Metal-organic frameworks (MOF-74) show distinct bio-effects to nitrogen-fixing bacterium Azotobacter vinelandii

Metal-organic framework (MOF) has been widely applied in many fields with unique structures and fantastic properties. Previous studies indicated that the toxicity of MOFs was determined by the metal centers. However, the distinct bio-effects of MOF materials in comparison with their central metals remained unknown. Therefore, four MOF-74 materials were synthesized using Co, Ni, Cu and Mg as central metals with the same organic ligands for toxicity evaluations on the nitrogen-fixing bacterium Azotobacter vinelandii. Mg-MOF-74 slightly stimulated the bacterial growth, while Co/Ni/Cu-MOF-74 materials inhibited the growth of A. vinelandii with higher concentrations. Particularly, free ions of Mg, Co and Cu showed higher toxicity than Mg/Co/Cu-MOF-74, but Ni²⁺ and Ni-MOF-74 had similar inhibition effects. Mg-MOF-74 had slight impact on physiological growth and metabolism pathways of A. vinelandii. Ni-MOF-74 and Cu-MOF-74 inhibited the nitrogen fixation process and down-regulated genes in glycolysis and regulation of organelle organization of A. vinelandii. Surprisingly, Co-MOF-74 enhanced the nitrogen fixation process by upregulating the ATP generation and metabolic process, organonitrogen compound metabolic process, oxidative phosphorylation and catalytic activity to supply adequate energy for nitrogen fixation process. Additionally, the released Co²⁺ from Co-MOF-74 synthesized cobalamin intracellularly to promote the electron transfer in nitrogen fixation process.