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Graham, Emily B ¹ ; McKnight, Diane ² ; Nemergut, Diana R. ³

¹ University of Colorado at ºù«ÍÞÊÓƵ, INSTAAR
² University of Colorado at ºù«ÍÞÊÓƵ, INSTAAR
³ University of Colorado at ºù«ÍÞÊÓƵ, INSTAAR

Biogeochemical cycling within sediments can lead to the bioaccumulation of mercury in a variety of plants and animals. While inorganic mercury deposition does not pose a direct threat to human health, its methylated form rapidly bioaccumulates up trophic levels. Dissolved organic matter (DOM) pools act as strong controls over mercury cycling, particularly in DOM-rich wetlands. Importantly, the chemical characteristics of the DOM pool may interact with the abundance and activity of the microorganisms that mediate mercury cycling to determine the dominant effect of DOM on mercury methylation. To examine the role of DOM in regulating mercury methylation, sediment from wild rice and unvegetated patches in the St. Louis River Estuary at the base of Lake Superior was amended with DOM leached from fresh or partially decomposed wild rice litter to simulate loadings from ecologically relevant portions of the DOM pool. Over a 28-day period, incubations were monitored for changes in DOM characteristics, extracellular enzyme activities, microbial community structure, and methylmercury concentrations. Methylmercury production appears to be carbon-limited in unvegetated areas, but not in vegetated areas, which produced higher overall concentrations of methylmercury. Our also suggested that mercury methylation may be facilitated by redox active portions of the DOM pool to a greater degree in vegetated areas than unvegetated areas. These results indicate a potential dual role for DOM in mercury methylation and suggest that the drivers of mercury methylation may differ between vegetated and unvegetated zones.