Microbes Reveal Hidden Natural Mercury Emissions from “Stable” Minerals

Elemental mercury (Hg^0^) is widely recognized as a pervasive and hazardous environmental pollutant, primarily due to its ability to volatilize and traverse long distances through the atmosphere. This characteristic facilitates its global circulation, contributing to widespread contamination far from original emission sources. Despite the extensive monitoring and inventorying of mercury emissions, a persistent discrepancy has been noted between observed atmospheric mercury concentrations and the quantities accounted for by known emission inventories. This gap has fueled ongoing scientific inquiry into potential, yet overlooked, contributors to mercury’s atmospheric presence.

In a groundbreaking study recently published in National Science Review, scientists have unveiled a novel natural mechanism that may explain a significant portion of this discrepancy. The research delineates how chemolithoautotrophic microbes — microorganisms that derive energy from inorganic chemical reactions — can interact with mercury sulfide nanominerals, using them as an energy source. This microbial activity subsequently results in the release of volatile elemental mercury (Hg^0^) into the atmosphere, suggesting a hitherto underestimated biogeochemical pathway in the mercury cycle.

Mercury sulfide, commonly viewed as a chemically stable and environmentally inert compound, especially in its macroscopic forms such as cinnabar, is rendered much more bioavailable and reactive when present as nanoparticles. These nanoscale particles, due to their high surface area to volume ratio and unique physicochemical properties, become accessible substrates for microbial metabolism. The study’s experimental data highlight that sulfur-oxidizing and iron-oxidizing chemolithoautotrophic microbes can thrive using mercury sulfide nanoparticles as their exclusive energy source, a process that simultaneously liberates substantial quantities of elemental mercury gas.

Crucially, the particle size of mercury sulfide emerges as a decisive factor in this mechanism. Nanoparticles possess the ability to penetrate microbial cells more efficiently compared to dissolved mercury species, which generally require tightly regulated, transporter-mediated uptake pathways to enter. This unusual uptake bypasses some of the traditional biochemical constraints, facilitating direct interaction between intracellular microbial metabolic systems and mercury sulfide minerals. Within the microbial cells, metabolic processes enzymatically degrade the mineral lattices, mobilizing mercury ions that undergo further transformation into the volatile elemental form.

The volatilized elemental mercury, Hg^0^, is then emitted into the atmosphere, constituting a potentially significant source of atmospheric mercury that had been unaccounted for in previous models. This biogenic emission pathway contrasts sharply with traditionally recognized sources such as fossil fuel combustion, mining activities, and cement production, highlighting the intricate complexity of mercury cycling and the need for integrative environmental models that encompass both anthropogenic and natural processes.

To quantify the potential global impact of this microbial nanomineral-mediated mercury release, the researchers integrated laboratory findings with extensive datasets concerning soil compositions, the prevalence and distribution of mercury sulfide nanoparticles, and chemolithoautotrophic microbial activity across diverse ecosystems. This comprehensive modeling effort estimates that approximately 272 ± 135 tonnes of elemental mercury are released annually via this newly identified pathway. Notably, this magnitude of emission rivals that attributed to cement production, which is currently identified as the fourth largest anthropogenic mercury source worldwide.

The revelation that widespread environmental microbes can act as bio-factories for mercury volatilization challenges longstanding assumptions in geochemistry and environmental toxicology. It underscores the dynamic interplay between microbial ecology and trace metal cycling, suggesting that naturally occurring microbial processes are critical regulators of mercury’s fate in the environment. Such insights compel a reevaluation of mercury emission budgets and demand that atmospheric mercury cycling models incorporate microbial and nanomineral interactions to improve predictive accuracy.

This innovative study also opens new frontiers in understanding environmental mercury risk and exposure. Regions rich in chemolithoautotrophic microbial communities, such as certain soils, sediments, and extreme environments, may be hotspots for this microbial mercury reduction and emission process. These findings provide a foundation for subsequent research aimed at mapping these emissions spatially and temporally, assessing variability under changing environmental conditions, such as temperature fluctuations and redox dynamics.

Moreover, these insights might have broader implications for environmental management and policy decisions. Recognizing the significance of microbial nanomineral interactions in mercury cycling can inform remediation strategies and pollution control measures. For example, interventions targeting the stabilization or sequestration of mercury in less bioavailable mineral forms might mitigate microbial access and, consequently, atmospheric emission, potentially reducing regional and global mercury pollution burdens.

The implications of this research extend beyond mercury alone, suggesting analogous microbial interactions with nanominerals of other toxic metals may also play critical roles in biogeochemical cycles. This highlights the importance of integrating nanoscience, microbiology, and environmental chemistry to holistically comprehend and manage contaminant dynamics in ecosystems.

In summary, this pioneering research demonstrates that chemolithoautotrophic microbes exploit mercury sulfide nanoparticles as unexpected energy sources, a process that not only sustains microbial life but also facilitates significant mercury recession back into the atmosphere as elemental mercury vapor. This discovery fundamentally transforms the understanding of mercury’s natural cycling and introduces a critical missing piece to the global mercury budget — one that has important ramifications for atmospheric science, environmental health, and regulatory frameworks aimed at mitigating mercury pollution.