Assessing mercury contamiantion in the Amazon

3. Discussion

3.5.2. Mercury methylation rates in the Amazon

The eco-toxicological effect of inorganic mercury inputs to the aquatic environment is dependent on its bioavailabilty. The water-soluble fraction of inorganic mercury is sometimes used as a measure for its bioavailabilty. The aqueous solubility of elemental mercury, i.e. from gold mining activities, is low (0.025 ppm at 20 oC). However, low pH and oxidizing agents in the aquatic environment may enhance mercury's solubility. The water-soluble fraction in soils is also thought to be low, however no studies addressing this issue were found for the Amazon region. In future studies, it would be interesting to relate the soluble fraction of inorganic mercury in soils to soil erosion.
Hg-methylation rates are usually associated with sediments. However, submerged roots of floating macrophyte mats are efficient traps for suspended particles and have a high surface area for the fixation of periphyton and bacteria. Guimaraes et al. (2000) showed that methylation potentials in untreated macrophyte roots were higher than in macrophyte roots stripped from associated solids, and methylation potentials were higher in solids stripped from macrophyte roots than in river sediments. Experiments with stimulation and inhibition of sulphate reduction activity suggested that SRB in macrophyte roots as well as in sediments are the main methylating bacteria (Guimaraes et al., 1999; Mauro et al., 1999).
Furthermore, data from various studies showed that on average methylation potentials in the submerged parts of macrophytes were much higher than in underlying lake sediments at the same sites (Guimaraes et al., 2000). Guimaraes et al., 2000 concluded that average net Hg-methylation in sediments and aquatic macrophytes were, 0.6% and 13.8%, respectively. However, part of the range between aquatic macrophytes and sediments could arise, from variations in the amount of added total Hg from study to study, caused mainly by the use of 203Hg solutions with different ages and belonging to different 203Hg lots. High methylmercury concentrations were also found in filtered water sampled in floating macrophyte mats (Guimaraes et al., 2000; Mauro et al., 1999). These findings are interesting because high methylmercury concentrations in filtered water are highly bioavailable compared to methylmercury bounded to sediment particles. Additionally, the roots of floating macrophytes are an essential source of food and shelter for large populations of fish and invertebrates. Commonly, methylmercury is bioaccumulated in the bottom level of the food chain and is then biomagnified up through the trophic levels of the aquatic food chain.
Guimaraes et al. (2000) also showed that flooded soils and semi aquatic sediments had higher hg-methylation potentials, than river sediments. The high net Hg-methylation potentials found in newly flooded soils are interesting, because vast areas of the Amazon are flooded in annual cycles. River impoundment is similar to flooded soils and also shows high Hg-methylation rates. Hg levels in reservoir fishes are frequently high, even in the absences of aquatic point source of mercury (Porvari, 1995).
In summery, our understanding of in situ Hg-methylation is still limited partly because of inadequate research techniques. The formation of methylmercury in the Amazon region seems to differ from temperate regions by having multiple substrates, e.g. sediments, floating macrophyte mats and flooded soils. Also, the unique nature of aquatic and semi-aquatic systems of the Amazon seems to favor net-methylmercury formation.

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