Bacterial communities from a mercury addition experiment

Permeable reactive barriers (PRBs) remove nitrogen from groundwater by enhancing microbial denitrification. The PRBs consist of woodchips that provide carbon for denitrifiers, but these woodchips also support other anaerobic bacteria, some of which have the ability to methylate inorganic mercury present in groundwater. Methylmercury is hazardous to human health, so it is essential to understand whether PRBs promote mercury methylation. We examined microbial communities and geochemistry in fresh water and sulfate-enriched PRB mesocosms by spiking replicates of both treatments with mercuric chloride. We hypothesized that mercury addition would alter bacterial community composition to favor higher abundances of known methylating taxa and that the sulfate-rich mesocosms would produce more methylmercury after Hg addition due mainly to an increase in abundance of sulfate reducing bacteria (SRB). However, methylmercury output at the end of the experiment was not different to output at the beginning, due in part to coupled Hg methylation and demethylation. Furthermore, nitrate removal became incomplete after mercury addition in the sulfate mesocosms, but became complete again after a few days, demonstrating resilience of the denitrifying community. Since methylmercury output did not increase and nitrate removal was not permanently affected, PRBs could be a low cost approach to combat eutrophication.