Uptake of molybdenum and vanadium by nitrogen-fixing soil bacterium using siderophores

Nitrogen fixation, the reaction that transforms atmospheric nitrogen into bioavailable ammonia and is responsible for the supply of nitrogen to Earth's ecosystems, is mediated by the enzyme nitrogenase. This reaction requires molybdenum (Mo) or vanadium (V) in addition to iron (Fe) (refs1, 2). Therefore, the availability of these trace metals may control the Earth's nitrogen cycle. Many bacteria release strong iron-binding compounds (siderophores) for iron acquisition, but the effect of these compounds on Mo and V availability to nitrogen-fixing organisms is not well understood. Here, we show that the siderophores produced in cultures of Azotobacter vinelandii while fixing atmospheric nitrogen under limitation by Mo or V form strong complexes with molybdate and vanadate, and that these complexes are available for uptake. We also show that addition of these siderophores rapidly reverses the effect of other natural binding compounds that make Mo and V unavailable for uptake. Our results resolve the long-standing debate regarding the existence of bacterial molybdophores, as well as the corollary question regarding vanadophores. We conclude that the production of strong binding compounds may be a widespread strategy for metal acquisition by bacteria, implying that the availability of Mo and V may be critical for the nitrogen cycle of terrestrial ecosystems.