Cathodes enhance Corynebacterium glutamicum growth with nitrate and promote acetate and formate production

The industrially important Corynebacterium glutamicum can only incompletely reduce nitrate into nitrite which then accumulates and inhibits growth. Herein we report that cathodes can resolve this problem and enhance glucose fermentation and growth by promoting nitrite reduction. Cell growth was inhibited at relatively high potentials but was significant when potentials were more reductive (-1.20 V with anthraquinone-2-sulfonate as redox mediator or -1.25 V vs. Ag/AgCl). Under these conditions, glucose was consumed up to 6 times faster and acetate was produced at up to 11 times higher yields (up to 1.1 mol/mol-glucose). Acetate concentrations are the highest reported so far for C. glutamicum under anaerobic conditions, reaching values up to 5.3 ± 0.3 g/L. Herein we also demonstrate for the first time formate production (up to 3.4 ± 0.3 g/L) by C. glutamicum under strongly reducing conditions, and we attribute this to a possible mechanism of CO2 bioreduction that was electrochemically triggered.