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

摘要

Comparative tests of net mercury methylation potentials, with cultivated and macrophyte-associated periphyton and using stable (~(200)HgCl_2 and CH_3~(199)HgCl) and labeled (~(203)HgCl_2) mercury, hae been conducted in the Everglades nutrient removal area (Florida, USA) and in a tropical coastal Brazilianlake (RJ, Brazil). More methyl-mercury was formed bymacrophyte-associated (up to 17 of added ~(203)Hg(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 ~(200)Hg(II)), confirming the results obtained with labeled mercury. Simultaneous addition of ~(200)HgCl_2 and CH_3~(199)HgCl indicated that CH_3~(199)HgCl 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 ~(203)Hg(II) and 0.8 ~(200)Hg(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.