Kerin, ElizabethMicrobial mercury methylation is an integral factor controlling methylmercury concentrations within aquatic ecosystems. This thesis explores the phylogenetic distribution and biochemistry of methylation among the dissimilatory iron-reducing bacteria (DIRB). When distribution of methylation capacity among DIRB was examined, strains of <em>Geobacter</em> spp., which are closely related to mercury-methylating, sulfate-reducing <em>Deltaproteobacteria</em>, methylated mercury while reducing iron or other substrates. In contrast, no tested strains of the <em>Gammaproteobacteria</em> genus <em>Shewanella</em> produced methylmercury above abiotic controls. Mercury methylation by the cobalamin-dependent methionine synthase (MetH) pathway was examined. Heterologous expression of <em>G. sulfurreducens metH</em> in <em>E. coli</em> was used to evaluate involvement of MetH in methylation. Methylation by a clone expressing MetH and a non-expressing control clone was tested in vivo and in vitro. Methylation by the expressing clone was not significantly higher than either the control or abiotic assays in either experiment, suggesting that MetH is not involved in methylation in <em>G. sulfurreducens</em>.en-USThesisEnvironmental Sciencesmercurymethylmercuryiron reducing bacteriamethionine synthase