Mechanistic insights into bidirectional extracellular electron transfer driving synchronous nitrogen fixation in...

Electricigens serve as anode catalysts to catalyze nitrogen fixation. At present, only a few electricigens can simultaneously perform both outward and inward extracellular electron transfer (EET) metabolic pathways during the process of biological nitrogen fixation. The mechanism of bidirectional EET coupling in nitrogen-fixing is still not well elucidated. In this study, a nitrogen-fixing bacterial strain Klebsiella variicola C1 that possesses the capacity for bidirectional EET was first reported. Scanning electron microscopy (SEM), three-dimensional fluorescence spectroscopy and cyclic voltammetry (CV) were employed to reveal the differences of biofilms formed at the anode and cathode. Fluorescence-based quantitative PCR and comparative proteomic approaches were used to explore key genes and proteins involve in bidirectional EET pathways and nitrogen fixation. The results showed the outer-membrane lipoprotein carrier proteins seemed to be primarily responsible for facilitating electron transfer from the cell to the anode, whereas pilus proteins may mainly perform electron uptake from the cathode. Additionally, an NAD(P)-dependent oxidoreductase located in the cytoplasm appears to play a critical role in ATP synthesis, which might contribute to efficient nitrogen fixation at the anode. Overall, these results suggested that bidirectional EET for K. variicola C1 led to different nitrogen fixation performance.