The possible involvement of methylcobalamin in the production of methyl iodide in the marine environment

Abstract

Methyl iodide (CH3I) is a biogenic iodocarbon of marine and estuarine waters, and is an important component in the biogeochemical cycling of iodine. A possible significant abiotic marine source of CH3I is the reaction of methylcobalamin (CH3B12) with either iodide or molecular iodine (I2) in seawater. These reactions were shown to occur in seawater and buffered water. The reaction between iodide and CH3B12 was apparently second order. The pH had a slight but significant effect on the reaction rate at pH between 7 and 8, and a more pronounced effect at a pH < 7. Chloride, bromide, iodate and bicarbonate had no significant effects on CH3I production at the standard reaction pH of 8.5. Iodate in the reaction mixture did not affect the reaction rate at pH > 7.5, however, at pH < 7.5 the presence of iodate increased the rate of CH3I production (9 fold at pH 5) due to the probable formation of I2. The addition of aquocobalamin to the reaction mixture (CH3B12 + iodide) enhanced CH3I production (63% at pH 5; 38% at pH 8.5) presumably by catalyzing the oxidation of iodide to I2. Molecular iodine also reacted with CH3B12 to form CH3I. The separate addition of CH3B12, I− or I2 to coastal seawater, for a final concentration of 0.22 mM, 4.8 mM and 25μM, respectively, resulted in the detectable formation of CH3I. The significance of these reactions to the formation CH3I in the oceans is discussed.