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Methylmercury (MeHg) is a neurotoxin that bioaccumulates to potentially harmful concentrations in Arctic and Subarctic marine predators and those that consume them. Monitoring and modeling MeHg bioaccumulation and biogeochemical cycling in the ocean requires an understanding of the mechanisms behind net mercury (Hg) methylation. The key functional gene pair for Hg methylation,hgcAB, is widely distributed throughout ocean basins and spans multiple microbial phyla. While multiple microbially mediated anaerobic pathways for Hg methylation in the ocean are known, the majority ofhgcAhomologs have been found in oxic subsurface waters, in contrast to other ecosystems. In particular, microaerophilicNitrospina, a genera of nitrite-oxidizing bacteria containing ahgcA-like sequence, have been proposed as a potentially important Hg methylator in the upper ocean. The objective of this work was therefore to examine the potential of nitrifiers as Hg methylators and quantify total Hg and MeHg across three Arctic and Subarctic seas (the Gulf of Alaska, the Bering Sea and the Chukchi Sea) in regions whereNitrospinaare likely present. In Spring 2021, samples for Hg analysis were obtained with a trace metal clean rosette across these seas. Mercury methylation rates were quantified in concert with nitrification rates using onboard incubation experiments with additions of stable isotope-labeled Hg and NH₄⁺. A significant correlation between Hg methylation and nitrification was observed across all sites (R²= 0.34,p< 0.05), with the strongest correlation in the Chukchi Sea (R²= 0.99,p< 0.001).Nitrospina-specifichgcA-like genes were detected at all sites. This study, linking Hg methylation and nitrification in oxic seawater, furthers understanding of MeHg cycling in these high latitude waters, and the ocean in general. Furthermore, these studies inform predictions of how climate and human interactions could influence MeHg concentrations across the Arctic in the future.

 

Environmental mercury (Hg) contamination is a global concern requiring action at national scales. Scientific understanding and regulatory policies are underpinned by global extrapolation of Northern Hemisphere Hg data, despite historical, political, and socioeconomic differences between the hemispheres that impact Hg sources and sinks. In this paper, we explore the primary anthropogenic perturbations to Hg emission and mobilization processes that differ between hemispheres and synthesize current understanding of the implications for Hg cycling. In the Southern Hemisphere (SH), lower historical production of Hg and other metals implies lower present-day legacy emissions, but the extent of the difference remains uncertain. More use of fire and higher deforestation rates drive re-mobilization of terrestrial Hg, while also removing vegetation that would otherwise provide a sink for atmospheric Hg. Prevalent Hg use in artisanal and small-scale gold mining is a dominant source of Hg inputs to the environment in tropical regions. Meanwhile, coal-fired power stations continue to be a significant Hg emission source and industrial production of non-ferrous metals is a large and growing contributor. Major uncertainties remain, hindering scientific understanding and effective policy formulation, and we argue for an urgent need to prioritize research activities in under-sampled regions of the SH.

 

Monomethylmercury (CH 3 Hg) is a neurotoxic pollutant that biomagnifies in aquatic food webs. In sediments, the production of CH 3 Hg depends on the bacterial activity of mercury (Hg) methylating bacteria and the amount of bioavailable inorganic divalent mercury (Hg II ). Biotic and abiotic reduction of Hg II to elemental mercury (Hg 0 ) may limit the pool of Hg II available for methylation in sediments, and thus the amount of CH 3 Hg produced. Knowledge about the transformation of Hg II is therefore primordial to the understanding of the production of toxic and bioaccumulative CH 3 Hg. Here, we examined the reduction of Hg II by sulfidic minerals (FeS (s) and CdS (s) ) in the presence of dissolved iron and dissolved organic matter (DOM) using low, environmentally relevant concentrations of Hg and ratio of Hg II :FeS (s) . Our results show that the reduction of Hg II by Mackinawite (FeS (s) ) was lower (<15% of the Hg II was reduced after 24 h) than when Hg II was reacted with DOM or dissolved iron. We did not observe any formation of Hg 0 when Hg II was reacted with CdS (s) (experiments done under both acidic and basic conditions for up to four days). While reactions in solution were favorable under the experimental conditions, Hg was rapidly removed from solution by co-precipitation. Thermodynamic calculations suggest that in the presence of FeS (s) , reduction of the precipitated Hg II is surface catalyzed and likely involves S −II as the electron donor. The lack of reaction with CdS may be due to its stronger M-S bond relative to FeS, and the lower concentrations of sulfide in solution. We conclude that the reaction of Hg with FeS (s) proceeds via a different mechanism from that of Hg with DOM or dissolved iron, and that it is not a major environmental pathway for the formation of Hg 0 in anoxic environments.