The Mechanism of Accumulation of Vanadium by Ascidians: Some Progress towards an Understanding of this Unusual Phenomenon

Since the discovery of vanadium in the blood cells (coelornic cells) of an ascidian by Henze in 1911, this unusual phenomenon has attracted the interest of many investigators. We started our studies by examining the vanadium contents of several tissues from 20 ascidian species, collected not only from Japanese waters but also from the Mediterranean since about 18 years ago, using an extremely sensitive method, namely, neutron-activation analysis. We found the highest concentration of vanadium, 350 mM, in the blood cells of Ascidia gemmata which belongs to the suborder Phiebobranchia. This concentration of vanadium is 107 times higher than that in seawater. Among the approximately ten types of blood cells, the signet ring cells were revealed to be the true vanadocytes by a combination of cell fractionation and neutron-activation analysis. Of the vanadium in these vanadocytes, 97.6% was in the +3 oxidation state while the rest was in the +4 oxidation state. The contents of the vanadocytes in A gemm...