NADH gene manipulation for advancing bioelectricity in Clostridium ljungdahlii microbial fuel cells

In this study, the heterogeneous expression of the formate dehydrogenase gene for nicotinamide adenine dinucleotide (NADH) regeneration is successfully achieved in Clostridium ljungdahlii. The specific activity of formate dehydrogenase is dependent on cell growth, reaching its highest value (3.16 U mg^-1 for the engineered strain, 1.59 U mg^-1 for the parent strain) when cells entered the log phase. The NADH concentration in engineered C. ljungdahlii (12.64 μm per g DCW) increased 4.3 fold compared with the parent strain (2.93 μm per g DCW). The genetically engineered bacterium C. ljungdahlii is employed in a microbial fuel cell to gauge the potential for bioelectricity generation improvement. The voltage of a sodium formate fed microbial fuel cell with C. ljungdahlii is enhanced to 290 mV ± 10, which is 3.8 times the parent strain (76 ± 8 mV). The higher NADH pool in engineered C. ljungdahlii can facilitate electron transfer in the system, which contributes to an increase in maximum power density from 15 mW m^-2 to 35 mW m^-2. On the basis of these results, the manipulation of intracellular cofactors is shown to be an efficient approach to improve bioelectricity generation in the microbial fuel cell, and indicates the great potential of metabolic engineering for improvements in bioelectricity.