Graphene quantum dots enhance the biological nitrogen fixation by up-regulation of cellular metabolism and electron transport

Graphene quantum dots (GQDs) are useful nanomaterials of excellent water solubility, biocompatibility and optical stability for various applications. In this study, we achieved the nitrogen fixation activity enhancement of autogenous azotobacters (Azotobacter vinelandii) by GQDs and revealed its mechanism. GQDs (1–10 mg/L) accelerated the logarithmic growth phase of A. vinelandii, making bacteria enter the plateau earlier. Biocompatible GQDs did not cause cell death and oxidative damage of A. vinelandii. The expression levels of metabolism and nitrogen fixation related genes were significantly up-regulated by GQDs, thereby enhancing the activity of nitrogenase. The nitrogen fixation activity of A. vinelandii reached 471.7 % of the control upon the exposure to GQDs. The intracellular polyhydroxyalkanoate vesicles were consumed to provide energy for nitrogen fixation, which involved with a chain reaction of carbohydrate metabolism, lipid metabolism, and energy metabolism pathways accompanied by significant increases in electron transfer rate. The synergistic effect of GQDs on A. vinelandii further facilitated the augmentation of soil nitrogen content in Qinghai-Tibet Plateau. Our results provided an effective approach to enhance the biological nitrogen fixation by GQDs.