Stable isotope fractionation of carbon and mercury during the experimental formation and degradation of methylmercury

Soutenance de thèse de Luisa Malberti Quintero.

Le jury est composé de:
Gwenael Imfeld, Research Director, CNRS, LIGES, Strasbourg (Referee)
Rosa Rodriguez Martin-Doimeadios, Professor, Toledo University, Spain (Referee)
Andrea Bravo, Professor, Institut de Ciències del Mar, Barcelona, Spain (Referee)
David Point, Research Director, IRD, GET, Toulouse (PhD advisor)
Jeroen E. Sonke, Research Director, CNRS, GET, Toulouse (PhD co-advisor)
Marc de Rafelis, Professor, Toulouse University, GET, Toulouse (Examiner)

Methylated mercury (MeHg) plays a critical role in linking the mercury (Hg) and carbon (C) biogeochemical cycles. Upon methylation, inorganic Hg becomes highly toxic, bioaccumulative, and biomagnifying, posing significant risks to human health. However, crucial gaps remain in understanding the pathways (e.g., biotic vs. abiotic methylation), environments (e.g., seawater vs. freshwater vs. sediments), and factors (e.g., chloride ions (Cl⁻), temperature, pH, dissolved organic matter) controlling MeHg formation and degradation in aquatic systems. Stable isotope analysis is a valuable tool for studying these cycles and pathways. Hg isotope studies have revealed mass-independent fractionation (MIF) during photodegradation but only mass-dependent fractionation (MDF) during MeHg formation. Conversely, C isotopes in MeHg and in its pathways are underexplored. The aim of this PhD is to employ C and Hg compound-specific isotope analysis to explore key MeHg formation and degradation pathways. We examined photodegradation of mono-methylmercury (MMHg), abiotic formation of MMHg and dimethylmercury (DMHg) with methylcobalamin, dark degradation and diffusion of DMHg, and biotic methylation of MMHg by the bacterial strain BerOc1. We found that the presence of Cl⁻ ions was a major controlling factor for reaction kinetics and isotope fractionation. For example, Hg MIF diminished in the presence of Cl⁻, potentially offering a way to differentiate between freshwater and seawater origins of photodegraded MMHg. All the studied pathways introduced C isotope fractionation and corresponding isotope fractionation factors were calculated when possible and are reported in this work. Furthermore, DMHg mobility and degradation introduce Rayleigh-like fractionation to the MMHg and DMHg pools. Overall, our findings provide new data on C and Hg isotope fractionation and highlight the importance of environmental controls in these processes. While caution is needed when extrapolating to natural systems, this work provides insights that could enhance models of MeHg cycling and its controlling factors in aquatic environments.