Lavoie, Noémie2025-01-162025-01-162025-01-16http://hdl.handle.net/10393/50099https://doi.org/10.20381/ruor-30862Mercury (Hg) is a pollutant of global concern and a potent neurotoxin that bioaccumulates and biomagnifies in terrestrial and aquatic food webs as methylmercury (MeHg). Though now heavily regulated, over a century of legacy anthropogenic emissions and releases of Hg into the environment still affect us today. Anaerobic microorganisms are principally responsible for producing MeHg from new and legacy pollution, a process which depends on the bioavailability of inorganic Hg. Hg redox reactions, such as HgII reduction to gaseous elemental Hg0, play a key role in determining the availability of Hg in the environment. In anoxic environments, certain anaerobic and phototrophic microbes can catalyze the reduction of HgII and may compete for other vital resources and nutrients with Hg methylators. Currently, the physiological mechanisms driving this anaerobic redox reaction are poorly understood. The main objective of my thesis is to uncover physiological and environmental controls of anaerobic and phototrophic HgII reduction pathways. I used a combination of microbial physiology, molecular biology, and trace Hg analytical techniques to study HgII reduction by anoxygenic phototrophs and fermenters from the Heliobacteria family. In Chapter 2, I discovered a novel HgII-reducing Heliobacteria from rice paddies. The magnitude of HgII reduced was enhanced by assimilating reduced sulphur sources, leading us to propose that anabolic pathways that consume reducing power can influence HgII reduction. In Chapter 3, I investigated the influence of nitrogen availability and metabolism on the magnitude of HgII reduction. Here, contrary to sulphur assimilation, nitrogen did not influence the magnitude of HgII reduced. In Chapter 4, I demonstrated how the ability to reduce HgII is tied to phototrophic and fermentative reducing power-generating machinery. In Chapter 5, I showed that HgII reduction depends on interactions between Hg and cellular thiols. Lastly, in Chapter 6, I explored the possibility of HgII reduction in phylogenetically distant anoxygenic phototrophs from the Chloroflexota phylum. At its core, my thesis underscores the relationships between HgII reduction and cellular pathways that produce or consume reducing power. Though still unresolved, my thesis significantly contributes to uncovering the cellular mechanism catalyzing anaerobic HgII reduction.enAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-nd/4.0/MercuryMicrobiologyBiogeochemistryPhysiologyBacteriaAnaerobeThesis