Shewanella oneidensis couples anaerobic oxidation of lactate, formate, and pyruvate to the reduction of vanadium pentoxide (V V ). The bacterium reduces V V (vanadate ion) to V IV (vanadyl ion) in an anaerobic atmosphere. The resulting vanadyl ion precipitates as a V IV -containing solid.Vanadium is a transition metal which, at neutral pH, can exist in two oxidation states, V IV (vanadyl ion, cationic species VO 2ϩ ), and V V (vanadate ion, anionic species, H 2 VO 4 Ϫ ) (10, 11).The environmental chemistry of vanadium is complex. Vanadium is an abundant element that has proven to be a valuable resource for different industrial applications such as vanadium alloys, oxidation catalysis in sulfuric acid manufacturing and automobile catalytic converters, photographic development, textile dyeing, and ceramic coloring. A number of bacteria are able to reduce metal compounds, most commonly iron and manganese, through anaerobic reduction. Some organisms are known to be able to reduce other metals such as arsenic, mercury, selenium, uranium, technetium, chromium, molybdenum, gold, silver, and copper (4,8,13). The microbial reduction of vanadate has also been reported (1,14). To date, two Pseudomonas strains have been described to be capable of reducing vanadium (5,15). In this study we demonstrate that the gram-negative facultative anaerobic nonfermenting bacterium Shewanella oneidensis (6) can also reduce vanadium V V . Vanadium uptake on solid media. Colonies of S. oneidensis were grown on Luria-Bertani 1% agar plates containing 5 mM vanadate at 28°C for 48 h. Plates were then exposed to hydrogen sulfide in a sealed container. Upon formation of metal sulfide, plates were examined for changes in the regions surrounding or inside bacterial colonies. A halo could be seen indicating possible sequestration of the metal. Darkening of the cells indicated accumulation or possible reduction of the metal (data not shown).Vanadate detection. To facilitate the detection and quantification of the reduction of vanadate to V IV , a vanadate detection assay was designed, based on a detection assay for chromium described by Sandel (12). Vanadate was thereby detected on the basis of its reaction with diphenylcarbazide (DPC) in acid. The assay solution was made by addition of 1% (wt/vol) DPC in acetone to an equal volume of 2 M H 2 SO 4 . A volume of 500 l of diluted sample was added to the same volume of assay solution. Absorbance was measured at 320 nm after 15 min. A standard curve was made with vanadium pentoxide dilutions. Vanadyl ions do not react in this assay.Anaerobic vanadium reduction. S. oneidensis was grown aerobically on a rotary shaker (150 rpm) at 28°C in LB medium containing 2 or 10 mM V 2 O 5 and anaerobically with a Coy anaerobic chamber (Coy Laboratories, Grass Lake, Mich.) in phosphate-buffered defined medium (7) containing 10 mM lactate as the electron donor and 2 or 10 mM V 2 O 5 as the electron acceptor. An increase in lag phase could be detected in cultures grown with 10 mM but not with 2 mM V 2 O 5 compared to c...