Editorial: Environmental chemistry of mercury: sources, pathways, transformations and impact vol II

Of all the inorganic pollutants that are globally distributed, mercury (Hg) is one of major concern because of its ability to be transported globally via the atmosphere, and through the ocean and via riverine transport from watersheds, and because of its toxicity as monomethylmercury (MMHg). Its environment concentration has been substantially increased by anthropogenic inputs, from its intentional use, such as in artisanal and small-scale goldmining (ASGM), and because it is a trace element in coal, ores and other primary resources extracted and used by humans. Anthropogenic emissions globally far exceed natural inputs of Hg and while it is primarily released as inorganic Hg, elevated levels of MMHg, the more toxic and bioaccumulative form, in seafood and other foods is a health concern for humans and wildlife. Mercury is taken up into vegetation and soils, and into ocean surface waters primarily from the atmosphere, and can be released back to the atmosphere due to natural processes, including those that are anthropogenically enhanced, such as biomass burning and deforestation, and after its reduction to elemental Hg (Hg0). The exchange of Hg between the reservoirs of the biosphere, and the aquatic and atmospheric chemistry of Hg are complex, as it can exist in many different inorganic and organic forms, including Hg0, ionic Hg, MMHg and dimethylmercury (DMHg), in the dissolved phase, while also binding to organic ligands in water or strongly attached to particles. In the atmosphere, Hg is mainly in the gaseous phase as Hg0 and DMHg. The conversion of inorganic Hg to the more toxic methylated Hg forms is primarily mediated by bacteria, and methylating genes have been recently identified in numerous microbes but the factors influencing the net methylation of Hg are not yet fully understood. While most studies examine the environment chemistry of Hg using measurements of concentrations and kinetic and other chemical characterizations of the fate, transport and transformation of Hg, there is a growing number of studies that use the environmental fractionation of Hg stable isotopes, and differences in isotopic composition in different source materials to examine the origin and fate of Hg from industrial pollution.

Given the complexities of the environmental chemistry of Hg, and the lack of knowledge, and the advancement of techniques for quantifying Hg species in the biosphere, this Research Topic was defined to invite scientific contributions that examined the complex environmental chemistry of Hg and was designed to enhance the publication of new and exciting results. The papers in the Research Topic include those published after the 16th International Conference on Mercury as a Global Pollutant (ICMGP) that was held in July 2024 in Cape Town, South Africa.

The six papers published in this Research Topic examined many aspects of Hg environmental chemistry. Haugk et al. examined the impact of permafrost thaw on Hg cycling and found that even after 17 years of thaw, there was no difference in the Hg stock or in MMHg production even though thawing enhanced the potential for Hg methylation by changing the microbial community. Irei. reviewed the information on the cycling of Hg in grasslands and highlighted the major sources and sinks for Hg and discussed the potential for Hg isotopes to help in understanding Hg cycling in these systems. Goss et al. examined the cycling of Hg in different soils (a pitch pine bog, a silver maple floodplain forest, and a wild rice marsh) of a river delta and documented the variability in the long-term retention of Hg supplied from atmospheric deposition, watershed inputs and plant uptake of Hg0. Weiss-Penzias et al. used in situ and transported lichens to quantify the release and transport of Hg from contaminated soils in the New Almaden Mining District and demonstrated that this was occurring as active uptake of Hg into the lichens occurred, and this Hg was strongly sequestered. Mason et al. detailed the air-sea exchange and mass balance for Hg and MMHg in the Gulf of Maine and concluded that external inputs of MMHg were not the primary driver of water column MMHg concentration, suggesting substantial water column methylation. Bakour et al. studied the impact of added cysteine and sulfide on Hg methylation and MMHg demethylation by a sulfate-reducing bacterium and found that the added ligands decreased methylation but had no impact on demethylation. Modeling suggested that the addition of reduced sulfur altered the rate of methylation by making Hg less bioavailable. These papers cover important research highlighting the complexity of the environmental chemistry of Hg and the complexities of the transformation between the different forms in the multiple compartments of the biosphere covered by these studies.

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Footnotes

Edited and reviewed by: Claude Fortin, Université du Québec, Canada