Sediment Microbial Community Composition and Methylmercury Pollution at Four Mercury Mine–Impacted Sites K.M. Batten, K.M. Scow Department of Land, Air, and Water Resources, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA Received: 14 January 2003; Accepted: 21 March 2003; Online publication: 18 September 2003 A B S T R A C T Mercury pollution presents a globally significant threat to human and ecosystem health. An important transformation in the mercury cycle is the conversion of inorganic mercury to methylmercury, a toxic substance that negatively affects neurological function and bioaccu- mulates in food chains. This transformation is primarily bacterially mediated, and sulfate-re- ducing bacteria (SRB) have been specifically implicated as key mercury methylators in lake and estuarine sediments. This study used phospholipid fatty acid (PLFA) analysis to investigate sediment microbial community composition at four abandoned mercury mine–impacted sites in the California Coast Range: the Abbott, Reed, Sulphur Bank, and Mt. Diablo mines. Differences in watershed and hydrology among these sites were related to differences in microbial com- munity composition. The Abbott and Sulphur Bank mines had the highest levels of methyl- mercury. Floc (a type of precipitate that forms when acid mine drainage contacts lake or river water) and sediment samples differed in terms of several important environmental variables and microbial community composition, but did not have statistically different methylmercury con- centrations. Quantification of PLFA biomarkers for SRB (10Mel6:0 for Desulfobacter and i17:1 for Desulfovibrio) revealed that Desulfobacter and Desulfovibrio organisms made up higher percentages of overall microbial biomass at the Sulphur Bank and Mt. Diablo mines than at the Abbott and Reed mines. Correlations between these SRB biomarker fatty acids and methyl- mercury concentrations suggest that Desulfobacter and Desulfovibrio organisms may contribute to methylmercury production in the Abbott, Reed, and Sulphur Bank mines but may not be important contributors to methylmercury in the Mt. Diablo Mine. Introduction Human activities such as gold and silver mining, coal combustion, and municipal waste incineration currently Correspondence to: K.M. Batten; E-mail: kmbatten@ucdavis.edu Microb Ecol (2003) 46:429–441 DOI: 10.1007/s00248-003-1005-z Ó 2003 Springer-Verlag New York Inc.