Bacteria as a Source of Electricity

The urgent need for viable strategies for the sustainable generation of energy is being tackled on a wide variety of fronts. Photosynthetic microorganisms or isolated elements of their photosynthetic machinery offer an attractive approach to the efficient, light-driven production of electrical energy. “However, isolated photosynthetic complexes are generally not sufficiently stable for long-term use,” as Dr. Felipe Conzuelo of the Ruhr University in Bochum explains. He and his colleagues therefore chose to explore an alternative system, which combines living photosynthetic bacteria with graphite and platinum electrodes to generate electricity. One advantage of using whole cells is that they possess a biochemical mechanism for the repair of components of photosynthesis that are particularly sensitive to light-induced damage. The main problem with such a system is that one requires some means of transporting the electrons out of the cell to provide the steady current required for technical applications. Conzuelo and his team have now solved this problem.

The researchers chose the cyanobacterium Synechocystis as the basis of their system. Cyanobacteria possess two biochemical systems for energy production in the light and in the dark. The first utilizes light energy to drive the conversion of water and atmospheric carbon dioxide into organic compounds and molecular oxygen. This process involves the light-driven excitation of electrons (ultimately derived from water), which are then sequentially passed to a series of pigment molecules that together form an electron transport chain. Some of these electrons contribute to the formation of an electrochemical gradient which powers the synthesis of the energy-rich molecule ATP. The rest are used to form sugars by enzymatic reduction of CO2. In the dark, stored sugar molecules are oxidized by molecular oxygen in a process known as respiration, which also involves the orderly transfer of electrons along an electron transport chain to eventually regenerate water. Using graphite electrodes coated with whole cyanobacterial cells, the team was able to extract electrons produced by both photosynthesis and respiration, thus producing an electric current outside the cells – in the absence of an added electron carrier and with a higher efficiency than had been attained in comparable systems previously reported. They went on to show that a soluble molecule which leaks out of the cells mediates the transport of the electrons to the surface of the electrode. In a successful attempt to boost the photocurrent, the researchers subjected the cells to mild pressure before applying them to the electrode. This increases the permeability of the cell walls and facilitates release of the endogenous electron carrier.

“To the best of our knowledge, this is the first time that such a mediator molecule has been found to diffuse through the cell walls of living bacteria and transports electrons to the exterior,” says Dr. Fangyuan Zhao of the Centre for Electrochemistry at the Ruhr University. The precise identity of the molecule is still unknown, but it must be a relatively small, water-soluble molecule that can cross both the bacterial cell membrane and the cell wall.

“We believe that the cyanobacterial system has the potential to become a ‘green’ energy source,” says Prof. Dr. Wolfgang Schuhmann, Head of the Institute of Analytical Chemistry in Bochum. “With some further modifications, it should be capable of supporting stable, long-term generation of electricity – because it contains everything necessary for self-regeneration.”

 

Original Publication

 

Gadiel Saper et al.: Live cyanobacteria produce photocurrent and hydrogen using both the respiratory and photosynthetic systems, in: Nature Communications, 2018, DOI: 10.1038/s41467-018-04613-x