Microbial Utilization of Electrically Reduced Neutral Red as the Sole Electron Donor for Growth and Metabolite Production

Electrically reduced neutral red (NR) served as the sole source of reducing power for growth and metabolism of pure and mixed cultures of H(2)-consuming bacteria in a novel electrochemical bioreactor system. NR was continuously reduced by the cathodic potential (−1.5 V) generated from an electric current (0.3 to 1.0 mA), and it was subsequently oxidized by Actinobacillus succinogenes or by mixed methanogenic cultures. The A. succinogenes mutant strain FZ-6 did not grow on fumarate alone unless electrically reduced NR or hydrogen was present as the electron donor for succinate production. The mutant strain, unlike the wild type, lacked pyruvate formate lyase and formate dehydrogenase. Electrically reduced NR also replaced hydrogen as the sole electron donor source for growth and production of methane from CO(2). These results show that both pure and mixed cultures can function as electrochemical devices when electrically generated reducing power can be used to drive metabolism. The potential utility of utilizing electrical reducing power in enhancing industrial fermentations or biotransformation processes is discussed.