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, δ 15 N 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.