The need for renewable energy sources has never been more urgent. Economically accessible petroleum reserves are rapidly running out, at the same time that global demand for energy is ballooning along with the world’s burgeoning population, and rising concentrations of atmospheric CO2 continue to threaten peace and prosperity.
Solar energy strikes the Earth’s surface at a rate of approximately 130,000TW (1TW= 1012W), a value that exceeds the current worldwide energy consumption rate of ca 15TW by more than three orders of magnitude. Although considerable progress has been made to develop photovoltaic devices that produce electricity from solar energy, electricity represents only a fraction of human energy consumption. Instead, the majority of energy is consumed as fuels: reduced compounds that can be oxidised in the presence of oxygen gas to release energy, and which have the advantage of being storable and transportable.
For the last 2.5bn years, Nature’s answer to sustainable solar fuel production has been photosynthesis, the biological process in which solar energy is transformed into fixed carbon in the form of biomass. Organisms across the biological spectrum, including cyanobacteria, algae and plants, use photosynthesis to power their metabolism while most other species use photosynthetic products as food and fuels.
The process of photosynthesis can be conveniently divided into two parts. In the first part, the light-dependent reactions, solar energy is captured and used to remove electrons from a stable source. Water is the electron source in ‘oxygenic’ photosynthesis, producing oxygen as a byproduct. In the second part, the light-independent reactions, the electrons promoted by the light reactions are used in reductive chemical reactions to produce fuels, often carbohydrates, by carbon dioxide fixation.
Photosynthesis parallels the use of photovoltaic solar panels to power electrolysers to produce hydrogen gas. In both processes, solar energy is used to create an electrochemical gradient, the dissipation of which is coupled to the production of a fuel. Photovoltaic-driven electrolysis of water, at 10-11% efficiency, is currently two to three times more efficient than photosynthesis under optimal conditions.1 However, biological systems have the advantages of being self-assembling and self-repairing; cheaper and environmentally cleaner than photovoltaics to manufacture and scale-up; and produce liquid fuels that can be directly exploited using existing infrastructure.
Small improvements in the efficiency of photosynthesis could provide a competitive route for producing fuels and high value chemicals – gains that could also dramatically increase global food supply.
The efficiency of photosynthesis depends on a variety of environmental factors, but the rates are often limited by the slowest reactions of CO2 fixation. The enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO), the catalyst for a crucial step in this process, is not only famously slow, but it also has relatively low specificity for CO2. As well as CO2, RuBisCO also fixes oxygen by a competing process of photorespiration that can consume up to 25% of the stored energy the enzyme produces as fuel.
Most organisms synthesise extremely large quantities of RuBisCO to compensate for its low catalytic rate. RuBisCO can constitute up to 30-50% of the soluble protein in chloroplasts, making it the most abundant soluble protein on the planet. However, the energy expended synthesising RuBisCO must be diverted from biosynthesis of other compounds. Under full light, the flux through the light-dependent reactions exceeds that of the light-independent reactions in spite of the abundance of RuBisCO.
As a consequence, organisms that use sunlight to generate energy absorb far more solar energy than they can use in downstream reductive chemical reactions. These phototrophs exploit an array of mechanisms to dissipate up to 80% of the absorbed solar energy as heat to prevent damage and to stop competitors from capturing that solar energy.
While the overall efficiency of photosynthesis could, in principle, be significantly enhanced by increasing the rate of carbon fixation, RuBisCO has proven a difficult enzyme to improve (C&I, 2011, 18, 12). The selectivity of RuBisCO for CO2 over O2 and the overall catalytic rates are negatively linked: improvement of one comes at the cost of the other. As a substitute, researchers are considering the development of alternative pathways for CO2 fixation in phototrophs.2
Although the vast majority of photosynthetic organisms use the RuBisCO-dependent Calvin-Benson cycle for carbon fixation, five alternative CO2 fixation pathways that do not rely on RuBisCO exist in a group of single celled organisms called Archaea.
Metabolic models also suggest that it may be possible to construct more efficient artificial carbon fixation pathways using existing enzymes. Most of these cycles, however, are extremely oxygen sensitive and may not be suitable for cells that perform aerobic photosynthesis since these organisms produce oxygen during the water splitting reaction. As an alternative to carbon fixation, excess electrons generated in the light-dependent reactions of photosynthesis could be shunted into hydrogen production, but this process is also sensitive to oxygen.
The solution to this problem could be a mechanism that spatially isolates the light-dependent reactions from the light-independent, fuel-producing reactions. In conventional photosynthesis, the light and dark reactions typically communicate via soluble electron carriers. However, this electron exchange could equally well be carried out by an electrically conductive ‘biowire’ between two isolated compartments or microbial cells in which the separate reactions are occurring.
In this ‘plug and play’ photosynthesis, the process is modularised so that the light and dark reactions can operate independently, preventing oxidative destruction of fuel-producing enzymes.
Ideally, these biowires should be ‘portable’ and capable of functioning in a variety of different microbial species. They could be used to plug the specialised fuel producing cells into the equally specialised photosynthetic electron source cells, analogous to linking a power plant to production factories through a power grid.
Moreover, they may also be optimised to address the mismatch in throughput between light-dependent and light-independent reactions that occurs in natural photosynthesis, by varying the ratio of different cells or numbers of electrical connections between them.
In short, the modularisation of photosynthesis could open the door to flexibility. It could pave the way for modular photosynthetic systems that harvest light more effectively and produce fuels more efficiently. Photosynthetic systems of the future may not even be completely biological. Microbial electrosynthesis, for example, can use electricity produced from solar panels to power the microbial production of fuel.
But is an electrically conductive connection between microbial cells possible? Cell membranes, after all, are largely insulating. In fact, a single tantalising experiment suggests that microbial communities may already take advantage of intercellular electronic communication as a means of energy exchange. Scanning electron micrograph images of Pelotomaculum thermopropionicum in co-culture with Methanothermobacter thermoautotropicus reveal conductive filaments connecting the two cell types that may allow direct exchange of electrons between the two species.3
So-called ‘electrogenic’ bacteria, meanwhile, have been found to transfer electrons to/from their exterior for the oxidation/reduction of external substrates such as oxyhydroxide minerals and electrode surfaces, and have been used to develop simple bioelectrochemical devices. Microbial fuel cells, one of the commonest bioelectrochemical systems, use microbes to oxidise fuel at the anode to produce current, without the aid of a precious metal catalyst.4 In these devices, electrons are transferred from the fuel, through the organism, to an electrode. Similarly, phototrophs, including both cyanobacteria and algae, have been employed in electrochemical systems to produce photocurrents in which the electrons are derived exclusively from water.5
The process in which an organism receives electrons from extracellular sources and uses these to produce chemical compounds is called ‘electrosynthesis’.6 For example, some acetogens have been shown to produce acetate from CO2 on electrode surfaces by directly employing electricity as an energy source to power metabolism. Interestingly, this process involves acetyl coenzyme A (acetyl-CoA) – a compound extensively used in metabolism to transfer carbon atoms between different pathways. This opens up the possibility of re-engineering metabolic pathways in such an organism to produce more attractive carbon fuels, such as alcohols or lipids, using only electricity as the energy input.7
Biochemical studies of electrogenicity have primarily focused on two organisms, Shewanella oneidensis and Geobacter sulfurreducens. Based on these studies, three distinct mechanisms have been proposed for the conduction of electrons between cells and extracellular objects. First, direct contact occurs between a redox active species in the outer membrane and the extracellular electron source/sink. Second, cells produce and excrete a redox active molecule that shuttles via diffusion between the cell and the extracellular object. Third, some organisms produce electrically conductive appendages, often referred to as bacterial nanowires, which facilitate the electrical communication.3, 8
While only a handful of natural organisms have been shown to be electrogenic, synthetic biology may allow researchers to transfer this capability to other species. The MtrA, MtrB and MtrC proteins are essential in the transfer of electrons from Shewanella oneidensis to extracellular electron acceptors. Introducing these proteins into Escherichia coli created an organism able to reduce extracellular metal ions and solid metal oxides, albeit slowly.9 In this way, synthetic biology may allow us to construct a transferable module to enable electrogenicity in the recipient organism. The work is preliminary, but it increases the probability that a ‘plug and play’photosynthesis platform could become a reality.
Creating a conductive unit to connect the light-dependent and light-independent reactions of photosynthesis would be the first in a series of modifications necessary to improve the efficiency of biofuel production using a ‘plug-and-play’ strategy.10 Mechanisms will also be required to ensure that electrons are transported only to desired products and side reactions are discouraged. Second, the metabolism of an engineered system must be adjusted to use ATP and NADPH at the same ratio that they are produced via photosynthesis. Finally, flux through the light dependent reactions may need to be reset by eliminating mechanisms that dissipate solar energy to accommodate for increased carbon fixation.
Thus, continued fundamental research into the natural physiology of photosynthetic organisms will continue to be essential to future enhancements in photosynthetic efficiency.
1 R. E. Blankenship et al., Science, 2011, 332, 805.
2 A. Bar-Even et al, Proc. Natl. Acad. Sci. USA, 2010, 107, 8889.
3 Y. A. Gorby et al, Proc. Natl. Acad. Sci. USA, 2006, 103, 11358.
4 B. E. Logan, Nature Reviews Microbiol., 2009, 7, 375.
5 A. J. McCormick et al, Energy & Environ.Sci., 2011, 4, 4699.
6 K. P. Nevin et al, Appl. Environ.l Microb., 2011, 77, 2882.
7 M. Kopke et al, Proc. Natl. Acad. Sci. USA, 2010, 107, 13087.
8 M. Y. El-Naggar et al, Proc. Natl. Acad. Sci. USA, 2010, 107, 18127.
9 H. M. Jensen et al, Proc. Natl. Acad. Sci. USA, 2010, 107, 19213.
10 D. M. Kramer and J. R. Evans, Plant Physiol., 2011, 155, 70.
Anne K. Jones is an assistant professor in the department of chemistry and biochemistry and the Center for Bioenergy and Photosynthesis at Arizona State University, US. Travis Bayer is a lecturer in synthetic biology at Imperial College London. Thomas Bibby is a lecturer in the School of Ocean and Earth Sciences at the University of Southampton. Leroy Cronin is the Gardiner Professor of Chemistry at the University of Glasgow. John Golbeck is professor of biochemistry and biophysics and professor of chemistry at The Pennsylvania State University, US. David M. Kramer is the Hannah Distinguished Professor in Photosynthesis and Bioenergetics at Michigan State University, US. Ichiro Matsumura is an associate professor of biochemistry at Emory University, US.