Abstract:

Sulfate-reducing bacteria (SRB) are the primary producers of methylmercury in freshwater and estuarine environments, although little is known about the physiology and biochemistry of mercury methylation in SRB. In the ocean, SRB only live in deep sediments, and due to the long time scales of ocean mixing, SRB are unlikely to be a significant source of MeHg to the ocean. This thesis is dedicated to investigating the mechanisms of mercury methylation in SRB, and to identifying a potential source of methylmercury in the ocean.

Previous research on one SRB strain implicated a corrinoid-containing enzyme (which contains cobalt in its active center) in the acetyl-CoA pathway as key to mercury methylation. Through the use of enzyme activity assays, mercury methylation assays, and specific inhibitor experiments, we have identified four incomplete-oxidizing SRB strains that clearly do not utilize the acetyl-CoA pathway, and possibly do not use corrinoid-containing enzymes, for mercury methylation. However, all complete-oxidizing SRB that use the acetyl-CoA pathway for major carbon metabolism can methylate mercury. To investigate the role of corrinoid-containing enzymes in mercury methylation, cobalt-limitation experiments were performed on the incomplete-oxidizing Desulfovibrio africanus and the complete-oxidizing Desulfococcus multivorans. D. multivorans became cobalt-limited, growing 40% slower than metal-replete treatments, and produced 3 times lower methylmercury concentrations per cell than metal-replete conditions. Lower cell concentration, growth rate, and Hg bioavailability couldn't account for the large decrease in methylmercury produced in cobalt-limited D. multivorans cultures. D. africanus , grown in the absence of added cobalt and vitamin B 12 , grew and methylated Hg at rates comparable to metal-replete treatments. These results are consistent with mercury methylation via different pathways in the two strains, catalyzed by a corrinoid-containing methyltransferase in D. multivorans and a corrinoid-independent methyltransferase in D. africanus .

Up to 4 pM methylmercury has been measured in high temperature vent fluid from a hydrothermal vent. Abiotic mercury methylation experiments under high pressure and high temperature conditions have produced up to 33 pM methylmercury, although the source of the methyl group in unknown. This work, along with recently published work, indicates that hydrothermal vents are a source of methylmercury to the ocean.