The production of the methylmercury in the aquatic environment is controlled in part by the bioavailability of inorganic mercury to anaerobic bacteria that methylate mercury. In sediment porewater, inorganic mercury speciation is controlled by sulfide and natural organic matter, resulting in a wide array of compounds that include NOM-coated mercury sulfide nanoparticles as reaction intermediates of heterogeneous mineral precipitation. Here, we show that sulfate-reducing bacteria can produce methylmercury when exposed to HgS nanoparticles. While bacteria cultures methylated nanoparticulate mercury at a slower rate compared to cultures exposed to dissolved forms of mercury, methylation of the nanoparticles was considerably faster than larger microscale HgS particles, even when normalized to specific surface area. Furthermore, the methylation potential of HgS nanoparticles decreased with storage time of the nanoparticles in their original stock solution. The methylation of mercury derived from nanoparticles (in contrast to the larger particles) would not be predicted by traditional models of mercury bioavailability and was probably caused by the high degree of surface contact between nanoparticles and bacteria. Additional characterization of the nanoparticles was performed to demonstrate that their structure is less crystalline and more chemically labile to thiol exchange when compared to HgS microparticles. These results suggested that dissociation of nanoparticles could be occurring directly outside cell surfaces. Our results provide a new paradigm for understanding mercury methylation potential in sediments and provide a basis for assessing and remediating methylmercury hotspots in the environment.

Production of methylmercury by sulfate-reducing bacteria exposed to mercuric sulfide nanoparticles

243rd National Meeting (2012)

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