Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Science



Collaborative Specialization

Environment and Sustainability


Branfireun, Brian A.


Mercury is a ubiquitous element with a complex geochemical cycle. Aquatic ecosystems such as wetland soils convert inorganic mercury to organic, neurotoxic methylmercury though the activity of sulphate-reducing bacteria (SRB). Sulphate stimulates the activity of SRB, and the production of methylmercury in these environments. My aim was to investigate the effect that legacy sulphate has on Hg methylation in northern peatlands through a laboratory sulphate addition experiment with differentially sulphate-exposed peats and a field study of peatlands subjected to different levels of sulphate. Results from the laboratory study indicate that peatlands in regions of higher atmospheric sulphate deposition show enhanced Hg methylation responses compared to pristine peatlands, while field results indicate that sulphate deposition increases Hg methylation dependence on other nutrients as opposed to sulphate supply. Management for peatlands impacted by industrial sulphate sources will have to consider legacy sulphate deposition within peatland geochemical context to mitigate potential Hg methylation.

Summary for Lay Audience

Mercury (Hg) is an element that occurs everywhere in the natural environment. Mercury in nature rarely reaches levels that would be harmful to the health of wildlife or humans. However, in wetland soils, Hg transforms into methylmercury (MeHg) which is the form of Hg that can easily build up in living tissue. Methylmercury is produced by small organisms known as sulphate-reducing bacteria (SRB) that live in wetland soils. Because SRB use sulphate (SO42-) to live and grow, giving SRB more SO42- has the potential to increase the amount of MeHg they produce. Human activities such as fossil fuel burning and mining can be a source of SO42- to wetlands such as peatlands, increasing MeHg production. Peatlands are wetlands that are covered with large amounts of built up dead and decaying plant matter known as peat. The first goal of my thesis was to link SO42-in peatlands from human sources to MeHg production in peatlands. My second goal was to determine if past SO42-release to peatlands affects the ability of peatlands to produce MeHg in the future. In my laboratory study, I found that when peat taken from an area that had higher levels of atmospheric SO42-deposition is given more SO42-, these peats are able to produce more MeHg compared to peats from areas with lower atmospheric SO42-deposition. In my field study, I found that in peatlands that have high levels of SO42-additions such as those surrounding a mine, SO42- does not have a large effect on MeHg production because the SRB are not limited by SO42-. The supply of other nutrients such as carbon that the bacteria need for growth become more important for MeHg production instead. These studies show that MeHg production in peatlands is not simply linked to the amount of sulphate in the environment but is also influenced by other factors that control the growth of SRB. Recovery plans need to consider not only the level of SO42-that has been added to these wetlands, but the balance of other nutrients as well and what this means for MeHg production in the future.