Mercury (Hg) and lead (Pb) are persistent, toxic trace metals whose pollution legacy from industrial activities can remain in the environment for centuries until reaching long-term sinks. This study reconstructs the history of Hg and Pb accumulation in Lake Estanya (NE Spain), a karstic lake system located 30 km downwind of the industrial complex of Monzón, known for hosting chlor-alkali, metallurgical and chemical industries. Two gravity cores retrieved in 2023 were analysed at sub-decadal resolution, spanning the last 330 years of evolution in Hg and Pb concentrations, enrichment factors, accumulation fluxes, and Pb isotopic composition. Analysis of atmospheric deposition samples from Monzón further enabled characterisation of the pollution processes currently affecting the Estanya catchment, allowing a comparison between legacy contamination and present-day trends. The sedimentary record reveals a rise in Hg and Pb accumulation during the second half of the twentieth century, synchronous with the establishment and expansion of the chemical industry in Monzón, the intensification of metallurgical activity, and the widespread use of leaded gasoline together with growing road traffic in Spain prior to its ban in 2001. Hg and Pb enrichment factors and accumulation fluxes increased exponentially from 1950 onward, followed by a marked decline at the turn of the 21 st century, attributed to the effectiveness of environmental regulations and the resulting reduction in primary Hg and Pb emissions. This trend is supported by the Pb isotopic signature, which shifts toward more radiogenic values in recent decades, indicating incipient system recovery. Nonetheless, Pb and especially Hg concentrations and enrichment factors remain elevated, reflecting the persistence of these trace metals and the formation of reservoirs that sustain their recycling decades after primary emissions ceased. Source apportionment of recent atmospheric deposition attributes Hg and Pb inputs to a mixture of industrial emissions, road traffic, and dust from regional sources and Saharan outbreaks. These results show that, while emission inputs are rapidly reflected in lake sediment records, system recovery is delayed by the long-term persistence and remobilisation of legacy Hg and Pb. This finding highlights the relevance of high-resolution lake records for tracking industrial pollution trajectories, enabling assessment of the effectiveness of environmental regulations and ecosystem recovery.

Artisanal and small-scale gold mining (ASGM) is prevalent in South America and the Amazon. It involves soil reworking, leading to an increase in downstream river turbidity and ecosystem degradation. In French Guiana, restoration, including landscaping and tree-replanting, is mandatory for legal gold mines, but the fate of postexploitation sites is poorly characterised. To investigate the extent of particle export originating from a restored gold-mining site, high frequency suspended solid concentrations (SSC) and discharge measurements were performed upstream and downstream of a site in French Guiana. Two flood events were followed with geochemical tracers involving multielemental concentrations and Fourier-transformed infrared spectroscopy (FTIR) data in the particulate and filtered fractions, to unravel the origin of water (overland flow and groundwater), and particles (forest topsoil, mine topsoil, or riverbed sediments) within the watershed. Results show that within the year following restoration, the mining site remains a significant source of particles and carbon to the river, with an excess 100-300 kg km(-2) (1.5-2.4 times increase) of particles exported due to the mining site, and specific carbon exports amounting to 9.8 +/- 0.6 t C yr(-1) km(-2), comparable to exports observed in larger watersheds in the Amazon region. Hydrogeochemical and mineralogical data in the particulate fraction hint towards riverbank erosion and riverbed sediment remobilisation as an important source of suspended particles rather than mining site surface erosion. This underlines the importance of restoring and stabilising the riverbed in the goldmine rehabilitation process.

Selenium plays a crucial role in estuarine biogeochemistry, balancing essential nutrient functions with potential environmental toxicity. This study examines the seasonal distribution of dissolved Se species, including volatiles, in the Adour estuary in relation to anthropogenic influences. To characterize major Se inputs from upstream watersheds to downstream tributaries, water samples were collected at low tide during three different seasons in upstream freshwaters, industrial/urban effluents and downstream estuarine waters. A tidal-cycle sampling campaign was conducted under low discharge conditions to assess Se dynamics during downstream estuarine mixing. Total dissolved Se (TDSe) concentrations ranged from 71 (pristine river) to 656 ng L-1 (industrial/urban-impacted tributaries). TDSe correlated strongly with nitrate (r = 0.84) in upstream waters, indicating significant agricultural and livestock contributions at the watershed scale. Selenate was the dominant species, followed by Se(-II+0) fraction and selenite. Volatile Se compound concentrations varied from 51 to 2757 pg L-1. Seasonal changes suggest that Se speciation is mainly controlled by watershed inputs derived from land use (agricultural and livestock practices) rather than downstream estuarine inputs. This speciation study further indicates that Se reactivity/bio-availability in estuarine systems can be largely influenced by anthropogenic activities, although further characterization of the aqueous reduced Se fraction is still needed.

Microbial mercury (Hg) methylation drives the formation of methylmercury (MeHg) hotspots in natural environments, but the cellular pathways that determine MeHg fate and isotopic signatures remain largely unresolved. In this study, we cultured the model sulfate-reducing bacterium Pseudodesulfovibrio hydrargyri BerOc1 anaerobically under fumarate respiration for 30 h and quantified Hg speciation and fraction- and species-specific Hg isotopic compositions. The results showed that intracellular MeHg was rapidly exported and dominated the extracellular pool by 30 h (> 80%). Exported MeHg bound to specific bioligands across size fractions, likely associated with its export mechanisms. Mass-dependent Hg isotope fractionation revealed temporal shifts in extracellular MeHg isotopic signatures (δ202HgMeHg= -1.10‰ to -0.82‰, 4-30 h), suggesting demethylation of bioligand-bound MeHg upon export. This isotopic observation refines previously reported suppressed mass-dependent fractionation signatures of MeHg in bulk bacterial cultures. Our results demonstrate that bacterially produced MeHg is rapidly exported from cells, potentially bound to specific biomolecules, and subsequently undergoes extracellular demethylation, thereby shaping aqueous MeHg isotopic signatures. These findings provide insight into the key factors that govern MeHg fate and accumulation during bacterial Hg methylation in aquatic ecosystems.

Globally, riverine inputs deliver more terrestrial mercury (Hg) to the coastal ocean than direct atmospheric deposition. Once in coastal environments, Hg can sorb to particles, become buried in sediments, and undergo methylation, facilitating its entry into food webs. However, at regional scales, Hg sources and transport pathways remain poorly constrained due to limited observations and uncertainties in source apportionment. Here, we combine Hg concentrations with Hg and organic carbon stable isotopic signatures in surface sediments to trace Hg sources along an estuarine-to-marine gradient in the southern Bay of Biscay (Atlantic Ocean). In-situ transformation processes had a negligible impact on Hg isotopic composition, supporting the use of Hg isotopes as a tracer for Hg source apportionment. Hg isotopic composition (δ 202 Hg) was significantly correlated to carbon (δ 13 C), revealing distinct terrestrial and marine endmembers, with an enrichment in lighter isotopes in estuarine sediments (δ 202 Hg -0.83 ± 0.15‰, δ 13 C -27.3 ± 0.50‰) compared to shelf and canyon marine sediments (δ 202 Hg -0.54 ± 0.16‰, δ 13 C -25.2 ± 0.80‰). A binary-mixing model constrained by the Hg-C isotope relationship suggests a progressive transition from a predominantly terrestrial Hg pool in estuarine samples (79%) towards decreasing terrestrial contributions in offshore marine samples (15%). Despite their lower terrestrial fraction, offshore sediments contained the largest terrestrial Hg stock, reflecting efficient terrestrial Hg export across the estuarine-to-marine boundary. Our results demonstrate that combined Hg and C stable isotope observations provide a powerful framework for Hg source apportionment, offering the potential to directly constrain the land-to-ocean transfer of Hg into coastal sediments.

The Amplex Red (AR) assay is a widely used method for the quantification of hydrogen peroxide (H2O2) in natural water due to its high sensitivity and specificity. However, the methodology developed for photochemistry may overestimate the H2O2 content estimated in sediment pore water, where H2O2 production results mainly from the oxidation of reduced species. Indeed, in previously published protocols, the measurement of H2O2 is not conducted a few minutes after the sample's mixing with the probe. We demonstrate that this delay is frequently sufficient for the reduced species present in pore water to produce a significant amount of H2O2. This study refines the AR technique by implementing high-frequency (HF, 10 Hz) measurements following probe addition. Moreover, the HF AR method gives access to three key parameters: I-the initial H2O2 concentration; II-the H2O2 production rate; and III-the total H2O2 production capacity of the sample upon oxidation. The accuracy and robustness of the method were demonstrated through a series of controlled experiments based on iron oxidation in the presence of citrate ligands. The efficiency of the method was demonstrated through its application in the field of sulfur- and iron-rich pore water. The findings represent a substantial enhancement in comparison to the prevailing protocols.

A quadruple isotope spiking methodology has been developed and applied for the study of Hg-species-specific transformations in incubation experiments. The method enables for the first time discrimination between interconversions reactions occurring during the incubation step from those occurring during the analytical procedure. Two different mathematical models for quadruple spiking were developed and applied here in model incubation experiments to track inorganic Hg(II) methylation and methylmercury demethylation. The results showed identical Hg compounds concentrations and interconversion reactions using both mathematical approaches so that an internal validation of the calculation procedure could be achieved. The methodology was also applied to determine Hg(II) methylation, methylmercury demethylation to Hg(II) and methylmercury loss rate in incubations performed with biofilms, sediments, freshwaters and pure phytoplankton cultures with Hg concentrations ranging from pg L-1 to μg L-1. At the initial incubation time, analytical and undesired Hg(II) methylation and/or methylmercury demethylation was fully corrected and eliminated after applying the quadruple spiking methodology. Methylation during analytical procedure was found negligible for all matrices (<0.2%) whereas significant demethylation during analytical procedure (from 3 to 10%) was observed for all samples. Natural Hg(II) methylation was only found in biofilms and sediments and, the discrimination of methylmercury demethylation and loss was achieved for all samples. The overall analytical uncertainty of the newly formed compounds was clearly affected by the concentration range. Specifically, the uncertainty of the experimental peak integration and the isotopic abundances of the isotopically enriched tracers were the main parameters contributing to the overall analytical uncertainty of the newly-formed Hg compounds concentration in natural samples. This work establishes a novel approach to study transformations of Hg compounds in biological and environmental matrices. The mathematical framework, based on multiple linear regression, is in principle transferable to other poly-isotopic elements with at least five stable isotopes and commercially available isotopically enriched species standards (e.g., tin), although such extension remains to be demonstrated experimentally.

Mercury is a globally distributed pollutant with significant ecological impacts. Terrestrial mosses have been widely used as Hg biomonitors, however, the mechanisms governing Hg accumulation, retention and release in these organisms remain poorly understood. This knowledge gap limits both the interpretation of biomonitoring data and the assessment of Hg ecotoxicological effects on mosses. This study investigates the deposition pathways, sources, and molecular mechanisms driving Hg accumulation in Pseudoscleropodium purum integrating isotopic, chemical, and molecular analyses. Our results showed that gaseous elemental Hg (Hg0) was the primary deposition pathway, accounting for 66% of total Hg inputs. However, following deposition, a substantial fraction of Hg0 appears to undergo oxidation within moss tissues. Together with post- depositional photoreduction, these processes reduce the remaining Hg0 fraction to only 5-35% of total Hg. Hg stable isotopes and chemical tracers provided no evidence for a major contribution from local sources such as forest fires and coal combustion, or AMDEs, suggesting that long-range atmospheric transport could be the prevailing source in the study area. Strong correlations between ∆201Hg and ∆199Hg (rho = 0.88) over a wide range of values is consistent with post-depositional photochemical reduction. Molecular analyses support that Hg accumulation in mosses is consistent with an increase in amide groups and a concurrent decrease in carbonyl groups within the extracellular space. The reverse process, amide hydrolysis, could lead to the release of previously bound Hg. These findings provide new mechanistic insights into Hg cycling in mosses, suggesting that functional groups in the cell wall and plasma membrane may influence Hg partitioning. These results challenge the assumption that mosses are reliable accumulative biomonitors, demonstrating instead that, at least for this species and under the studied environmental conditions, Hg content reflects a dynamic balance between deposition and photoreductive losses.

Understanding microbial transformations of the group VIa/16 metalloids tellurium and selenium is important for the remediation of contaminated environments and has been proposed as a green route for Se/Te nanoparticle synthesis. We previously isolated several strains of aerobic tellurite resistant marine yeast and bacteria. Here, we explored the capability of these strains to metabolize selenite and mixtures of tellurite and selenite to quantify fate and identify volatile metabolic products. The experimental results indicate that selenite is metabolized differently than tellurite by the yeast Rhodotorula mucilaginosa and bacteria Bacillus spp. and Virgibacillus halodenitrificans. The production of volatile Se compounds appears to be positively correlated with selenite resistance. However, selenite fate, e.g., the proportion of volatilized or precipitated Se, was not predictable from tellurite resistance or fate of the same strain. Under non-aerated conditions, when cultures were provided mixtures of selenite and tellurite, tellurite strongly influenced the fate of selenite and the types of volatile products made. Tellurite in oxyanion mixtures appears to strongly inhibit Se volatilization and drive speciation to less complex Se volatiles. Mixtures boosted the production of Te and Se precipitates by Bacillus sp. strain 6A and the production of Te precipitates by Rhodotorula mucilaginosa strains 13B and decreased the production of both volatile Te and Se compounds. Dimethylselenide and dimethyltelluride are acutely toxic by inhalation and oral exposure, so understanding their production is a key consideration in any biologically based manufacture of Se/Te containing nanoparticles.IMPORTANCEMany microbes are remarkably resistant to high concentrations of both selenite and tellurite while producing less toxic and bioavailable elemental forms, providing opportunities for the remediation of contaminated environments and green biosynthesis of Se/Te nanoparticles. The toxicity of volatile tellurite and selenite compounds produced during microbial processing may limit the development of remediation and biosynthesis technologies. The precise biochemical mechanisms governing Te and Se fate are still unclear. The data presented here demonstrate that combining Se and Te influenced the tolerance of marine microbes (Rhodotorula mucilaginosa 13B and Bacillus sp. strain 6A) to tellurite, significantly increasing precipitation as a product while limiting volatilization with the implication that combined Se/Te microbial remediation and/or nanoparticle synthesis may be less problematic than single element processes.

Biotic methylation of inorganic mercury (iHg) in aquatic systems is largely driven by microorganisms such as sulfate-reducing bacteria (SRB). Using the SRB model strain Pseudodesulfovibrio hydrargyri BerOc1 we investigated biotic iHg methylation aiming to assess the rates of mono-methylmercury (CH3Hg) production and to characterize the carbon (C) isotopic signatures (δ13C) of the CH3Hg product. BiogenicCH3Hg exhibited δ13C values averaging −23.1 ± 2.0‰, representing a 13C-depletion of 14.4‰ compared to the pyruvate carbon source used for the growing of the strain and a 9‰ depletion relative to the microbial biomass. The maximum methylation yield observed in our samples was around 15

Artisanal and small-scale gold mining is responsible for deforestation and habitat degradation in Amazonian ecosystems due to soil reworking and increased turbidity in surrounding rivers. In French Guiana, legal mines are required to perform rehabilitation after gold extraction. To assess the successfulness of this rehabilitation, the temporal evolution of particles and metal(loid)s (mercury, lead, arsenic) export was measured, as well as several parameters linked to soil functions (soil carbon and nitrogen, microbial diversity, enzymatic activities). Results show a rapid decrease in particle export (i.e., 3-fold reduction within the first 8 months, and up to 2000-fold reduction after 4 years), linked to an increase in spontaneous colonisation by herbaceous plants. This decrease in particle export induced an efficient decrease in metal(loid)s (Hg, Pb, As) export predominantly associated to the particulate fraction. The parameters associated with ecological functions did not show any significant increase within 5 years after rehabilitation, possibly due to depletion of nitrogen stocks by the herbaceous growth inducing a competition for resources with soil microbial communities. Microbial communities on the mining site displayed significant evolutions during the 5 years after restoration, with no visible convergence with forest-soil communities, but underlining the adaptability to climatic conditions and a changing environment. These results highlight that current rehabilitation practices are efficient in limiting particle and major contaminants export while more attention needs to be addressed to the recovery of soil functions and biological activities.

The coastal ocean receives more mercury from riverine input (land-derived Hg) than from direct atmospheric deposition. Land-derived Hg is primarily particle-bound and buried in coastal sediment, where it may undergo methylation. However, its contribution to coastal sediment Hg loads remains poorly constrained. Here, we investigate Hg sources along an estuarine-to-marine gradient in the Bay of Biscay (Atlantic Ocean) combining Hg and carbon (C) stable isotopic composition in sediments. We find a significant (p<0.05) difference in δ202Hg and δ13C between estuarine (δ202Hg -0.83 ± 0.15‰, δ13C -27.3 ± 0.50‰) and marine (δ202Hg -0.54 ± 0.16‰, δ13C -25.2 ± 0.80‰) sediments, and a significant correlation (p<0.05) between δ13C and δ202Hg. While in-situ processes had a negligible impact on Hg isotopic composition, the observed trends could be explained by the mixing of land-derived Hg (e.g. vegetational uptake, anthropogenic) and direct atmospheric deposition to the marine environment. Applying a binary-mixing model with regional endmembers to δ13C and δ202 signatures reveals a near 1:1 relationship between land-derived Hg and terrestrial C along our estuarine-to-marine gradient. This relationship, if confirmed for other regions, suggests that joint observations of C stable isotopes and Hg concentrations could directly constrain the transfer of land-derived Hg into coastal sediments.

Phytoplankton species influence mercury cycling through bioaccumulation and Hg(II) reduction, however their contribution to oxidation of Hg(0) in aquatic systems remains largely overlooked. The present study aims at investigating the oxidation of Hg(0) by two phytoplankton species: the diatom Cyclotella meneghiniana and the green alga Chlamydomonas reinhardtii. The algae were exposed to gaseous Hg(0) at concentrations in the range of 6-12 μg m-3, representative for contaminated environments, under various experimental conditions (open vs. closed systems, light vs dark, and alive vs dead cells). The obtained results revealed, for a first time, that Hg(0) oxidation in C. meneghiniana cultures was light-dependent and limited to live cells, whereas C. reinhardtii maintained similar oxidation rates in both live and dead cells. C. reinhardtii cultures exhibited nearly tenfold higher Hg(0) oxidation efficiency than C. meneghiniana, demonstrating a strong species-dependent effect. Both species facilitated Hg(0) uptake from air into water, demonstrating a potential route for atmospheric Hg(0) to enter aquatic food webs. This novel evidence of phytoplankton-mediated Hg(0) oxidation highlights the importance of species identity and environmental conditions in governing Hg transformations and bioavailability. The results could have significant implications for understanding mercury bioaccumulation and toxicity in aquatic ecosystems. Further research is needed to clarify their contribution to Hg(0) oxidation in aquatic systems and to elucidate the underlying mechanisms driving the process.

Selenoneine, a naturally occurring form of selenium (Se) in biota, has recently raised considerable interest in the fields of nutrition, drug development and environmental research. Selenoneine has a strong antioxidant capacity which makes it a potential Se supplement for humans or farm animals. There is a lack of a reliable and cost-effective online speciation methodology for the determination of such a biologically relevant Se species. In this study, a method has been developed for the simultaneous analysis of selenoneine and other organic Se species. It is based on the online coupling of reversed-phase liquid chromatography, ultraviolet radiation, thermal treatment, hydride generation and atomic fluorescence spectrometry (HPLC-UV-TT-HG-AFS). Quantitative determination of selenoneine by external calibration and its separation from commonly found organic Se species (selenomethionine, Se-methyl-selenocysteine) was achieved. The proposed methodology resulted in a linear response (R-2 > 0.999) for a concentration range of 5-100 mu gSe L-1 and a limit of detection of 0.5 mu gSe L-1. The repeatability was 0.8 %, with a reproducibility of 1.6 %. This analytical approach is versatile, cost-effective, and fast with a chromatographic runtime of less than 12 min. The high selectivity of HPLC-ICP-MS, combined with the species identification power of HPLC-ESI-MS/MS was found invaluable for gaining further speciation insights and confirming the results obtained by HPLC-UV-TT-HG-AFS. The developed approach could serve as useful tool for routine selenoneine analysis in biological samples as shown here with the analysis of seabird liver extracts.

The National Institute for Environmental Studies (NIES) developed the NIES CRM No. 13-a, a new certified reference material for human hair, using scalp hair from Asian females. This CRM represents a significant advancement in support of global mercury exposure assessments and offers unparalleled reliability and scope compared with existing materials. We aimed to provide a comprehensive overview of the preparation, certification, and application of NIES CRM No. 13-a. In total, 806 bottles (3 g each) were produced, with thorough homogenization ensured through sieving and blending. Certified values for total mercury (1.06 ± 0.07 mg/kg), methylmercury (0.858 ± 0.075 mg/kg), and key trace elements (arsenic, cadmium, lead, selenium, and zinc) were determined through extensive collaborative analyses involving 20 laboratories. Additional reference values were provided for calcium, magnesium, sodium, sulfur, antimony, barium, copper, iron, and manganese. Rigorous stability and homogeneity assessments demonstrated the stability of the CRM for over 10 years and consistency across sample units, even for challenging elements such as selenium. The CRM also includes information values of stable mercury isotope ratios, reflecting their growing importance as exposure tracers. This enhancement in accuracy and traceability facilitates accurate mercury and trace element assessments in human hair, enabling improved biomonitoring of mercury exposure, dietary studies, toxicological evaluations, human health risk evaluations, and regulatory compliance.

Atmospheric mercury (Hg) emissions represent a persistent global threat to ecosystems and human health. Stable Hg isotopes have emerged as powerful tools to trace historical pollution sources and reconstruct depositional pathways in natural archives. In this study, we present a 4000-year reconstruction of Hg isotopic composition from two Pyrenean lake sediment records (Lake Marboré and Lake Estanya) located along an altitudinal gradient and compare them with those of a nearby ombrotrophic peatland (Estibere mire). Both lakes exhibit a long-term increase in Hg accumulation rates and shifts in isotope values since the onset of the Modern Period (∼16th century), consistent with intensified anthropogenic emissions. However, the isotopic patterns differ: Lake Estanya, located in a lowland area with historical land-use changes, reflects a more localized Hg signal, whereas the high-elevation, remote Lake Marboré preserves a broader regional atmospheric imprint, dominated by wet deposition. The comparison with Estibere mirepristine and situated within the same air mass trajectory as Marboréreveals a consistent offset in Δ199Hg values yet strikingly similar temporal trends, indicating a shared regional source signal modulated by ecosystem-specific processes. This multiarchive and multialtitude framework provides a rare opportunity to disentangle Hg source signatures from depositional and postdepositional transformations. Moreover, variations in even-MIF (Δ200Hg) in the alpine lake show the potential to reflect past climate phases, highlighting the additional value of Hg isotopes as paleoclimatic proxies. Our results underscore the importance of integrating different ecosystem archives to improve reconstructions of atmospheric Hg dynamics and to refine interpretations of legacy pollution and climate interactions.

The rapid growth of cities and mines in developing countries has a major impact on the environment, through the discharge of untreated wastewater and mining waste. In the high-altitude tropical Andes, the Katari watershed combines a fast-developing urban area, upstream mining sites, and a downstream agricultural area, which are drained by the Katari River to Lake Titicaca. To assess the respective contribution of these areas to the watershed mercury (Hg) budget, Hg and monomethyl-Hg (MMHg) were measured in soil, sediment, surface water, groundwater, and the atmosphere (airborne fallout and total gaseous Hg). Further, two well-dated sediment cores collected in Lake Titicaca were used to reconstruct Hg accumulation rates over the last centuries. From instrumental river discharge, air and rainfall monitoring, the contribution of Hg originating from the atmosphere and the river was assessed. Results show that the mining area is a locally restricted main Hg hotspot. Although less contaminated, the urban area is the main source of MMHg for surface water through wastewater discharge, and the atmosphere the main source of total Hg through particulate and gaseous Hg emissions. Total Hg inventories in lake sediment from a bay at the outlet of the watershed show that at least one half of the Hg input originated from the river discharge, the rest being deposited from the atmosphere. The increasing development of the urban area without improvement of wastewater treatment implies that river discharge will be a source of Hg for the lake for many years.

Methylmercury is a bioaccumulative neurotoxin that poses severe risks to marine ecosystems and human health worldwide. Hydrothermal systems and submarine volcanoes are natural sources of mercury, yet the magnitude of emissions, their transport, and their impact on marine ecosystems remain poorly understood. Quantifying natural mercury fluxes is essential to understanding anthropogenic perturbations and guiding effective reduction strategies. We investigate hydrothermal mercury inputs at the Tonga volcanic arc and their impact on the local ecosystem. Our results show that hydrothermal and volcanic activity in the Tonga Arc increases mercury concentrations in seawater. Comprehensive surveys identified mercury-rich plumes (up to 22.7 pmol L-1) associated with high mercury fluxes (4763 pmol m-2 day-1) reaching productive surface waters, resulting in an estimated total flux of 4.23 t y-1 for the entire Tonga Arc. Despite these significant inputs, mercury concentrations in phytoplankton remain unexpectedly low. We demonstrate that phytoplankton blooms, stimulated by natural iron fertilization from hydrothermal sources, dilute mercury at the cellular level, reducing the impact of hydrothermal mercury. Additionally, we provide a revised global estimate of hydrothermal mercury inputs with a maximum of 120 t y-1, which is considerably lower than atmospheric and riverine inputs to the ocean.

Phytoplankton are traditionally viewed as simple bioaccumulators and key entry points for mercury (Hg) into aquatic trophic chain. However the more recent findings suggest that they can function as dynamic biological systems capable of enhancing Hg cycling reactivity and altering its speciation. Nevertheless, the role of phytoplankton species in mercury transformations remains largely overlooked. The present study examined inorganic mercury (Hg(II)) methylation, monomethylmercury (MeHg) demethylation, and the production of dissolved gaseous mercury (Hg(0)) following exposure to sub-nanomolar concentrations Hg(II) or MeHg, representative of contaminated environments. Diatom Cyclotella meneghiniana was selected as a representative phytoplankton species due to its widespread presence in diverse aquatic ecosystems. To track transformation pathways, isotopically labeled Hg species were used to distinguish between methylation and demethylation processes. The results demonstrated rapid accumulation of both Hg(II) and MeHg in the diatom cells. A cellular demethylation of MeHg into Hg(II), primarily occurring within the cell debris fraction, but no detectable Hg(II) methylation was observed. The reduction of Hg(II) to Hg(0) was found to be biologically mediated and independent of the photosynthetic system. No significant production of Hg(0) after MeHg exposure was determined. Overall, these findings imply that phytoplankton species could actively contribute to mercury cycling in aquatic environments through cellular transformation processes, including MeHg demethylation and Hg(II) reduction.