Bioavailability of mineral-associated trace metals as cofactors for nitrogen fixation by Azotobacter vinelandii

Life on Earth depends on N-2-fixing microbes to make ammonia from atmospheric N-2 gas by the nitrogenase enzyme. Most nitrogenases use Mo as a cofactor; however, V and Fe are also possible. N-2 fixation was once believed to have evolved during the Archean-Proterozoic times using Fe as a cofactor. However, delta N-15 values of paleo-ocean sediments suggest Mo and V cofactors despite their low concentrations in the paleo-oceans. This apparent paradox is based on an untested assumption that only soluble metals are bioavailable. In this study, laboratory experiments were performed to test the bioavailability of mineral-associated trace metals to a model N-2-fixing bacterium Azotobacter vinelandii. N-2 fixation was observed when Mo in molybdenite, V in cavansite, and Fe in ferrihydrite were used as the sole sources of cofactors, but the rate of N-2 fixation was greatly reduced. A physical separation between minerals and cells further reduced the rate of N-2 fixation. Biochemical assays detected five siderophores, including aminochelin, azotochelin, azotobactin, protochelin, and vibrioferrin, as possible chelators to extract metals from minerals. The results of this study demonstrate that mineral-associated trace metals are bioavailable as cofactors of nitrogenases to support N-2 fixation in those environments that lack soluble trace metals and may offer a partial answer to the paradox.

Automated summary

Life on Earth depends on N-2-fixing microbes to convert atmospheric N-2 gas into ammonia. This study shows that mineral-associated trace metals, such as Mo, V, and Fe, can serve as cofactors for nitrogenases and support N-2 fixation when soluble trace metals are lacking. The results suggest a partial solution to the paradox of N-2 fixation.