Seasonal changes in bromoperoxidase activity in coralline algae (Corallinaceae) are responsible for the production of volatile halogenated compounds. SDS-polyacrylamide gel electrophoresis (PAGE) of a crude protein extract showed that the concentration of this enzyme was almost constant throughout the year. Therefore, the enzyme activity in vivo changed seasonally due to a structural alteration. To elucidate this, the metal content of this enzyme at different states of activity was measured. The results revealed that the enzyme activity is controlled by the incorporation of vanadate ions, less than 1.2 mol mol enzyme - 1 , in the active site of the enzyme.