Assessing mercury contamiantion in the Amazon
3.5. Methylation of mercury in the Amazon
2 issues will be addressed in this section. First, I will comment on a common analytical method for net methylation rates of mercury in sediments, as this has been a subject to some criticism. Then, I will refer to the literature on net methylation rates in the aquatic environment of the Amazon.
3.5.1. Common analytical method
Despite the toxicological significance of Hg methylation, this step of the Hg cycle is still poorly understood.
Net Hg-methylation is dependant on the balance between methylation and demethylation, which are influenced in a
complex and variable manner by an array of biological and physio-chemical parameters like, pH, oxygen, sulphate,
Hg and methylmercury concentration and availability, and bacterial activity.
Typically, researchers add 203Hg++ to biologically active untreated sediment and sterilized
sediment. They observe that methyl-203Hg formation occurs predominantly in biologically active sediments.
Experiments under anoxic conditions with molybdate (MnO4--), a specific inhibitor of sulphate reducing
bacteria (SRB), suggest that SRB are the main methylators of bioavailable Hg++. However, critics argue
that the mercury species added may not reflect the natural species found in sediments. The added mercury
could be more or less bioavailable than natural mercury species. Also, solutions added could change the
biology and chemistry of sediments, e.g. MnO4-- may act as an oxidizing agent or react with