Research progress on biot-mercury interactions in the periphyton: environmental processes, ecological effects and restoration applications

Pan Li¹Ming Xu²Yufeng Xu¹Wenyu Zhao^1*LuYao Miao¹周洁 周¹

• ¹Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology; Engineering Research Center of Watershed Protection and Green Development, Guangxi, Guilin University of Technology, Guilin, China
• ²Xi’an Water Group Planning and Design Research Institute Co., Ltd., Xian, China, Xian, China

Mercury (Hg) is a widespread global pollutant, found throughout the natural environment, posing a health threat to millions of people worldwide. In 2010, the World Health Organization identified mercury as one of the top ten chemicals of major concern for public health (Zhang, 2022). Once mercury enters the ecosystem, its extremely low biodegradability means it can remain in environmental media for decades, accumulating over time and causing irreversible organ damage and cognitive impairments in those exposed. Mercury naturally occurs in the Earth's crust and is mainly released into the surface environment through natural geological processes such as rock weathering and volcanic eruptions. Human activities are also the primary source of mercury release, with key contributors including the steel, cement, gold, non-ferrous metal smelting, chlorine-alkali industries, and sectors directly involved in mercury production (Pavithra et al., 2023).Mercury is an element that exists in multiple forms, commonly known as quicksilver. It can be classified into three main types: elemental mercury (metal), inorganic mercury (such as the highly toxic mercury chloride), and organic mercury (such as the extremely toxic methylmercury). Different forms of mercury exhibit significant differences in toxicity and can severely damage critical human organs, including the central nervous system, digestive tract, immune system, lungs, kidneys, skin, and eyes. In aquatic systems, mercury exists in organic, inorganic, and elemental forms (Zhang, 2023). In the atmosphere, mercury is mostly found in its elemental form, with divalent mercury being the most common oxidation state of the elemental mercury. Mercury is typically released into the environment in its inorganic form, while methylmercury (MeHg) is one of the most neurotoxic substances and predominates in fish. This occurs because inorganic mercury is transformed into methylmercury through various biological and abiological processes in aquatic environments. While microbial methylation is the dominant pathway, abiological processes, such as the photochemical degradation of methylmercury, also significantly influence its environmental cycling and persistence (Donovan et al., 2016). Thus, mercury methylation is a crucial step in the mercury environmental cycle, and it determines the toxicological consequences of mercury release (Wu et al., 2024). Methylmercury has a higher bioavailability than both elemental and inorganic mercury, and it can diffuse from cells and be adsorbed by benthic organisms. These benthic organisms can include zooplankton, small fish, and other small organisms or nonliving entities. Through the food chain, mercury accumulates biologically and is passed on to predators at the top of the food chain (Donadt et al., 2021;Minet et al., 2022). Additionally, studies on wetlands have shown that the production and accumulation of methylmercury are significantly present in floating mats of large aquatic plants (such as Eichhornia crassipes) and waterlogged forest soils in tropical regions (Murphy and Guentzel, 2022).Compared to other metals, mercury is much harder to degrade in ecosystems. Remediation of mercury will typically involve processes like immobilization or removal (Mu, 2023). Techniques such as oxidation, reduction, and adsorption can remove mercury from contaminated sites and also convert harmful mercury into less toxic forms (Veeraswamy et al., 2024). Therefore, understanding the mechanisms behind mercury production and accumulation is crucial. It is urgent to find safe and effective ways to control mercury pollution, protecting both the environment and human health. Periphyton is a microbial community made up of various organisms, including algae, bacteria, fungi, animals, and both organic and inorganic debris. These organisms attach together to organic or inorganic substrates (Zhao et al., 2023), held together by extracellular polymeric substances (EPS) secreted by the matrix. EPS is widely present in various aquatic systems and forms the basis of the food web in aquatic ecosystems. As the main component of surrounding organisms, EPS is a crucial source of carbon and energy that helps maintain the integrity of the periphyton community and enhances microbial activity, including that of mercury methylating bacteria (Xiang et al., 2022). Crucially, the same EPS matrix that supports the periphyton community also governs its interaction with environmental contaminants.Periphyton, as a bioaccumulator or a sensitive receptor of environmental pollutants (Shahzad et al., 2025), has been shown to play a critical role in binding and adsorbing heavy metals due to the various functional groups (such as carboxyl, phospho, amine, sulfhydryl, hydroxyl, etc.) in its EPS (Wang L. et al., 2020).Given the escalating severity of mercury pollution, understanding its biogeochemical cycling in critical environmental compartments is paramount. Periphyton, a ubiquitous and complex microbial-algal consortium, represents such a compartment where the fate of mercury is decisively shaped. This community not only efficiently concentrates and adsorbs mercury ions via its extracellular polymers but also hosts a dynamic microbial consortium that actively transforms them. For instance, sulfate-reducing bacteria (SRB), known mercury methylators found within periphyton, see their composition and methylation rates influenced by the community's nutrient status (Gambardella et al., 2025;Leclerc et al., 2021). Interestingly, under certain conditions, methanogens can emerge as the primary methylators, working synergistically with other microorganisms in this process (Lei et al., 2021). This intricate interplay between adsorption and microbial transformation within periphyton establishes it as a pivotal micro-ecosystem governing mercury speciation, bioavailability, and risk in aquatic environments.As the economy and society continue to develop, the increasing emissions of mercury and its compounds from human activities are making mercury pollution more severe. Periphyton, located at the lowest trophic level in the ecosystem, is a highly dynamic micro-ecological environment composed of algae, archaea, bacteria, fungi, microorganisms, and organic and inorganic substances. The extracellular polymers it secretes not only concentrate and adsorb mercury but also have a high potential for mercury methylation. As primary producers in the ecosystem, periphyton serves as the primary food source for small consumers, contributing more than half of the primary production in the ecosystem. By studying the interactions between periphyton and mercury from a microbial ecology perspective, we can gain further insight into the biological mechanisms and clarify how periphyton adsorbs and transforms different bioavailable forms of mercury. The high methylation potential within periphyton and its important role in the food chain of aquatic ecosystems highlight its significance for research on and removal of methylmercury pollution. Therefore, further understanding of the migration and transformation mechanisms of mercury under the influence of periphyton is essential for mercury pollution prevention and control. To establish a robust evidence base for this review, a systematic literature search was conducted focusing on studies from the year 2000 onward. Primary databases, including Web of Science and Google Scholar, were utilized with targeted keywords such as "mercury biosorption," "periphyton," "extracellular polymeric substances (EPS)," alongside specific terms for "bacteria," "fungi," and "algae." The synthesis prioritizes peer-reviewed studies that provide quantitative adsorption capacities and elucidate binding mechanisms, which directly inform the following discussion structured around the three key microbial constituents of periphyton: bacteria, fungi, and algae. The mercury biosorption capability of periphyton is largely dependent on the chemical makeup of its microbial constituents and their extracellular polymeric substances (EPS). Cell surfaces and the EPS matrix are rich in polysaccharides, proteins, and lipids, which offer a multitude of functional groups including amino, carboxyl, and thiol groups for effective complexation with metal ions like mercury. This structural and compositional diversity underpins the high affinity and selectivity of periphyton for various mercury species, forming the basis for its remarkable adsorption capacity. This is corroborated by research indicating that the EPS matrix is rich in these components and displays significant biological activity, with its organic components containing diverse metalbinding functional groups such as amino, carboxyl, thiol, phosphate, and sulfhydryl (Balíková et al., 2022). Since the 21st century, the bioadsorption potential of microorganisms for metals has become a focus of attention due to its broad applicability and environmental adaptability (Zhou et al., 2023). Consequently,The selection of bioadsorption materials for mercury has mainly focused on algae, bacteria, and fungi.Compelling evidence exists for bacterial biosorption, which often hinges on specific functional groups and demonstrates high remediation potential. Studies on bacteria have shown that the thiol group (-SH) on the surface of sulfate-reducing bacteria binds with Hg(II), promoting the adsorption and methylation process of Hg(II) by these bacteria (Thomas et al., 2020). Notably, Research on mucilaginous Bacillus as a bioadsorbent shows that it has good adsorption capacity for Hg(II), with a maximum adsorption of 393 mg (Hg)/L (Baran et al., 2022). Of methodological importance, In studies quantifying the adsorption of Hg(II) by Bacillus subtilis, the Surface Complexation Model (SCM) was introduced to effectively predict the adsorption effects of bioadsorbents on Hg(II) at the regional scale (Mitra et al., 2023). Further refining our understanding of environmental influences, Sharma further investigated how the presence of chloride ions in different pH environments affects the mercury adsorption capacity of Bacillus subtilis, Ochrobactrum, and sulfate-reducing bacteria (Sharma P. et al., 2021). Supporting the broad applicability of this approach, Zhao characterized the cell walls and extracellular polymers of seven bacterial strains, demonstrating that the biomass on the cell surfaces and EPS of these strains (Bacillus cereus, Bacillus lysiniformis, Bacillus, Roseococcus, Microbacterium, Serratia marcescens, and Ochrobactrum) have great potential as bioadsorbents for mercury bioremediation (Zhao M. M. et al., 2021).Fungal biosorbents, particularly from species like Aspergillus niger, exhibit distinct adsorption characteristics and practical resilience. Studies on fungal biosorbents have shown that Aspergillus niger spores can effectively adsorb Hg(II). Cui et al. reported optimal adsorption at pH 4.0-6.0 with a capacity of 27.2 mg/g (Cui H. et al., 2020). Separately, Oliveira et al. observed that UV-irradiated Aspergillus niger exhibited strong cell wall reorganization and DNA repair abilities, supporting its potential use in water treatmen (Oliveira et al., 2021).Algal biosorption is marked by significant uptake capacities and demonstrated in vivo relevance, highlighting its practical promise. Chlorella vulgaris demonstrated high mercury uptake due to its polar functional group-rich cell wall, with adsorption reaching 125 mg•g -1 , especially under simulated intestinal conditions (Fekry et al., 2024). Additionally, Yadav et al. found that its powdered form reduced mercury retention in mice and promoted fecal excretion, underscoring its applicability in bioremediation (Yadav et al., 2020). The bioadsorption of mercury onto periphyton is governed by a suite of physicochemical mechanisms, primarily mediated by the diverse functional groups within extracellular polymeric substances (EPS) and on microbial cell walls (Zhang et al., 2021;Cui D. et al., 2020). Key processes include chemical complexation (e.g., with carboxyl, hydroxyl, and thiol groups), ion exchange, and physical adsorption, which operate synergistically to achieve efficient mercury sequestration. The adsorption behavior of periphyton toward mercury typically occurs in two key stages: rapid extracellular adsorption and slower intracellular accumulation (Figure 1). The first stage involves rapid adsorption onto the cell wall surface and ion exchange reactions. The second stage is an active absorption process where metal ions are transported across the cell membrane, primarily via chemical adsorption. This second stage is characterized by significantly slower kinetics but is crucial for the intracellular sequestration of mercury.Most algae in periphyton have cell walls, which are generally divided into outer and inner layers. The outer layer is composed of a porous structure made up of multiple layers of microfibrils, including pectin, ammonium alginate, algal polysaccharides, and polygalacturonic acid esters, while the inner layer mainly consists of cellulose. The composition of the cell wall varies based on the classification of the algae. The extracellular wall is rich in extracellular products, mainly consisting of polypeptides and polysaccharides (such as alginates and salt-tolerant mucopolysaccharides) released by algal cells. For example, the cell wall of green algae like Chlorella contains 24%-74% polysaccharides, 2%-16% proteins, and 1%-24% uronic acid (Spain et al., 2021). These organisms provide polymer complexes, and functional groups such as amino, amide, carbonyl, aldehyde, hydroxyl, phosphate, and sulfate groups, which can extensively bind with metal ions, playing an important role in the adsorption of mercury ions by periphyton.For example, the cell wall structure of brown algae contains alginates and fucoidans, which primarily function in chelation with heavy metals (Al Monla et al., 2022). Further analysis reveals that the unique advantage of brown algae in the periphyton community lies in the pronounced wrinkled morphology of its cell wall surface, creating a large specific surface area. This structure provides ideal conditions for the ordered distribution of functional groups, ensuring that these groups can efficiently anchor to the algal cell wall, thereby maximizing the contact probability with metal ions.Experimental data shows that typically 80%-90% of heavy metal ions can be successfully captured on the surface of algal cells (Znad et al., 2022). This efficient adsorption process is mainly attributed to the extracellular polysaccharides (EPS) commonly found on the surface of periphyton and the synergistic action of various functional groups, such as phosphate, carboxyl, thiol, and amine groups, enriched in the cell wall. During the adsorption process, some functional groups can release negative charges through protonation, relying on electrostatic forces to adsorb metal ions.Meanwhile, functional groups on other algal species form stable complexes with metal ions through the donation of lone pair electrons to establish coordination bonds, thereby achieving efficient chelating adsorption (Fang et al., 2023). The chemical adsorption mechanism of periphyton for mercury (Hg) is a complex multi-component, multi-process system involving the synergistic action of extracellular polymers (EPS) in the biofilm, microbial cell surface functional groups, and metabolic products. Extracellular polymers are the core components of periphyton's mercury adsorption, primarily composed of polysaccharides, proteins, nucleic acids, and lipids. The functional groups responsible for mercury adsorption mainly include carboxyl (-COOH) and hydroxyl (-OH) groups, which dissociate into negative charges under near-neutral pH conditions, capturing Hg(II) via electrostatic attraction and subsequently forming stable coordination complexes (e.g., Hg-O or Hg-COO-); Taking advantage of mercury's high affinity for sulfur and the stability of the Hg-S bond, amino acids containing thiol groups (-SH) in EPS (such as cysteine) and sulfides directly chelate Hg(II), forming Hg-SR 2 complexes; In acidic environments, the lower protonation level enhances the coordination bond's ability to bind with Hg(II) through amine (-NH 2 ) groups (Figure 2). In addition to the chemical complexation and ion exchange mechanisms described above, physical adsorption constitutes a third fundamental pathway for mercury sequestration by periphyton. This process predominantly occurs through van der Waals interactions, exhibiting spontaneous and exothermic characteristics. While typically secondary in binding strength to chemical bonds in biological systems, recent studies confirm its significant contribution, particularly informed by research on engineered materials. Chen's study systematically demonstrated that iron-based MOFs achieve 93.4% methylmercury removal via physical adsorption, with a capacity of 4.91 mg/g and 90% uptake within 500 min. Structural characterization revealed that preserved carboxyl/Fe-O groups and hierarchical porosity (0.6 nm micropores, 1.95 nm mesopores) collectively enable this performance through enhanced reversibility and accessibility (Chen Y. et al., 2025). This Two-stage chemical mechanism of mercury adsorption by periphyton and the role of functional groups.Frontiers in Environmental Science frontiersin.orgunderscores the critical role that the inherently porous architecture of EPS plays in providing abundant sites for the physical entrapment of mercury, complementing the chemisorption activity and forming an integral part of the periphyton's multi-faceted adsorption strategy. The efficacy of the adsorption mechanisms described in Section 3.3 is critically modulated by a suite of environmental factors. Furthermore, the physical architecture of the EPS matrix itself, which facilitates these mechanisms, can be influenced by ambient conditions. The key parameters governing Hg adsorption and transformation are summarized in Table 1. The pH of the solution is a key factor in regulating the adsorption of heavy metals by bioadsorbents. It affects not only the surface complexation sites of the bioadsorbents but also significantly alters the chemical form and overall properties of the heavy metal solution. In the study of Cladosporium biomass for Hg(II) adsorption(II) (Martinez-Juarez et al., 2012), functional groups like amino, carboxyl, and hydroxyl significantly affected Adsorption of mercury by bacteria, fungi, and algae in periphyton. Increased kinetic energy and diffusion rates; process can be endothermic (e.g., for algae) or exhibit a distinct optimal temperature (e.g., for fungi) Dunham-Cheatham.'s study on the bacterial adsorption model for Hg(II) found that at low bacterial-to-metal ratios and in the absence of chloride ions, Hg(II) adsorption reached its maximum when pH increased from 3.0 to 6.0, then stabilized at pH > 8.0. However, at higher bacterial-to-metal ratios, the changes in these pH trends were less pronounced. The study also observed that the pH value was related to the adsorption amount of Hg(II), suggesting that the adsorption of Hg(II) is partially controlled by the surface morphology of the bacteria. For example, when the pH increased from 6.0 to 8.0, the adsorption of Hg(II) decreased; In the presence of chloride ions, when pH < 5.0, Hg(II) existed as complexes such as HgCl 2 and HgOHCl, further affecting its adsorbable state (Dunham-Cheatham et al., 2014).The adsorption of Hg(II) onto algal bioadsorbents is highly dependent on solution pH. For instance, Chlorella vulgaris exhibited increasing Hg(II) uptake as the pH rose from 2.0 to 7.0, with capacity plateauing under neutral to alkaline conditions (Solisio et al., 2019). This trend is driven by the cell surface charge: under strongly acidic conditions (pH 2.0-3.5), the algal surface is protonated, conferring a net positive charge that electrostatically repels cationic Hg(II) species (Kumar M. et al., 2020). As the pH increases, deprotonation of functional groups (e.g., carboxyl, phosphate) creates a negatively charged surface, facilitating metal ion adsorption. When Chlorella was used as a bioadsorbent for Hg(II) (Alvis et al., 2018), it was found that Hg(II) adsorption increased as the pH rose from 2.0 to 6.0. The results indicate that under strongly acidic conditions (pH 2.0-3.5), the algal cell surface carries a net positive charge, which is electrostatically unfavorable for the adsorption of positively charged Hg(II) ions. The temperature of the bioadsorption medium may be an important parameter for the energy-dependent mechanisms of microorganisms in metal adsorption. Generally, energy-independent mechanisms are unlikely to be influenced by temperature, as the processes responsible for bioadsorption are primarily within the realm of physical and chemical phenomena. The effect of temperature, however, varies significantly with the type of biosorbent. For instance, in studies of black Aspergillus as a bioadsorbent for Hg(II), the bioadsorption appeared to be largely temperature-independent across a broad range (10 °C-50 °C), yet a distinct peak in binding capacity was observed at 40 °C (Cui et al., 2018). In contrast, studies on algal biomass often demonstrate a clear temperature dependence. For instance, the biosorption of Hg(II) onto Ulva lactuca has been reported to be an endothermic process, where the adsorption capacity is enhanced by temperature, as supported by positive enthalpy change (ΔH ° > 0) (Çetintaş et al., 2022). Beyond directly altering adsorption interfaces, environmental redox potential (Eh) exerts a profound indirect influence on mercury sequestration by periphyton through its control over speciation. Mercury methylation, the process of forming neurotoxic methylmercury (MeHg) from inorganic mercury, is critically controlled by redox potential (Eh) in anaerobic environments such as paddy soils and sediments. Fluctuations in Eh directly regulate the microbial communities and biogeochemical conditions that drive this transformation.Research indicates that elevated Eh can significantly enhance MeHg production. A key mechanism involves the oxidation of reduced sulfur species into sulfate under higher Eh conditions, which stimulates sulfate-reducing bacteria (SRB) and promotes methylation (Tang et al., 2022). Concurrently, the decrease in acid-volatile sulfide (AVS) under these conditions increases MeHg mobility and phytoavailability. The role of sulfur is further underscored by studies showing that sulfur amendments enhance Hg bioavailability and methylation in paddy soils, as sulfur reduction facilitates the formation of Hg-polysulfide complexes (Li et al., 2023).The influence of Eh on methylation is observable across ecosystems. In estuarine sediments, SRB and iron-reducing bacteria (FeRB) are identified as primary methylators, with MeHg concentrations positively correlated with Eh variations (Zhao et al., 2024). Moreover, Eh significantly shapes the composition of methylating microbial communities. For example, in subalpine peatlands, FeRB dominate mercury methylation under specific Eh regimes, highlighting how redox conditions determine the dominant methylators in different environments (Liu et al., 2021).Therefore, by governing the microbial synthesis of MeHg, redox potential acts as a pivotal environmental switch regulating the speciation, pool, and consequent adsorption of mercury within periphyton systems. Periphyton, a ubiquitous microbial biofilm in aquatic systems (Merbt et al., 2022), represents a complex consortium of algae, bacteria, and other microorganisms embedded in a self-produced matrix of extracellular polymeric substances (EPS). This section examines the physiological impacts and adaptive strategies of this critical biofilm community under mercury stress. Under mercury stress, periphyton experience significant disruption in their overall physiological and chemical activities (Chen and Zhu, 2022). The physiological responses mainly manifest as certain impacts or hindrances to their physiological processes and biochemical reactions within the cells. The effect of mercury stress on the growth and reproduction of periphyton stems directly from the toxicity of mercury and its derivatives. Research shows that after mercury contamination, the cell density of Scenedesmus decreases significantly, and its cell volume also shrinks notably as mercury ion concentration increases (Zhao Q. et al., 2021). Furthermore, continuous cultivation of Chlorella in a phosphorus-limited environment showed that both short-term mercury shocks and long-term pollution lead to a continuous decrease in the algae's total biomass from the onset of pollution (Ding et al., 2019a).The proliferation dynamics and adaptive mechanisms of periphyton are closely related to the sensitivity and resistance of algal communities in the environment to mercury stress. Various algae exhibit distinct physiological responses under mercury stress, with their tolerance levels showing notable species specificity. Studies have confirmed that when algae are exposed to high concentrations of mercury pollutants, their growth and metabolism are significantly hindered, yet the defense abilities against mercury toxicity vary greatly among species. For example, Micrasterias shows a high resistance to mercury during the inhibition of mercury methylation (Ding et al., 2019b). Similarly, the non-methylating microalga Chlorella pyrenoidosa exhibits considerable adaptive capacity to survive in eutrophic waters containing elevated mercury concentrations (1-20 μg/L). Research demonstrates that under solar irradiation, C. pyrenoidosa reduces Hg(II) to elemental Hg 0 via both surface adsorption and intracellular assimilation, further transforming it into low-bioavailability species such as β-HgS and Hg-phytochelatin complexes. These mechanisms support its persistence in mercurycontaminated aquatic environments (Liang et al., 2022). In contrast, the growth of Convoluta at higher mercury concentrations ([HgCl 2 ] ≥25 μg/L) is severely inhibited, and its tolerance is noticeably weaker. Therefore, under mercury pollution, algal growth and reproduction face direct harm and are influenced by their unique sensitivity to mercury stress and complex tolerance mechanisms. Under the negative influence of mercury, the internal composition of algal cells undergoes notable uneven changes. Specifically, the levels of certain core biomolecules fluctuate significantly, showing distinct variability, while other components remain relatively balanced. For example, soluble sugars and nucleic acids remain stable under stress, with no significant deviations observed; in contrast, the levels of soluble proteins, hydrogen peroxide (H 2 O 2 ), and oxygen (O 2 ) exhibit strong fluctuations. According to Wu Juan and her team's in-depth research (Wu et al., 2023), as mercury concentration increases, the oxygen (O 2 ) production rate shows a sharp, non-linear increase, strongly correlating with mercury concentration; Meanwhile, the level of hydrogen peroxide (H 2 O 2 ) accumulates significantly, reaching a highly significant statistical difference, while the soluble protein content decreases significantly and shows a notable negative dependence on mercury levels. These findings clearly show that the dynamic adjustment of intracellular component levels is closely tied to algae's survival strategies and adaptive capabilities to cope with adverse environments. Research indicates that the microalga Isochrysis galbana demonstrates notable physiological adaptations under mercury (Hg) stress. At Hg(II) concentrations exceeding 0.2 mg/L, algal growth is markedly suppressed, whereas lower concentrations enhance antioxidant enzyme activities (Zhang et al., 2025). Mercury facilitates detoxification by binding to functional groups on the cell surface and intracellular macromolecules, highlighting the alga's considerable adaptability to mercury-contaminated environments. Such adaptations involve not only morphological alterations but also precise modulation of intracellular biochemical pathways.Under mercury stress, algae's chlorophyll fluorescence parameters show varying downward trends as mercury concentration increases, but the effect differs significantly between parameters, reflecting their specificity. Currently, precise measurement of these parameters using advanced chlorophyll fluorescence technology has become an important research method in this field. According to the study conducted by Chen et al., mercury stress markedly inhibits the photosynthetic system of algae within periphyton communities. Chlorophyll fluorescence imaging analysis demonstrated that mercury stress significantly reduced the maximum photochemical quantum yield (Fv/Fm) of algae to a range of 0.48-0.50, accompanied by a decline in actual photochemical efficiency (Y(II)), indicating severe impairment of the photosystem II (PSII) reaction center. Further investigations revealed that, under stress conditions, algal cells initiate a photoprotective mechanism by elevating non-photochemical quenching (Y(NPQ)) to a range of 0.02-0.08, thereby dissipating excess light energy and mitigating further damage to the photosynthetic apparatus. Moreover, confocal Raman imaging analysis indicated that while mercury stress did not markedly alter the distribution or content of key intracellular components, such as proteins (1,003 cm -1 ), lipids (1,445 cm -1 ), and carotenoids (1,520 cm -1 ), its effects were predominantly observed in compositional changes of extracellular polymeric substances (EPS). Three-dimensional fluorescence spectroscopy revealed notable alterations in the intensity of the tryptophan-like fluorescence peak (Ex/Em = 280/330 nm) in the EPS of stressed groups, suggesting a potential adaptive response of algae to heavy metal toxicity. This response likely involves the complexation and passivation of mercury ions by functional groups (e.g., carboxyl and hydroxyl) within EPS (Chen H. et al., 2025). Algae play a key role in the biogeochemical cycling of mercury, particularly in its adsorption and subsequent transformations. For instance, Microcystis aeruginosa effectively adsorbs Hg(II) from water. Studies have shown a significant correlation between intracellular mercury content in single algal cells and various physiological responses, including cell growth, photosynthetic pigments, fluorescence parameters, reactive oxygen species content, superoxide dismutase activity, and cell membrane permeability (Tang et al., 2023). This highlights how algae's adaptation involves mercury uptake and how this accumulation can impact their physiological wellbeing.Upon algal decay, their biomass settles on the sediment surface, creating a unique microenvironment that influences mercury biogeochemistry. Algal-derived dissolved organic matter (ADOM) has been shown to accelerate mercury methylation under cyanobacterial blooms in the sediments of eutrophic lakes. This is because ADOM components are closely linked to the Hg methylation process, playing roles in mercury absorption or complexation, participating in redox reactions, and modulating microbial activity. Furthermore, the dsrB gene in sulfate-reducing bacteria (SRB) has been identified as a key factor in metabolic pathways related to mercury reduction and methylation under anoxic conditions, underscoring the influence of decaying algae on microbial-driven mercury transformations (Wang et al., 2024).Algal decomposition products also promote the generation of sulfides, which in turn affect elemental cycling in sediments. Research indicates that algae decomposition leads to high sulfide production (ΣS 2-) in anaerobic water columns, with a significant portion derived from "algae-derived ΣS 2-" in addition to sulfate reduction. These substantial amounts of ΣS 2-can diffuse from the water column into sediments, altering iron and phosphorus cycles (Zhao et al., 2019). Such high sulfide conditions can influence mercury's speciation and bioavailability in the sediment environment.Addressing the challenges of methylmercury (MeHg) pollution, particularly in eutrophic systems, novel biotechnological approaches are being developed. For instance, the potential for methylmercury production in excessive sludge from wastewater treatment plants can be eliminated by elemental sulfur (S0) addition. This treatment effectively mitigates and potentially eliminates MeHg formation by simultaneously reducing mercury bioavailability and mercurymethylating bioactivity, partly through converting bioavailable Hg(II) to insoluble HgS and promoting the growth of Hgreducers with the merA (OuYang et al., 2024). This provides a promising strategy to combat MeHg pollution, which could be relevant for environments impacted by algal decay.6 Mercury transformation mediated by periphyton Mercury methylation by microorganisms, especially anaerobic bacteria, is the most common pathway for producing methylmercury (Bravo and Cosio, 2020;Dranguet et al., 2017;Yin et al., 2022). Anaerobic bacteria involved in mercury bioremediation include: (1) sulfate-reducing bacteria (SRB), (2) iron-reducing bacteria, (3) methanogenic bacteria, and (4) other types (e.g., fermentative, acetate-producing, and cellulose-degrading microbes (Bravo et al., 2020;Joshi et al., 2021;Al-Ansari, 2022). The biosynthesis of methylmercury is influenced by various environmental factors which ultimately regulate the activity and dominance of different methylating microbial groups. Key parameters such as pH, redox potential, and sulfate availability create distinct ecological niches that favor specific anaerobic bacteria. For instance, sulfate-reducing bacteria (SRB) typically dominate and drive methylation in anoxic, sulfate-rich environments. In contrast, when sulfate is depleted, ironreducing bacteria (IRB) and methanogens become more prominent methylators, with their activity being influenced by pH and the competition for electron donors. Other factors like temperature, salinity, and the presence of sulfur-containing ligands (e.g., dissolved organic matter, cysteine) further modulate the bioavailability of Hg(II) to these microorganisms (Li et al., 2022a;Chételat et al., 2020;Wang et al., 2021). Given the complex and sometimes conflicting interplay of these environmental parameters, further research is needed to fully clarify the niche partitioning and methylation mechanisms among these microbial groups. Several studies have confirmed that sulfate-reducing bacteria (SRB), iron-reducing bacteria (IRB), and anaerobic sulfur-oxidizing bacteria (MA) are the key microorganisms responsible for mercury methylation. Among these efficient mercury-methylating strains, over 90% carry the hgcAB gene cluster, which is essential for mercury methylation. The hgcA gene within this core gene cluster encodes a cobalamin-like protein, whose functional properties are similar to those of cobalamin-containing proteins involved in CO 2 reduction to acetyl-CoA (Mcdaniel et al., 2020). Its main role is to efficiently convert the toxic Hg(II) into the highly toxic CH 3 Hg + , and the subsequent hgcB gene encodes a ferric redox protein, which is responsible for precisely regenerating the cofactor of the cobalamin-like protein (Lin et al., 2021).In natural environments, the microbial methylation of mercury is accompanied by the microbial demethylation of MeHg. These two processes are interdependent and co-regulated, maintaining the dynamic balance of MeHg concentration in the environment and playing a critical role in the global mercury cycle. The demethylation of MeHg primarily involves two pathways: indirect degradation via photochemical reactions and direct metabolism via microbial transformation (Barkay and Gu, 2021), with microbial transformation being particularly prominent. Microorganisms degrade MeHg through demethylation, which can be divided into two major mechanisms: single-electron reduction (RD) and doubleelectron oxidation (OD) (Hu et al., 2024;McCarthy, 2020). The OD pathway is primarily driven by anaerobic methanotrophic archaea (MA) and sulfate-reducing bacteria (SRB), which use complex biological oxidation reactions to convert MeHg into less toxic Hg(II) ions. RD is closely associated with the MerA and MerB enzymes encoded by mercury-resistant microorganisms (Du et al., 2019). The MerB enzyme can efficiently cleave the C-Hg bond, releasing more active Hg(II) ions; and the MerA enzyme further reduces Hg(II) ions into the highly volatile Hg 0 gas. This process not only reduces the toxicity of MeHg, but also facilitates the escape of Hg 0 from cells and its diffusion into the atmosphere, thus enabling effective mercury release (Nanda et al., 2019) (Figure 3). Sulfate-Reducing Bacteria (SRB) are the most extensively studied group in mercury methylation research, with their core mechanism linked to the sulfate reduction metabolic pathway.Under anaerobic conditions, SRB obtain energy through sulfate reduction, producing cytochrome c3 and hydrogenase, which catalyze the methylation of Hg(Ⅱ). The key intermediate, methyltetrahydrofolate, transfers the methyl group to Hg(Ⅱ) through methyltransferases such as the HgcA/HgcB complex, resulting in the neurotoxin methylmercury (MeHg). The close association with the Desulfobacteraceae family is likely due to the high expression of the hgcA gene cluster in species like Desulfobacter postgatei, where the hgcA gene copy number is 3-5 times higher than in other SRBs, thus enhancing the methylation efficiency (Peng et al., 2024).Iron-Reducing Bacteria (IRB) are another important group of mercury-methylating bacteria, almost as essential as SRB. IRBs, such as Geobacter spp, indirectly promote methylation through the dissimilatory iron reduction pathway. The quinone-type electron shuttles secreted by IRBs reduce Hg(Ⅱ) to Hg 0 , increasing mercury bioavailability. Studies have shown that in the presence of Fe 3+ , Geobacter sulfurreducens PCA strain increases MeHg production by 2.3 times, with its outer membrane protein OmcB directly facilitating electron transfer to mercury ions (Tang et al., 2025). A strain of IRB Geobacter isolated from lake sediments demonstrated Hg(Ⅱ) methylation rates comparable to Desulfobulbus propiicus strain 1pr3, a highly active methylating SRB factor (Regnell and Watras, 2019).Methanogens play a significant role in the environment as major methylators of Hg(Ⅱ). The methylcobalamin (Me-B 12 ) they produce is crucial for converting inorganic Hg(Ⅱ) into methylmercury (MeHg), a key process in microbial environmental chemistry. Methanogens rely on Me-B 12 for Hg(Ⅱ) methylation, regulated by the Wood-Ljungdahl pathway. Furthermore, a recent groundbreaking study has demonstrated that the methyl group for this methylation in methanogens is directly supplied by the Wolfe cycle of methanogenesis, revealing a pathway distinct from the acetyl-CoA pathway utilized by sulfate-reducing and iron-reducing bacteria (Gao et al., 2024). The inhibitor 2bromoethanesulfonate blocks the methyl coenzyme M reductase (MCR), preventing methyl donor formation and completely inhibiting methylation. However, after sodium molybdate inhibits SRB, methanogens show increased activity due to reduced competition for common substrates, such as acetate, thereby boosting MeHg production by 70%. In temperate lakes, the abundance of the hgcA gene in Methanoregula boonei is significantly correlated with MeHg concentrations in sediments. Studies found that mercury methylation was completely inhibited by a methanogen inhibitor (Zhang et al., 2022), 2-bromoethanesulfonate, but highly promoted by sodium molybdate (a sulfate reduction inhibitor). These findings suggest that methanogens, rather than SRBs, are likely the primary mercury-methylating microorganisms in temperate river-lake ecosystems and are associated with the most common gene sequences.Microbial-mediated demethylation mechanisms mainly include oxidative demethylation and photochemical degradation. Studies have shown that aerobic bacteria, such as Pseudomonas putida, degrade MeHg into Hg 0 and CH 4 via the MerB lyase encoded by the mer operon. The MerA reductase further reduces Hg(Ⅱ) to volatile Hg 0 , completing the detoxification cycle (Krout et al., 2022). In aquatic environments, dissolved organic matter (DOM) generates reactive oxygen species (ROS) under UV excitation, breaking Hg-C bonds through free radical reactions. Experiments have shown that in water with high humic acid, the half-life of MeHg can decrease from 200 days to 20 days (Han et al., 2017;Wang Q. et al., 2020). Algal blooms are a common occurrence in eutrophic waters. For years, it was believed that algae in lakes reduce MeHg Frontiers in Environmental Science frontiersin.org concentrations, bioaccumulation, and trophic transfer (e.g., through growth dilution). However, research has shown that during algal growth, increased sedimentation and decomposition lead to higher MeHg content in the sediments (by 54%-514%) (Wang, 2021). This is primarily due to extracellular polymers produced by lake algae, which are rich in small-molecular thiol groups capable of adsorbing mercury while hosting numerous mercury-methylating microorganisms, forming a hotspot for mercury methylation (Lei et al., 2019;Sharma R. K. et al., 2021). Additionally, the decomposition of algae contributes small-molecule aromatic proteins and microbial communities to sediments, enhancing the abundance of mercury-methylating microorganisms and mercury bioavailability, thus promoting mercury methylation (Liang et al., 2024;Lei et al., 2025).The impact of algal activity on mercury transformation exhibits significant environmental dependency. A representative investigation involving 15 marine microalgal species detected minimal inorganic mercury methylation but observed substantial MeHg demethylation in multiple species, with degradation rates comparable to those of photodemethylation (Li et al., 2022b). This finding indicates that in marine ecosystems, specific microalgae together with their extracellular metabolites and associated bacterial consortia can function primarily as MeHg sinks, emphasizing the contrasting ecological functions that algal communities may perform across different aquatic environments (Figure 4). The functional role of periphyton in the mercury cycle is context-dependent, shifting between that of a net producer and a net decomposer of methylmercury (MeHg). This transition is governed by environmental conditions that select for distinct microbial functional groups and their associated biochemical pathways.Periphyton acts as a net producer of MeHg under environmental regimes that promote anaerobic metabolism. A prime example is eutrophic freshwater systems, where the decay of algal biomass depletes oxygen, establishes anoxic microenvironments, and supplies labile organic carbon. These conditions selectively enrich for sulfate-reducing and iron-reducing bacteria, many of which harbor the essential hgcAB gene cluster. The efficacy of this methylating consortium is further modulated by key physicochemical parameters, primarily a low redox potential and adequate sulfate availability, which collectively establish a niche optimal for methylator activity.Conversely, the community transitions to a net decomposer of MeHg in habitats characterized by oxidative metabolism, such as well-oxygenated waters or specific marine settings. In these environments, the community restructures, often favoring aerobic bacteria that encode the mer operon, such as Pseudomonas species, which catalyze the reductive demethylation of MeHg to gaseous Hg 0 . This microbial process can be complemented by the activity of demethylating microalgae and abiotic photodegradation, collectively enhancing the overall capacity for MeHg breakdown.In summary, the local redox gradient, primarily determined by organic matter input and oxygen availability, serves as the principal regulator governing the transition between these two functional states. By selecting for dominant microbial functional guilds, it controls the critical balance between hgcAB-driven methylation and mer-facilitated demethylation. This equilibrium ultimately determines whether the periphyton matrix functions as a site of net MeHg production or attenuation. The pivotal role of periphyton microorganisms in mercury migration and transformation.Frontiers in Environmental Science frontiersin.org 10 Li et al. 10.3389/fenvs.2025.1693766 7 Bioremediation of mercury: microbial and phytoremediation approaches Bioremediation technologies, leveraging microorganisms to eliminate environmental pollutants, have garnered significant interest as environmentally friendly, cost-effective, and sustainable solutions. These methods include genetically modifying microorganisms to enhance mercury (Hg) remediation at contaminated sites. Such approaches often focus on molecular mechanisms like the mercury resistance (Mer) operon, which encodes proteins pivotal for microbial mercury detoxification and its biogeochemical cycling (Zhang et al., 2020). Overall, microbial remediation of mercury encompasses diverse mechanisms, offering a wide range of applications: biosorption, bioaccumulation, biotransformation, biovolatilization, and biomineralization (Kumar et al., 2023).Microbial interactions with mercury involve distinct physical and biochemical processes. Biosorption facilitates the passive removal of mercury ions onto microbial cell surfaces, utilizing functional groups from cell wall components and extracellular polymeric substances (EPS) (Singh and Kumar, 2020). Bioaccumulation refers to the active, metabolism-dependent uptake and intracellular sequestration of mercury by living microbial cells. These initial steps concentrate mercury, making it amenable to further processing.Enzymatic biotransformation and biovolatilization are critical active detoxification pathways. The bacterial mer operon is a prime example of a genetically controlled system for mercury detoxification. This operon encodes proteins that facilitate the conversion, transport, and detoxification of mercury. Specifically, the merA gene encodes mercuric reductase (MerA), which reduces toxic inorganic Hg(II) to volatile, significantly less toxic elemental mercury (Hg(0)), leading to biovolatilization. For organic mercury, the merB gene encodes organomercurial lyase (MerB), which converts it to Hg(II) for subsequent MerA reduction (Bhat et al., 2024).Microbial biomineralization provides an effective immobilization strategy. This process involves microorganisms inducing the formation of stable mineral precipitates (e.g., carbonates, sulfides, phosphates, iron/manganese oxides) from dissolved metal ions. This effectively encapsulates and immobilizes contaminants, transforming them into low-toxicity mineral forms (Liu et al., 2025). Such diverse microbial capacities, both inherent and genetically engineered, are fundamental to developing efficient and sustainable strategies for mercury pollution control. A study showed that large algae species, such as Ulva lactuca (green algae), Gracilaria (red algae), and Sargassum (brown algae), can also be used to remove mercury from saline waters. Three mercury solutions with different concentrations (50, 200, and 500 μg/dm 3 ) were used in the experiment, and the treatment was conducted over a 72-h period. The results revealed that all algae displayed significant mercury removal efficiency at these concentrations. Furthermore, the study demonstrated that among macroalgae species, Ulva intestinalis showed an absorption capacity for mercury (II) of up to 1888 μg g -1 , with a bioconcentration factor (BCF) as high as 3823. This property strongly demonstrates that Ulva intestinalis has considerable application potential in the remediation of aquatic mercury pollution (Fabre et al., 2021).Qian et al. demonstrated the toxic effect of mercury on Vallisneria natans, where root activity was significantly inhibited when pore-water Hg concentrations reached levels as low as 0.052-0.059 mg/L (Qian et al., 2019). Furthermore, small floating aquatic plants are increasingly recognized as a key research area in this field, owing to their distinct growth traits and remarkable pollutant accumulation capabilities. Research indicates that designing multi-species assemblages can substantially enhance remediation performance. For instance, Spencer and colleagues demonstrated that co-culturing Lemna minor and Spirodela polyrhiza not only increased biomass production compared to monoculture but also improved mercury uptake and stress tolerance, highlighting the synergistic role of biodiversity in pollution management (Spencer et al., 2024). In investigations involving Azolla species, Sharma et al. reported that Azolla filiculoides displayed exceptional mercury accumulation in fly ashcontaminated water, achieving notably high bioconcentration factor values. Importantly, this species maintained normal growth under oxidative stress conditions, suggesting unique physiological adaptive traits (Sharma et al., 2023). A separate study on Azolla pinnata revealed a concentration-dependent response in treating mercury-laden industrial effluent, with optimal heavy metal removal efficiency observed at 60% wastewater concentration, offering valuable insights for practical implementation (Kumar V. et al., 2020). Similarly, Salvinia natans exhibited strong potential for mercury remediation. Sitarska's group systematically evaluated its root-mediated mercury absorption and accumulation, documenting a removal rate of up to 94%. Notably, cultivated plants showed increased biomass and protein content under mercury stress, reinforcing the practical utility of aquatic vegetation in mitigating mercury pollution (Sitarska et al., 2023). Periphyton, a ubiquitous microbial-algal consortium, and its extracellular polymeric substances (EPS) play pivotal and multifaceted roles in the environmental fate of mercury, functioning as "adsorption filters, microzone reactors, and ecological pathways." The functional groups of EPS quickly bind to and temporarily immobilize Hg(Ⅱ). In the anaerobic microenvironments within periphyton, the associated microbial communities facilitate mercury methylation and demethylation through the hgcAB and merAB pathways. Furthermore, methylmercury, which is produced within and settles with periphytic biomass, can biomagnify in the food web, thus increasing ecological and health risks. Moving forward, the critical task is to understand the quantitative link between these micro-scale periphyton-mercury interactions and the larger watershed-scale mercury cycle. This involves the development of in situ dynamic monitoring systems and targeted regulation technologies, along with verifying the low-carbon remediation potential of periphyton-based strategies in natural systems such as wetlands and algal ponds. These efforts will provide more precise and sustainable scientific support for global mercury pollution management.