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Chapter 2 - Energy conversion by photosynthetic organisms

2.1 Photosynthetic capture of solar energy
2.2 Photosynthesis mechanisms
2.3 Hydrogen production through solar energy conversions

Adverse environmental effects of the use of fossil fuels have been thoroughly discussed in Chapter 1. Energy demands are nevertheless expected to continue increasing with global improvements in living standards. There is however a limit to the amount of energy consumption that can be tolerated by nature. Excessively large amounts of energy produced by nuclear fusion for example, have the potential of altering terrestrial climates. Renewable sources of energy which do not alter global energy levels are therefore desirable.

2.1 Photosynthetic capture of solar energy

2.1.1 Solar energy
2.1.2 Why is biotechnology now applied to energy technology?

2.1.1 Solar energy

Approximately 5.7 x 1024 J of solar energy are irradiated to the earth's surface on an annual basis. Plants and photosynthetic organisms utilize this solar energy in fixing large amounts of CO2 (2x1011 t = 3x1021 J/year), while amounts consumed by human beings are relatively small, (3 x 1020 J/year) (1), representing only 10% of the energy converted during photosynthesis.

Although large amounts of solar energy are irradiated to the earth's surface, the effective energy concentration (energy/unit area) of solar energy at any one point on the earth's surface is small - only about 1 kW/m2 at most, even at noon. Such low effective energy concentrations, limit the use of solar energy as a primary energy source, and elevate the costs associated with its accumulation and transmission.

Technologies for the utilization of low-density energy sources must be developed in order to facilitate the use of solar energy. Solar energy conversions through the use of photosynthetic microorganisms do not incorporate the use of complex systems or large quantities of factory manufactured products, and indeed have relatively minimal investment and resource requirements. Additionally, these technologies are largely dependent on the use of renewable resources, thereby generating minimal amounts of waste. Recent advances in biotechnology have made possible studies on the utilization of biological processes such as photosynthesis for energy production.

2.1.2 Why is biotechnology now applied to energy technology?

The use of natural energy involves the control of entropy. Prior to the industrial revolution, wood served as a major energy source. However, the industrial revolution gave rise to the widespread use of both coal and petroleum as energy sources. Since coal and petroleum were often produced at sites far from their consumption points, recuperation of their production costs involved mass production, thus precipitating the formation of an integrated industrial society. This integrated system soon reached its limits, making it necessary to consider discrete social systems that utilize delocalized abundant energy sources.

Integration is necessary in any system where energy, goods, and information are dispersed. Integration of systems involves the use of entropy-reducing processes. Application of mechanical methods to the integration of systems incorporates large amounts of energy expenditure. Existing problems cannot therefore be resolved without modification of existing social systems. Consequently, energy production technology must be reformed. Microorganisms have the ability to reduce entropy through energy utilization, and can potentially simplify the conversion and accumulation of solar energy and energy utilization over large areas.

Following the industrial revolution, energy-releasing techniques were developed. The development of techniques which control entropy are essential for human survival. To this end, there is a need for the development of industrial technology which makes use of biological principles in a sophisticated manner.

2.2 Photosynthesis mechanisms

2.2.1 Plant photosynthesis
2.2.2 Bacterial photosynthesis

Biological energy conversions can be categorized into two groups: i) photosynthesis (the process whereby solar energy is fixed to yield energy useful to organisms and industry), and ii) biomass conversion (the product of photosynthesis) into energy. Photosynthesis occurs in plants, algae and photosynthetic bacteria, while biomass conversion reactions often occur in non-photosynthetic microorganisms. This Chapter focuses on photosynthetic processes.

2.2.1 Plant photosynthesis

Photosynthesis is often regarded as a CO2 anabolic reaction, whereby glucose is formed from CO2 and water. CO2 anabolism is an energy-consuming reaction in that it utilizes chemical energy produced by photosynthesis. In its narrowest sense, photosynthesis can be regarded as a process whereby energy is supplied for CO2 anabolism. In a broader sense, photosynthesis, including CO2 anabolism, can be divided into several steps: i) photoelectric charge isolation using photon energy (conversion to electrical energy), ii) fixation of electrical energy in the form of chemical energy (ATP synthesis), and iii) chemical reactions involving ATP (fixation of CO2, and hydrogen production).

The supply of energy for CO2 anabolism is common to all photosynthetic organisms which exhibit photosynthesis. Energy conversion, ATP synthesis and the production of both CO2 and hydrogen on the other hand, are not unique to photosynthetic organisms, but occur in all types of microorganisms, and are in fact similar to the respiratory processes which occur in mitochondria of higher organisms.

Two types of photosynthesis are distinguishable on the basis of source of the electrons used as energy carriers. In plants such as green algae, and cyanobacteria (blue-green algae), water is the electron source, while in photosynthetic bacteria, organic or sulfur compounds provide electron sources.

Photosynthetic mechanisms which occur within plant photosynthetic membranes are schematically presented in Figure 2-1. Two Photosystem II water molecules are initially decomposed by four incident photons, to yield one oxygen molecule and four excited electrons. Excited electron energy is subsequently utilized in ATP synthesis. Unlike in the case of ordinary chemical reactions, ATP synthesis cannot be stoichiometrically analyzed (2). The ratio of excited photons to ATP produced is still a somewhat debatable issue. Although it has generally been thought that two photons give rise to the formation of two ATP molecules, some researchers claim that three photons are involved (3). Furthermore, other researchers have suggested a loose coupling between proton transport and ATP synthesis (4, 5):

4 photons + 2H2O + 2(or more)ADP = 2(or more)ATP + 4H+ + 4e-+O2


Subsequent to their energy release in ATP production, photosystem II electrons are transported to photosystem I, where they are again excited to a higher energy level, allowing them to be utilized for NADP reduction. NADP serves both as an electron carrier and an oxidizing and reducing agent in vivo. Two photons are utilized per molecule of NADP reduced:

4 photons + 4e- + 2NADP + 4H+ = 2NADPH.
Photosystem I may also be involved in ATP synthesis. In cases where it is involved, excited photosystem I electrons are recycled:

4 photons + 2(or more)ADP = 2(or more)ATP.
Fixation of one molecule of CO2, involves the following reaction:

CO2 + 3ATP + 2NADPH = CH2O + 3ADP + 2NADP.
If two ATP molecules are obtained through Photosystem II excitation (Eq. 2-1), the net reaction, following equations 2-1 through 2-4 is:

CO2 + 10 photons + H2O = CH2O + 1/2O2.

Figure 2.1 - Schematic representation of mechanisms involved in plant photosynthesis

Experimental data indicates that between 8 and 12 photons are required for fixation of one molecule of CO2. Since the energy equivalent of one photon (700 nm) is approximately 170 kJ/E, and the change in free energy during the fixation of CO2 is approximately 450 U/mol, the energy efficiency of this process for monochromatic light of a wavelength of 700 nm is estimated to be approximately 21-33%. However, owing to the quantum nature of photosynthetic reactions, energy efficiency decreases if light of shorter wavelengths (i.e. higher quantum energy) is used. Additionally, energy losses, energy requirements for plant growth, and the distribution of solar energy wavelengths need to be considered.

Plant photosynthesis takes place only in the presence of visible light (400-700 nm). However, solar light contains both visible and infrared components. Since visible light accounts for about 45% of all solar energy, the maximum achievable energy efficiency for CO2 fixation using solar radiation is approximately 13%.

2.2.2 Bacterial photosynthesis

Bacterial photosynthesis is thought to be a relatively old form of photosynthesis. It incorporates the use of either organic or sulfur compounds as electron donors in photosystem I (Figure 2-2). Unlike in the case of plant photosynthesis, cyclic photophosphorylation takes place in bacterial photosynthesis, i.e. electrons are repeatedly excited in a cyclic manner, with ATP being generated in each cycle. Photosynthetic bacteria are also capable of reducing electron carriers such as NAD, via a linear reaction similar to the electron transmission which occurs during plant photosynthesis (Figure 2-2).

Figure 2.2 - Schematic representation of mechanisms involved in bacterial photosynthesis

CO2-fixing reactions do not produce energy during bacterial photosynthesis (i.e equimolar amounts of organic compounds are produced through decomposition of organic compounds), except when sulfur compounds serve as electron carriers. The energy conversion efficiency for this type of photosynthesis is more fully described in Chapter 5.

2 photons + 1 (or more)ADP = 1 (or more)ATP.
Electrons are donated as follows:

CH2O + H2O = 4e- + 4H+ + CO2.

The structure of the photosynthetic reaction center (RC), involved in the early steps of photosynthesis, has been elucidated for certain photosynthetic bacteria (Fig. 2-3). Such chlorophyll- containing bacteria which include Rhodopseudomonas viridis and Rhodobacter sphaeroides, show similarities with respect to the arrangement of chlorophyll, and the three-dimensional structures of major portions of the proteins possessing that pigment. Such structural similarities between photosynthetic bacteria, seem to suggest the acquisition of an optimal structure by these bacteria, over a long evolutionary period.

Pigments such as bacteriochlorophyll are also present within the RC. Photoelectric charge isolation takes place within dimers of these bacteriochlorphyll pigments, resulting in the release of high-energy electrons, via the action of bacteriochlorophyll monomers such as bacteriopheophytin, quinone A, and quinone B. These high-energy electrons are subsequently conjugated with proton transportation in the cytochrome b/c1 complex.

Figure 2.3 - Initial steps of photosynthesis in bacterial photosynthetic membranes

A noteworthy feature of the RC function is that photon involvement in photoelectric charge isolation, resembles that which occurs in photo-semiconductors. These RC centres can thus be regarded as molecular elements produced by nature. The fact that photoelectric charge isolation is observed in protein molecules will greatly influence future research relevant to molecular elements and solar batteries.

2.3 Hydrogen production through solar energy conversions

2.3.1 Cyanobacterial hydrogen production (plant-type photosynthesis)
2.3.2 Bacterial hydrogen production (bacterial-type photosynthesis)
2.3.3 Use of photosynthesized proteins in photoelectric conversion elements

Although large amounts of solar energy are irradiated to the earth's surface, the actual concentration (energy/unit area) of solar energy at the earth's surface is relatively low. The accumulation of solar energy for practical use, therefore necessitates collection of solar energy over large areas. This could potentially be a costly venture, particularly with respect to the use of solar batteries for solar energy concentration. Photosynthetic bacteria however have the potential to eliminate requirements for such large batteries for solar collection.

Hydrogen appears to be the most useful form of solar-converted energy, in that it can be easily substituted for petroleum-based fuels. The use of hydrogen is also advantageous in that when burnt it yields only water, and hence does not contribute to environmental pollution.

2.3.1 Cyanobacterial hydrogen production (plant-type photosynthesis)

Cyanobacteria (blue-green algae) and photosynthetic bacteria are representative photosynthetic hydrogen producers. In vivo hydrogen production by these microorganisms is catalyzed by either nitrogenase or hydrogenase enzymes. Nitrogenases require ATP for catalytic action:

2H+ + 2e-*(Fd) + 4ATP = H2 + 4ADP.
Hydrogenases which catalyze hydrogen production in organisms such as Clostridium, on the other hand, do not require ATP for their activity. However, since hydrogenase activity involves some hydrogen uptake, the efficiency of hydrogen production through hydrogenase activity is low. Relatively little hydrogen is produced by these organisms in the absence of strong electron donors such as ferredoxin (Fd):

2H+ + e-* = H2

(e-*: high-energy electrons in NADH, Fdred, etc.).

Where hydrogenase enzymes are used for hydrogen production and the reducing potential, yielded by photosynthetic reactions is completely used for reducing hydrogen ions, two photons can theoretically produce one hydrogen molecule:

2 photons + H2O + 1(or more)ADP = 1(or more)ATP + 2H+ + 2e- + 1/2O2


2 photons + 2e- = 2e-*

2H+ + 2e-* = H2 (by hydrogenase).

Hydrogenase-mediated reactions in cyanobacteria are not usually biased in favour of hydrogen production. Instead, such reactions are likely to cause hydrogen uptake and are consequently impractical. It is highly likely that the high-energy electrons, yielded as a result of Clostridium) or by the use of these organic substances as electron donors. The efficiency of the latter reaction can be determined by multiplying the efficiency of CO2; anabolism by the efficiency of hydrogen production in photosynthetic bacteria.

Although there are few reports of precisely determined hydrogen production efficiencies of cyanobacteria, Miyamoto et al. determined that outdoor solar incubation for a period exceeding one month in California, resulted in an average energy conversion efficiency (energy yielded by combustion of produced hydrogen/incidence solar energy) of 0.2% (6).

2.3.2 Bacterial hydrogen production (bacterial-type photosynthesis)

Bacterial mechanisms for photosynthetic hydrogen production are summarized in Figure 2-4. Upon exposure of ammonia-free media containing photosynthetic bacteria to light, nitrogenase activity is induced, resulting in hydrogen production. Organic substances such as lactic acid (Eq. 2-13) serve as electron donors in photosynthetic bacteria. In such reactions, ”G is positive, indicating that the use of solar energy allows photosynthetic bacteria to produce hydrogen through complete decomposition of organic substances. Anaerobes such as Clostridium also produce hydrogen, but are incapable of completely utilizing energy or decomposing organic substances (Eq. 2-14):

C3H6O3 + 3H2O = 12H+ + 3CO2 + 12e- = 6H2 + 3CO2
”G = 51.2 kJ

C6H12O6 + 2H2O = 4H2 + 2CH3COOH + 2CO2
”G = -184.2 kJ.

Photosynthetic bacteria are capable of completely decomposing organic substances. Studies on hydrogen production through the exposure of organic wastes (waste fluids from food factories, pulp factories, etc.) to photosynthetic bacteria and light, have been conducted in a number of countries. Research into hydrogen production using a combination of photosynthetic bacteria and anaerobes has also been conducted.

A key factor in determining the commercial applicability of hydrogen production processes, is the rate at which hydrogen is produced. Bacteria have been widely investigated for their rates of hydrogen production. To date, R. sphaeroides has been identified as the bacterium having the highest hydrogen-producing rate (260 ml/mg/h) (7), with a photoenergy conversion efficiency (energy yielded by combustion of produced hydrogen/incident solar energy) of 7%, determined using a solar simulator (7, 8). Further strain development will potentially elevate the energy conversion efficiency of photosynthetic bacteria to levels comparable to those of solar batteries.

Figure 2.4 - Mechanism for hydrogen production by photosynthetic bacteria

Organic substances are utilized as electron donors by photosynthetic bacteria. The energy required for extracting electrons from these molecules is much lower than that required for the hydrolysis of water. Photo-energy is more often used for nitrogenase activation, i.e. ATP reproduction (Eq. 2-10), than for the decomposition of organic substances. The number of photons required for ATP reproduction by photosynthetic bacteria has not been theoretically elucidated, though an experimental number of 1.5 has been determined (9). A value of 1 has also been suggested (10).

In addition to ATP, nitrogenase-mediated reactions, require Fd as an electron carrier. Reaction mechanisms involved have not however been completely elucidated (10, 11). The reducing potential, created in the RC of purple, non-sulfur photosynthetic bacteria such as R. sphaeroides and R. capsulatus is approximately 100 mV at most (12). Such bacteria would be incapable of directly reducing Fd, which has an oxidizing and reducing potential of approximately 400 mV. ATP may however be capable of reducing Fd. Assuming that one ATP molecule is used for reducing one Fd molecule, at least six ATP molecules are needed for the production of one hydrogen molecule, resulting in a net requirement for nine photons. Since an 850-nm photon has an energy content of 141 kJ energy, the photon energy required for the production of hydrogen is 1269 kJ/mol H2 if monochromatic light of 850 nm is used. Energy is also required for the decomposition of organic substances (Equation 2-13). This energy is however, much lower (8.5 kJ/mol H2 in the case of lactic acid) than the energy required for nitrogenase-mediated reactions.

It is unlikely that improvements can be made in the photo-energy conversion efficiencies of nitrogenases. On the other hand, hydrogenases catalyze hydrogen-producing reactions without ATP requirements (Eq. 2-9). Hydrogenase-catalyzed reactions are reversible, and are either biased in favour of hydrogen production or hydrogen uptake. Hydrogenases in Clostridium and other bacteria work primarily to produce hydrogen, while hydrogenases in photosynthetic bacteria work toward hydrogen uptake. Hydrogen-producing efficiency is known to be higher in hydrogenase-deplete strains of photosynthetic bacteria.

Hydrogen-producing hydrogenases, such as those which occur in species such as Clostridium are required for hydrogen production. However, problems such as the necessity for complex control are encountered with the use of hydrogenases. Technologies for the incorporation of hydrogenases into reaction systems must therefore be developed.

2.3.3 Use of photosynthesized proteins in photoelectric conversion elements

As stated above, RC is capable of converting photo-energy into electrical energy. It can therefore be utilized in either photoelectric converters or photo-semiconductors. Solar batteries containing chromatophore membranes or an RC are now available. The RC is a relatively stable protein which retains its photoelectric conversion function even in the dry state (13). Unlike ordinary enzymes, the RC neither binds to the substrate nor undergoes chemical reactions, except for its binding to, or dissociation from quinones. Upon exposure to photons within membranes, the RC only releases electrons and has no mobile portions. Unlike enzymes, the RC does not need to vibrate within water molecules. Using the Langmuir-Blodgett method, it is possible to prepare dried RC photoelectric devices (14).



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