Simultaneous radioassays of bacterial production and mercury methylation in the periphyton of a tropical and a temperate wetland.

Laboratory radioassays were made to study mercury (Hg) methylation together with bacterial production in the periphyton of two aquatic macrophytes, the submerged Myriophyllum spicatum, from a constructed wetland in Sweden and the floating Eichhornia crassipes, from a eutrophied tropical lake in Brazil. Time course incubations were made by addition of (203)HgCl(2) and the methylmercury formed was extracted at pre-defined time intervals. Bacterial production ((14)C-leucine incorporation) was measured at the same time intervals, with plants removed from parallel incubations made with and without addition of cold HgCl(2). For E. crassipes, higher methylmercury production was observed at elevated bacterial production, whereas for M. spicatum, the bacterial production was significantly lower, and Hg methylation was below the detection limit. The combined results confirm the importance of microbial processes for Hg methylation, although other factors are known to influence this process in complex ways. The addition of Hg did not significantly influence bacterial production, while the incubation temperatures used (25 and 35 degrees C) resulted in different methylation rates. Radiotracer techniques for measurements of bacterial production such as (14)C-leucine uptake can provide useful insights into the Hg cycle in aquatic environments, and our data suggest that they may be used as a proxy of mercury methylation potentials.

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 ((200)HgCl(2) and CH(3)(199)HgCl) and labeled ((203)HgCl(2)) 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 (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.