas when oxygen is insufficient. In a photoreactor, partial inhibition of the cell's photosynthetic system PSII produces anaerobic conditions because only a small amount of water is oxidized to oxygen, and the remaining oxygen is consumed by respiration.
The chemical reaction formula is
2H2O+hν→O2↑+4H++Fd(red)(4e-)→Fd(red)(4e-)+4H+→Fd(ox)+2H2
Indirect biological photosynthesis is the process of efficiently separating oxygen from hydrogen, especially most common in cyanobacteria, where stored carbohydrates are oxidized to produce hydrogen.
chemical reaction
12H2O+6CO2→C6H12O6+6O2;C6H12O6+12H2O→12H2+6CO2
Under anaerobic dark conditions, pyruvate ferredoxin oxidoreductase loses the carbonic acid group of pyruvate, and acetyl-CoA generates hydrogen through the reduction of ferredoxin. Pyruvate dehydrogenase (PDH) produces NADH during the metabolism of pyruvate, and ferredoxin is reduced by NADH in places with less sunlight. Nitrogen-fixing cyanobacteria produce hydrogen gas (fixing N2 to NH3) primarily by nitrogenase, not by hydrogenation, which has a two-way effect. However, in many cyanobacteria that do not fix nitrogen, hydrogen production can also be observed by a bidirectional hydrogenase.
The two main types of photosynthetic bacteria are purple bacteria and green bacteria, which utilize only one photosystem for photosynthesis. Green bacteria have PS I type reaction centers. Inorganic/organic substrates are oxidized, donating electrons, reducing ferredoxin via FeS protein. The reduced ferredoxin acts directly as the electron donor for the dark reaction as well as the hydrogen production reaction. In contrast, purple bacteria contain a reaction center-like PSII system that cannot reduce ferredoxin, but can generate ATP through cyclic electron flow. The electrons required for the nitrogenase-promoted hydrogen evolution process come from inorganic/organic substrates. Bacteriochlorophyll enters the "benzoquinone pool" through the reaction center. The energy barrier of benzoquinone is not sufficiently negative to directly reduce NAD+. Therefore, electrons from the quinone pool are forced to reduce NAD+ to NADH in turn. The electrons required for this process are called reverse electron flow. No oxygen is produced during the entire process, and the net total amount of hydrogen produced is affected by the hydrogenase activity. Studies have shown that the photosynthesis of purple bacteria is considered to be the best in terms of hydrogen production by biological photosynthesis, because purple can utilize industrial wastes and the products of fermentation processes (such as organic acids, etc.).
2. Photobioreactor
In order to improve the hydrogen production rate of biological hydrogen production, various photobioreactors have been designed. In a photobioreactor, light energy is converted into biochemical energy. The most basic factors that distinguish the photobioreactor from other ordinary reactors are: the reactor is transparent, so that the light can pass through to the maximum extent; the energy is instantaneous and cannot be stored in the reactor; the cells are self-shading. Self-shading leads to loss of additional absorbed energy, fluorescence and heat increase temperatures, and bioreactors require additional cooling systems. The thickness of the reactor is usually small, thereby increasing the reactor area to volume ratio and avoiding the effects of cell self-shading.
There are many factors affecting the hydrogen production efficiency of photobioreactors. In order to improve the hydrogen production and biomass of photosynthesis, a variety of bioreactors have been designed. Bioreactor. Various photobioreactors have their own advantages and disadvantages. For example, in the tubular photoreactor, a larger light irradiation area can be obtained, but the dissolved oxygen concentration generated by photosynthesis is higher and the energy input by the pump is also higher. The scale of such a reactor is limited. In the column-type photobioreactor, the irradiation area of light is small, but due to its small size, low price, easy operation, and the ability to stir by bubbling, it is widely used in microalgae and photosynthetic bacteria. When hydrogen is used, column-type photobioreactors are still widely used. In the plate reactor, the photosynthesis efficiency is high and the air pressure is controllable. Compared with other reactors, the cost is low, but it is difficult to maintain a constant culture temperature and proper agitation during the hydrogen production process.
In addition to the effect of the shape of the photobioreactor on the hydrogen production efficiency, the physicochemical parameters of the photobioreactor also affect the hydrogen production, such as the pH of the solution, temperature, light intensity, depth of light penetration, dissolved oxygen , dissolved CO2, agitation, gas exchange, carbon and nitrogen sources and the ratio of the two, etc. Whether the photobioreactor is open or closed will affect the setting of these physicochemical parameters.
The main factor limiting large-scale hydrogen production by biological methods is that there is only limited transmission of light energy deep into the reactor; external light sources are placed into the deeper regions of the reactor through fiber optic cables, and the light sources are uniformly distributed. This dispersed light source plays a key role in hydrogen production.
3. Biological hydrogenase
In the process of biological hydrogen production, there are two main enzymes, namely hydrogenase and nitrogenase, which play a catalytic role in biological hydrogen production, reducing proton H+ to generate molecular H2. Both of these enzymes are metalloproteins.
1) Hydrogenase: A series of hydrogen production behaviors that occur in organisms by decomposing water are based on the continuous arrangement of hydrogenase subunits with catalytic functions; according to research, there are 3 different types of hydrogenases, namely [NiFe] hydrogenase, [ FeFe]hydrogenase and Fe-S hydrogenase, of which Fe-S hydrogenase was later named [Fe]hydrogenase; a nucleus with a specific function is present in each enzyme. The hydrogen production of [FeFe]hydrogenase is higher, while the oxygen tolerance of [NiFe]hydrogenase is stronger, and [Fe]hydrogenase does not participate in hydrogen production. [Fe]hydrogenase catalyzes the reduction of CO2 to methane via H2, and most bacteria that rely on this enzyme are alkane-producing organisms.
2) Nitrogenase: Nitrogenase is very important for agricultural production, because through nitrogen fixation, N2 in the atmosphere can be converted into NH3, which naturally fertilizes the soil. Nitrogenase can also produce hydrogen while mainly fixing nitrogen. The study found that nitrogenase can produce more hydrogen under nitrogen-limited conditions.
Biohydrogenases play an important role in the process of biohydrogen production, but studies have found that most biohydrogenases are very sensitive to oxygen. In the process of splitting water to produce hydrogen, the generation of oxygen is inevitable, so the continuity of hydrogen generation will be affected. So far, only a few hydrogenases have shown tolerance to oxygen, and many approaches have been tried to develop oxygen-tolerant variants. It is observed that algal hydrogenase is the smallest and simplest hydrogenase, which is most sensitive to oxygen inactivation. Since only the H group where the active site is located, algal hydrogenase is more sensitive to oxygen inactivation. The F group code protects the active site from oxygen inactivation. The amino acid sequence of the H group or the amino acid composition of the active site region is very critical, which is directly related to the protection of the active site from the influence of oxygen.
Nitrogenase is also sensitive to oxygen, but it is relatively less sensitive to oxygen than hydrogenase. Nitrogenase is mainly found in anaerobic prokaryotes, but also in cyanobacteria. Through heterokaryotic cells, anaerobic hydrogen production is isolated from aerobic photosynthesis. Under nitrogen-limited conditions, hydrogen production is favored, and hydrogen production is an energy-intensive process.
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