Enhancing and Redirecting Cyanobacterial Electron Flow (Bioelectricity Spotlight)

The movement of electrons within cells is fundamental to life, and drives the processes that happen inside cells. For photosynthetic organisms, the movement of electrons can be powered by sunlight, and for all organisms it can be powered by the metabolism of compounds in food. The electrons can be used within cells in the synthesis of useful compounds. Some of the electrons can leave the cells, resulting in electric currents that can be harvested and used as a source of renewable energy. We have shown this is possible with photosynthetic bacteria (also known as 'blue-green algae' and 'cyanobacteria'). They can use sunlight to produce external electric current. We have succeeded in using the currents from these bacteria to drive a microprocessor - highlighted in the news media as 'algae-powered computing'. One of our long-term goals is to find ways to increase the electrical currents produced by photosynthetic bacteria, inside the cell or outside. However, we know little about what determines how many electrons are available, and how the electrons get to the outside of the cell. This makes it difficult to improve the process in a directed, rational way. We have recently developed a way of screening large numbers of photosynthetic bacterial cells for ones that have genetic changes (mutations) increasing electron availability. This exciting step forward allows us to find cells with improved electron availability without having to make assumptions in advance about how the process works. We will obtain improved cells, and then determine what the mutations responsible are. We will measure the ability of the improved cells to produce currents outside the cell, and to synthesis novel compounds inside the cell. This will be be important for biotechnological exploitation of this 'microbial electricity', and also help us to answer biological questions about what limits the movement of electrons.

Movement of electrons is essential for oxygenic photosynthetic bacteria (cyanobacteria), whose thylakoid membranes contain respiratory and photosynthetic electron transfer chains. There is rapidly growing interest in redirecting electron flow within the cell for biosynthesis and for export outside the cell (exoelectrogenesis). The latter area is particularly timely as it offers a possible source of renewable energy for small devices in off-grid locations. It would be highly desirable to increase electron availability for these applications. Progress has been made in understanding how some well-known electron sinks in the cell influence availability of electrons for heterologous electron transfer reactions (and thus many novel biosynthetic pathways) and for export outside the cell. However, our broader understanding of what determines availability of electrons, and especially for transfer outside the cyanobacterial cell, is limited. We know little of how electrons pass out of the cell, for example. This makes it difficult to design targeted mutational strategies to enhance availability. We have recently developed methods that allow high-throughput screening for mutants with increased electron availability, using redox-sensitive dyes and FACS sorting, and shown proof of concept. We have also developed parallel electrode arrays for screening for increased exoelectrogenic activity. We will use these systems to isolate mutant strains with increased electron availability and exoelectrogenic activity, without the need to make assumptions about the underlying limiting factors and pathways. We will study analytically the electron transfer processes within the cells, and their electron export. Identifying the mutations involved will help us understand much better what limits electron availability in the cell and outside it. The work will provide us with novel strains with enhanced ability to power electronic devices, and increased electron availability for biosynthesis.