Reversible environmental impacts of iron-based metal-organic framework MIL-53(Fe) on nitrogen-fixing bacterium Azotobacter vinelandii

With the widespread use of metal organic framework materials (MOF), they inevitably enter the environment and pose potential threats to diverse organisms. Herein, we investigated the impacts of iron-based metal organic framework MIL-53(Fe) on the growth and the activity of nitrogen-fixing bacterium Azotobacter vinelandii. MIL-53(Fe) inhibited bacterial growth at a relatively low concentration (50 mg/L) and almost complete inhibition occurred at 140 mg/L. However, MIL-53(Fe) did not cause cell death even at high concentrations up to 130 mg/L. MIL-53(Fe) did not penetrate the cell wall and no cellular uptake was observed. MIL-53(Fe) lowered nifH gene expression of A. vinelandii, resulting in the attenuation of nitrogen fixation activity. MIL-53(Fe) did not arouse meaningful oxidative damage, but there were significant membrane leakage and pinholes in cell wall, indicating the mechanical damage. In addition, MIL-53(Fe) was stable in aqueous systems without obvious Fe release. After removing MIL-53(Fe) from the culture system, there was recovery of growth and nitrogen fixation activity of A. vinelandii that had initially been exposed to 10–130 mg/L of MIL-53(Fe), implying that the damage caused by MIL-53(Fe) to A. vinelandii was reversible. These findings highlighted the reversible environmental toxicity of MOF materials to microorganisms in the nitrogen cycle.