| Mercury methylation is the most critical mercury transformation in nature because the end product, methylmercury, is a potent neurotoxin that can be a threat to wildlife reproduction and human health. It is well accepted that dominant environmental mercury methylation process is mediated biologically, and sulfate-reducing bacteria (SRB) have been identified as the primary mercury methylators in most aquatic ecosystems. While previous mercury methylation studies so far have been conducted with planktonic SRB cultures, it is generally believed that the majority of bacteria in nature indeed grow as surface-attached communities, or biofilms. To our knowledge, little work has been undertaken to investigate the role of SRB biofilms in mercury cycling, especially in the formation of methylmercury.;In this dissertation, effects of chemical speciation on uptake and methylation of mercury in bacteria were investigated. Variable chloride and sulfide chemistry experiments were carried out to modify mercury speciation under the oxic and anoxic conditions. By using model organisms (two specific strains of E. coli and SRB that were able to form biofilms) it was observed that the presence of biofilms did not drastically change the relative availability of the dominant mercury species for uptake in bacteria. However, the results suggested that E. colli biofilm cultures showed more resistance to mercury toxicity than planktonic cultures, and, more interestingly, SRB biofilms showed greater potential in mercury methylation. These results led to a further investigation regarding whether metabolic differences between biofilm- and planktonic-growth caused such differential methylation rates. Data from the inhibition tests using an inhibitor (i.e., chloroform) specific to the acetyl-CoA pathway (which has been implicated as a key mechanism for in vivo methylmercury synthesis in SRB) revealed that difference in metabolic pathways appeared between planktonic and biofilm cultures: while addition of chloroform impacted mercury methylation in biofilm cultures, planktonic cultures were not inhibited. The results indicated that two mechanisms for mercury methylation are likely in use. |
