Biogeochemical Controls on Environmental Mercury Methylation

Contamination of food chains by methyl mercury (meHg) is a complex, global issue that can have an impact even on relatively pristine areas. Sulfate-reducing bacteria (SRB) have been identified as the primary mercury methylators in the environment in a wide range of aquatic systems. It is critical to understand the chemical and physiological factors that control mercury methylation in order to be able to predict or mitigate the effects of mercury contamination, however these factors are extremely complex. Though most studies dealing with biotic mercury methylation to date have dealt with pure, planktonic cultures of SRB, it is now known that the majority of bacteria live in attached communities, or biofilms, and often coexist with other bacterial communities.;This thesis addressed several aspects of mercury contamination and methylation. First, mercury and methyl mercury concentrations in different organs of four different species of sea turtles, which may serve as an indicator organism for ecosystem health, were measured. Some evidence of biomagnification and bioaccumulation was observed through correlations of mercury concentration with size of the individuals, as well as differences in concentrations measured in herbivorous versus carnivorous species.;Metabolic differences in mercury methylation by planktonic versus biofilm cultures of sulfate-reducing bacteria were investigated by using specific inhibitors known to effect enzymatic pathways believed to be important to mercury methylation. Biofilm cultures were found to methylate at a higher rate than planktonic cultures of the same organism. In addition, enzyme inhibitors that did not affect methylation in planktonic cultures were found to depress methylation rates in biofilm cultures, indicating that methylation pathways may be different between the two growth modes.;The importance of sulfur chemistry, in particular the presence of elemental sulfur, to mercury methylation was investigated in pure cultures of SRB, as well as mixed cultures of SRB and sulfur-oxidizing bacteria. The presence of elemental sulfur was generally found to decrease methylation rates in both pure and mixed cultures. The presence of sulfur-oxidizers was found to increase methylation rates of SRB, possibly by maintaining favorable chemical speciation in the environment, as well as by stimulating growth.