Understanding Biophotocurrent Generation in Photosynthetic Purple Bacteria

Establishing an efficient extracellular electron transfer (EET) process between photoelectroactive microorganisms and an electrode surface is critical for the development of photobioelectrocatalysis. Soluble and immobilized redox mediators have been applied with the purple bacterium Rhodobacter capsulatus for this purpose. However, detailed information on its EET with an electrode surface is not available and, therefore, choice of mediators has been by trial and error. Herein, we experimentally evaluated the capability of different soluble, quinone-based redox mediators to support EET and compared the experimental data with a computational model based on density functional theory calculations. We show that computed electrochemical redox properties of redox mediators in a lipophilic environment correlate to EET processes of Rhodobacter capsulatus, suggesting that intermembrane mediator characteristics are more diagnostic than redox properties of the mediators in an aqueous solution, and that the limiting electron transfer step takes place in the lipophilic membrane of the bacterial cells. This knowledge provides critical insight into designing future mediated bioelectrocatalysis systems.