Photodemethylation of Methylmercury in Eastern Canadian Arctic Thaw Pond and Lake Ecosystems

Abstract

Permafrost thaw ponds of the warming Eastern Canadian Arctic are major landscape constituents and often display high levels of methylmercury (MeHg). We examined photodegradation potentials in high-dissolved organic matter (DOC) thaw ponds on Bylot Island (BYL) and a low-DOC oligotrophic lake on Cornwallis Island (Char Lake). In BYL, the ambient MeHg photodemethylation (PD) rate over 48 h of solar exposure was 6.1 × 10 -3 m 2 E -1 , and the rate in MeHg amended samples was 9.3 × 10 -3 m 2 E -1 . In contrast, in low-DOC Char Lake, PD was only observed in the first 12 hours, which suggests that PD may not be an important loss process in polar desert lakes. Thioglycolic acid addition slowed PD, while glutathione and chlorides did not impact northern PD rates. During an ecosystem-wide experiment conducted in a covered BYL pond, there was neither net MeHg increase in the dark nor loss attributable to PD following reexposure to sunlight. We propose that high-DOC Arctic thaw ponds are more prone to MeHg PD than nearby oligotrophic lakes, likely through photoproduction of reactive species rather than via thiol complexation. However, at the ecosystem level, these ponds, which are widespread through the Arctic, remain likely sources of MeHg for neighbouring systems.3

Global Meta‐Analysis on the Relationship Between Mercury and Dissolved Organic Carbon in Freshwater Environments

“…In total, 108 studies were excluded through that screening process. A total of 44 studies were kept from this search (Bergman & Bump, ; Berndt et al, ; Berndt & Bavin, ; Braaten et al, ; Bravo et al, ; Brazeau et al, ; Burns et al, , , ; Burton et al, ; Chaves‐Ulloa et al, ; de Wit et al, ; Demers et al, ; Denkenberger et al, ; Dranguet et al, ; Eckley et al, ; Eklöf et al, ; Ekstrom et al, ; Fagnani et al, ; Gerson & Driscoll, ; Girard et al, ; Guedron et al, ; Hamelin et al, ; Haverstock et al, ; Houben et al, ; Jeremiason et al, ; Leopold et al, ; Loftin et al, ; Luo et al, ; MacMillan et al, ; Moreno et al, ; Navratil et al, ; Noh et al, ; Oswald & Branfireun, ; Painter et al, ; Picado & Bengtsson, ; Poste et al, ; Pyhtila et al, ; Riscassi et al, ; Sherman & Blum, ; Tjerngren et al, ; Ukonmaanaho et al, ; Vidon et al, ; Voss et al, ). We included additional studies that have been cited by the abovementioned pool of studies more than five times specifically in reference to the relationship (or lack thereof) between Hg and DOC (eight studies; Brigham et al, ; Dittman & Driscoll, ; Dittman et al, ; Dittman et al, ; Driscoll et al, ; Eklöf et al, ; Porvari & Verta, ; Riscassi & Scanlon, ; Sørensen et al, ).…”

“…In total, 108 studies were excluded through that screening process. A total of 44 studies were kept from this search (Bergman & Bump, 2014;Berndt et al, 2016;Berndt & Bavin, 2012;Braaten et al, 2014;Bravo et al, 2017;Brazeau et al, 2013;Burns et al, 2012Burns et al, , 2013Burns et al, , 2014Burton et al, 2013;Chaves-Ulloa et al, 2016;de Wit et al, 2014;Demers et al, 2013;Denkenberger et al, 2014;Dranguet et al, 2017;Eckley et al, 2015;Eklöf et al, 2013;Ekstrom et al, 2016;Fagnani et al, 2012;Gerson & Driscoll, 2016;Girard et al, 2016;Guedron et al, 2014;Hamelin et al, 2015;Haverstock et al, 2012;Houben et al, 2016;Jeremiason et al, 2016;Leopold et al, 2012;Loftin et al, 2012;Luo et al, 2014;MacMillan et al, 2015;Moreno et al, 2016;Navratil et al, 2015;Noh et al, 2016;Oswald & Branfireun, 2014;Painter et al, 2015;Picado & Bengtsson, 2012;Poste et al, 2015;Pyhtila et al, 2012;Riscassi et al, 2016;Sherman & Blum, 2013;…”

“…Indeed, Hg–DOC relationships have been repeatedly observed across space (Braaten et al, ; Luo et al, ; MacMillan et al, ) and time (Burns et al, ; de Wit et al, ; Gerson & Driscoll, ) scales. MeHg–DOC relationships have been found to be weaker than THg–DOC relationships in the literature (Burns et al, ; Hall et al, ; Lescord et al, ), which could be due to the origin and molecular composition of DOC (Bravo et al, , ) or to photodemethylation of MeHg in presence of DOC (Girard et al, ). Stronger relationships with Hg have been found for the aromatic fraction of DOC as measured by SUVA 254 or UV 254 (Burns et al, ; Riscassi & Scanlon, ) or by specific families of compounds measured by fluorescence and analyzed by parallel factor analysis (Jiang et al, ) than for bulk DOC concentrations.…”

Recent advances in the study of mercury methylation in aquatic systems

“…Increased temperature will also result in the loss of sea ice and larger areas of open ocean susceptible to increased rates of photochemically mediated reactions in the Hg biogeochemical cycles (Stern et al 2012). Interestingly, Girard et al (2016) found that the rates of photodemethylation were higher in high DOC thaw ponds than in oligotrophic lakes, suggesting that some level of DOC is necessary for photodegradation to occur. Finally, increased temperatures will likely result in increased forest fires, which are a source of atmospheric deposited Hg sequestered in plants and soils (Kelly et al 2006).…”

The interactive effects of stratospheric ozone depletion, UV radiation, and climate change on aquatic ecosystems

“…The toxicity and transport of mercury up the food web to fish occurs through uptake of methylmercury. UV-B radiation dominates the photodemethylation process, 84 and the fraction that is photo-demethylated varies between 25% to 80% depending on water transparency. 166 Exposure to UV-B radiation thus reduces the uptake of mercury to fish.…”