Assessing mercury contamiantion in the Amazon

3. Discussion

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 Hg⁺⁺.
High concentrations of Hg⁺⁺ are sometimes added to samples, allowing only mercury resistant species, which do not predominate in the aquatic environment, to survive. Sterilization methods may also change chemistry as well as biology in sediments. Interestingly, the 28 mM sulphate concentration in seawater makes it difficult to explain the ubiquitous occurrence of methylmercury in the marine biota as results indicate that methylmercury production, by SRB, stops at 5 mM sulphate (Weber, 1993).

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