tons of bromomethane annually. Most naturally occurring organobromine compounds are produced by this enzyme, catalyzing the following reaction (R-H is hydrocarbon substrate): A vanadium nitrogenase is used by some nitrogen-fixing micro-organisms, such as Azotobacter. In this role, vanadium serves in place of the more common molybdenum or iron, and gives the nitrogenase slightly different properties. Vanadium accumulation in tunicates Vanadium is essential to tunicates, where it is stored in the highly acidified vacuoles of certain blood cell types, designated vanadocytes. Vanabins (vanadium binding proteins) have been identified in the cytoplasm of such cells. The concentration of vanadium in the blood of ascidian tunicates is as much as ten million times higher than the surrounding seawater, which normally contains 1 to 2 µg/L. The function of this vanadium concentration system and these vanadium-bearing proteins is still unknown, but the vanadocytes are later deposited just under the outer surface of the tunic, where they may deter predation. Fungi Amanita muscaria and related species of macrofungi accumulate vanadium (up to 500 mg/kg in dry weight). Vanadium is present in the coordination complex amavadin in fungal fruit-bodies. The biological importance of the accumulation is unknown. Toxic or peroxidase enzyme functions have been suggested. Mammals Deficiencies in vanadium result in reduced growth in rats. The U.S. Institute of Medicine has not confirmed that vanadium is an essential nutrient for humans, so neither a Recommended Dietary Intake nor an Adequate Intake have been established. Dietary intake is estimated at 6 to 18 µg/day, with less than 5% absorbed. The Tolerable Upper Intake Level (UL) of dietary vanadium, beyond which adverse effects may occur, is set at 1.8 mg/day. Research Vanadyl sulfate as a dietary supplement has been researched as a means of increasing insulin sensitivity or otherwise improving glycemic control in people who are diabetic. Some of the trials had significant treatment effects, but were deemed as being of poor study quality. The amounts of vanadium used in these trials (30 to 150 mg) far exceeded the safe upper limit. The conclusion of the systemic review was "There is no rigorous evidence that oral vanadium supplementation improves glycaemic control in type 2 diabetes. The routine use of vanadium for this purpose cannot be recommended." In astrobiology, it has been suggested that discrete vanadium accumulations on Mars could be a potential microbial biosignature, when used in conjunction with Raman spectroscopy and morphology. Safety All vanadium compounds should