Biotic formation of methylmercury: A bio-physico-chemical conundrum

Warming inhibits HgII methylation but stimulates methylmercury demethylation in paddy soils

Inorganic mercury (HgII) can be transformed into neurotoxic methylmercury (MeHg) by microorganisms in paddy soils, and the subsequent accumulation in rice grains poses an exposure risk for human health. Warming as an important manifestation of climate change, changes the composition and structure of microbial communities, and regulates the biogeochemical cycles of Hg in natural environments. However, the response of specific HgII methylation/demethylation to the changes in microbial communities caused by warming remain unclear. Here, nationwide sampling of rice paddy soils and a temperature-adjusted incubation experiment coupled with isotope labeling technique (202HgII and Me198Hg) were conducted to investigate the effects of temperature on HgII methylation, MeHg demethylation, and microbial mechanisms in paddy soils along Hg gradients. We showed that increasing temperature significantly inhibited HgII methylation but promoted MeHg demethylation. The reduction in the relative abundance of Hg-methylating microorganisms and increase in the relative abundance of MeHg-demethylating microorganisms are the likely reasons. Consequently, the net Hg methylation production potential in rice paddy soils was largely inhibited under the increasing temperature. Collectively, our findings offer insights into the decrease in net MeHg production potential associated with increasing temperature and highlight the need for further evaluation of climate change for its potential effect on Hg transformation in Hg-sensitive ecosystems.

Temporal mercury dynamics throughout the rice cultivation season in the Ebro Delta (NE Spain): An integrative approach

During the last few decades, inputs of mercury (Hg) to the environment from anthropogenic sources have increased. The Ebro Delta is an important area of rice production in the Iberian Peninsula. Given the industrial activity and its legacy pollution along the Ebro river, residues containing Hg have been transported throughout the Ebro Delta ecosystems. Rice paddies are regarded as propitious environments for Hg methylation and its subsequent incorporation to plants and rice paddies' food webs. We have analyzed how Hg dynamics change throughout the rice cultivation season in different compartments from the paddies' ecosystems: soil, water, rice plants and fauna. Furthermore, we assessed the effect of different agricultural practices (ecological vs. conventional) associated to various flooding patterns (wet vs. mild alternating wet and dry) to the Hg levels in rice fields. Finally, we have estimated the proportion of methylmercury (MeHg) to total mercury in a subset of samples, as MeHg is the most bioaccumulable toxic form for humans and wildlife. Overall, we observed varying degrees of mercury concentration over the rice cultivation season in the different compartments. We found that different agricultural practices and flooding patterns did not influence the THg levels observed in water, soil or plants. However, Hg concentrations in fauna samples seemed to be affected by hydroperiod and we also observed evidence of Hg biomagnification along the rice fields' aquatic food webs.

Organ-specific mercury stable isotopes, speciation and particle measurements reveal methylmercury detoxification processes in Atlantic Bluefin Tuna

Identifying metabolism and detoxification mechanisms of Hg in biota has important implications for biomonitoring, ecotoxicology, and food safety. Compared to marine mammals and waterbirds, detoxification of MeHg in fish is understudied. Here, we investigated Hg detoxification in Atlantic bluefin tuna Thunnus thynnus using organ-specific Hg and Se speciation data, stable Hg isotope signatures, and Hg and Se particle measurements in multiple tissues. Our results provide evidence for in vivo demethylation and biomineralization of HgSe particles, particularly in spleen and kidney. We observed a maximum range of 1.83‰ for δ202Hg between spleen and lean muscle, whereas Δ199Hg values were similar across all tissues. Mean percent methylmercury ranged from 8% in spleen to 90% in lean muscle. The particulate masses of Hg and Se were higher in spleen and kidney (Hg: 61% and 59%, Se: 12% and 6%, respectively) compared to muscle (Hg: 2%, Se: 0.05%). Our data supports the hypothesis of an organ-specific, two-step detoxification of methylmercury in wild marine fish, consisting of demethylation and biomineralization, like reported for waterbirds. While mass dependent fractionation signatures were highly organ specific, stable mass independent fractionation signatures across all tissues make them potential candidates for source apportionment studies of Hg using ABFT.

Methylmercury cycling in the Bohai Sea and Yellow Sea: Reasons for the low system efficiency of methylmercury production

Coastal seas contribute the majority of human methylmercury (MeHg) exposure via marine fisheries. The terrestrial area surrounding the Bohai Sea and Yellow Sea (BS and YS) is one of the mercury (Hg) emission "hot spots" in the world, resulting in high concentrations of Hg in BS and YS seawater in comparison to other marine systems. However, comparable or even lower Hg levels were detected in seafood from the BS and YS than other coastal regions around the word, suggesting a low system bioaccumulation of Hg. Reasoning a low system efficiency of MeHg production (represented by MeHg/THg (total Hg) in seawater) may be present in these two systems, seven cruises were conducted in the BS and YS to test this hypothesis. MeHg/THg ratios in BS and YS seawater were found to be lower than that in most coastal systems, indicating that the system efficiency of MeHg production is relatively lower in the BS and YS. The low system efficiency of MeHg production reduces the risk of Hg in the BS and YS with high Hg discharge intensity. By measuring in situ production and degradation of MeHg using double stable isotope addition method, and MeHg discharge flux from various sources and its exchange at various interfaces, the budgets of MeHg in the BS and YS were estimated. The results indicate that in situ methylation and demethylation are the major source and sink of MeHg in the BS and YS. By comparing the potential controlling processes and environmental parameters for MeHg/THg in the BS and YS with the other coastal seas, estuaries and bays, lower transport efficiency of inorganic Hg from water column to the sediment, slower methylation of Hg, and rapid demethylation of MeHg were identified to be major reasons for the low system efficiency of MeHg production in the BS and YS. This study highlights the necessity of monitoring the system efficiency of MeHg production, associated processes, and controlling parameters to evaluate the efficiency of reducing Hg emissions in China as well as the other countries.

Bioconcentration of Inorganic and Methyl Mercury by Algae Revealed Using Dual-Mass Single-Cell ICP-MS with Double Isotope Tracers

Algae are an entry point for mercury (Hg) into the food web. Bioconcentration of Hg by algae is crucial for its biogeochemical cycling and environmental risk. Herein, considering the cell heterogeneity, we investigated the bioconcentration of coexisting isotope-labeled inorganic (199IHg) and methyl Hg (201MeHg) by six typical freshwater and marine algae using dual-mass single-cell inductively coupled plasma mass spectrometry (scICP-MS). First, a universal pretreatment procedure for the scICP-MS analysis of algae was developed. Using the proposed method, the intra- and interspecies heterogeneities and the kinetics of Hg bioconcentration by algae were revealed at the single-cell level. The heterogeneity in the cellular Hg contents is largely related to cell size. The bioconcentration process reached a dynamic equilibrium involving influx/adsorption and efflux/desorption within hours. Algal density is a key factor affecting the distribution of Hg between algae and ambient water. Cellular Hg contents were negatively correlated with algal density, whereas the volume concentration factors almost remained constant. Accordingly, we developed a model based on single-cell analysis that well describes the density-driven effects of Hg bioconcentration by algae. From a novel single-cell perspective, the findings improve our understanding of algal bioconcentration governed by various biological and environmental factors.

Field-aged rice hull biochar stimulated the methylation of mercury and altered the microbial community in a paddy soil under controlled redox condition changes

Mercury (Hg) contaminated paddy soils are hot spots for methylmercury (MeHg) which can enter the food chain via rice plants causing high risks for human health. Biochar can immobilize Hg and reduce plant uptake of MeHg. However, the effects of biochar on the microbial community and Hg (de)methylation under dynamic redox conditions in paddy soils are unclear. Therefore, we determined the microbial community in an Hg contaminated paddy soil non-treated and treated with rice hull biochar under controlled redox conditions (< 0 mV to 600 mV) using a biogeochemical microcosm system. Hg methylation exceeded demethylation in the biochar-treated soil. The aromatic hydrocarbon degraders Phenylobacterium and Novosphingobium provided electron donors stimulating Hg methylation. MeHg demethylation exceeded methylation in the non-treated soil and was associated with lower available organic matter. Actinobacteria were involved in MeHg demethylation and interlinked with nitrifying bacteria and nitrogen-fixing genus Hyphomicrobium. Microbial assemblages seem more important than single species in Hg transformation. For future directions, the demethylation potential of Hyphomicrobium assemblages and other nitrogen-fixing bacteria should be elucidated. Additionally, different organic matter inputs on paddy soils under constant and dynamic redox conditions could unravel the relationship between Hg (de)methylation, microbial carbon utilization and nitrogen cycling.

New Insights into MeHg Accumulation in Rice (Oryza sativa L.): Evidence from Cysteine

The intake of methylmercury (MeHg)-contaminated rice poses immense health risks to rice consumers. However, the mechanisms of MeHg accumulation in rice plants are not entirely understood. The knowledge that the MeHg-Cysteine complex was dominant in polished rice proposed a hypothesis of co-transportation of MeHg and cysteine inside rice plants. This study was therefore designed to explore the MeHg accumulation processes in rice plants by investigating biogeochemical associations between MeHg and amino acids. Rice plants and underlying soils were collected from different Hg-contaminated sites in the Wanshan Hg mining area. The concentrations of both MeHg and cysteine in polished rice were higher than those in other rice tissues. A significant positive correlation between MeHg and cysteine in rice plants was found, especially in polished rice, indicating a close geochemical association between cysteine and MeHg. The translocation factor (TF) of cysteine showed behavior similar to that of the TF of MeHg, demonstrating that these two chemical species might share a similar transportation mechanism in rice plants. The accumulation of MeHg in rice plants may vary due to differences in the molar ratios of MeHg to cysteine and the presence of specific amino acid transporters. Our results suggest that cysteine plays a vital role in MeHg accumulation and transportation inside rice plants.

Non-mercury methylating microbial taxa are integral to understanding links between mercury methylation and elemental cycles in marine and freshwater sediments

The goal of this study was to explore the role of non-mercury (Hg) methylating taxa in mercury methylation and to identify potential links between elemental cycles and Hg methylation. Statistical approaches were utilized to investigate the microbial community and biochemical functions in relation to methylmercury (MeHg) concentrations in marine and freshwater sediments. Sediments were collected from the methylation zone (top 15 cm) in four Hg-contaminated sites. Both abiotic (e.g., sulfate, sulfide, iron, salinity, total organic matter, etc.) and biotic factors (e.g., hgcA, abundances of methylating and non-methylating taxa) were quantified. Random forest and stepwise regression were performed to assess whether non-methylating taxa were significantly associated with MeHg concentration. Co-occurrence and functional network analyses were constructed to explore associations between taxa by examining microbial community structure, composition, and biochemical functions across sites. Regression analysis showed that approximately 80% of the variability in sediment MeHg concentration was predicted by total mercury concentration, the abundances of Hg methylating taxa, and the abundances of the non-Hg methylating taxa. The co-occurrence networks identified Paludibacteraceae and Syntrophorhabdaceae as keystone non Hg methylating taxa in multiple sites, indicating the potential for syntrophic interactions with Hg methylators. Strong associations were also observed between methanogens and sulfate-reducing bacteria, which were likely symbiotic associations. The functional network results suggested that non-Hg methylating taxa play important roles in sulfur respiration, nitrogen respiration, and the carbon metabolism-related functions methylotrophy, methanotrophy, and chemoheterotrophy. Interestingly, keystone functions varied by site and did not involve carbon- and sulfur-related functions only. Our findings highlight associations between methylating and non-methylating taxa and sulfur, carbon, and nitrogen cycles in sediment methylation zones, with implications for predicting and understanding the impact of climate and land/sea use changes on Hg methylation.

Microbial antagonistic mechanisms of Hg(II) and Se(IV) in efficient wastewater treatment using granular sludge

The antagonistic effects of mercury (Hg) and selenium (Se) have been extensively studied in higher animals and plants. In this study, the microbial antagonistic effects of Hg and Se were utilized for wastewater treatment. We developed and optimized a new granular sludge approach to efficiently remove Hg(II) and Se(IV) from wastewater. Under anaerobic-oxic-anaerobic (AOA) conditions, the removal rates of Hg(II) and Se(IV) reached up to 99.91±0.07 % and 97.7 ± 0.8 %, respectively. The wastewater Hg(II) was mostly (97.43±0.01 %) converted to an inert mineral called tiemannite (HgSe) in the sludge, and no methylmercury (MeHg) was detected. The HgSe in sludge is less toxic, with almost no risk of secondary release, and it can be recovered with high purity. An inhibition experiment of mercury reduction and the high expression of the mer operon indicated that most Hg(II) (∼71 %) was first reduced to Hg0, and then Hg0 reacted with Se0 to synthesize HgSe. Metagenomic results showed that the final sludge (day 182) was dominated by two unclassified bacteria in the orders Rhodospirillales (27.7 %) and Xanthomonadales (6.3 %). Their metagenome-assembled genomes (MAGs) were recovered, suggesting that both of them can reduce Hg(II) and Se(IV). Metatranscriptomic analyses indicate that they can independently and cooperatively synthesize HgSe. In summary, granular sludge under AOA conditions is an efficient method for removing and recovering Hg from wastewater. The microbial transformation of Hg2+to Hg0 to HgSe may occur widely in both engineering and natural ecosystems.

Revisiting the mercury cycle in marine sediments: A potential multifaceted role for Desulfobacterota

Marine sediments impacted by urban and industrial pollutants are typically exposed to reducing conditions and represent major reservoirs of toxic mercury species. Mercury methylation mediated by anaerobic microorganisms is favored under such conditions, yet little is known about potential microbial mechanisms for mercury detoxification. We used culture-independent (metagenomics, metabarcoding) and culture-dependent approaches in anoxic marine sediments to identify microbial indicators of mercury pollution and analyze the distribution of genes involved in mercury reduction (merA) and demethylation (merB). While none of the isolates featured merB genes, 52 isolates, predominantly affiliated with Gammaproteobacteria, were merA positive. In contrast, merA genes detected in metagenomes were assigned to different phyla, including Desulfobacterota, Actinomycetota, Gemmatimonadota, Nitrospirota, and Pseudomonadota. This indicates a widespread capacity for mercury reduction in anoxic sediment microbiomes. Notably, merA genes were predominately identified in Desulfobacterota, a phylum previously associated only with mercury methylation. Marker genes involved in the latter process (hgcAB) were also mainly assigned to Desulfobacterota, implying a potential central and multifaceted role of this phylum in the mercury cycle. Network analysis revealed that Desulfobacterota were associated with anaerobic fermenters, methanogens and sulfur-oxidizers, indicating potential interactions between key players of the carbon, sulfur and mercury cycling in anoxic marine sediments.

Recent advance of microbial mercury methylation in the environment

Methylmercury formation is mainly driven by microbial-mediated process. The mechanism of microbial mercury methylation has become a crucial research topic for understanding methylation in the environment. Pioneering studies of microbial mercury methylation are focusing on functional strain isolation, microbial community composition characterization, and mechanism elucidation in various environments. Therefore, the functional genes of microbial mercury methylation, global isolations of Hg methylation strains, and their methylation potential were systematically analyzed, and methylators in typical environments were extensively reviewed. The main drivers (key physicochemical factors and microbiota) of microbial mercury methylation were summarized and discussed. Though significant progress on the mechanism of the Hg microbial methylation has been explored in recent decade, it is still limited in several aspects, including (1) molecular biology techniques for identifying methylators; (2) characterization methods for mercury methylation potential; and (3) complex environmental properties (environmental factors, complex communities, etc.). Accordingly, strategies for studying the Hg microbial methylation mechanism were proposed. These strategies include the following: (1) the development of new molecular biology methods to characterize methylation potential; (2) treating the environment as a micro-ecosystem and studying them from a holistic perspective to clearly understand mercury methylation; (3) a more reasonable and sensitive inhibition test needs to be considered. KEY POINTS: • Global Hg microbial methylation is phylogenetically and functionally discussed. • The main drivers of microbial methylation are compared in various condition. • Future study of Hg microbial methylation is proposed.

World war munitions as a source of mercury in the southwest Baltic Sea

Mercury (Hg) fulminate was used as a primary fuse in World War (WW) munitions, and may consequently be a Hg source for impacted environments. Mercury is a conspicuous and persistent pollutant, with methylmercury (MeHg) acting as a notorious neurotoxin. Considerable amounts of munitions were intentionally dumped in the North Sea and Baltic Sea following the First and Second WWs. After more than 70 years on the seafloor many munitions have corroded and likely release explosive compounds, including Hg fulminate. The Germany coastal city of Kiel was a manufacturing centre for submarines, and accordingly a prominent target for bombing and post-war disarmament. We collected water and sediment samples around Kiel Bay to assess regional levels and quantify any Hg contamination. The munition dump site Kolberger Heide (KH) and a former anti-aircraft training center Dänisch-Nienhof are situated in Kiel Bay, and were targeted for sampling. Sediment Hg concentrations around KH were notably elevated. Average Hg concentrations in KH sediments were 125 ± 76 ng/g, compared to 14 ± 18 ng/g at background (control) sites. In contrast, dissolved Hg in the water column exhibited no site variations, all ranging between 0.8 and 2.1 pM. Methylmercury in sediments and waters did not have enhanced concentrations amongst sites (<30 pg/g and <50 fM, respectively). Sediment-water exchange experiments showed elevated Hg and MeHg fluxes (i.e. >400 pmol m-2 d-1 MeHg) at one KH location, however remaining cores had low to no Hg and MeHg output (<0-27 pmol m-2 d-1 MeHg). Thus, sediments in Kiel Bay proximate to WW munitions could harbor and form a source of Hg, however water column mixing and removal processes attenuate any discharge from the seafloor to overlying waters.

Microbial community function and methylmercury production in oxygen-limited paddy soil

Methylmercury is a neurotoxic compound that can enter rice fields through rainfall or irrigation with contaminated wastewater, and then contaminate the human food chain through the consumption of rice. Flooded paddy soil has a porous structure that facilitates air exchange with the atmosphere, but the presence of trace amounts of oxygen in flooded rice field soil and its impact on microbial-mediated formation of methylmercury is still unclear. We compared the microbial communities and their functions in oxygen-depleted and oxygen-limited paddy soil. We discovered that oxygen-limited paddy soil had higher methylmercury concentration, which was strongly correlated with soil properties and methylation potential. Compared with oxygen-depleted soil, oxygen-limited soil altered the microbial composition based on 16 S rRNA sequences, but not based on hgcA sequences. Moreover, oxygen-limited soil enhanced microbial activity significantly, increasing the abundance of more than half of the KEGG pathways, especially the metabolic pathways that might be involved in methylation. Our study unveils how microbial communities influence methylmercury formation in oxygen-limited paddy soil. ENVIRONMENTAL IMPLICATIONS: This study examined how low oxygen input affects microbial-induced MeHg formation in anaerobic paddy soil. We found that oxygen-limited soil produced more MeHg than oxygen-depleted soil. Oxygen input altered the microbial community structure of 16 S rRNA sequencing in anaerobic paddy soil, but had little impact on the hgcA sequencing community structure. Microbial activity and metabolic functions related to MeHg formation were also higher in oxygen-limited paddy soil. We suggest that oxygen may not be a limiting factor for Hg methylators, and that insufficient oxygen input in flooded paddy soil increases the risk of human exposure to MeHg from rice consumption.

Assessing comparability and uncertainty of analytical methods for methylated mercury species in seawater

BACKGROUND

The relative distribution and importance of monomethylmercury (MMHg) and dimethylmercury (DMHg) in seawater is still under debate. A lack of comparability between measurements at sub-picomolar levels hampered the further understanding of the biogeochemical Hg cycle. To overcome this, we assessed the relative standard measurement uncertainties (Uex,r) for direct measurements of MMHg and DMHg by species-specific isotope dilution ICP-MS and cryo-focusing GC-ICP-MS at femtomolar concentrations. Furthermore, Uex,r was determined for the indirect determination of DMHg (DMHgcalc = MeHg - MMHg) and MeHg (MeHgcalc = MMHg + DMHg) to compare the two methodologies.

RESULTS

Expanded Uex,r (confidence interval of 95%) for cryo-focusing GC-ICP-MS was 14.4 (<50 fM) and 14.2% (>50 fM) and for SS-ID GC-ICP-MS 5.6 (<50 fM) and 3.7% (>50 fM). For concentrations above 50 fM, Uex,r for DMHgcalc was always lower than for direct measurements (14.2%). For MeHgcalc, on the other hand, Uex,r was always higher for concentrations above 115 fM (range: 3.7-13.9%) than for direct measurements (3.7%). We evaluated the comparability of directly measured and calculated DMHg and MeHg concentrations based on Hg speciation measurements for two vertical profiles in the Mediterranean Sea. We show that directly measured and indirectly determined DMHg and MeHg concentrations yield comparable results.

SIGNIFICANCE

Our results validate the application of the indirect method for the determination of DMHg if a direct measurement method with a low Uex,r such as isotope dilution is used for MMHg and MeHg measurements. The validation of the indirect measurement approach opens new possibilities to generate more precise and accurate DMHg data in the global ocean.

Gaseous mercury exchange between air and highly dynamic tidal flats: A laboratory incubation experiment

Gaseous mercury (mainly elemental mercury, Hg(0)) exchange between air and Earth's surfaces is one of the most critical fluxes governing global Hg cycle. As an important and unique part of intertidal ecosystem, tidal flat is characterized by periodic inundation and exposure due to tidal cycle, generating varying hydrological, photochemical and biogeochemical processes. However, quantitative and mechanistic understanding of Hg(0) dynamics between air and exceptionally dynamic tide flats has remained limited to date. In this study, we select five representative tidal flat sediments from typical coastal habits of Chinese coastlines to perform laboratory incubation experiments for deciphering the effect of the interaction of tidal cycle and solar radiation on Hg(0) dynamics over tidal flats with different sediment compositions. We show that sediment Hg concentration, tidal cycle and solar radiation collectively modulate the air-surface Hg(0) exchange over tidal flats and highlight that the photochemistry dominates the Hg(0) production and emission over tidal flats. We find that the daytime inundation presents highest Hg(0) emission fluxes for Hg-poor sediment, but the daytime exposure is the hot moment of Hg(0) emission from Hg-rich sediments and substantially contributes to daily Hg(0) emission fluxes. In the treatment to mimic semidiurnal tide, the daily Hg(0) fluxes are positively correlated to sediment Hg concentrations. Combining our mechanistic insights on air-surface Hg(0) exchange over tidal flats and related data and knowledge reported by other studies, we discuss the implications of our study for field measurement and model development of Hg(0) dynamics over highly dynamic tidal flats. We conclude that the air-surface Hg(0) dynamics over tidal flats are extremely complex and highly variable, and a greater understanding the interactions between natural processes, human impacts and climate forcings will better constrain current and future Hg biogeochemical cycle in global tidal flats.

Geochemical mercury pools regulate diverse communities of hgcA microbes and MeHg levels in paddy soils

Rice paddies are unique artificial wetlands generating methylmercury (MeHg), a highly potent neurotoxin. However, the impact of diverse mercury (Hg) pools on the Hg-methylating communities during rice growth is unclear. This study investigates soil treated with five mercury forms (HgCl2, α-HgS, β-HgS, nano-HgS, and Hg-DOM) at two levels (5 mg/kg and 50 mg/kg). The results showed a varying abundance of sulphate-reducing bacteria, Geobacteraceae, methanogens, and hgcA microbes in the soils across rice grown under different mercury treatments and concentrations. Soils treated with HgCl2, nano-HgS and β-HgS had higher than average levels of hgcA-methanogen abundance, and the abundance significantly and positively correlated with MeHg concentration in all samples (p < 0.05). The shifting trends in Hg-methylating microbial structure following treatment with α-HgS, β-HgS, nano-HgS and Hg-DOM at both 5 and 50 mg/kg Hg levels were diverse compared with the control group. HgCl2 treatment showed contrasting trends in community distribution of Hg methylators at 5 and 50 mg/kg Hg levels during rice growth. Dissolved organic carbon, redox potential and sulphate levels significantly correlated with variation in the Hg-methylating microbial community structure and MeHg production in soils.

Metabolically diverse microorganisms mediate methylmercury formation under nitrate-reducing conditions in a dynamic hydroelectric reservoir

Brownlee Reservoir is a mercury (Hg)-impaired hydroelectric reservoir that exhibits dynamic hydrological and geochemical conditions and is located within the Hells Canyon Complex in Idaho, USA. Methylmercury (MeHg) contamination in fish is a concern in the reservoir. While MeHg production has historically been attributed to sulfate-reducing bacteria and methanogenic archaea, microorganisms carrying the hgcA gene are taxonomically and metabolically diverse and the major biogeochemical cycles driving mercury (Hg) methylation are not well understood. In this study, Hg speciation and redox-active compounds were measured throughout Brownlee Reservoir across the stratified period in four consecutive years (2016-2019) to identify the location where and redox conditions under which MeHg is produced. Metagenomic sequencing was performed on a subset of samples to characterize the microbial community with hgcA and identify possible links between biogeochemical cycles and MeHg production. Biogeochemical profiles suggested in situ water column Hg methylation was the major source of MeHg. These profiles, combined with genome-resolved metagenomics focused on hgcA-carrying microbes, indicated that MeHg production occurs in this system under nitrate- or manganese-reducing conditions, which were previously thought to preclude Hg-methylation. Using this multidisciplinary approach, we identified the cascading effects of interannual variability in hydrology on the redox status, microbial metabolic strategies, abundance and metabolic diversity of Hg methylators, and ultimately MeHg concentrations throughout the reservoir. This work expands the known conditions conducive to producing MeHg and suggests that the Hg-methylation mitigation efforts by nitrate or manganese amendment may be unsuccessful in some locations.

Non-conservative mixing behaviors of mercury in subterranean estuary: Coupling effect of hydrological and biogeochemical processes and implications for rapidly changing world

Coastal ecosystem is an important zone of mercury (Hg) storage and hotspot of neurotoxic methylmercury (MMHg) production and bioaccumulation. The releases of Hg from coastal aquifer or subterranean estuary (STE) via submarine groundwater discharge (SGD) to coastal waters provide an important source of Hg from land to seas. However, the transport and biogeochemical transformation of Hg in STEs are less quantified. In this study, we documented total Hg (THg) and MMHg dynamics in two distinct STEs (a sandflat at an open coast versus a mudflat at a bay) during two different seasons (December versus August) in the temperate coast of northern China, and further applied the salinity-based conservative mixing model (CMM) to quantify the coupling effect of hydrological and biogeochemical processes on STE Hg cycle. Our field data presented large variations of THg and MMHg concentrations and%MMHg/THg of groundwater and sediment in both STEs over time and space. The CMM results clearly displayed substantial divergences of dissolved THg and MMHg from salinity in groundwater between sites and seasons, and the concentration and percent deviations in the Hg-rich mudflat were significantly higher than those in the Hg-poor sandflat. Our findings indicate the non-conservative mixing behaviors of Hg along the groundwater flow paths of both STEs, and the Hg-rich intertidal zone could be hotspot for the production and source of dissolved THg and MMHg to coastal waters via SGD. Our results provide field evidence to highlight that the hydrological shifts and biogeochemical processes collectively drive complex transport and biogeochemical transformation of Hg in STEs. The non-conservative mixing behaviors of Hg in STEs also highlight that, for more accurately calculating SGD-derived Hg fluxes to coastal seas, we need to carefully select the groundwater zonation of STE to better represent the output endmember. Our findings also address that human activities and climate change will profoundly alter the Hg biogeochemical cycle and toxicology in global coastal aquifers.

Coastal streams and sewage outfalls: Hot spots of mercury discharge, pollution and cycling in nearshore environments

The coastal streams (CSs) and sewage outfalls (SOs) are widely distributed and direct anthropogenic stress on global coastal ecosystems. However, the CS/SO-associated mercury (Hg) discharge, pollution and cycle in nearshore environment are less quantified. Here, we report that total Hg (THg) and methylmercury (MMHg) concentrations in waters of CSs (n = 8) and SOs (n = 15) of the northern China were ∼102 to 103 times of coastal surface waters and 10 to 102 times of major rivers in China and other regions. The CS/SO discharges resulted in the increase of total organic carbon (TOC) contents, THg and MMHg concentrations and TOC-normalized THg and MMHg concentrations in sediments of CS/SO-impacted coasts. The laboratory experiments further illustrated that the CS/SO-impacted sediments characterized with high potentials of dissolved THg and MMHg productions and releases. Our findings indicate that the layout optimization of SOs is able to reduce the Hg risk in coastal environment.

Variation in habitat use and its consequences for mercury exposure in two Eastern Ontario bat species, Myotis lucifugus and Eptesicus fuscus

The St. Lawrence River in Eastern Ontario, Canada, has been a designated an area of concern due to past industrial contamination of sediment in some areas and transport of mercury from tributaries. Previous research using bats as sentinel species identified elevated concentrations of total mercury (THg) in fur of local bats and species-specific variation between little brown bats (Myotis lucifugus) and big brown bats (Eptesicus fuscus). Here, we investigated the mercury exposure pathways for these two species by testing the hypothesis that diet variation, particularly the reliance on aquatic over terrestrial insects, is a determinant of local bat mercury concentrations. We analyzed THg concentration and stable isotope ratios of δ15N and δ13C in fur of little and big brown bats, and in aquatic and terrestrial insects. Big brown bats, especially males, accumulated significantly higher THg concentrations in their fur compared to little brown bats. However, this difference was not related to diet because big brown bats consumed terrestrial insects, which were lower in mercury than aquatic insects, the primary prey for little brown bats. We also evaluated whether fur THg concentrations translate into molecular changes in tissues linked to (methyl)mercury toxicity by quantifying tissue changes in global DNA methylation and mitochondrial DNA abundance. No significant changes in DNA molecular markers were observed in relation to fur THg concentration, suggesting mercury exposure to local bats did not impact molecular level changes at the DNA level. Higher mercury in bats was not associated with local aquatic contamination or genotoxicity in this study area.

Redox gradient shapes the abundance and diversity of mercury-methylating microorganisms along the water column of the Black Sea

In the global context of seawater deoxygenation triggered by climate change and anthropogenic activities, changes in redox gradients impacting biogeochemical transformations of pollutants, such as mercury, become more likely. Being the largest anoxic basin worldwide, with high concentrations of the potent neurotoxic methylmercury (MeHg), the Black Sea is an ideal natural laboratory to provide new insights about the link between dissolved oxygen concentration and hgcAB gene-carrying (hgc+) microorganisms involved in the formation of MeHg. We combined geochemical and microbial approaches to assess the effect of vertical redox gradients on abundance, diversity, and metabolic potential of hgc+ microorganisms in the Black Sea water column. The abundance of hgcA genes [congruently estimated by quantitative PCR (qPCR) and metagenomics] correlated with MeHg concentration, both maximal in the upper part of the anoxic water. Besides the predominant Desulfobacterales, hgc+ microorganisms belonged to a unique assemblage of diverse-previously underappreciated-anaerobic fermenters from Anaerolineales, Phycisphaerae (characteristic of the anoxic and sulfidic zone), Kiritimatiellales, and Bacteroidales (characteristic of the suboxic zone). The metabolic versatility of Desulfobacterota differed from strict sulfate reduction in the anoxic water to reduction of various electron acceptors in the suboxic water. Linking microbial activity and contaminant concentration in environmental studies is rare due to the complexity of biological pathways. In this study, we disentangle the role of oxygen in shaping the distribution of Hg-methylating microorganisms consistently with MeHg concentration, and we highlight their taxonomic and metabolic niche partitioning across redox gradients, improving the prediction of the response of marine communities to the expansion of oxygen-deficient zones. IMPORTANCE Methylmercury (MeHg) is a neurotoxin detected at high concentrations in certain marine ecosystems, posing a threat to human health. MeHg production is mainly mediated by hgcAB gene-carrying (hgc+) microorganisms. Oxygen is one of the main factors controlling Hg methylation; however, its effect on the diversity and ecology of hgc+ microorganisms remains unknown. Under the current context of seawater deoxygenation, mercury cycling is expected to be disturbed. Here, we show the strong effect of oxygen gradients on the distribution of potential Hg methylators. In addition, we show for the first time the significant contribution of a unique assemblage of potential fermenters from Anaerolineales, Phycisphaerae, and Kiritimatiellales to Hg methylation, stratified in different redox niches along the Black Sea gradient. Our results considerably expand the known taxonomic diversity and ecological niches prone to the formation of MeHg and contribute to better apprehend the consequences of oxygen depletion in seawater.

Interaction of Naturally Occurring Phytoplankton with the Biogeochemical Cycling of Mercury in Aquatic Environments and Its Effects on Global Hg Pollution and Public Health

The biogeochemical cycling of mercury in aquatic environments is a complex process driven by various factors, such as ambient temperature, seasonal variations, methylating bacteria activity, dissolved oxygen levels, and Hg interaction with dissolved organic matter (DOM). As a consequence, part of the Hg contamination from anthropogenic activity that was buried in sediments is reinserted into water columns mainly in highly toxic organic Hg forms (methylmercury, dimethylmercury, etc.). This is especially prominent in the coastal shallow waters of industrial regions worldwide. The main entrance point of these highly toxic Hg forms in the aquatic food web is the naturally occurring phytoplankton. Hg availability, intake, effect on population size, cell toxicity, eventual biotransformation, and intracellular stability in phytoplankton are of the greatest importance for human health, having in mind that such Hg incorporated inside the phytoplankton cells due to biomagnification effects eventually ends up in aquatic wildlife, fish, seafood, and in the human diet. This review summarizes recent findings on the topic of organic Hg form interaction with natural phytoplankton and offers new insight into the matter with possible directions of future research for the prevention of Hg biomagnification in the scope of climate change and global pollution increase scenarios.

Mining-impacted rice paddies select for Archaeal methylators and reveal a putative (Archaeal) regulator of mercury methylation

Methylmercury (MeHg) is a microbially produced neurotoxin derived from inorganic mercury (Hg), which accumulation in rice represents a major health concern to humans. However, the microbial control of MeHg dynamics in the environment remains elusive. Here, leveraging three rice paddy fields with distinct concentrations of Hg (Total Hg (THg): 0.21-513 mg kg-1 dry wt. soil; MeHg: 1.21-6.82 ng g-1 dry wt. soil), we resorted to metagenomics to determine the microbial determinants involved in MeHg production under contrasted contamination settings. We show that Hg methylating Archaea, along with methane-cycling genes, were enriched in severely contaminated paddy soils. Metagenome-resolved Genomes of novel putative Hg methylators belonging to Nitrospinota (UBA7883), with poorly resolved taxonomy despite high completeness, showed evidence of facultative anaerobic metabolism and adaptations to fluctuating redox potential. Furthermore, we found evidence of environmental filtering effects that influenced the phylogenies of not only hgcA genes under different THg concentrations, but also of two housekeeping genes, rpoB and glnA, highlighting the need for further experimental validation of whether THg drives the evolution of hgcAB. Finally, assessment of the genomic environment surrounding hgcAB suggests that this gene pair may be regulated by an archaeal toxin-antitoxin (TA) system, instead of the more frequently found arsR-like genes in bacterial methylators. This suggests the presence of distinct hgcAB regulation systems in bacteria and archaea. Our results support the emerging role of Archaea in MeHg cycling under mining-impacted environments and shed light on the differential control of the expression of genes involved in MeHg formation between Archaea and Bacteria.

Mercury and Sulfur Redox Cycling Affect Methylmercury Levels in Rice Paddy Soils across a Contamination Gradient

Methylmercury (MeHg) contamination in rice via paddy soils is an emerging global environmental issue. An understanding of mercury (Hg) transformation processes in paddy soils is urgently needed in order to control Hg contamination of human food and related health impacts. Sulfur (S)-regulated Hg transformation is one important process that controls Hg cycling in agricultural fields. In this study, Hg transformation processes, such as methylation, demethylation, oxidation, and reduction, and their responses to S input (sulfate and thiosulfate) in paddy soils with a Hg contamination gradient were elucidated simultaneously using a multi-compound-specific isotope labeling technique (200HgII, Me198Hg, and 202Hg0). In addition to HgII methylation and MeHg demethylation, this study revealed that microbially mediated reduction of HgII, methylation of Hg0, and oxidative demethylation-reduction of MeHg occurred under dark conditions; these processes served to transform Hg between different species (Hg0, HgII, and MeHg) in flooded paddy soils. Rapid redox recycling of Hg species contributed to Hg speciation resetting, which promoted the transformation between Hg0 and MeHg by generating bioavailable HgII for fuel methylation. Sulfur input also likely affected the microbial community structure and functional profile of HgII methylators and, therefore, influenced HgII methylation. The findings of this study contribute to our understanding of Hg transformation processes in paddy soils and provide much-needed knowledge for assessing Hg risks in hydrological fluctuation-regulated ecosystems.

The Combined Effect of Hg(II) Speciation, Thiol Metabolism, and Cell Physiology on Methylmercury Formation by Geobacter sulfurreducens

The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0-2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)₂ complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)₂, to complexes with lower availability, Hg(PEN)₂, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2-6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.

Global change effects on biogeochemical mercury cycling

Past and present anthropogenic mercury (Hg) release to ecosystems causes neurotoxicity and cardiovascular disease in humans with an estimated economic cost of $117 billion USD annually. Humans are primarily exposed to Hg via the consumption of contaminated freshwater and marine fish. The UNEP Minamata Convention on Hg aims to curb Hg release to the environment and is accompanied by global Hg monitoring efforts to track its success. The biogeochemical Hg cycle is a complex cascade of release, dispersal, transformation and bio-uptake processes that link Hg sources to Hg exposure. Global change interacts with the Hg cycle by impacting the physical, biogeochemical and ecological factors that control these processes. In this review we examine how global change such as biome shifts, deforestation, permafrost thaw or ocean stratification will alter Hg cycling and exposure. Based on past declines in Hg release and environmental levels, we expect that future policy impacts should be distinguishable from global change effects at the regional and global scales.

A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

Recent studies demonstrate a short 3-6-month atmospheric lifetime for mercury (Hg). This implies Hg emissions are predominantly deposited within the same hemisphere in which they are emitted, thus placing increasing importance on considering Hg sources, sinks and impacts from a hemispheric perspective. In the absence of comprehensive Hg data from the Southern Hemisphere (SH), estimates and inventories for the SH have been drawn from data collected in the NH, with the assumption that the NH data are broadly applicable. In this paper, we centre the uniqueness of the SH in the context of natural biogeochemical Hg cycling, with focus on the midlatitudes and tropics. Due to its uniqueness, Antarctica warrants an exclusive review of its contribution to the biogeochemical cycling of Hg and is therefore excluded from this review. We identify and describe five key natural differences between the hemispheres that affect the biogeochemical cycling of Hg: biome heterogeneity, vegetation type, ocean area, methylation hotspot zones and occurence of volcanic activities. We review the current state of knowledge of SH Hg cycling within the context of each difference, as well as the key gaps that impede our understanding of natural Hg cycling in the SH. The differences demonstrate the limitations in using NH data to infer Hg processes and emissions in the SH.

Mercury in wetlands over 60 years: Research progress and emerging trends

Wetlands are considered the hotspots for mercury (Hg) biogeochemistry, garnering global attention. Therefore, it is important to review the research progress in this field and predict future frontiers. To achieve that, we conducted a literature analysis by collecting 15,813 publications about Hg in wetlands from the Web of Science Core Collection. The focus of wetland Hg research has changed dramatically over time: 1) In the initial stage (i.e., 1959-1990), research mainly focused on investigating the sources and contents of Hg in wetland environments and fish. 2) For the next 20 years (i.e., 1991-2010), Hg transformation (e.g., Hg reduction and methylation) and environmental factors that affect Hg bioaccumulation have attracted extensive attention. 3) In the recent years of 2011-2022, hot topics in Hg study include microbial Hg methylators, Hg bioavailability, methylmercury (MeHg) demethylation, Hg stable isotope, and Hg cycling in paddy fields. Finally, we put forward future research priorities, i.e., 1) clarifying the primary factors controlling MeHg production, 2) uncovering the MeHg demethylation process, 3) elucidating MeHg bioaccumulation process to better predict its risk, and 4) recognizing the role of wetlands in Hg circulation. This research shows a comprehensive knowledge map for wetland Hg research and suggests avenues for future studies.

"Trojan Horse" Type Internalization Increases the Bioavailability of Mercury Sulfide Nanoparticles and Methylation after Intracellular Dissolution

Mercury sulfide nanoparticles (HgS_NP), as natural metal-containing nanoparticles, are the dominant Hg species in anoxic zones. Although the microbial Hg methylation of HgS_NP has been previously reported, the importance of this process in Hg methylation has yet to be clarified due to the lack of knowledge on the internalization and transformation of HgS_NP. Here, we investigated the internalization and transformation of HgS_NP in microbial methylator Geobacter sulfurreducens PCA through total Hg analysis and different Hg species quantification in medium and cytoplasm. We found that the microbial uptake of HgS_NP, via a passive diffusion pathway, was significantly higher than that of the Hg²⁺-dissolved organic matter (Hg²⁺-DOM) complex. Internalized HgS_NP were dissolved to Hg²⁺ in cytoplasm with a maximal dissolution of 41%, suggesting a "Trojan horse" mechanism. The intracellular Hg²⁺ from HgS_NP exposure at the initial stage (8 h) was higher than that in Hg²⁺-DOM group, which led to higher methylation of HgS_NP. Furthermore, no differences in methylmercury (MeHg) production from HgS_NP were observed between the hgcAB gene knockout (ΔhgcAB) and wild-type strains, suggesting that HgS_NP methylation may occur through HgcAB-independent pathways. Considering the possibility of a broad range of hgcAB-lacking microbes serving as methylators for HgS_NP and the ubiquity of HgS_NP in anoxic environments, this study highlights the importance of HgS_NP internalization and methylation in MeHg production and demonstrates the necessity of understanding the assimilation and transformation of nutrient and toxic metal nanoparticles in general.

Using mercury stable isotope fractionation to identify the contribution of historical mercury mining sources present in downstream water, sediment and fish

Ecosystems downstream of mercury (Hg) contaminated sites can be impacted by both localized releases as well as Hg deposited to the watershed from atmospheric transport. Identifying the source of Hg in water, sediment, and fish downstream of contaminated sites is important for determining the effectiveness of source-control remediation actions. This study uses measurements of Hg stable isotopes in soil, sediment, water, and fish to differentiate between Hg from an abandoned Hg mine from non-mine-related sources. The study site is located within the Willamette River watershed (Oregon, United States), which includes free-flowing river segments and a reservoir downstream of the mine. The concentrations of total-Hg (THg) in the reservoir fish were 4-fold higher than those further downstream (>90 km) from the mine site in free-flowing sections of the river. Mercury stable isotope fractionation analysis showed that the mine tailings (δ²⁰²Hg: -0.36‰ ± 0.03‰) had a distinctive isotopic composition compared to background soils (δ²⁰²Hg: -2.30‰ ± 0.25‰). Similar differences in isotopic composition were observed between stream water that flowed through the tailings (particulate bound δ²⁰²Hg: -0.58‰; dissolved: -0.91‰) versus a background stream (particle-bound δ²⁰²Hg: -2.36‰; dissolved: -2.09‰). Within the reservoir sediment, the Hg isotopic composition indicated that the proportion of the Hg related to mine-release increased with THg concentrations. However, in the fish samples the opposite trend was observed-the degree of mine-related Hg was lower in fish with the higher THg concentrations. While sediment concentrations clearly show the influence of the mine, the relationship in fish is more complicated due to differences in methylmercury (MeHg) formation and the foraging behavior of different fish species. The fish tissue δ¹³C and Δ¹⁹⁹Hg values indicate that there is a higher influence of mine-sourced Hg in fish feeding in a more sediment-based food web and less so in planktonic and littoral-based food webs. Identifying the relative proportion of Hg from local contaminated site can help inform remediation decisions, especially when the relationship between total Hg concentrations and sources do not show similar covariation between abiotic and biotic media.

Anaerobic mercury methylators inhabit sinking particles of oxic water columns

Increased concentration of mercury, particularly methylmercury, in the environment is a worldwide concern because of its toxicity in severely exposed humans. Although the formation of methylmercury in oxic water columns has been previously suggested, there is no evidence of the presence of microorganisms able to perform this process, using the hgcAB gene pair (hgc⁺ microorganisms), in such environments. Here we show the prevalence of hgc⁺ microorganisms in sinking particles of the oxic water column of Lake Geneva (Switzerland and France) and its anoxic bottom sediments. Compared to anoxic sediments, sinking particles found in oxic waters exhibited relatively high proportion of hgc⁺genes taxonomically assigned to Firmicutes. In contrast hgc⁺members from Nitrospirae, Chloroflexota and PVC superphylum were prevalent in anoxic sediment while hgc⁺ Desulfobacterota were found in both environments. Altogether, the description of the diversity of putative mercury methylators in the oxic water column expand our understanding on MeHg formation in aquatic environments and at a global scale.

The relationship between zooplankton vertical distribution and the concentration of aqueous Hg in boreal lakes: A comparative field study

The production of the highly toxic monomethylmercury (MeHg) is heterogenous throughout the water column. Multiple factors have been identified to significantly affect this process, such as an extended anoxic water layer and a deep-water phytoplankton maximum. However, the role of water column heterogeneity on mercury (Hg) cycling is still poorly known, especially concerning the role of zooplankton grazers. Here, four boreal lakes with contrasting characteristics were sampled (i.e., transparency and the presence/absence of fish) at both day and night in order to maximize the heterogeneity in zooplankton abundance both among and within lakes, and to investigate their potential links with Hg vertical heterogeneity. Diel variation of the concentrations of both dissolved total Hg (DTHg) and total Hg (THg) were observed, with night samples significantly higher than day samples. Although this pattern was not related to diel changes in the vertical distribution of zooplankton, results showed that the presence of large copepods (>1.2 mm) and medium-sized (0.6 to 1.2 mm) cladocerans was significantly associated with lower concentrations of DTHg in the water at a given depth, whereas the presence of medium-sized copepods was significantly associated with the concentration of THg. The presence of cladocerans was significantly associated with the ratio between the dissolved MeHg and DTHg (conventionally used as a proxy of methylation potential). Phytoplankton biomass was directly correlated with the concentration of both dissolved and total MeHg and the methylation potential. At the same time, phytoplankton biomass was inversely related to the fraction of DTHg. These results suggest a potential key role of the heterogeneity of biotic factors in the water column, especially of phytoplankton and zooplankton, in the cycling of total Hg and MeHg in boreal lakes.

Effects of methylmercury and food stress on migratory activity in song sparrows, Melospiza melodia

Avian migration is a challenging life stage susceptible to the adverse effects of stressors, including contaminants like methylmercury (MeHg). Although birds often experience stressors and contaminants concurrently in the wild, no study to date has investigated how simultaneous exposure to MeHg and food stress affects migratory behavior. Our objectives were to determine if MeHg or food stress exposure during summer, alone or combined, has carry-over effects on autumn migratory activity, and if hormone levels (corticosterone, thyroxine) and body condition were related to these effects. We tested how exposure to dietary MeHg and/or food stress (unpredictable temporary food removal) affected migratory behavior in captive song sparrows, Melospiza melodia. Nocturnal activity was influenced by a 3-way interaction between MeHg × stress × nights of the study, indicating that activity changed over time in different ways depending on prior treatments. Thyroxine was not affected by treatment or sampling date. During the migratory season, fecal corticosterone metabolite concentrations increased in birds co-exposed to MeHg and food stress compared to controls, suggesting an additive carry-over effect. As well, during the period of behavioral recording, body condition increased with time in unstressed birds, but not in stressed birds. Fecal corticosterone metabolite concentrations were positively correlated to duration of nocturnal activity, but thyroxine levels and body condition were not. The differences in nocturnal activity between groups suggest that food stress and MeHg exposure on breeding grounds could have direct and indirect carry-over effects that have the potential to affect the fall migration journey.

Long-term mercury contamination does not affect the microbial gene potential for C and N cycling in soils but enhances detoxification gene abundance

Soil microorganisms are key transformers of mercury (Hg), a toxic and widespread pollutant. It remains uncertain, however, how long-term exposure to Hg affects crucial microbial functions, such as litter decomposition and nitrogen cycling. Here, we used a metagenomic approach to investigate the state of soil functions in an agricultural floodplain contaminated with Hg for more than 80 years. We sampled soils along a gradient of Hg contamination (high, moderate, low). Hg concentrations at the highly contaminated site (36 mg kg–1 dry soil on average) were approximately 10 times higher than at the moderately contaminated site (3 mg kg–1 dry soil) and more than 100 times higher than at the site with low contamination (0.25 mg kg–1 dry soil; corresponding to the natural background concentration in Switzerland). The analysis of the CAZy and NCyc databases showed that carbon and nitrogen cycling was not strongly affected with high Hg concentrations, although a significant change in the beta-diversity of the predicted genes was observed. The only functional classes from the CAZy database that were significantly positively overrepresented under higher Hg concentrations were genes involved in pectin degradation, and from the NCyc database dissimilatory nitrate reduction and N-fixation. When comparing between low and high Hg concentrations the genes of the EggNOG functional category of inorganic ion transport and metabolism, two genes encoding Hg transport proteins and one gene involved in heavy metal transport detoxification were among those that were highly significantly overrepresented. A look at genes specifically involved in detoxification of Hg species, such as the mer and hgc genes, showed a significant overrepresentation when Hg contamination was increased. Normalized counts of these genes revealed a dominant role for the phylum Proteobacteria. In particular, most counts for almost all mer genes were found in Betaproteobacteria. In contrast, hgc genes were most abundant in Desulfuromonadales. Overall, we conclude from this metagenomic analysis that long-term exposure to high Hg triggers shifts in the functional beta-diversity of the predicted microbial genes, but we do not see a dramatic change or breakdown in functional capabilities, but rather functional redundancy.

Expression Levels of hgcAB Genes and Mercury Availability Jointly Explain Methylmercury Formation in Stratified Brackish Waters

Neurotoxic methylmercury (MeHg) is formed by microbial methylation of inorganic divalent Hg (Hg^II) and constitutes severe environmental and human health risks. The methylation is enabled by hgcA and hgcB genes, but it is not known if the associated molecular-level processes are rate-limiting or enable accurate prediction of MeHg formation in nature. In this study, we investigated the relationships between hgc genes and MeHg across redox-stratified water columns in the brackish Baltic Sea. We showed, for the first time, that hgc transcript abundance and the concentration of dissolved Hg^II-sulfide species were strong predictors of both the Hg^II methylation rate and MeHg concentration, implying their roles as principal joint drivers of MeHg formation in these systems. Additionally, we characterized the metabolic capacities of hgc⁺ microorganisms by reconstructing their genomes from metagenomes (i.e., hgc⁺ MAGs), which highlighted the versatility of putative Hg^II methylators in the water column of the Baltic Sea. In establishing relationships between hgc transcripts and the Hg^II methylation rate, we advance the fundamental understanding of mechanistic principles governing MeHg formation in nature and enable refined predictions of MeHg levels in coastal seas in response to the accelerating spread of oxygen-deficient zones.

Decadal trends of mercury cycling and bioaccumulation within Everglades National Park

Mercury (Hg) contamination has been a persistent concern in the Florida Everglades for over three decades due to elevated atmospheric deposition and the system's propensity for methylation and rapid bioaccumulation. Given declines in atmospheric Hg concentrations in the conterminous United States and efforts to mitigate nutrient release to the greater Everglades ecosystem, it was vital to assess how Hg dynamics responded on temporal and spatial scales. This study used a multimedia approach (water and biota) to examine Hg and methylmercury (MeHg) dynamics across a 76-site network within the southernmost portion of the region, Everglades National Park (ENP), from 2008 to 2018. Hg concentrations across matrices showed that air, water, and biota from the system were inextricably linked. Temporal patterns across matrices were driven primarily by hydrologic and climatic changes in the park and no evidence of a decline in atmospheric Hg deposition from 2008 to 2018 was observed, unlike other regions of the United States. In the Shark River Slough (SRS), excess dissolved organic carbon and sulfate were also consistently delivered from upgradient canals and showed no evidence of decline over the study period. Within the SRS a strong positive correlation was observed between MeHg concentrations in surface water and resident fish. Within distinct geographic regions of ENP (SRS, Marsh, Coastal), the geochemical controls on MeHg dynamics differed and highlighted regions susceptible to higher MeHg bioaccumulation, particularly in the SRS and Coastal regions. This study demonstrates the strong influence that dissolved organic carbon and sulfate loads have on spatial and temporal distributions of MeHg across ENP. Importantly, improved water quality and flow rates are two key restoration targets of the nearly 30-year Everglades restoration program, which if achieved, this study suggests would lead to reduced MeHg production and exposure.

Understanding the risks of mercury sulfide nanoparticles in the environment: Formation, presence, and environmental behaviors

Mercury (Hg) could be microbially methylated to the bioaccumulative neurotoxin methylmercury (MeHg), raising health concerns. Understanding the methylation of various Hg species is thus critical in predicting the MeHg risk. Among the known Hg species, mercury sulfide (HgS) is the largest Hg reservoir in the lithosphere and has long been considered to be highly inert. However, with advances in the analytical methods of nanoparticles, HgS nanoparticles (HgS NPs) have recently been detected in various environmental matrices or organisms. Furthermore, pioneering laboratory studies have reported the high bioavailability of HgS NPs. The formation, presence, and transformation (e.g., methylation) of HgS NPs are intricately related to several environmental factors, especially dissolved organic matter (DOM). The complexity of the behavior of HgS NPs and the heterogeneity of DOM prevent us from comprehensively understanding and predicting the risk of HgS NPs. To reveal the role of HgS NPs in Hg biogeochemical cycling, research needs should focus on the following aspects: the formation pathways, the presence, and the environmental behaviors of HgS NPs impacted by the dominant influential factor of DOM. We thus summarized the latest progress in these aspects and proposed future research priorities, e.g., developing the detection techniques of HgS NPs and probing HgS NPs in various matrices, further exploring the interactions between DOM and HgS NPs. Besides, as most of the previous studies were conducted in laboratories, our current knowledge should be further refreshed through field observations, which would help to gain better insights into predicting the Hg risks in natural environment.

A consensus protocol for the recovery of mercury methylation genes from metagenomes

Mercury (Hg) methylation genes (hgcAB) mediate the formation of the toxic methylmercury and have been identified from diverse environments, including freshwater and marine ecosystems, Arctic permafrost, forest and paddy soils, coal-ash amended sediments, chlor-alkali plants discharges and geothermal springs. Here we present the first attempt at a standardized protocol for the detection, identification and quantification of hgc genes from metagenomes. Our Hg-MATE (Hg-cycling Microorganisms in Aquatic and Terrestrial Ecosystems) database, a catalogue of hgc genes, provides the most accurate information to date on the taxonomic identity and functional/metabolic attributes of microorganisms responsible for Hg methylation in the environment. Furthermore, we introduce "marky-coco", a ready-to-use bioinformatic pipeline based on de novo single-metagenome assembly, for easy and accurate characterization of hgc genes from environmental samples. We compared the recovery of hgc genes from environmental metagenomes using the marky-coco pipeline with an approach based on co-assembly of multiple metagenomes. Our data show similar efficiency in both approaches for most environments except those with high diversity (i.e., paddy soils) for which a co-assembly approach was preferred. Finally, we discuss the definition of true hgc genes and methods to normalize hgc gene counts from metagenomes.

Chronic Toxic Effects of Waterborne Mercury on Silver Carp (Hypophthalmichthys molitrix) Larvae

Mercury (Hg) is a kind of heavy metal pollutant widely existing in the aquatic environment, and it is also recognized to have a highly toxic effect on fish. In this study, silver carp (Hypophthalmichthys molitrix) larvae were exposed to 0 (control), 1, 5, and 10 μg/L Hg2+ for 2 weeks. Antioxidant ability, neurotoxicity, and thyroid hormones (THs) content were evaluated. In comparison with the control, the superoxide dismutase (SOD) activity and the glutathione (GSH) activity were lower in silver carp exposed to 10 μg/L Hg2+. The lowest catalase (CAT) activity was found in the 10 μg/L Hg2+, while malondialdehyde (MDA) content was not significantly different among all groups. Compared with the control, monoamine oxidase (MAO) activity and nitric oxide (NO) content were significantly higher in the 10 μg/L Hg2+, while acetylcholinesterase (AChE) activity significantly decreased. Compared with the control, triiodothyronine (T3) content was significantly higher in the 1 μg/L Hg2+ and significantly lower in the 10 μg/L Hg2+; the 1 μg/L and 5 μg/L Hg2+ groups had significantly higher thyroxine (T4) content than the other groups. In the 1 μg/L Hg2+, the integrated biomarker response (IBR) index value was the highest. In summary, exposure to Hg could decrease the antioxidant ability, cause changes in neurotoxic parameters, and induce disorders of the thyroid hormone system in silver carp larvae. The results of this study may contribute to the understanding of the adverse effects of chronic mercury poisoning on fish.

Microbial mercury transformations: Molecules, functions and organisms

Mercury (Hg) methylation, methylmercury (MeHg) demethylation, and inorganic redox transformations of Hg are microbe-mediating processes that determine the fate and cycling of Hg and MeHg in many environments, and by doing so influence the health of humans and wild life. The discovery of the Hg methylation genes, hgcAB, in the last decade together with advances in high throughput and genome sequencing methods, have resulted in an expanded appreciation of the diversity of Hg methylating microbes. This review aims to describe experimentally confirmed and recently discovered hgcAB gene-carrying Hg methylating microbes; phylogenetic and taxonomic analyses are presented. In addition, the current knowledge on transformation mechanisms, the organisms that carry them out, and the impact of environmental parameters on Hg methylation, MeHg demethylation, and inorganic Hg reduction and oxidation is summarized. This knowledge provides a foundation for future action toward mitigating the impact of environmental Hg pollution.

Evidence of Covid-19 lockdown effects on riverine dissolved organic matter dynamics provides a proof-of-concept for needed regulations of anthropogenic emissions

The fast spread of SARS-CoV-2 virus in Italy resulted in a 3-months lockdown of the entire country. During this period, the effect of the relieved anthropogenic activities on the environment was plainly clear all over the country. Herein, we provide the first evidence of the lockdown effects on riverine dissolved organic matter (DOM) dynamics. The strong reduction in anthropogenic activities resulted in a marked decrease in dissolved organic carbon (DOC) concentration in the Arno River (-44%) and the coastal area affected by its input (-15%), compared to previous conditions. The DOM optical properties (absorption and fluorescence) showed a change in its quality, with a shift toward smaller and less aromatic molecules during the lockdown. The reduced human activity and the consequent change in DOM dynamics affected the abundance and annual dynamics of heterotrophic prokaryotes. The results of this study highlight the extent to which DOM dynamics in small rivers is affected by secondary and tertiary human activities as well as the quite short time scales to return to the impacted conditions. Our work also supports the importance of long-term research to disentangle the effects of casual events from the natural variability.

Zooplankton as Mercury Repository in Lake Maggiore (Northern Italy): Biomass Composition and Stable Isotope Analysis

Total mercury (THg) and methylmercury (MeHg) concentrations were analyzed in zooplankton (≥450 and ≥850 µm size fractions) collected seasonally over 6 years in Lake Maggiore (Northern Italy), characterized by a legacy mercury contamination. Analysis of δ 15N and δ13C stable isotopes was carried out to trace how taxa with different trophic levels and carbon sources contributed to mercury concentrations and trends. THg ranged between 44–213 µg kg−1 d.w. and MeHg 15–93 µg kg−1 d.w., representing 24–61% of THg. Values showed strong seasonal variations, with peaks in winter, due to the high biomass of predator taxa (Bythotrephes longimanus, Leptodora kindtii) and of Daphnia longispina-galeata gr. A positive correlation between THg and MeHg and δ15N signature was observed. D. longispina-galeata gr. prevailed in both size fractions, substantially contributing to THg and MeHg concentrations. Δ13C signature was strictly bound to lake thermal circulation dynamics. Mercury stock in the zooplankton compartment ranged between 19–140 ng THg m−2 and 6–44 ng MeHg m−2 for the ≥450 µm size fraction and between 2–66 ng THg m−2 and 1–7 ng MeHg m−2 for the ≥850 µm fraction, with the highest values in spring when zooplanktivorous fish actively prey in the pelagic zone. The results highlighted the crucial role of zooplankton as a repository of mercury, easily available to higher trophic levels.

The underappreciated role of natural organic matter bond Hg(II) and nanoparticulate HgS as substrates for methylation in paddy soils across a Hg concentration gradient

Rice consumption is the major pathway for human methylmercury (MeHg) exposure in inland China, especially in mercury (Hg) contaminated regions. MeHg production, a microbially driven process, depends on both the chemical speciation of inorganic divalent mercury, Hg(II), that determines Hg bioavailability for methylation. Studies have shown that Hg(II) speciation in contaminated paddy soils is mostly controlled by natural organic matter and sulfide levels, which are typically thought to limit Hg mobility and bioavailability. Yet, high levels of MeHg are found in rice, calling for reconsideration of the nature of Hg species bioavailable to methylators in paddy soils. Here, we conducted incubation experiments using a multi-isotope tracer technique including ¹⁹⁸Hg(NO₃)₂, natural organic matter bond Hg(II) (NOM-¹⁹⁹Hg(II)), ferrous sulfide sorbed Hg(II) (≡FeS-²⁰⁰Hg(II)), and nanoparticulate mercuric sulfide (nano-²⁰²HgS), to investigate the relative importance of geochemically diverse yet relevant Hg(II) species on Hg methylation in paddy soils across a Hg concentration gradient. We show that methylation rates for all Hg(II) species tested decreased with increasing Hg concentrations, and that methylation rates using NOM-¹⁹⁹Hg(II) and nano-²⁰²HgS as substrates were similar or greater than rates obtained using the labile ¹⁹⁸Hg(NO₃)₂ substrate. ≡FeS-²⁰⁰Hg(II) yielded the lowest methylation rate in all sites, and thus the formation of FeS is likely a sink for labile ¹⁹⁸Hg(NO₃)₂ in sulfide-rich paddy soils. Moreover, the variability in the methylation data for a given site (1 to 5-fold variation depending on the Hg species) was smaller than what was observed across the Hg concentration gradient (10³-10⁴ fold variation between sites). These findings emphasize that at broad spatial scales, site-specific characteristics, such as microbial community structure, need to be taken into consideration, alongside the nature of the Hg substrate available for methylation, to determine net MeHg production. This study highlights the importance of developing site-specific strategies for remediating Hg pollution.

Species-specific isotope tracking of mercury uptake and transformations by pico-nanoplankton in an eutrophic lake

The present study aims to explore the bioaccumulation and biotic transformations of inorganic (iHg) and monomethyl mercury (MMHg) by natural pico-nanoplankton community from eutrophic lake Soppen, Switzerland. Pico-nanoplankton encompass mainly bacterioplankton, mycoplankton and phytoplankton groups with size between 0.2 and 20 μm. Species-specific enriched isotope mixture of ¹⁹⁹iHg and ²⁰¹MMHg was used to explore the accumulation, the subcellular distribution and transformations occurring in natural pico-nanoplankton sampled at 2 different depths (6.6 m and 8.3 m). Cyanobacteria, diatoms, cryptophyta, green algae and heterotrophic microorganisms were identified as the major groups of pico-nanoplankton with diatoms prevailing at deeper samples. Results showed that pico-nanoplankton accumulated both iHg and MMHg preferentially in the cell membrane/organelles, despite observed losses. The ratios between the iHg and MMHg concentrations measured in the membrane/organelles and cytosol were comparable for iHg and MMHg. Pico-nanoplankton demethylate added ²⁰¹MMHg (~4 and 12% per day depending on cellular compartment), although the involved pathways are to further explore. Comparison of the concentrations of ²⁰¹iHg formed from ²⁰¹MMHg demethylation in whole system, medium and whole cells showed that 82% of the demethylation was biologically mediated by pico-nanoplankton. No significant methylation of iHg by pico-nanoplankton was observed. The accumulation of iHg and MMHg and the percentage of demethylated MMHg correlated positively with the relative abundance of diatoms and heterotrophic microorganisms in the pico-nanoplankton, the concentrations of TN, Mg²⁺, NO₃^-, NO₂^-, NH₄⁺ and negatively with the concentrations of DOC, K⁺, Na⁺, Ca²⁺, SO₄^2-. Taken together the results of the present field study confirm the role of pico-nanoplankton in Hg bioaccumulation and demethylation, however further research is needed to better understand the underlying mechanisms and interconnection between heterotrophic and autotrophic microorganisms.

Algal Organic Matter Drives Methanogen-Mediated Methylmercury Production in Water from Eutrophic Shallow Lakes

Algal blooms bring massive amounts of algal organic matter (AOM) into eutrophic lakes, which influences microbial methylmercury (MeHg) production. However, because of the complexity of AOM and its dynamic changes during algal decomposition, the relationship between AOM and microbial Hg methylators remains poorly understood, which hinders predicting MeHg production and its bioaccumulation in eutrophic shallow lakes. To address that, we explored the impacts of AOM on microbial Hg methylators and MeHg production by characterizing dissolved organic matter with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy and quantifying the microbial Hg methylation gene hgcA. We first reveal that the predominance of methanogens, facilitated by eutrophication-induced carbon input, could drive MeHg production in lake water. Specifically, bioavailable components of AOM (i.e., CHONs such as aromatic proteins and soluble microbial byproduct-like materials) increased the abundances (Archaea-hgcA gene: 438-2240% higher) and activities (net CH₄ production: 16.0-44.4% higher) of Archaea (e.g., methanogens). These in turn led to enhanced dissolved MeHg levels (24.3-15,918% higher) for three major eutrophic shallow lakes in China. Nevertheless, our model results indicate that AOM-facilitated MeHg production could be offset by AOM-induced MeHg biodilution under eutrophication. Our study would help reduce uncertainties in predicting MeHg production, providing a basis for mitigating the MeHg risk in eutrophic lakes.

Mobilization, Methylation, and Demethylation of Mercury in a Paddy Soil Under Systematic Redox Changes

Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation-reduction potential (E_h) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing E_h stepwise from -300 to +300 mV using a sophisticated biogeochemical microcosm. At the E_h range from -300 to -100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., Desulfitobacterium spp.), acidification, and reductive dissolution of Fe(oxyhydr)oxides. At the E_h range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with E_h (-300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of Desulfitobacterium spp. decreased by 50 and 96%, respectively, at E_h of +200 mV as compared to E_h of -300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (Clostridium spp.) also decreased over 93% at E_h of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.

Nutrient Exposure Alters Microbial Composition, Structure, and Mercury Methylating Activity in Periphyton in a Contaminated Watershed

The conversion of mercury (Hg) to monomethylmercury (MMHg) is a critical area of concern in global Hg cycling. Periphyton biofilms may harbor significant amounts of MMHg but little is known about the Hg-methylating potential of the periphyton microbiome. Therefore, we used high-throughput amplicon sequencing of the 16S rRNA gene, ITS2 region, and Hg methylation gene pair (hgcAB) to characterize the archaea/bacteria, fungi, and Hg-methylating microorganisms in periphyton communities grown in a contaminated watershed in East Tennessee (United States). Furthermore, we examined how nutrient amendments (nitrate and/or phosphate) altered periphyton community structure and function. We found that bacterial/archaeal richness in experimental conditions decreased in summer and increased in autumn relative to control treatments, while fungal diversity generally increased in summer and decreased in autumn relative to control treatments. Interestingly, the Hg-methylating communities were dominated by Proteobacteria followed by Candidatus Atribacteria across both seasons. Surprisingly, Hg methylation potential correlated with numerous bacterial families that do not contain hgcAB, suggesting that the overall microbiome structure of periphyton communities influences rates of Hg transformation within these microbial mats. To further explore these complex community interactions, we performed a microbial network analysis and found that the nitrate-amended treatment resulted in the highest number of hub taxa that also corresponded with enhanced Hg methylation potential. This work provides insight into community interactions within the periphyton microbiome that may contribute to Hg cycling and will inform future research that will focus on establishing mixed microbial consortia to uncover mechanisms driving shifts in Hg cycling within periphyton habitats.

Microbial Diversity and Mercury Methylation Activity in Periphytic Biofilms at a Run-of-River Hydroelectric Dam and Constructed Wetlands

Periphytic biofilms have the potential to greatly influence the microbial production of the neurotoxicant monomethylmercury in freshwaters although few studies have simultaneously assessed periphyton mercury methylation and demethylation rates and the microbial communities associated with these transformations. We performed a field study on periphyton from a river affected by run-of-river power plants and artificial wetlands in a boreal landscape (Québec, Canada). In situ incubations were performed on three sites using environmental concentrations of isotopically enriched monomethylmercury (MM198Hg) and inorganic mercury (200Hg) for demethylation and methylation rate measurements. Periphytic microbial communities were investigated through 16S rRNA gene analyses and metagenomic screenings for the hgcA gene, involved in mercury methylation. Positive mercury methylation rates ([5.9 ± 3.4] × 10-3 day-1) were observed only in the wetlands, and demethylation rates averaged 1.78 ± 0.21 day-1 for the three studied sites. The 16S rRNA gene analyses revealed Proteobacteria as the most abundant phylum across all sites (36.3% ± 1.4%), from which families associated with mercury methylation were mostly found in the wetland site. Metagenome screening for HgcA identified 24 different hgcA sequences in the constructed wetland site only, associated with 8 known families, where the iron-reducing Geobacteraceae were the most abundant. This work brings new information on mercury methylation in periphyton from habitats of impacted rivers, associating it mostly with putative iron-reducing bacteria.IMPORTANCE Monomethylmercury (MMHg) is a biomagnifiable neurotoxin of global concern with risks to human health mostly associated with fish consumption. Hydroelectric reservoirs are known to be sources of MMHg many years after their impoundment. Little is known, however, on run-of-river dams flooding smaller terrestrial areas, although their numbers are expected to increase considerably worldwide in decades to come. Production of MMHg is associated mostly with anaerobic processes, but Hg methylation has been shown to occur in periphytic biofilms located in oxic zones of the water column. Therefore, in this study, we investigated in situ production of MMHg by periphytic communities in habitats impacted by the construction of a run-of-river dam by combining transformation rate measurements with genomic approaches targeting hgcAB genes, responsible for mercury methylation. These results provide extended knowledge on mercury methylators in river ecosystems impacted by run-of-river dams in temperate habitats.