Extracellular electron transfer drives ATP synthesis for nitrogen fixation by Pseudomonas stutzeri

Biological nitrogen fixation is a key step in the reduction of N2 to available nitrogen in the global nitrogen cycle. Pseudomonas stutzeri A1501 is an electroactive diazotroph and previous studies have shown that its nitrogen fixation performance is better in a micro-oxygen environment than in an oxygen-free environment. In this study, a bioelectrochemical system (nitrogen fixation in an anode chamber) was set up to explore whether extracellular electrodes can replace oxygen in acting as electron acceptors to drive ATP synthesis for nitrogen fixation by P. stutzeri under oxygen-free conditions. Nitrogenase activity, extracellular NH4+ production, increase of total nitrogen, 15N/14N atom ratio and the genes related to nitrogen fixation by P. stutzeri in the anodic bioelectrochemical group under oxygen-free conditions were at least 1.64 times higher than the corresponding values without electron output to the anode. The planktonic cells in the anode chamber were responsible for most of the electron output via an electron shuttle–electron transfer pathway. The transmembrane proton motive force produced by the transfer of electrons from the intracellular environment to the anode drives ATP synthesis to meet the high energy demand of the nitrogen fixation reaction in the absence of O2. These findings provide a basis for optimization of the nitrogen fixing performance of P. stutzeri in an oxygen-free environment.