1 Geomicrobiology Journal, 29:667–674, 2012 Sulfate reducing bacteria and mercury methylation in the water column of the Lake 658 of the Experimental Lake Area Darío Achá 1 , Holger Hintelmann 2 and Cecilia A. Pabón 2 1 Environmental and Life Sciences Graduate Program, Trent University, 1600 West Bank Drive, Peterborough, Ontario, K9J 7B8 Canada 2 Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario, K9J 7B8 Canada Abstract Sulfate reducing bacteria (SRB) appear to be the main mediators of mercury methylation in sediments, which are deemed to be major sites of methylmercury (MMHg) production. However, recent studies have also found significant MMHg formation in the water column of lakes across North America. To investigate the potential involvement of SRB in mercury methylation in the water column of a stratified oligotrophic lake, two of the main families of SRB (Desulfobacteraceae and Desulfovibrionaceae) were quantified by Real-Time Polymerase Chain Reaction of the 16S rRNA gene. MMHg production was measured applying a stable isotope technique using 198 HgCl. Methylation assays were conducted at different water depths and under stimulation with lactate, acetate or propionate and inhibition with molybdate. Desulfobacteraceae and Desulfovibrionaceae16S rRNA gene copies in control samples accounted for 0.05% to 33% and <0.01% to 1.12% of the total bacterial 16S rRNA, respectively. MMHg formation was as high as 0.3 ng L -1 day -1 and largest in lactate amended samples. Strain isolation was only achieved in lactate amended media with all isolated strains being SRB belonging to the Desulfovibrio genus according to their 16S rRNA gene sequence. Isolated strains methylated between 0.06 and 0.2% of 198 HgCl per day. Acetate and propionate did not stimulate mercury methylation as much as lactate. Two strains were identified as Desulfovibrio sp. 12ML1 (FJ865472) and Desulfovibrio sp. 12ML3 (FJ865473), based on partial sequences of their 16S rRNA and DSR gene. Methylation assays and bacteria characterization suggest that Desulfovibrionaceae is an important mercury methylators in Lake 658. Keywords: mercury, methylmercury, sulfate reducing bacteria, water column, mercury methylation, Desulfovibrionaceae Introduction Mercury contamination of fish is a major problem across pristine and polluted lakes in North America. Fish having mercury contents above health advisory levels suggested by the World Health Organization and the U.S. Environmental Protection Agency are reported all across North America up to the Arctic region. Although different anthropogenic and natural sources of mercury contamination are known, mercury concentrations in lakes are often relatively low. The contamination levels in fish appear to be closely related to the amount of methylmercury (MMHg) produced in the lakes (Gilmour and others 1998). Most mercury in fish is found as MMHg (Harris and others 2003) and is incorporated mainly through food. Since MMHg bioaccumulates in organisms and biomagnifies through the food chain (Watras and others 1998), the highest concentrations of MMHg are generally present in predatory fish at the top of the food chain. MMHg in aquatic ecosystems is linked to bacterial activity (e.g. Pak and Bartha 1998). In sediments and slurries, sulfate reducing bacteria (SRB) have repeatedly been found to be major mercury methylators (Compeau and Bartha 1985; King and others 2000). However, recent findings suggest that iron reducing bacteria and other unknown groups may also be involved (Fleming and others 2006; Loseto and others 2004). Although most research on mercury methylation has focused in the past on sediments, recent studies also reported significant mercury methylation in the water column of lakes (Eckley and others 2005; Eckley and Hintelmann 2006; Watras and others 2005). As well, it has been suggested that a large portion of MMHg is produced in the water column and sediments being mostly a sink for this MMHg (Qureshi and others 2009). It has been proposed that SRB are involved in this process (Eckley and Hintelmann 2006; Watras and others 2005), but since SRB are a phylogeneticaly diverse and methabolically heterogeneous group (Rabus