229 - Exploring Plankton-mediated Photochemical Transformations of Methylmercury Using Isotope Approaches

"Photochemical reduction of divalent mercury (Hg2+) and photodemethylation of methylmercury (MeHg) in the environment play an important role in the fate and transport of inorganic Hg and methylmercury (MeHg) in aquatic systems. The application of Hg stable isotope tracers has demonstrated that these photodegradation reactions can be enhanced in the presence of marine phytoplankton, suggesting that both abiotic and biotic photochemical processes affect aquatic transformations of MeHg prior to bioaccumulation. During photodegradation, Hg isotopes undergo both mass-dependent (MDF) and mass-independent fractionation (MIF) which can be utilized to estimate Hg sources and biota foraging locations. The isotopic fractionation of Hg is also dependent on dissolved organic matter (DOM), though this variable has not been examined in the context of phytoplankton-mediated reactions. This study investigates how DOM alters rates of MeHg uptake and photodegradation as well as the corresponding isotope fractionation in the presence of the freshwater phytoplankton species, Raphidocelis subcapitata. We conducted outdoor abiotic and biotic photochemical experiments with waters of varying DOM composition from contrasting natural systems (e.g., peatland, estuary, freshwater, marine) using an ambient MeHg/thiol molar ratio (0.01) across all conditions. Less MIF was observed in the presence of phytoplankton when compared to abiotic reactors. These experiments also indicate that planktonic uptake of MeHg and photo-induced isotopic fractionation are system-specific due to MeHg-DOM interactions. This study provides both a refined and extended examination of Hg isotope fractionation markers of key processes affecting chemical transformations occurring in the lower food web prior to bioaccumulation.

Ultimately, these findings are critical to accurately account for photochemical processes on Hg isotope signatures across a wide spectrum of environmental systems including inland freshwater environments and protected ecosystems."

Authors: Grace Armstrong, Sarah Janssen, Mike Tate, Brett Poulin, James Hurley, David Krabbenhoft