Synergistic interaction between microbial nitrogen fixation and iron reduction in the environment.

Nitrogen and iron are essential yet often limiting nutrients in many ecosystems. Microbial nitrogen fixation by diazotrophs and dissimilatory ferric iron reduction are key processes that sustain nitrogen and iron availability. However, their interactions are not well understood. Here, we demonstrate a synergistic relationship between microbial nitrogen fixation and ferric iron reduction, observed in both laboratory cultures and environmental samples. In diazotrophic ferric iron-reducing bacteria, including Klebsiella grimontii N7 and Geobacter sulfurreducens PCA, nitrogen fixation enhanced heterotrophic ferric iron-reducing rates by 14.7- and 2.69-fold, respectively, and ferric iron reduction concurrently increased 15N2 fixation by up to 100%. A similar synergy was observed in an interspecies system comprising the diazotroph Azospirillum humicireducens SgZ-5 T and the dissimilatory ferric iron-reducing bacterium Shewanella oneidensis MR-1. Transcriptomic analysis revealed that nitrogen fixation upregulated pathways involved in carbon and nitrogen metabolism, including amino acid biosynthesis, glycolysis, and the tricarboxylic acid cycle (P < 0.01), thereby accelerating ferric iron reduction through nitrogen supply. In turn, ferric iron reduction stimulated organic carbon oxidation, generating the energy and reducing equivalents needed for microbial nitrogen fixation. These findings were further validated through microcosm experiments and meta-omics analyses of environmental samples from aquifers, marine sediments, hot springs, and soils, providing new insights into the coupled nitrogen, iron, and carbon cycles in natural ecosystems.