The concern over mercury stems from two intertwined properties. First, it can travel enormous distances: released into the atmosphere, it circulates globally and returns to the surface through deposition onto vegetation, soils and ocean waters. Second, once in the environment, inorganic mercury can be converted by bacteria into monomethylmercury, a toxic and bioaccumulative form that concentrates in seafood and other foods, posing health risks to humans and wildlife. Anthropogenic emissions now far exceed natural inputs, driven both by the intentional use of mercury in artisanal and small-scale gold mining and by its presence as a trace contaminant in coal, ores and other primary resources extracted and used by humans.

Technically, mercury exists in many different inorganic and organic forms, and this speciation is central to its fate. In the dissolved phase, it can occur as elemental mercury vapor, ionic mercury, monomethylmercury and dimethylmercury, while also binding to organic ligands in water or attaching strongly to particles. In the atmosphere, mercury is mainly present in the gaseous phase as elemental mercury and dimethylmercury. The element can be taken up into vegetation and soils and into ocean surface waters primarily from the atmosphere, and it can be released back to the air after reduction to elemental mercury, a process enhanced by anthropogenically influenced activities such as biomass burning and deforestation. This exchange between the reservoirs of the biosphere makes the overall cycling extraordinarily intricate.

The conversion of inorganic mercury to the more toxic methylated forms is primarily mediated by bacteria. Recent advances have identified methylating genes in numerous microbes, yet the factors controlling the net methylation of mercury in natural systems are not fully understood. This gap in knowledge is significant because monomethylmercury is the form that bioaccumulates in aquatic food webs, ultimately reaching seafood consumed by people. Understanding what drives methylation, and equally what drives demethylation, is therefore a priority for assessing exposure risks across the globe.

A growing methodological frontier in mercury research is the use of stable isotope fractionation. Rather than relying solely on concentration measurements and kinetic characterizations of transport and transformation, scientists increasingly exploit differences in the isotopic composition of mercury in different source materials to trace the origin and fate of industrial pollution. This environmental fractionation of mercury stable isotopes offers a forensic tool for distinguishing atmospheric deposition from watershed inputs and plant uptake, and several of the papers in the Research Topic highlight its potential for understanding cycling in specific ecosystems.

Six papers make up the volume, each probing a different compartment of the biosphere. Haugk and colleagues examined the impact of permafrost thaw on mercury cycling and found that even after 17 years of thaw, there was no difference in mercury stock or in monomethylmercury production, even though thawing enhanced the potential for methylation by changing the microbial community. This result suggests that structural changes in microbial communities can alter methylation potential without immediately translating into measurable changes in mercury stocks or methylmercury levels, a nuance with implications for Arctic and subarctic regions undergoing rapid thaw.

Irei reviewed the cycling of mercury in grasslands, highlighting the major sources and sinks for the element and discussing how mercury isotopes could help unravel cycling in these systems. Goss and colleagues examined mercury cycling in different soils of a river delta, including a pitch pine bog, a silver maple floodplain forest and a wild rice marsh, documenting the variability in the long-term retention of mercury supplied from atmospheric deposition, watershed inputs and plant uptake of elemental mercury. Their work demonstrates that even within a single delta, different soil environments can retain mercury supplied from the same sources with markedly different efficiency.

Weiss-Penzias and colleagues used in situ and transported lichens to quantify the release and transport of mercury from contaminated soils in the New Almaden Mining District, a historically significant mercury mining area. They demonstrated that mercury was being released and transported while active uptake into the lichens occurred, and that this mercury was strongly sequestered within the lichen tissue. Lichens, which absorb nutrients directly from the atmosphere, thus serve as both sentinels of contamination and active participants in the local mercury cycle, capturing mercury released from legacy mining soils.

In the marine realm, Mason and colleagues detailed the air-sea exchange and mass balance for mercury and monomethylmercury in the Gulf of Maine. Their analysis concluded that external inputs of monomethylmercury were not the primary driver of water column concentrations, suggesting substantial methylation occurring within the water column itself. This finding is important for coastal seas, where methylmercury in seafood is a persistent exposure concern, because it implies that local production within the water, rather than transport from outside sources, controls the levels of the toxic form available for bioaccumulation.

Finally, Bakour and colleagues studied the impact of added cysteine and sulfide on mercury methylation and monomethylmercury demethylation by a sulfate-reducing bacterium. They 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 mercury less bioavailable, a result that underscores how the chemical speciation of mercury, governed by the ligands present in the environment, directly controls the accessibility of the element to methylating microbes. Together, these six studies highlight the complexity of mercury’s environmental chemistry and the intricate transformations between its different forms across the multiple compartments of the biosphere, from thawing permafrost and grassland soils to river deltas, mining districts and coastal seas, reinforcing why mercury remains a pollutant of major global concern.

Subject of Research: Environmental chemistry of mercury

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

Article References: Mason, R. P., Bjorn, E., & Amyot, M. (2026). Editorial: Environmental chemistry of mercury: sources, pathways, transformations and impact vol II. Frontiers in Environmental Chemistry, 7, Article 1833826. https://doi.org/10.3389/fenvc.2026.1833826

Image Credits: AI Generated

DOI: 10.3389/fenvc.2026.1833826

Keywords: mercury, methylmercury, environmental chemistry, permafrost thaw, stable isotopes, air-sea exchange, soil contamination, Frontiers in Environmental Chemistry, Editorial, Environmental, chemistry, sources