Mercury (Hg) is a global pollutant that poses serious risks to marine ecosystems and human health. In oceanic systems, deep-pelagic species can serve as key prey and major vectors of Hg for large predators such as whales, sharks, and tunas, thereby affecting the health of both wildlife and humans. Mercury accumulation in deep-pelagic communities remains poorly understood, with scarce data and unresolved ecological, biogeochemical, and anthropogenic drivers. Here, we test the hypothesis that ocean productivity drives Hg accumulation in deep-pelagic species. We investigate total Hg concentrations and trophic structure (based on δ15N and δ13C values) in deep-pelagic species across two ecologically distinct regions of the western tropical Atlantic: the oligotrophic waters of the Fernando de Noronha Ridge (FNR) and the more productive off the Amazon River mouth (AMZ). Stable isotope analyses reveal that ocean productivity significantly influences food web architecture. FNR assemblages exhibit a broader δ15N range, consistent with elongated, vertically structured food chains or multiple trophic baselines. In contrast, species from AMZ show compressed δ15N values, indicative of shorter food chains. These trophic differences are mirrored in Hg dynamics. Mercury concentrations were significantly higher in FNR, even when accounting for species size and δ15N values. This elevated Hg burden is likely associated with longer food chains, increased reliance on detritus-based pathways, and synergistic effects linked to low productivity. Across both areas, deep-pelagic species generally accumulate higher Hg levels than ecologically similar epipelagic taxa. Mercury accumulation was also structured along trophic guilds and patterns of vertical distribution and migration. We discuss these results in the context of a changing ocean and their implications for pelagic food web dynamics, providing directions for future monitoring and risk assessment in ocean ecosystems.

Abstract. The western tropical Atlantic off Northeast Brazil is a dynamically complex region where major current systems contribute to interhemispheric exchanges, yet it remains one of the least observed ocean areas in terms of integrated, depth-resolved, and ecosystem-wide measurements. The two ABRACOS (Acoustics along the BRAzilian COaSt) surveys, conducted during austral spring 2015 (ABRACOS 1) and austral autumn 2017 (ABRACOS 2) aboard the R/V ANTEA, were designed to address this gap through coordinated multidisciplinary observations spanning the continental shelf, slope, seamounts, and open ocean along the Fernando de Noronha Ridge. Although each survey represents a synoptic seasonal snapshot, hydrographic and current observations from both campaigns have been shown to be representative of canonical spring and autumn conditions in the region. The dataset combines continuous underway measurements — thermosalinograph, ship-mounted ADCP, multifrequency active acoustics (38, 70, 120, and 200 kHz), and meteorological data — with station-based physical, biogeochemical, and biological sampling at 117 oceanographic stations. Station-based observations include 96 CTD-O₂ profiles, 881 discrete rosette bottle samples (salinity, dissolved oxygen, nutrients, pigments, cytometry, and phytoplankton abundance), 266 plankton net deployments, and 116 trawl operations targeting demersal and deep-pelagic communities from 10 to 1,100 m depth. Biological sampling yielded 172,241 taxonomically identified specimens representing 1,532 taxa across planktonic, nektonic, and demersal compartments. The dataset is further complemented by 3,466 stable isotope measurements (δ¹³C and δ¹⁵N) across particulate organic matter, zooplankton, and demersal and deep-pelagic organisms, as well as mercury concentrations and microplastic occurrence data for 194 and 381 individuals, respectively, collected during ABRACOS 2. All datasets are quality-controlled, harmonised across surveys using consistent station identifiers and taxonomic referencing against WoRMS and Eschmeyer's Catalog, and publicly archived in the SEANOE repository under dedicated DOIs. Having already supported more than 80 peer-reviewed publications across nine thematic domains — from physical oceanography and taxonomy to trophic ecology, anthropogenic contamination, and ecosystem connectivity — the ABRACOS dataset constitutes a unique multi-trophic, multi-compartment, and multi-contaminant baseline for the western tropical Atlantic, designed to support comparative analyses, ecosystem modelling, and synthesis efforts at regional to global scales. The datasets described in this paper are publicly available through SEANOE (https://www.seanoe.org).

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.

Diffusive Gradients in Thin Films (DGT) are widely used for in situ measurement of trace metals and mercury species in natural waters. Long deployments, required for ultratrace analytes, often suffer from biofouling and particle accumulation, compromising measurement accuracy. This study presents an automated in situ miniwiper (MW) system designed to clean DGT surfaces during deployment. The MW was evaluated in two contrasting environments: (i) productive Peruvian coastal waters, to assess its impact on mercury speciation, and (ii) the particle-rich Nugu River (India), for trace metal and rare earth element (REE) measurements. In Peruvian waters, MW-equipped DGTs showed minimal biofilm over up to 68 days, while uncleaned DGTs exhibited significant biofouling, leading to 14-68% underestimation of mercury species. In the Nugu River, MW-DGTs effectively limited particle deposition, yielding concentrations for Al, Mn, Fe, Co, Cd, and REE in close agreement with discrete water samples, whereas uncleaned DGTs underestimated these values. The MW had little effect on Ni, Cu, and U. This study demonstrates that biofouling and particle accumulation can significantly bias DGT measurements, and the MW system provides a robust, operational solution for long-term monitoring.

Monomethylmercury (MMHg) is a potent neurotoxin to which humans are exposed via fish consumption. However, the relative importance of planktonic and benthic biomagnification pathways to fish MMHg concentrations in marine food webs is challenging to quantify. Here, we apply compound-specific isotope analysis (CSIA) of Hg to identify fish MMHg biomagnification pathways across nearshore bay (NB), marine continental shelf (MCS), and pelagic ocean (PO) regions. We observe significant differences in Δ199Hg between MMHg and total mercury (THg), highlighting the limitations of using THg isotopes to resolve MMHg dynamics in the environment. In NB fish, Δ199Hg of MMHg closely matches that of benthic invertebrates, while in MCS and PO fish, it aligns with phytoplankton. According to the MMHg isotope binary mixing model, about 85% of MMHg in NB fish derives from the benthic biomagnification pathway, whereas over 90% of MMHg in MCS and PO fish originates from seawater-phytoplankton trophic transfer. These findings reveal that the benthic biomagnification pathway in near-shore regions has been underestimated in previous models, leading to potential uncertainties in evaluating marine Hg cycling and human exposure risks. This study highlights the importance of the benthic biomagnification pathway in coastal environments and demonstrates the potential of the CSIA of Hg for investigating MMHg biomagnification pathways in marine food webs, which provides new insights for global Hg pollution management under the Minamata Convention.

This study evaluated trace metal pollution in sediments and fishes from two tropical estuarine systems in Northeast Brazil: the Santa Cruz Channel Estuary (ITAP) and the Sirinhaém River Estuary (SIR). An integrative approach combining several environmental and toxicological assessment indices was employed. In general, concentrations of trace metals in sediments-including cadmium (Cd), chromium (Cr), lead (Pb), and zinc (Zn) in ITAP and Cd, Cr, Pb, Zn, and total mercury (THg) in SIR-were below the Threshold Effect Levels (TEL), suggesting a low likelihood of adverse effects on benthic biota. However, elevated THg concentrations exceeding the Probable Effect Levels (PEL) were observed at specific sites within the ITAP system, indicating a high potential for ecological harm. Environmental quality indices identified THg and Copper (Cu) pollution in ITAP and Cu pollution in both estuaries. Ecological risk and Nemerow multi-factor indices suggested that SIR is subject to low ecological risk, with Cu being the dominant pollutant. In contrast, ITAP exhibited high ecological risk driven primarily by Cu and THg. Among the species examined, Bairdiella ronchus and Centropomus undecimalis, which are frequently consumed by local populations in ITAP, exhibited Hg concentrations exceeding established safety thresholds for human consumption. The application of multiple assessment indices provided a comprehensive understanding of environmental degradation caused by anthropogenic pressures, particularly in the Santa Cruz Channel. These findings underscore the need for effective public policy enforcement aimed at long-term pollution monitoring and safeguarding food security for local communities.

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

Humans are exposed to toxic methylmercury mainly by consuming marine fish, in particular top predator species like billfishes or tunas. In seafood risk assessments, mercury is assumed to be mostly present as organic methylmercury in predatory fishes; yet high percentages of inorganic mercury were recently reported in marlins, suggesting markedly different methylmercury metabolism across species. We quantified total mercury and methylmercury concentrations in muscle of four billfish species from the Indian and the Pacific oceans to address this knowledge gap. We found low percentages of methylmercury in blue and black marlins (15 ± 7 %) compared to swordfish and striped marlin (89 ± 13 %), with no significant differences among ocean regions. This illustrates that billfishes exhibit species-specific methylmercury bioaccumulation patterns, likely related to unique selenium-dependent in vivo methylmercury demethylation capacities in muscle. Blue and black marlins therefore appeared generally safer for human consumption than swordfish and striped marlin regarding MeHg toxicological effects. Yet, no matter the species, the frequency of recommended weekly billfish meals decreased with increasing fish size, given that mercury naturally accumulates over time. When assessing potential risks of billfish consumption, we therefore recommend measuring methylmercury, rather than total mercury, and relying on a large number of samples to cover a broad range of fish sizes. This study calls for additional characterization of mercury speciation and bioavailability in billfishes to better understand the mechanisms driving species-specific differences of methylmercury detoxification, and to refine dietary advices associated to marine top predators consumption.

Stable isotopes are well established as routine and reliable tracers of nutrient flux and trophic structure. However, inferring trophic ecology from isotopic data is challenging due to variability at the food web base and systematic differences in biochemical fractionation during metabolism. Analyses of isotope systems from multiple elements with contrasting fractionation drivers may resolve some sources of variance, strengthening connections between measured isotopic variations and inferred ecological processes. This study combines carbon (δ13C), nitrogen (δ15N), sulfur (δ34S), and mercury (Δ199Hg/δ202Hg) isotopes to investigate trophic niches of coastal and oceanic elasmobranchs across two ecosystems in northwestern Mexico. In the Pacific Ocean, similar δ13C, δ15N, Δ199Hg, and δ202Hg values suggest that elasmobranchs relied on common pelagic resources, likely from upwelling events. In the Gulf of California, coastal species with higher δ13C and δ15N values and lower Δ199Hg and δ202Hg values fed on prey isotopically distinct from those offshore, allowing classification trees to identify foraging habitats more accurately than in the Pacific. Meanwhile, δ34S values systematically decreased from oceanic to coastal species at both sites and were highlighted as the most discriminative isotopic tracer by random forests. This study advocates for integrating complementary isotopic analyses to better comprehend biogeochemical and ecological mechanisms.

Paracas Bay, located in the Humboldt Current system, is a highly variable coastal environment where hypoxia (dissolved oxygen concentrations <2 mg L-1) has been reported as a persistent feature of bottom conditions. In addition to hypoxia, milky water events have been reported in the bay, most likely associated with the presence of sulfides (i.e. sulfidic events), including toxic hydrogen sulfide (H2S). This study is the first report of sulfide concentrations in the water column of Paracas Bay, with concentrations up to 6.79 mu mol L-1 measured in the bottom water layer using Diffusive Gradient in Thin film (DGT) passive samplers. Sulfides showed a marked seasonal pattern, while ENSO phases did not seem to affect their occurrence. The presence of sulfides in the water column is closely linked to hypoxic conditions. Indeed, the analysis of the relationship between the duration of severe bottom hypoxia (dissolved oxygen <= 0.15 mg L-1) and sulfides concentrations revealed a threshold of 18 h for sulfides accumulation in the overlying waters. Also, a gradient in sulfides concentration was observed from the sediment to the upper water layer. This gradient coupled with the variability of environmental variables (temperature, dissolved oxygen and currents velocity) highlights the complexity of this system, and suggests that a combination of advective and local physical and biogeochemical processes are responsible for the sulfidic events to occur in Paracas Bay. This study provides a baseline for assessing the potential toxicity of sulfides to cultured Peruvian scallops in upwelling bays.

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.

The photodemethylation of monomethylmercury (CH3Hg) is one of the most important natural degradation processes of this toxic compound and is therefore key to understanding Hg exposure. The isotopic composition of CH3Hg contains information about its sources and transformation pathways. The stable isotopes of Hg during photodemethylation of CH3Hg bound to dissolved organic matter (DOM) have been shown to undergo unique mass-independent fractionation (MIF), as well as mass-dependent fractionation (MDF). Here we present different photodegradation experiments (DOM, Cl- ligands) where, in addition to Hg isotopes and degradation kinetics, the delta 13C of CH3Hg was analyzed by compound-specific isotope analysis (CSIA): purge and trap-gas chromatography-combustion-isotope ratio mass spectrometry (PT-GC-C-IRMS). Our results show covariation of odd Hg MIF and light C isotope enrichment in CH3Hg photodemethylation products, with delta 13C fractionation factors from -5 to -16 parts per thousand, depending on the presence of Cl- and DOM. We also find a linear relationship between C MDF and Hg MIF, indicating that both isotope effects occur during C-Hg bond breaking. We suggest that biota Hg MIF can potentially be used to correct for photochemical C MDF, bringing us one step closer to exploring the origin of the methyl group contributing to CH3Hg formation.

A combination of mercury (Hg) speciation and compound-specific stable isotope analyses was employed to trace the origin and fate of monomethylmercury (MMHg) in the high-altitude food webs of Lakes Titicaca (TTKK) and Uru Uru (UU). Significant MMHg biomagnification was observed, with concentrations reaching up to 2 μg.g-1 Hg in top predators. Hg isotopes lake-specific trends were identified in relation to trophic position (δ15N) and MMHg fractions. In particular, Δ199MMHg increased from 0 ‰ in UU epiphytic biofilm to ≈2 ‰ and ≈4 ‰ in UU and TTKK piscivorous fish, respectively. Both δ202MMHg and Δ199MMHg signatures indicate that the sediment and/or the epibenthic environment is the primary MMHg source in both food webs. However, an additional MMHg pool, associated with photodegraded MMHg, was identified entering the two food webs at a different trophic level. Photodemethylation was estimated to account for 21 % of MMHg degradation before it entered TTKK Lake food web at the fish level, and 16 % before reaching UU Lake food web at the invertebrate level. Even-Hg MIF (Δ200Hg) shows that both Hg(0) atmospheric deposition and geogenic inputs contributed to Hg accumulation in sediments, while the food web adds significant atmospheric Hg(II) signatures, with up to 94 % of Hg(II) contributing to Lake TTKK fish MMHg. These findings underscore the high potential of Hg-CSIA revealing the dominant role of atmospheric Hg(II) deposition and distinct MMHg pools in driving MMHg bioaccumulation in high-altitude lake food webs.

Monomethylmercury (MMHg) is a potent neurotoxin causing neurodevelopmental delays and cardiovascular and immunological issues. Human exposure primarily occurs through seafood consumption due to MMHg bioaccumulation and biomagnification from seawater into marine organisms. Determining MMHg in seawater at ultratrace concentrations poses logistical and analytical challenges. Diffusive Gradient in Thin-film (DGT) samplers represent a promising solution, which captures time-averaged concentrations by preconcentrating in situ MMHg over a defined exposure time. DGT manufactured with 3-mercaptopropyl-functionalized silica (3MFS) in agarose and polyacrylamide gels were tested and compared for the determination of MMHg present in open ocean and coastal waters. Different elution methods using acidic thiourea were tested to reach precise, accurate and quantitative elution of MMHg from the binding gel. We found that polyacrylamide-3MFS binding gels display a higher elution efficiency (94 ± 3 %), precision and better handling compared to agarose-3MFS gels (41 ± 6 %). A unique mooring line installed in the South Western Tropical Pacific Ocean, provided monthly DGT-MMHg concentrations over a year showing potential seasonal differences in MMHg concentrations ranging between 18 and 106 fM. DGT were also deployed in shallow Peruvian coastal waters, exhibiting higher MMHg concentrations (170 ± 97, n = 26) with typical benthopelagic gradients. DGT-MMHg concentrations were in good agreement with discrete water samples analyzed by reference methods using isotope dilution. DGTs offer complementary advantages over oceanographic cruises, notably in situ preconcentration, low blanks, minimal logistical requirements and cost-effectiveness. DGTs represent a valuable tool for studying the marine MMHg cycle for evaluating the implementation of the Minamata Convention.

The diffusive gradient in thin film technique (DGT) represents an in situ passive sampling method designed to preconcentrate various compounds, including sulfides, for detection at low concentrations. While DGT applications for sulfides have been studied in freshwater, this research extends its use to marine environments. A detailed methodology is presented for synthesizing, assembling, calibrating, and field-deploying DGT samplers to measure sulfides in the low micromolar range in marine waters. The in-house DGT samplers developed in this study demonstrated improved performance, with more homogeneous binding gels and smaller silver iodide particles (0.51 +/- 0.34 mu m) compared to commercial alternatives. Grayscale imaging enabled accurate quantification of sulfide accumulation in the gels, confirming the method's reliability for detecting trace-level sulfides in marine environments. Comparative analysis showed in-house and commercial samplers performed similarly in estimating sulfide concentrations. Field deployments along the Peruvian coast revealed significant vertical and spatial sulfide gradients. In the Callao coastal area (12 degrees S) (July-August 2022), concentrations ranged from 0.03 to 0.45 mu M across a 35 m depth profile. In Paracas bay (13.8 degrees S) (March-April 2023), a shallower coastal station, concentrations ranged from 1.17 to 6.46 mu M, reflecting increased benthic production. These results highlight the utility of DGT samplers as cost-effective tools for biogeochemical monitoring, enabling studies of the ocean sulfur cycle. The findings emphasize the growing application of DGTs in marine and coastal water column research.

Deep-pelagic species are central to marine ecosystems and increasingly vulnerable to global change and human exploitation. To date, our understanding of these communities remains limited mainly due to the difficulty of observations, calling for complementary innovative tools to better characterize their ecology. We used mercury (Δ199Hg, δ202Hg, Δ201Hg, and Δ200Hg), carbon (δ13C), and nitrogen (δ15N) stable isotope compositions to segregate deep-pelagic species caught on the continental slope of the Bay of Biscay (NE Atlantic) according to their foraging depth and trophic ecology. Decreasing fish Δ199Hg values with corresponding depth estimates from the surface to down to 1,800 m confirmed that mercury isotopes are able to segregate deep species over a large vertical gradient according to their foraging depth. Results from isotopic compositions also identified different mercury sources, likely reflecting different trophic assemblages over the continental slope, in particular, the demersal influence for some species, compared to purely oceanic species. Overall, our results demonstrate how mercury stable isotopes can inform the vertical foraging habitat of little-known species and communities feeding in the deep.

Humans are exposed to toxic methylmercury mainly by consuming marine fish. The Minamata Convention aims at reducing anthropogenic mercury releases to protect human and ecosystem health, employing monitoring programs to meet its objectives. Tunas are suspected to be sentinels of mercury exposure in the ocean, though not evidenced yet. Here, we conducted a literature review of mercury concentrations in tropical tunas (bigeye, yellowfin, and skipjack) and albacore, the four most exploited tunas worldwide. Strong spatial patterns of tuna mercury concentrations were shown, mainly explained by fish size, and methylmercury bioavailability in marine food web, suggesting that tunas reflect spatial trends of mercury exposure in their ecosystem. The few mercury long-term trends in tunas were contrasted and sometimes disconnected to estimated regional changes in atmospheric emissions and deposition, highlighting potential confounding effects of legacy mercury, and complex reactions governing the fate of mercury in the ocean. Inter-species differences of tuna mercury concentrations associated with their distinct ecology suggest that tropical tunas and albacore could be used complementarily to assess the vertical and horizontal variability of methylmercury in the ocean. Overall, this review elevates tunas as relevant bioindicators for the Minamata Convention, and calls for large-scale and continuous mercury measurements within the international community. We provide guidelines for tuna sample collection, preparation, analyses and data standardization with recommended transdisciplinary approaches to explore tuna mercury content in parallel with observation abiotic data, and biogeochemical model outputs. Such global and transdisciplinary biomonitoring is essential to explore the complex mechanisms of the marine methylmercury cycle.