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-consumin g bacteria in a novel electrochemical bioreactor system. NR was continuousl y reduced by the cathodic potential (-1.5 V) generated from an electric cur rent (0.3 to 1.0 mA), and it was subsequently oxidized by Actinobacillus su ccinogenes or by mixed methanogenic cultures. The A. succinogenes mutant st rain FZ-6 did not grow on fumarate alone unless electrically reduced NR or hydrogen was present as the electron donor for succinate production. The mu tant strain, unlike the wild type, lacked pyruvate formate lyase and format e dehydrogenase. Electrically reduced NR also replaced hydrogen as the sole electron donor source for growth and production of methane from CO2. These results show that both pure and mixed cultures can function as electrochem ical devices when electrically generated reducing power can be used to driv e metabolism. The potential utility of utilizing electrical reducing power in enhancing industrial fermentations or biotransformation processes is dis cussed.