Antioxidant CeO₂ doped with carbon dots enhance ammonia production by an electroactive Azospirillum humicireducens SgZ-5^T

Microbial nitrogen fixation is a fundamental process in the nitrogen cycle, providing a continuous supply of biologically available nitrogen essential for life. In this study, we combined cerium oxide-doped carbon dots (CeO₂/CDs) with electroactive nitrogen-fixing bacterium Azospirillum humicireducens SgZ-5^T to enhance nitrogen fixation through ammonium production. Our research demonstrates that treatment of SgZ-5^T cells with CeO₂/CDs (0.2 mg mL^-1) resulted in a 265.70% increase in ammonium production compared to SgZ-5^T cells alone. CeO₂/CDs facilitate electron transfer in the biocatalytic process, thereby enhancing nitrogenase activity. Additionally, CeO₂/CDs reduce the concentration of reactive oxygen species in SgZ-5^T cells, leading to increased ammonium production. The upregulation of nifD, nifH and nifK gene expression upon incorporation of CeO₂/CDs (0.2 mg mL^-1) into SgZ-5^T cells supports this observation. Our findings not only provide an economical and environmentally friendly approach to enhance biological nitrogen fixation but also hold potential for alleviating nitrogen fertilizer scarcity.