Environmental Stability Determines the Cytotoxicity of Metal–Organic Frameworks to a Nitrogen-Fixing Bacterium Azotobacter vinelandii

During widespread applications of metal-organic frameworks (MOFs), the environmental hazards and risks of MOFs have aroused great concerns. In this study, we aimed to reveal the importance of the environmental stability of MOFs on their toxicity. Two Zn-MOFs, namely, ZIF-8 with high aqueous stability and Zn-BDC with low aqueous stability, were compared directly in the toxicological evaluations of a nitrogen-fixing bacterium Azotobacter vinelandii. Zn-BDC showed strong cytotoxicity at 100 mg/L and higher, inducing growth inhibition, cell apoptosis, structural changes, oxidative damage, and, consequently, loss of nitrogen fixation ability. In contrast, ZIF-8 was nearly nontoxic to A. vinelandii. The transcriptome analysis showed that Zn-BDC directly disturbed the ribosome pathway and lowered the expression level of nitrogen-fixing nif cluster genes. On the other hand, ZIF-8 stress could regulate the flagellar assembly, siderophore group nonribosomal peptide biosynthesis, bacterial chemotaxis, and amino sugar and nucleotide sugar metabolism pathways to promote the cell growth of A. vinelandii. Beyond that, the toxicity of Zn-MOFs to A. vinelandii was associated with the release of Zn²⁺, but Zn-MOFs were less toxic than the mixtures of their starting materials. Overall, our results suggested that the environmental stability of Zn-MOFs determined their environmental toxicity through different molecular pathways. Designing stable MOFs is preferred due to environment-friendly considerations.