Unexpected Sink [+]Enlarge Credit: Rose-Marie Kronberg

A particularly toxic form of mercury may get destroyed in a place where researchers previously thought it was generated. A recent study of wetlands in Sweden has found that some, instead of creating methylmercury, serve as a sink for it (Environ. Sci. Technol., DOI: 10.1021/es303543k).

Past research in wetlands in Canada, the Florida Everglades, and elsewhere has pinpointed wetlands such as bogs, fens, and swamps as a source of methylmercury. This form of the metal is neurotoxic and can be readily taken up by fish and passed along to birds and humans. The mucky freshwater bodies receive inorganic mercury from the air, rain, and stream runoff. In the wetlands, sulfate-reducing microorganisms transform the mercury by adding a methyl group, making the metal bioavailable to fish and other creatures.

But a study of swamps in southern Sweden shows that wetlands that are home to black alder trees (Alnus glutinosa) may treat incoming mercury differently. Researchers led by Ulf Skyllberg of the Swedish University of Agricultural Sciences in Umeå measured various forms of mercury in the water flowing into the wetlands from upstream sources, in the wetland soils, and in water flowing downstream of the sites.

They found that one alder swamp released one- to two-thirds as much methylmercury as flowed into it, according to water samples taken from 2006 to 2010. They also collected soil samples five times over that period, which showed methylmercury was not accumulating there. The researchers also looked at an hour or so of flow at eight other alder-dominated wetlands during a May rainstorm and determined that more methylmercury flowed in than out.

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But over the same four years, seven other wetland sites without alders released more methylmercury downstream than had flowed in. The team does not yet know what caused the difference, but they suspect that the nutrients present in alder swamps influence the growth of microbes that break down methylmercury.