Molybdenum is an essential trace element - in plants, in enzymes which catalyse nitrogen fixation (nitrogenase) and nitrate reduction (nitrate reductase); in animals, in enzymes involved in, e.g., nitrogen metabolism (e.g., xanthine oxidase) and sulfur metabolism (sulfite oxidase). Some soils, especially acid soils require supplemental molybdenum for healthy plant life. For human beings molybdenum features as a component of over-the-counter vitamin and mineral supplements. Molybdenum is generally regarded as non toxic for humans.
The vital role of molybdenum in biology derives from its affinity for sulfur, its finely balanced oxidation states, and its ability in its higher oxidation states to transfer oxygen atoms.