A new pathway of mercury methylation occurring via a common fumigant, methyl iodide, in sunlight and water, has been identified this week in Nature Communications. This research suggests that the fumigant is not as safe as previously thought and that comprehensive risk assessments are now necessary.
Addition of methyl iodide (CH3I) to the environment has been rapidly increasing since the 1990s, after it was suggested as a replacement for the ozone-depleting fumigant methyl bromide (CH3Br). Methyl iodide was believed to be a safe alternative with only a minimal impact on the environment. Despite being able to methylate some metals in aquatic environments, methyl iodide was thought incapable of directly methylating the dominant species of mercury, inorganic Hg2+, into the more reactive and damaging methylmercury.
Guibin Jiang and colleagues show that methylation of inorganic mercury by methyl iodide can in fact occur in natural waters via a previously-unrecognised two-step process mediated by sunlight. This photo-methylation, due to the presence of methyl iodide, may lead to increased methylmercury in aquatic environments. The future use of methyl iodide as a fumigant should be carefully considered in the light of these findings.