While solar bioenergy is potentially an abundant and environmentally benign energy source, natural photosynthesis is relatively inefficient owing to slow steps in the conversion of carbon dioxide into biomass. This project seeks to increase energy capture by photosynthesis by diverting energy away from the normal slow steps to potentially more efficient processes. The first major goal of the project is to demonstrate that energy, in the form of electrical current produced by photosynthetic organisms, can be transmitted from a photosynthetic, energy capturing cell to an energy-storing "factory cell" via biological nanowires (biowires). The second major goal of the proposed work is to show that the factory cell can be engineered to use energy to produce useful fuel compounds.
Broader Impacts: This project will create the foundation for new directions in bioenergy research, with potential for dramatic increases in the efficiency of solar energy capture and storage, while training the next generation of scientists and engineers needed to compete in emerging areas of bioenergy and biotechnology. The project will demonstrate that energy can be transferred directly between cells as bio-electricity. Furthermore, the biowire to be developed will serve as a future generic connector for electrically connecting distinct cell types to create novel, functional biofilms. The photosynthetic components constructed in this project will serve as prototypes to establish a new design paradigm. In addition to these benefits, the project will 1) provide valuable resources to catalyze other important research projects, 2) train undergraduate and graduate students and postdoctoral fellows in areas of technology relevant to critical national needs, 3) establish productive international collaborations, and 4) disseminate information relevant to basic and applied research and development in energy and biotechnology.
Certain bacteria such as Shewanella oneidensis produce electrically conductive nanowires that allow them to transfer electrons generated inside the cell by respiration to iron oxide and manganese oxide particles located on the outside the cell. They do this when the normal electron acceptor, oxygen, is absent, as frequently occurs in the bottom of lakes. This requires that electrons be transferred across the cell's lipid bilayer, which is an electrical insulator, and this requires specialized biochemical machinery. The nature of this machinery is unknown, although for years, it had been assumed that the electrically conductive nanowires consist of Type IV pilins, which are rod-like appendages composed solely of protein. Because pilins do not contain metals, the mechanism of electron transfer mechanism was thought to be similar to that of semiconductors, which have valence and conduction band structures. Our studies, however, show that at the onset of nanowire formation due to oxygen deprivation, the messenger RNA transcript that codes for Type IV pilin proteins PilA and PilE are unchanged. In contrast, the messenger RNA transcript that codes for decaheme cytochrome proteins are over-expressed 6-fold for MtrC, MtrA and OmcA (which transfer electrons to iron oxide and manganese oxide), and over-expressed 20-fold for DmsE (which transfers electrons to dimethylsulfoxide). Light microscope studies show that the nanowires stained with lipid stains as well as protein stains, thereby eliminating any possibility that they are Type IV pilins. Antibodies against the decaheme cytochromes MtrC and OmcA indicate that these two proteins are located on the nanowires, and mutants that lack MtrC and OmcA show that they are indeed absent, even though the nanowires were still formed. High resolution atomic force microscopic images show that the nanowires are, in fact, composed of connected vesicles of the outer cell membrane, configured much like pearls on a string. We conclude that S. oneidensis nanowires are extensions of the outer membrane and periplasm, rather than pilin-based structures. The electrically conductive mechanism is proposed to consist of outer membrane multiheme cytochromes MtrC and OmcA that localize to these membrane extensions, directly supporting one of the two proposed models of electron transport through the nanowires. The data point to a general strategy wherein bacteria extend their outer membrane and periplasmic electron transport components, including multiheme cytochromes, many micrometers away from the inner membrane in the search for suitable oxidants in anaerobic environments.
