BioPhotovoltaics

Biological Photovoltaic Cell

Biological photovoltaic cells use photosynthetic cells, or parts of cells, to produce electricity. A BPV cell is split into anodic and cathodic half chambers - with the anodic chamber containing the photosynthetic material. At the beginning of the light-dependent stage of photosynthesis, light energy is used to split water into protons and electrons in photolysis: 2H2O → 4H+ + 4e- + O2 These electrons are then transported along a transport chain, and the energy released from their movement used to create energy. However some electrons from photolysis escape the cell, and it is these electrons that reduce the anode of the BPV cell, producing a current. Chloroplasts on their own can be used in BPV cells but these cells often do not possess the required longevity - chloroplasts break down over a period of hours, and require low operating temperatures. As such the most viable BPV cells employ whole photosynthetic organisms - algae or cyanobacteria - as these can repair themselves and as such present a more sustainable option. The inherent problem with this is that the cell walls/membranes prevent sufficient exoelectrogenesis(electrons leaving the cell). We decided to use cyanobacteria(a species called Synechocystis, a model cyanobacterium) for our BPV cell prototype - they generally offer higher efficiency as their membrane systems are simpler, and from a synthetic biology perspective cyanobacteria are easier to work with(in theory). Work has been done on increasing efficiency of these BPV cells by adding an exogenous redox mediator into the anodic chamber. This increases the rate of electron transport into the extracellular space, thus making the system more efficient. Various mediators have been studied and shown to increase efficiency, such as phenazines, quinones, and riboflavin. All of these mediators have been shown to have a positive effect on efficiency, but this approach is not a sustainable one - these mediators are added externally, not produced within the cell. As such we saw the opportunity here to modify the cyanobacteria to produce mediators, making the whole setup more sustainable - it could be left for several months with no drop in efficiency and no need to add mediators manually. We planned to modify Synechocystis to achieve this goal of improved efficiency in two ways - amplifying genes coding for riboflavin, a redox mediator already found in cyanobacteria, and introducing genes for porins - membrane channel proteins that we postulate could increase rate of electron export.