Gadi Schuster – faculty of Biology
Abstract
Nonrenewable fossil fuel combustion continues to be the major source of electrical power and transportation fuels. At the same time, fossil fuels continue to be identified as the major cause of global pollution and probably climate changes. Alternative, renewable sources of energy are ex- tremely important for the future of the planet. The most ample renewable energy resource is sun- light. In an era of progressive depletion of fuel resources, we develop a means to harness the natu- ral process of energy conversion of photosynthetic organisms, to produce electricity and hydrogen, the cleanest of all fuels, from abundant natural resources: water and sunlight. The goal of this pro- posal is to capitalize on our recent progress in engineering native photosynthetic systems to develop a bio-generator for the conversion of solar energy to hydrogen and electricity. Elec- trons withdrawn in the light from the photosynthetic electron transport chain can drive an electrical current that is further used for the production of hydrogen. The energy conversion system to be de- veloped will contain photosynthetic membranes from cyanobacteria or higher plants. Using genetic engineering, we produced a cyanobacterial strain that can reduce the electrode of bio-electric pho- tovoltaic cell using electrons derived from its photosynthetic apparatus that further produce electric- ity and hydrogen. We plan to merge our novel biological electron transfer pathway with a photo- electrochemical cell able to produce hydrogen, as well as an electric current. Harnessing biological photosynthesis to produce electricity and hydrogen as a clean energy resource is in high demand. The outcome of this project will be an efficient bio-electric photovoltaic cell producing a long-standing photocurrent that can operate low-current electrical devices. Upon addition of a silicon cell in tandem to the bio-electric photovoltaic cell, the combined apparatus will pro- duce H2 gas that can be collected and stored as a clean and green energy source.
Introduction
Nonrenewable fossil fuel combustion continues to be the major source of electrical power and transportation fuels. At the same time, fossil fuels continue to be identified as the major cause of global pollution and probably climate changes. Alternative, renewable sources of energy are ex- tremely important for the future of the planet. The most ample renewable energy resource is sun- light and much of the present alternative energy research is focused on solar energy conversion to either electrical current and/or storable fuels. The “classical” photovoltaic cell directly converts light to electrical current using semiconductor materials (such as silicon) or by equivalent light-absorbing dyes to thin films of appropriate semiconductors. Newer technologies utilizing quantum dots or per- ovskite have created new generations of photovoltaic cells. In parallel, exploitation of natural photosynthesis is a popular means of converting solar energy for human purposes. In this approach, organisms grow using sunlight, and the resulting biomass is then converted to fuel. In or- der to continually provide energy, one of the most important areas of research is the combining of solar energy conversion with energy storage technologies. Many such studies focus on using sun- light energy to oxidize water and produce hydrogen gas, which can be stored and used as fuel.
Photosynthesis is the main source of chemical energy in the biosphere and is used by plants, algae, cyanobacteria, as well as other species of bacteria. Photosynthesis utilizes sunlight and water as a source of electrons to generate ATP and reduced electron carrier NADPH, which is
used to reduce CO2. Water oxidation results in O2 release and carbon reduction results in the syn- thesis of glucose by the Calvin cycle. Cyanobacteria is a broad class of photosynthetic organisms that is extensively studied as a model for extracting electrons from photosynthesis for the purpose of alternative energy and renewable fuels. While in the light cyanobacteria use photosynthesis, in the dark, cyanobacterial cells oxidize carbon sources via the respiratory system. Cyanobacteria lack the separation afforded by eukaryotic organelles, and it has been previously shown that electrons can flow between the photosynthetic and respiratory systems. For example, quinones from the pho- tosynthetic plastoquinone (PQ) pool can oxidize NADH and FADH2, both products of glucose oxida- tion.
Various attempts have been made to extract electrons from photosynthetic systems for the purpose of solar energy conversion. Some studies focus on isolated photosystems (PS), whereas others prefer to work with whole cells or thylakoid membranes. Isolation of PS complexes is time- consuming and costly and requires complicated biochemical processes. Therefore, we aim to use fast and simple methods, such as working with live cells or with thylakoid membranes prepared in a simple procedures. In our previous work, we demonstrated photo-induced electron transfer from spinach and cyanobacteria thylakoids using several electrode compositions and electron mediators. Using spinach thylakoids, we recorded a high electric current of 0.5 mA/cm2, and with the addition of a silicon cell, connected in tandem with the biological electric photovoltaic cell (BEPC), illumination with the same light, led to H2 generation and accumulation in the BEPC.
One of the main limitations of use of isolated photosynthetic complexes or thylakoid mem- branes for solar energy conversion is the relatively short life time of the biological components of the system, due to damage caused by radicals formed within the reaction centers. The use of living cells could mitigate this problem, thanks to the presence of repair systems that can replace photo- damaged photosynthetic proteins. However, extracting electrons from living organisms may be more complicated, as it likely requires an exogenous electron carrier that can penetrate the cell wall and membrane. Recently, we have found that following a gentle low-pressure treatment, live cyanobac- teria cells produced an unmediated electric current for several hours in a BEPC. The photocurrent was found to initiate at the respiratory chain by glucose consumption, transferred to photosystem I, and finally reduced the graphite electrode via a yet unknown internal mediator. The system pro- duced a stable high electric photocurrent of about 30 mA/cm2 for several hours, while the cells can continue to multiply. We now aim to by upscale and streamline this BEPC to more efficiently pro- duce electricity and H2 in a green and cleaner process.
Objective: To improve the amount and duration of the electric current in the established BEPC
This objective will be approached in four stage:
- Constructing a 10-times larger BEPC (in volume, area of the graphite electrode and amount of live cyanobacteria), that generate electricity for several hours
- Constructing a glucose-secreting strain of cyanobacteria to be added to the BEPC system.
- Identification of the mediator and its addition to the BEPC in order to increase the electrical current.
- Constructing a self-standing BEPC, with the addition of a silicon cell, in tandem, for H2 production.
Research plan
1. Building of x10 scaled-up BEPC. We recently succeeded in establishing a BEPC in which living and multiplying cyanobacteria (Synechocystis PCC6803) produced a long-lived electric cur- rent of about 30 mA/cm2, following a gentle pressure treatment and illumination using a graphite electrode. The electrons were derived at the respiratory chain, and reduced PSI, that can further photo-reduce the graphite electrode. The photoexcitation of PSI is necessary to drive the electric current, rendering it a photo-induced reaction. PSII does not take part in this process and accord- ingly, the herbicide DCMU did not inhibit the photocurrent. The first objective of the proposal is to build a cell approximately ten times larger than the first one (which is a 1.8 cm-diameter graphite electrode), that can contain ten times more cyanobacteria cells and ten times larger graphite elec- trode contact area (18-20 diameter). Since the photocurrent intensity was linear with the number of cyanobacteria cells in the BEPC, we expect to obtain a significantly higher photocurrent than that which has been obtained in the small cell. Of note, the chlorophyll concentration must be main- tained within a range that does not interfere with the intensity of the illumination light. The larger BEPC will enable us to operate small electronic devices, such as led lights and small digital watches, and upon supplementation of glucose to the medium, the steady electric current should last, like in the small cell, for several hours.
2. Constructing a glucose-secreting strain of cyanobacteria. While addition of glucose to the medium of the BEPC, to achieve a higher and longer-lived photocurrent, is simple, it is not en- ergy-efficient. Accordingly, we would like to harness photosynthesis for the production of the photo- current using the energy of light and PSII activity. To this end, a glucose-secreted strain of Syn- echocystis or Synechococcus will be constructed and then added to the BEPC, together with the gently treated Synechocystis cells (iSyn) described above that produce the electricity. Synecho- cystis or Synechoccus strains will be engineered to express genes encoding an invertase and a glu- cose facilitator, which mediate secretion of glucose and fructose, as described in. Sucrose produc- tion occurs under conditions of salt stress, i.e., > 100 mM NaCl, which is the current concentration of the BEPC medium. Therefore, the possibility of elevating the NaCl concentration in order to in- crease the production of sucrose will be examined. If this fails, the cyanobacteria will first be ex- posed to salt stress (300 mM NaCl) and then added to the BEPC, where the glucose will be continu- ously secreted from the engineered cells for several hours and consumed by the iSyn that produce the electricity. The easiest and most simple approach would be to make these cells iSyn by a gentle microfluidizer treatment and then place in the BEPC. If this approach will not work, both the con- structed cells and iSyn will be placed in the BEPC. The genetic constructed cells will use the illumi- nating light to drive glucose formation by the ordinary photosynthetic pathway and secrete it to the medium of the BEPC. Then, the iSyn will use this glucose to drive the newly described pathway of the “respiration chain – PSI – graphite electrode”, generating the photocurrent. The BEPC will be established in such a way that the iSyn is first layered on the graphite electrode, following by a sec- ond layer of the glucose-secreting cells on top of the iSyn layer. The number of cells, light intensity and the fraction of each cell type, will be optimized to generate the highest and longest photocurrent possible.
3. Identification of the internal mediator. Our preliminary results identified the existence of an internal mediator driving the current between PSI and the graphite electrode. The mediator be- came active when the Synechocystis cells were exposed to the gentle treatment of passing the cells through a microfluidizer operating at low pressure. Initial characterization efforts disclosed a soluble material that was present in the filtrate of the centricon containing a membrane, with cut-off size of 3000 Dalton. Since we hypothesize that the amount of this internal mediator could be the limiting factor of the amount of electricity produced by the BEPC, we plan to identify and then add the inter- nal mediator to the BEPC. Cells will be treated by the microfluidizer and verified to produce the high electric current (iSyn). Then, the filtrate of these cells will be tested by adding it to non-treated cells in order to verify that it can induce the electricity in the BEPC, as described in (Saper et al., 2018). The component present in the filtrate will be identified by mass spectroscopy and/or spectral analy- sis. When identified, this material will be added to the BEPC and the mediator concentrations nec- essary to increase the electricity will be determined.
4. Constructing a self-standing BEPC for hydrogen production. To generate hydrogen in the BEPC, a bias voltage must be added to reach the reversible voltage of water electrolysis (1.23 V under standard conditions). We have previously shown that the addition of a silicon photovoltaic module in tandem to the spinach thylakoids-based PEPC, produced H2 that accumulated in the BEPC. Thus, H2 can be produced by this hybrid tandem cell from oxidized water molecules, without any external power source. In a similar manner, a silicon cell will be connected in tandem in such a way that the same illumination light will drive the electric current in both the PEBC and the silicon photovoltaic module. The accumulation of H2 gas will be monitored using gas chromatography, as we have previously done with the spinach thylakoid BEPC. However, while the spinach thylakoid BEPC activity declined and the thylakoids had to be replaced every 10 min of illumination, here, we expect the BEPC to continuously produce H2 under illumination for several hours.
Harnessing biological photosynthesis to produce electricity and hydrogen as a clean energy resource is in high demand in order to replace the polluting and CO2-elevating processes that are generally used at present. The outcome of this project will be an efficient BEPC in which one popu- lation of cyanobacteria produces glucose by ordinary photosynthesis and secretes it to the medium. A second, gently treated cyanobacteria will exploit this glucose to drive electrons in a light-depend- ent manner, from the respiratory pathway, via PSI, to the graphite electrode, producing a long- standing photocurrent that can operate low-current electrical devices. Upon addition of the silicon cell in tandem to the BEPC, the combined apparatus will produce H2 gas that can be collected and stored as a clean and green energy source.
