How a Common Fungus Eliminates Toxic Mercury from Soil and Water

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Mercury contamination is one of the most persistent and dangerous forms of environmental pollution affecting soil and water worldwide. Released through industrial activities such as coal combustion, gold mining, and chemical manufacturing, mercury can accumulate in ecosystems and enter the food chain, posing serious health risks to humans and wildlife. Even small amounts can transform into highly toxic compounds like methylmercury, which bioaccumulates in fish and threatens neurological development in children and adults alike.

Mercury Pollution Problem: A Crisis Spreading

Mercury is one of the few heavy metals that does not stay in place once it enters the environment. It moves, transforms, and accumulates, and it does so in ways that conventional cleanup technologies struggle to follow. According to the U.S. Toxic Release Inventory data published in 2024, industrial facilities in the United States alone released approximately 1,751 metric tons of mercury compounds into soils in 2022.

That figure covers only facilities required to report. The global number is considerably higher, and it does not account for the unregulated and informal sources that dominate mercury pollution in the developing world. Mercury enters the environment through several pathways.

Coal-fired power plants are the single largest anthropogenic source, releasing mercury vapor into the atmosphere that eventually deposits into soil and water. Artisanal and small-scale gold mining, known as ASGM, uses liquid mercury to extract gold from ore and then burns off the mercury in open air.

Industrial waste from chlor-alkali plants, battery manufacturing, and fluorescent lamp production adds further contamination. Once mercury reaches soil or a water body, it does not simply sit there inert. Soil microbes convert inorganic mercury into methylmercury, the most biologically dangerous form, which then enters the food chain.

The scale of the problem demands solutions that are practical, affordable, and deployable at the field level. That is precisely where fungi enter the picture. A common fungus eliminates toxic mercury from soil and water not by adding chemicals but by deploying enzymes it already produces as part of its natural biology. Understanding how this works, and why it matters for crop farmers and environmental practitioners, is the subject of this article.

Understanding Mercury Toxicity

Three Forms of Mercury, Three Levels of Danger

Mercury exists in three chemically distinct forms, each with different behavior in the environment and different levels of harm to living organisms.

• Elemental mercury (the liquid silver metal, chemical symbol Hg0) is relatively stable at room temperature, but when heated or volatilized, it becomes a vapor that is easily inhaled.
• Inorganic mercury refers to mercury salts such as mercuric chloride, which dissolve in water and bind readily to soil particles.
• Methylmercury (written as MeHg) is an organomercury compound formed when inorganic mercury binds to a methyl group through microbial activity in sediments and wetlands. Methylmercury is the most dangerous form because it crosses both the blood-brain barrier and the placental barrier with ease.

How Mercury Moves Through Ecosystems

Mercury does not stay where it lands. In soil, inorganic mercury binds to organic matter and clay particles, but a fraction remains soluble and migrates through soil pores into groundwater and surface water.

Sulfate-reducing bacteria in anaerobic sediments convert this inorganic mercury into methylmercury through a process called microbial methylation (the biological addition of a carbon-hydrogen cluster to a mercury atom). From sediment, methylmercury dissolves into the overlying water column, where phytoplankton absorb it directly from solution.

Bioaccumulation (the gradual buildup of a substance inside one organism over its lifetime) and biomagnification (the increasing concentration of that substance as you move up the food chain) mean that mercury concentrations amplify dramatically from one trophic level to the next.

Methylmercury concentrations in large predatory fish can be 10 million times higher than in the surrounding water, according to data from UNEP’s Global Mercury Assessment. Pregnant women who eat contaminated fish regularly pass methylmercury to developing fetuses, causing

• neurological damage,
• hearing loss, and
• impaired motor development.

For crop farmers, the pathway of concern is somewhat different but equally serious. Rice, in particular, accumulates methylmercury in its grain more efficiently than almost any other food crop, because flooded paddy soils create the anaerobic conditions that favor microbial methylation.

Studies published in Environmental Science and Technology in 2024 have documented methylmercury concentrations in rice grain from mercury-mining areas exceeding safe consumption thresholds by a factor of three to five.

The Fungus: Nature’s Detox Specialist

Introducing Metarhizium robertsii

Metarhizium robertsii is not an exotic laboratory organism. It is one of the most common soil-dwelling fungi on the planet, found naturally in agricultural fields, forests, grasslands, and wetlands across every inhabited continent. Most farmers who grow maize, rice, soybean, or sugarcane are likely cultivating soil that already contains Metarhizium species.

The fungus is best known in agricultural circles as a biological insecticide because it parasitizes soil-dwelling insect larvae, including the larvae of beetles and flies that attack crop roots. This dual function as both a plant symbiont and an insect pathogen makes it particularly well suited for agricultural deployment.

From a biological standpoint, Metarhizium robertsii is a filamentous fungus (a fungus that grows in long, thread-like strands rather than as single cells). These strands, called mycelium (the network of thin, root-like filaments that make up the main body of a fungus), spread through soil in three dimensions, reaching pores and particle surfaces that roots and bacteria cannot access.

The cell walls of filamentous fungi are rich in chitin, glucan, and proteins that carry strong negative electrical charges. These charges attract and bind positively charged heavy metal ions, including mercury, in a process that begins even before the fungus does anything metabolically active. The cell wall is, in effect, the first line of mercury capture.

Mechanisms: How the Fungus Removes Mercury

A. Biosorption: The Cell Wall as a Mercury Trap

Biosorption (the passive binding of metal ions to the surface of biological material without any energy expenditure by the organism) is the fastest of the three mercury-removal mechanisms that Metarhizium uses.

When mercury ions in soil water come into contact with fungal mycelium, they bind immediately to functional groups on the cell wall surface. Carboxyl groups, amino groups, and phosphate groups all carry partial negative charges that attract divalent mercury ions.

This binding is rapid, occurring within minutes of contact. The mercury is effectively immobilized on the fungal surface, removed from the soil solution, and prevented from being taken up by plant roots or moving into groundwater.

B. Bioaccumulation: Pulling Mercury Inside

Bioaccumulation in this context refers to the active transport of mercury ions across the fungal cell membrane and into the cytoplasm (the interior of the cell). This process requires cellular energy, unlike biosorption, but it moves mercury deeper into the fungal body where it can be processed further.

Research published in the Journal of Environmental Chemical Engineering in 2024 confirmed that Metarhizium species accumulate mercury in intracellular compartments, including vacuoles, where it is sequestered away from sensitive cellular machinery through complexation with thiol compounds such as glutathione.

This compartmentalization prevents mercury from interfering with the fungal cell’s own enzymes while holding the metal in a stable, immobile form.

C. Biotransformation: Converting Mercury to Its Least Dangerous Form

The most remarkable mechanism that this fungus uses, and the one that truly sets it apart from other bioremediation agents, is enzymatic biotransformation. The landmark PNAS study by Wu, Tang, Dai, and colleagues at Zhejiang University and the University of Maryland (2022) identified two specific enzymes responsible for this process.

1. The first enzyme is methylmercury demethylase, abbreviated MMD. This enzyme cleaves the carbon-mercury bond in methylmercury, breaking down the most dangerous form of the metal into the less mobile inorganic mercury ion. The reaction: CH3Hg+ (methylmercury) is converted to Hg2+ (divalent mercury) plus a methyl fragment.

2.  The second enzyme is mercury ion reductase, abbreviated MIR. This enzyme then takes the inorganic mercury ion produced by MMD and reduces it from Hg2+ to Hg0, the volatile elemental form. Elemental mercury at soil temperature gradually volatilizes out of the soil matrix into the atmosphere, where its concentration is diluted to safe levels.

3. The combined action of MMD and MIR constitutes a complete two-step transformation pathway that converts the most toxic form of mercury into a volatilizable, chemically inert elemental form that the soil can release gradually rather than accumulate.

This two-enzyme system is not just theoretically elegant. It works continuously as long as the fungus is alive and active in the soil, which means it functions as a persistent, self-renewing detoxification system rather than a one-time treatment.

Wu, Tang, Dai et al. (Zhejiang University and University of Maryland, PNAS, 2022) demonstrated that Metarhizium robertsii nourished by maize roots removed methylmercury and divalent mercury from contaminated soil, significantly reducing mercury accumulation in plant tissues and restoring normal plant growth in mercury-polluted conditions.

When the MMD and MIR genes were overexpressed through genetic engineering, the fungus achieved substantially higher mercury removal rates from both fresh water and sea water even in the absence of added nutrients. Farmers growing maize or other crops in mercury-contaminated soils can potentially inoculate roots with Metarhizium to protect the crop from mercury uptake while simultaneously cleaning the soil over successive growing seasons.

Mercury Removal From Soil With Mycelium

The architecture of fungal mycelium gives it a decisive advantage over other bioremediation organisms when it comes to treating soil. A single gram of healthy soil can contain more than 100 meters of fungal hyphae (individual mycelial threads), according to estimates published in the journal Science.

This extraordinary surface area means that the fungal network contacts an immense volume of soil per unit of fungal biomass. Mercury bound to soil particles, trapped in micro-pores, or dissolved in soil pore water all come within reach of fungal hyphae as the network extends.

In mercury-contaminated agricultural soil, Metarhizium does not need to be uniformly distributed. The hyphal network grows toward nutrient sources, including plant roots, and in doing so it explores the soil volume around the rhizosphere (the narrow zone of soil directly surrounding and influenced by plant roots).

This rhizosphere zone is precisely where mercury is most likely to be absorbed by crops. By saturating this zone with active mycelium, the fungus intercepts mercury before roots do. There is an important distinction between two remediation strategies that the fungus enables.

• Stabilization is the process of binding mercury firmly to fungal cell walls or organic matter so it cannot move, without necessarily removing it from the soil.
• Extraction is the process of converting mercury to a form that leaves the soil entirely, either through volatilization or through harvesting mercury-laden fungal biomass.

Metarhizium performs both. In the short term, biosorption and bioaccumulation stabilize mercury and protect crops immediately. In the medium to long term, the MMD-MIR enzyme system extracts mercury from the soil through volatilization.

Lab experiments conducted at Zhejiang University showed that maize plants inoculated with Metarhizium and grown in mercury-spiked soil showed measurably lower mercury concentrations in shoots and roots compared to uninoculated control plants. The inoculated plants also showed improved biomass and root development, confirming that mercury removal by the fungus translated directly into plant health benefits.

Mercury Removal From Water Systems

How Fungi Perform in Aquatic Environments

One of the most striking findings of the PNAS 2022 study was that Metarhizium removed methylmercury and divalent mercury from water even when no plant was present to nourish it and even when the water contained no added nutrients. This was tested in

• regular distilled water,
• in sea water, and
• in nutrient-rich culture medium.

The fungus performed across all three, demonstrating that mercury removal in aquatic systems does not depend on the conditions that support the fungus in soil. The mycelium uses stored energy reserves to sustain the detoxification enzymes for a period even in nutrient-limited water.

This property opens the door to practical water treatment applications. Dead fungal biomass, called fungal necromass, also retains significant mercury-binding capacity through the biosorption mechanism, even though it cannot perform enzymatic biotransformation.

Research published in the journal Chemosphere in 2022 showed that fungal necromass presents a high potential for mercury immobilization in soil and sediment, meaning that even spent fungal material after a treatment cycle continues to serve as a mercury sink.

Performance Versus Conventional Methods

Conventional mercury removal from wastewater relies heavily on chemical precipitation using sulfide reagents, ion exchange resins, or activated carbon adsorption. Each of these methods is effective but carries significant drawbacks.

• Chemical precipitation generates large volumes of mercury-bearing sludge that requires hazardous waste disposal, often creating a secondary contamination problem that is equally difficult to manage.
• Ion exchange resins are highly effective at trace mercury removal but are expensive to manufacture, require regeneration with strong acids, and foul quickly when organic matter is present in the water.
• Activated carbon adsorption removes mercury efficiently at low concentrations but performs poorly when mercury is present alongside competing metal ions, and spent carbon must be managed as hazardous waste.
• Fungal biosorption systems, by contrast, use biomass that can be produced cheaply through fermentation, operate across a wide pH range, and in the case of live fungal systems, actively transform rather than merely trap mercury.

Vácar and colleagues, reviewing filamentous fungi as mercury bioremediators in the Journal of Fungi (2021), documented that several Ascomycota species achieved measurable mercury removal from 100 mg/L Hg2+ aqueous solutions over 48 hours, demonstrating that fungal systems function even at relatively high contamination concentrations that would challenge conventional ion exchange systems.

Endophytic fungi including Aspergillus sp. A31, Curvularia geniculata P1, Lindgomycetaceae P87, and Westerdykella sp. P71, studied by Durán and colleagues (Chemosphere, 2019), removed up to 100% of mercury from culture medium in species-dependent trials and simultaneously promoted maize growth in mercury-contaminated substrate.

Multiple fungal species already present in agricultural soils can be selected and deployed in combination, giving practitioners the option to match fungal strains to local soil and contamination conditions rather than relying on a single organism.

Advantages of Fungal Bioremediation

Fungal bioremediation earns its place in the remediation toolkit not because it is new but because it addresses the practical constraints that have prevented chemical and physical methods from scaling up in the field.

1. The cost of producing fungal inoculant is a fraction of the cost of chemical treatment agents. Metarhizium robertsii is already produced commercially as a biopesticide in several countries, meaning the production infrastructure exists and the supply chain is established.

2. The treatment is self-sustaining in the presence of a living plant host. As long as the crop grows and nourishes the fungal symbiont through root exudates, the fungus continues to grow, spread, and detoxify mercury without any additional input from the farmer.

3. Fungal bioremediation generates no toxic byproducts from the treatment process itself. The volatile elemental mercury released is diluted into a very large atmospheric volume, and the organic residues of fungal metabolism are biodegradable and non-hazardous.

4. The same inoculant that cleans the soil can simultaneously protect the crop from insect root pests through Metarhizium’s insecticidal properties, giving the farmer a dual return on a single biological input.

5. Mycelial networks adapt to soil heterogeneity. Unlike chemical injections that require uniform distribution to be effective, fungal hyphae grow toward contaminated zones through a process called chemotropism (directed growth in response to chemical gradients), actively seeking out mercury-rich areas in the soil profile.

Limitations and Challenges What It Has

Honest assessment of fungal bioremediation requires acknowledging the constraints that currently limit deployment. None of these challenges are insurmountable, but they do define the conditions under which current technology is reliably effective.

1. Soil pH, temperature, and moisture all affect fungal activity significantly. Metarhizium robertsii performs optimally in soils with a pH between 5.5 and 7.5 and at temperatures between 15 and 30 degrees Celsius. Highly acidic, waterlogged, or extremely arid soils reduce fungal establishment and therefore reduce remediation efficiency.

2. The time required for meaningful mercury reduction is measured in growing seasons, not days. Biosorption begins immediately on fungal-mercury contact, but enzymatic biotransformation and volatilization of the full mercury load in a heavily contaminated soil may take multiple crop cycles to achieve regulatory thresholds. This is a slower timeline than chemical precipitation in a treatment plant setting.

3. Harvesting mercury-laden fungal biomass after a biosorption-focused treatment represents a waste management challenge. Biomass that has concentrated mercury must be disposed of as hazardous waste or treated to recover the metal, and the logistics and cost of doing this at field scale are not yet standardized.

4. The regulatory status of genetically engineered fungal strains with overexpressed MMD and MIR genes is unclear in many jurisdictions. While wild-type Metarhizium strains are approved as biopesticides in numerous countries, the pathway to regulatory approval for genetically modified environmental release strains will require time and region-specific data packages.

These limitations point to areas where research investment is most needed: better formulation of fungal inoculants for challenging soil conditions, improved understanding of biomass handling after field application, and development of regulatory frameworks suited to biological remediation agents.

Real-World Applications and Research Developments

Where the Science Stands Today

The 2022 PNAS study by Wu and colleagues was a pivotal moment because it moved the conversation about fungal mercury removal from generic biosorption into the realm of specific enzymatic mechanisms. That mechanistic clarity is what allows researchers to now target improvements through genetic engineering with precision rather than trial and error.

The overexpression of the MMD and MIR genes in the PNAS study produced a strain with demonstrably higher mercury removal capacity in both soil and water, proving that enhancement through biotechnology is feasible.

The real power of fungal bioremediation lies not in replacing chemistry but in deploying biology as a self-replicating, self-directing cleanup system that works continuously across the same soil volume that grows the food we eat.

Beyond Metarhizium, the fungal bioremediation research field has expanded significantly. A 2024 review in Environmental Chemistry and Ecotoxicology by Dinakarkumar and colleagues surveyed the full landscape of fungal bioremediation mechanisms and identified multiple Ascomycota species across genera including Aspergillus, Penicillium, Trichoderma, and Cladosporium as active mercury-tolerant organisms with demonstrated removal capacity.

The same review noted that multi-strain approaches, deploying communities of fungi rather than single species, show higher remediation consistency across variable field conditions. Pilot-scale applications are beginning to appear in regions with legacy mercury contamination.

Research teams in China, Brazil, and parts of sub-Saharan Africa, where artisanal gold mining has left extensive mercury contamination in agricultural soils, are now conducting field trials with both wild-type Metarhizium strains and engineered variants. The results from these trials, expected to be published through 2025 and 2026, will provide the first real-world performance data beyond controlled laboratory and greenhouse conditions.

The Role of Genetic Engineering Going Forward

The genetic toolkit now available to fungal bioremediation researchers extends well beyond simple gene overexpression. Researchers are investigating several directions.

1. Promoter engineering: replacing the native promoters of MMD and MIR with mercury-responsive promoters that activate enzyme production only when mercury concentrations exceed a defined threshold, preventing wasteful enzyme synthesis in clean soils.

2. Synthetic microbial communities: combining engineered Metarhizium with mercury-sensing bacteria that signal to the fungal network where contamination is highest, effectively giving the system a spatial intelligence that directs fungal growth toward polluted zones.

3. Surface display technology: anchoring additional metal-binding peptides on the outer surface of fungal cell walls through protein engineering, increasing biosorption capacity without altering any of the fungus’s natural enzyme pathways.

4. Biomass valorization: developing post-treatment processes to recover elemental mercury from harvested fungal biomass using thermal desorption, converting the remediation byproduct into a recoverable resource rather than a disposal problem.

Conclusion

The story of how a common fungus eliminates toxic mercury from soil and water is ultimately a story about the depth of biological engineering that already exists in nature. Metarhizium robertsii does not merely absorb mercury and hold it in place. It recognizes the two most dangerous forms of mercury, breaks them down chemically through dedicated enzymes, and converts the toxic metal into a form that the soil can shed naturally. This is not remediation by accumulation. It is remediation by transformation.

A biological tool that can protect crops from mercury uptake, improve plant growth, control soil insect pests, and clean the soil over successive seasons represents a qualitatively different kind of intervention than any single-function chemical treatment. Its cost profile is favorable, its ecological footprint is minimal, and the underlying science is now mechanistically understood at the molecular level. The challenge ahead is field validation at scale, regulatory alignment for engineered strains, and the development of formulation and delivery systems that make inoculation straightforward for farmers in the communities that need it most.