Mercury methylation in macrophytes, periphyton, and water - comparative studies with stable and radio-mercury additions

Comparative tests of net mercury methylation potentials, with cultivated and macrophyte-associated periphyton and using stable ((HgCl2)-Hg-200 and (CH3HgCl)-Hg-199) and labeled ((HgC12)-Hg-203) mercury, have been conducted in the Everglades nutrient removal area (Florida, USA) and in a tropical coastal Brazilian lake (RJ, Brazil). More methylmercury was formed by macrophyte-associated (up to 17% of added Hg-203(II)) than cultivated (up to 1.6%) periphyton and methylmercury formation was lower in periphyton exposed to light (0.2%). High methylation was also observed for samples incubated with stable mercury isotopes (1.5-7.7% of added Hg-200(II)), confirming the results obtained with labeled mercury. Simultaneous addition of (HgCl2)-Hg-200 and (CH3HgCl)-Hg-199 indicated that (CH3HgCl)-Hg-199 had no inhibitory effect on Hg methylation. The elevated methylation potentials observed in macrophytes, because of their root-associated periphyton, might contribute significantly to the high levels of methylmercury observed in Everglades biota. Comparative mercury methylation tests were also conducted in the water of a stratified temperate lake (Wisconsin, USA). Similar trends were observed for both stable and radioisotopes, with increasing mercury methylation along the depth profile. The highest levels (0.9% Hg-203(II) and 0.8% Hg-200(II)) were obtained below the oxic/anoxic boundary, where sulfide starts to increase, probably as a result of the intense activity of sulfate-reducing bacteria in the anoxic layer.