Molybdenum can exist in multiple oxidation states (III to VI), but is most frequently found as Mo(VI) in molybdate (MoO₄^2-). Molybdenum is an essential trace element and is found in microbial enzymes more frequently than the other group V elements. Enzyme-bound molybdenum has been demonstrated to exist in only two forms, both of them as distinct cofactors that can be removed from their respective enzymes. One form is the MoFe cofactor of nitrogenase; the other is the molybdopterin cofactor of other molybdenum-containing enzymes (reviewed by Kisker et al, 1997).

The regulation of molybdate transport has been studied, and molybdenum import is known to occur via inducible ABC-type transporters, with transport also occurring through sulfate uptake systems and nonspecific anion transporters (reviewed by Self et al, 2001). A variety of bacteria and archaea can reduce molybdenum(VI) to molybdenum(V) under aerobic and anaerobic conditions (reviewed by Lovley, 1993) . The mechanisms of prokaryotic molybdenum reduction are not known.

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Database of Molybdenum in Human Health and the Environment