# Diffusive gradient in thin film for ultra-trace methylmercury measurements in the coastal and open sea

Isalyne Blondet, Fanny Rioual, Michelle I Graco, Jesús Ledesma, Kevin Ruiz-Diaz, Arturo Aguirre-Velarde, Jonathan Flye-Sainte-Marie, Laure Laffont, Lars-Eric Heimbürger-Boavida, Natalia Torres-Rodriguez, et al.

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## To cite this version:

Isalyne Blondet, Fanny Rioual, Michelle I Graco, Jesús Ledesma, Kevin Ruiz-Diaz, et al. "Diffusive gradient in thin film for ultra-trace methylmercury measurements in the coastal and open sea. Marine Pollution Bulletin, 2026, 222 (2), pp.118813. 10.1016/j.marpolbul.2025.118813. hal-05326793

HAL Id: hal-05326793

https://hal.science/hal-05326793v1

Submitted on 22 Oct 2025

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# Diffusive gradient in thin film for ultra-trace methylmercury measurements in the coastal and open sea

Isalyne Blondet<sup>a, b</sup>, Fanny Rioual<sup>c, d</sup>, Michelle I. Graco<sup>e</sup>, Jesús Ledesma<sup>e</sup>, Kevin Ruiz-Diaz<sup>e</sup>, Arturo Aguirre-Velarde<sup>h</sup>, Jonathan Flye-Sainte-Marie<sup>c</sup>, Laure Laffont<sup>a</sup>, Lars-Eric Heimbürger-Boavida<sup>f</sup>, Natalia Torres-Rodriguez<sup>f</sup>, Sophie Bonnet<sup>f</sup>, Cécile Guieu<sup>g</sup>, Vincent Raimbault<sup>b</sup>, David Point<sup>a, *</sup>

## ARTICLE INFO

Keywords:

DGT
methylmercury
passive samplers
coastal ocean
open ocean

## ABSTRACT

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 ultra-trace 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.

## 1. Introduction

Mercury (Hg) is a global pollutant and one of the most toxic trace metals in the environment (Karagas et al., 2012), released by natural sources (erosion, volcanism) and anthropogenic activities (mining, fossil fuel combustion) (Outridge et al., 2018; UNEP, 2020). Atmospheric Hg concentrations are 2.6 times higher than before the industrial revolu-

lution (Amos et al., 2013; Outridge et al., 2018; Y. Zhang et al., 2014), while surface ocean Hg levels have likely tripled (C. Lamborg et al., 2014). In suboxic environments, such as sediments and subsurface seawater, inorganic Hg can be methylated into monomethylmercury (MMHg) and dimethylmercury (DMHg) (Cabrol et al., 2023; Mason and Fitzgerald, 1990), most likely by microorganisms carrying the HgcAB gene pair (Parks et al., 2013; Villar et al., 2020). In the Peruvian up-

<footnote><sup>a</sup> Géosciences Environnement Toulouse (GET) - Institut de Recherche pour le Développement (IRD), CNRS, Université de Toulouse, Toulouse, 31 400, France</footnote>

<footnote><sup>b</sup> Laboratoire d'Analyse et d'Architecture des Systèmes - (LAAS) - CNRS, Université de Toulouse, CNRS, Toulouse, 31 400, France</footnote>

<footnote><sup>c</sup> Laboratoire des sciences de l'Environnement MARin (LEMAR), (UBO/CNRS/IRD/Ifremer), Technopôle Brest-Iroise, Plouzané, 29 280, France</footnote>

<footnote><sup>d</sup> Laboratorio de Ecosiología Acuática, Instituto del Mar del Perú (IMARPE), Esquina Gamarra y General Valle S/N Chucuito, Callao, Peru</footnote>

<footnote><sup>e</sup> Dirección General de Investigaciones Oceanográficas y cambio Climático, Instituto del Mar del Perú (IMARPE), P.O. Box 22, Callao, Peru</footnote>

<footnote><sup>f</sup> CNRS/INSU, Aix-Marseille-Université, Ird, Mediterranean Institute of Oceanography (MIO), Marseille, France</footnote>

<footnote><sup>g</sup> Laboratoire d'Océanographie de Villefranche (LOV), Institut de la Mer de Villefranche, CNRS, Sorbonne Université, Villefranche-sur-Mer, 06 230, France</footnote>

<footnote><sup>h</sup> Facultad de Pesquería, Universidad Nacional Agraria La Molina, Av. La Molina S/N, La Molina, Lima, 15024, Peru</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: david.point@ird.fr (D. Point).</footnote>

<footnote>https://doi.org/10.1016/j.marpolbul.2025.118813</footnote>

<footnote>Received 23 July 2025; Received in revised form 30 September 2025; Accepted 4 October 2025
0025-326/© 20XX</footnote>
welling region, one of the world's largest fisheries (FAO, 2024), MMHg can account for up to 20 % of the total Hg flux from coastal upwelling (Bowman et al., 2016). MMHg is the most toxic of Hg species, and a potent neurotoxin with unique bioconcentration, bioaccumulation and biomagnification properties in aquatic food webs (Médieu et al., 2022; Sunderland et al., 2018; Tesán-Onrubia et al., 2023). Humans are mainly exposed through seafood consumption (Petrova et al., 2020; Sunderland, 2007). Health effects include neurodevelopmental delays in children, cardiovascular problems in adults, and immune dysfunction (Karagas et al., 2012; Tan et al., 2009). Biogeochemical processes at ultra-trace levels have global implications for Hg contamination in the marine food web and human health (Barbosa et al., 2022; Médieu et al., 2022), yet most risk assessment lack key information on the conversion of Hg emissions to MMHg in marine systems (Dietz et al., 2022; Kojadinovic et al., 2006; Médieu et al., 2024). A major limitation is the scarcity of seawater MMHg data at large and sub-regional scales.

The femtomolar (fM) levels of MMHg in seawater pose significant logistical and analytical challenges that only a few laboratories can meet (Živković et al., 2025). Sampling during oceanographic campaigns follow trace metal clean (TMC) procedures, using acid-cleaned Teflon or calcinated glass bottles (Cutter et al., 2017; Torres-Rodriguez et al., 2024), and often requires several liters of seawater for sample rinsing, processing and analysis (C. H. Lamborg et al., 2012). MMHg analysis on purged and acidified seawater samples is most often performed by ethylation and purge-and-trap gas-chromatographic atomic fluorescence spectrometry (PT-GC-AFS) (Bowman and Hammerschmidt, 2011; Kleindienst et al., 2023; C. H. Lamborg et al., 2012; Mansfield and Black, 2015), a method occasionally affected by matrix effects lowering derivatization yields or causing MMHg degradation (Demuth and Heumann, 2001; Jackson et al., 2009; Leermakers et al., 2005; Monperrus et al., 2005). Alternatively, Species Specific Isotope dilution gas chromatography inductively coupled plasma mass spectrometry (SS-ID-GC-ICP-MS) corrects for matrix bias and non-quantitative yields, enabling more accurate MMHg measurements (Demuth and Heumann, 2001; Sharif et al., 2013; Heimbürger et al., 2015). This method was later improved for the measurement of both MMHg and DMHg species (Kleindienst et al., 2023). However, few laboratories have access to this advanced analytical technique, highlighting the need for alternative, more accessible tools.

Passive sampling, such as the Diffusive Gradients in Thin Films (DGT) technique (Davison and Zhang, 1994), offers a promising in situ approach for MMHg determination in freshwater (Bratkič et al., 2019, 2019; Muresan et al., 2018; Neal-Walthall et al., 2022; Pelcová et al., 2022; Tafurt-Cardona et al., 2015). However, no application studies have yet considered DGT in challenging environments such as the open ocean, since Hg compounds are present at ultratrace concentration. Few studies have focused on coastal waters (Bratkič et al., 2019; Clarisse and Hintelmann, 2006; Dommergue et al., 2010; Hong et al., 2011). DGT samplers are low cost in situ preconcentration devices composed of two hydrogel layers and a protective filter membrane hold together by a plastic support (Davison and Zhang, 1994). A diffusive gel, usually made of 1.5 % agarose (AG), overlay a binding gel containing an ion-exchange resin such as 3-mercaptopropyl functionalized silica (3MFS), in 1.5 % AG gel or in polyacrylamide cross-linked with agarose (APA) gel (Bratkič et al., 2019; Clarisse and Hintelmann, 2006; Fernández-Gómez et al., 2015; Gao et al., 2014). Mercury compounds such as inorganic Hg (HgII) or MMHg accumulate in the binding gel of DGTs linearly as a function of time thanks to Fick's law, given a specific diffusion coefficient, the surface area of exchange, and their relative concentrations in water (Davison and Zhang, 1994). Since binding gels with 3MFS capture both MMHg and HgII, obtaining an accurate, selective and quantitative elution of MMHg prior to analysis remains challenging. A recent review (Galceran et al., 2021) comparing HNO₃, HCl and acidic thiourea (TU) solutions reported highly variable MMHg elution yields (31–97 %). Clarisse and Hintelmann (2006) showed that TU

was relatively specific and efficient for MMHg recovery but interfering with ethylation prior to PT-GC-AFS. Gao et al. (2014) attempted this procedure, but no MMHg could be detected by Headspace-Gas Chromatography Atomic Fluorescence (HS-GC-AFS) (Gao et al., 2014). Clarisse et al. later shifted to SS-ID-GC-ICP-MS for MMHg analysis (Clarisse et al., 2009). This literature review suggests that there is yet no consensus for the procedural analysis of MMHg using DGT samplers.

In this work, we (i) revisited and optimized the procedure proposed by Clarisse and Hintelmann (2006) to achieve quantitative MMHg elution from DGT binding gels while minimizing ethylation interference during PT-GC-AFS analysis, (ii) applied the optimized procedure to DGT for ultra-trace MMHg determination in Southwestern Pacific and Peruvian coastal waters. (iii) compared DGT-MMHg concentrations with discrete seawater samples analyzed by SS-ID-GC-ICPMS. (iv) provide recommendations for the use of DGTs for marine Hg research and implications for their application to comply with the Minamata Convention.

## 2. Material and methods

### 2.1. Reagents, standards and DGT

DGT samplers and separate binding gels were purchased from DGT Research®. DGTs consisted of a polyethersulphone filter, a 0.8 mm 1.5 % AG diffusive gel and a 0.5 mm binding gel employing a 3MFS resin in a 1.5 % AG gel before 2021, and in APA gel after 2021 and now on.

Sodium Acetate (anhydrous ≥99.0 %, Baker analyzed® ACS, J.T.Baker®), Acetic acid (≥99.0 %, ULTREX® II, Ultrapure, J.T.Baker®), Sodium Hydroxide (Fisher BioReagents™), Sodium tetraethylborate (98 %, pure, Strem Chemicals Inc.), Antifoaming (RESTEK), Thiourea (min 99 %, ACS reagent, Sigma Aldrich), Hydrochloric acid (HCl, 34–37 %, Trace Metal™, Fisher Chemical) were used in this study. Methylmercury standard solution (MeHgCl) was purchased from Brooks Rand (Seattle, USA). All solutions were prepared using 18.2 MΩ cm ultrapure water in calcinated glass vials.

### 2.2. Instruments

Elution of the binding gels was carried out using a Fisherbrand FB15050 ultrasonic tank or an orbital heated shaker (Mini shake lite VWR). Monomethylmercury on blanks, standard solutions and DGT eluent solution was analyzed by ethylation, purge and trap gas chromatography atomic fluorescence spectrophotometer (PT-GC-AFS, Brooks Rand M-MERX, USA) using 40 mL glass vials. The conditions of analysis of MMHg by PT-GC-AFS consisted of setting the photomultiplicator to 800, and adjusting the argon gas flow to 60, 45 and 35 mm for the purging, drying, and the chromatography gas, respectively. The GC temperature was set to 34.5 °C leading to runtime of 7 min. Per sample. The limit of detection (LOD) for MMHg analysis by PT-GC-AFS, estimated on *n* = 9 ultra pure water blanks, was 0.3 pg, with a limit of quantification (LOQ) of 1 pg.

### 2.3. MMHg analysis

#### 2.3.1. DGT-MMHg analysis

Blanks, standard solution and DGT-MMHg eluent solutions were prepared accordingly to reference procedures prior to PT-GC-AFS analyses. Briefly, MMHg standards and blanks were prepared by first adding 30 mL of 18.2 MΩ cm ultrapure water in a 40 mL glass vial, then adding 0.6 mL of 2 M acetate/acetic acid Buffer (pH 4.9), prepared with antifoaming (0.01 %), and 0.05–0.3 mL of 1 ng/g MMHg (as Hg) standard for calibration. The 40 mL vials were equipped with a Si/PTFE-lined septa and homogenized gently by hand. The sample vials were reopened, ultrapure water was added to the top of the vials and
50 µL of 1 % NaBEt₄ was added prior to analysis. The analysis of MMHg in binding gel eluates consisting of adding the 3.75 mL of 0.2 N HCl/TU solution in a 40 mL glass vial prefilled with 30 mL of MQ water, followed by the addition of 1.8 mL of 2 M acetate/acetic acid Buffer (pH 4.9) and 0.4 mL of NaOH (2 M) to set the pH to 4.9. Samples were then finally prepared and derivatized the same way as MMHg standards. Typical sample batches included: 3 ultrapure water blanks, 3 procedural blanks, 3 MMHg calibration standards prepared in ultrapure water and 3 MMHg calibration standards prepared in HCl/TU solutions ran every 10 samples. Measurement precision on calibration standards prepared in ultrapure water and in 3.75 mL of 0.2 N HCl/TU elution solution was consistently better than 5 % (n = 3) for each analytical batch.

### 2.3.2. Discrete water samples-MMHg analysis

Seawater samples were collected into 155 mL polyethylene terephthalate (PET) bottles and purged onboard 15 times the volume with Hg-free Argon. Upon purging the gaseous species, DMHg is removed from the sample leaving MMHg as the only methylated Hg species. Samples were then acidified with bidistilled HCl to 0.4 % v/v, and stored for laboratory analysis. MMHg concentrations were measured by SS-ID-GC-ICP-MS (Heimbürger et al., 2015; Kleindienst et al., 2023; Monperrus et al., 2005). Briefly, 110 mL of sample was transferred to pre-combusted glass vials, spiked at an excess ratio of 5.1 for i¹⁹⁹Hg and 4.2 for MM²⁰¹Hg (ICS, Spain), and buffered to pH 3.9 with a sodium acetate solution and NH₃ (ULTREX II Ultrapure Reagent, J.T. Baker). After the pH adjustment, 5 % v/v tetrapopylborate (Merseburger Spezialchemikalien) and 150 µL isooctane (Sigma Aldrich) were added, the vials were sealed, shaken, and the organic phase extracted for injection on a GC (Thermo GC 1300) coupled to sector field ICP-MS (Thermo Element XR). The detection limit was 1 fM (3:1 signal-to-noise ratio). The spiked concentration (97 ± 8 %) was calibrated by reverse isotopic dilution against a MeHgCl standard traceable to NIST1641E.

## 2.4. Optimization of the elution procedure

The aim of this study consisted of determining the optimum TU concentration, volume and agitation conditions for which MMHg is quantitatively eluted from a 3MFS binding gel, while minimizing its interference during MMHg derivatization during PT-GC-AFS analysis.

We first tested the influence of TU concentrations on the derivatization yields of MMHg standards. We compared the intensity of the MMHg peak at different TU concentrations (0, 1.5, 5 and 10 mM TU, prepared in 0.2 N HCl) in ultrapure water samples spiked with 300 pg MMHg in a 40 mL vial. Following Clarisse and Hintelmann (2006) recommendations, this TU concentration range was chosen to show low to moderate matrix effects.

The flow chart summarizing the different experimental conditions is illustrated in Fig. 1. We have optimized the procedure for APA gels, as these are the gels used in commercial DGTs since 2021. Binding gels were spiked with 300 pg of MMHg, placed in 2 mL Eppendorf tubes (Fisher Scientific), and equilibrated overnight prior to extraction. Three thiourea (TU) concentrations (1.5, 5, and 10 mM TU, all prepared in 0.2 N HCl) were tested. For each TU concentration, 1.25 mL of elution solution was added to the Eppendorf tube and the samples were subjected either to ultrasonication (US) in a water bath (20 °C, 3 h) or to agitation (A) on an orbital heated shaker (45 °C, 1 h, 1250 rpm). To assess whether a single elution was sufficient to efficiently extract the accumulated MMHg, some gels underwent three successive elution cycles (both US and A conditions), in which the elution solution was renewed at each step (3 × 1 h). In these cases, the three eluates were pooled prior to analysis. The water in the ultrasonic bath was changed between each cycle to avoid overheating. For each treatment, *n* = 5 replicate gels were analyzed. We evaluated the elution efficiency of 3MFS-APA and 3MFS-AG binding gels using the same optimized experimental pro-

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Overview of the different elution conditions tested for the quantitative elution of MMHg spiked in polyacrylamide cross-linked with agarose binding gels (APA), and agarose binding gels (AG). These conditions consisted of testing the influence of different TU concentrations, comparing different elution methods (orbital heated shaker (A), and ultrasonic bath (US) and using single or pooled successive elutions. All conditions were analyzed in triplicate at minimum.</figcaption></figure>

cedure in order to allow a direct comparison of the results. Finally, the optimized elution procedure was latter applied to DGTs deployed in both open-ocean and coastal environments to validate its performance under field conditions.

## 2.5. Statistical analysis

All statistical analyses were performed using Python (version 3.11). The *pandas* library (imported as *pd*) was used for data handling and preprocessing. Statistical tests were conducted using the *scipy.stats* module (imported as *stats*), including parametric and non-parametric tests depending on data distribution. Specifically, one-way ANOVA was applied for normally distributed data (Shapiro-Wilk test) with homogeneity of variances (Levene test), while the Kruskal-Wallis test was used for non-parametric comparisons. The chi-squared distribution (χ²) function from *scipy.stats* was also used where applicable. Statistical significance was set at *p* < 0.05. All results are given with confident level of 95 %. Due to logistical constraints related to the THOE prototype (SWPO, Open Ocean) and the configuration of the mooring line deployed along the Peruvian coast, individual DGT samplers were used in field deployments. Uncertainty for DGT-MMHg concentrations shown in the graph for field samples corresponds to the analytical uncertainty of PT-GC-AFS measurements.

## 2.6. Field deployments

DGTs manufactured in 2022 by DGT Research (using 3MFS-APA binding gels, *n* = 26) were deployed to investigate MMHg depth water profiles in the Peruvian coastal zone (Fig. 2), in Paracas bay, South of Lima, at two stations (E1 and EM) with an approximate depth of 5 and 10 m, respectively. Individual DGT samplers were installed at different depths on mooring lines (See Fig. 2), for approximately 15 days, with 2 consecutive sampling periods (09/03–24/03/2022 and 24/03–04/04/2022, see Table S1 for details). Field DGT samples were eluted and analyzed using the optimized procedure developed in this work. We measured dissolved MMHg in nearby surface waters, out of
<figure><img src="image_5.png" /><figcaption>Fig. 2. Location of the DGT sampling sites in the open ocean in the South Western Tropical Pacific Ocean (SWTPO) (Orange), and in the Coastal Peruvian Upwelling (Pink, Green) in Paracas Bay, South of Lima. The DGTs were installed on specific mooring during 11 months in the SWTPO, at 230 m depth, below the photic zone, and at different depths during a period of 11–15 days in the Peruvian Coast. Water samples collected using GEOTRACES Trace Metal Clean conditions during the TONGA (Blue) and MERTOX (Red) oceanographic cruises were also sampled. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

the bay, during the MERTOX cruise (August 2019) and we use the discrete measurements for comparison.

Early DGTs manufactured in 2019 by DGT Research (using 3MFS-AG binding gel, n = 12) were deployed in the open ocean, in the South Western Tropical Pacific Ocean (SWTPO), near the Tonga Arc Region at 230 m depth on a vertical instrumented mooring line (1940 m depth). This mooring was installed in December 2019 during the GEOTRACES GPpr14 TONGA cruise (Bonnet et al., 2023; GUIEU Cécile and BONNET Sophie, 2019; Tilliette et al., 2023), and recovered during a second specific cruise in November 2020. The DGT samplers were placed in a prototype automated deployment device (THOE) for 11 months (01/12/2019–11/11/2020) (Fig. 2), installed on a trace metal clean cable (Kevlar), below the photic zone in order to prevent biofouling (Fig. 2). The automated deployment device was programmed sequentially to expose each DGT for 30 days (see table S2 for details). During the TONGA cruise, MMHg vertical profiles were measured (see Section 2.8).

## 2.7. MMHg concentrations estimated by DGT (DGT-MMHg)

MMHg concentrations estimated by DGT (DGT-MMHg) were determined using Fick's law (Davison and Zhang, 1994; Hao. Zhang and Davison, 1995), considering the following equation:

$$C_{\text{DGT}} = M \Delta_g / D A t$$

Where D is the diffusion coefficient of MMHg in the diffusive gel, A the exposure area of the gel (3.14 cm²); t the deployment time; and Δg the thickness of the diffusive layer (0.094 cm: diffusive gel and filter). M represents the absolute amount of MMHg accumulated in the binding gel, determined from the concentration of MMHg in the eluted gel solution, measured by PT-GC-AFS. MMHg concentrations were corrected

for procedural and/or DGT field blanks, and subsequently adjusted to account for the recovery factor, as determined from spiked binding gels (94 % for 3MFS-APA and 41 % for 3MFS-AG see Section 3.3). For DGTs deployed in Peru, the diffusion coefficients of MMHg reported by DGT research were used considering the average temperature obtained at each depth during the profiles performed at the time of deployment and recovery. Autonomous temperature data loggers (HOBO Pendant, Onset) were distributed on the mooring at the same depth as the DGTs. At station E1, the water temperature ranged from 17 (bottom) to 19 °C (surface) during the first deployment campaign. It varied from 15 (bottom) to 16 °C (surface) during the second deployment campaign. At the EM station, the water temperature ranged from 14 (bottom) to 15 °C (surface) during the first deployment campaign. It was between 14 (bottom) and 16 °C (surface) during the second deployment campaign.

For DGTs deployed on the mooring in the SWPO we used the average monthly temperature during the time of deployment, recorded by a data logger deployed at the same depth. The average temperature during the deployment was 18 ± 1 °C (n = 12). A slightly modified elution protocol was applied to the 3MFS-AG binding gel recovered from the SWPO experiment. Upon opening the DGT, the 3MFS-AG gel was found to be partially denatured, with a fraction of resin beads adhering strongly to the plastic support. Consequently, it was neither possible to manually recover all the resin beads nor to precisely determine the resin mass, as part of it remained attached to the inner surface of the support. To circumvent this issue, a 30 mL acid-cleaned PFA Savillex vial prefilled with 1.25 mL of 5 mM thiourea (TU) was tightly fitted onto the inner part of the plastic support, ensuring direct contact between the eluent and the binding gel (See Fig. S1). The vial was then placed in an ultrasonic bath for 60 min. This elution step was repeated three times, and the sequential eluates were pooled prior to analysis,
following the optimized procedure established in this work. Spike-and-recovery tests performed on these 3MFS-AG gels yielded a recovery of 41 ± 6%.

## 2.8. Detection limits and blanks

Individual 3MFS-APA binding gels and 3MFS-APA binding gel removed from pre-assembled DGT, eluted using the optimized procedure developed in this work, displayed low blank values of 0.3 ± 0.1 pg and 0.4 pg ± 0.06 pg (n = 3), nearly identical to the instrumental LOD. These values show that no MMHg cross-contamination occurred during the assembly of DGTs by DGT Research. These low blank values show that commercially available DGTs using 3MFS-APA binding gels display a procedural MMHg LOD and LOQ of 0.3 and 1 pg, respectively when measured by PT-GC-AFS, similar to the MMHg LOD and LOQ of water blanks measured by PT-GC-AFS. In the case of 3MFS-AG gels, the blanks value was 0.3 ± 0.4 pg (n = 3), with a MMHg LOD and LOQ of 0.4 and 1.2 pg, respectively. DGT blank concentrations represented less than 5% of the average DGT-MMHg concentration determined in Peruvian coastal waters and 6% in the SWTPO relative to their respective deployment time (see Section 5).

## 2.9. Discrete water samples collection

Seawater from the SWTPO was collected as full water column profiles at 11 stations during the GEOTRACES GPpr14 TONGA expedition onboard the RV L'Atalante, from 31 October 2019 to 06 December 2019. Sampling was conducted using GOFLO bottles mounted on a trace metal-clean rosette (General Oceanics Inc., Model 1018 Intelligent Rosette) attached to a 6 mm Kevlar line. Sampling was done following GEOTRACES clean protocols (Cutter et al., 2017).

Seawater out of the Paracas Bay was collected as full water column profiles at 13 stations onboard the RV Olaya, from 15 to 21 August 2019, IMARPE biannual survey cruises of the Peruvian upwelling system. The same sampling procedure, as for TONGA samples, was applied for MERTOX water samples.

# 3. Results and discussion

## 3.1. Influence of TU concentration on MMHg derivatization

The influence of TU concentrations on MMHg recovery after derivatization relative to standards prepared in MQ water is illustrated in Fig. 3. MMHg derivatization showed a slightly reduced recovery from 100 ± 4% to 84 ± 3% with increasing TU concentration from 1.5 to 10 mM. Our results are in line with those obtained by Clarisse and Hintelmann (2006), showing that matrix effects are more pronounced at higher TU concentrations, although our results were three times more precise than in that previous study. Since MMHg recovery from binding gels is also dependent on the TU concentration of the elution solution (discussed in the following section) we conclude that a TU concentration of 5 mM or less represents an acceptable compromise, in line with Clarisse and Hintelmann (2006) findings. These conditions provided both a high elution efficiency while maximizing the sensitivity of MMHg analysis by PT-GC-AFS, with peak intensities better than 93% compared to the same MMHg standard concentration prepared in MQ water. A Kruskal-Wallis test revealed no significant differences (H = 7.966, | $\chi^2$| Critical = 11.070, p = 0.158) for intra-day (mean ± SD, RSD, n: 93 ± 4%, 5%, 11) and inter-days for the derivatization of MMHg using 5 mM TU (mean ± SD, RSD, n: 91 ± 1%, 1%, 6). This indicates that this effect can be precisely and reliably controlled and corrected. This effect can be directly accounted for by preparing the MMHg calibration standards with the same TU concentration as for the eluted samples.

<figure><img src="image_6.png" /></figure>

Fig. 3. Effect of thiourea (TU) concentration on MMHg derivatization (Blue) on a 300 pg MMHg standard. Comparison with data from Clarisse and Hintelmann (2006) (Grey). Results correspond to Av. ± SD (n = 3). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

## 3.2. Comparison of different elution conditions for MMHg extraction in APA binding gels

The influence of different elution conditions tested on spiked APA binding gels is illustrated in Fig. 4. These conditions included the influence of the different TU concentrations tested above for MMHg derivatization yields, while also comparing single versus successive elutions, performed either in an ultrasonic bath (US) or on an orbital heated shaker (A).

The influence of different TU concentrations on MMHg elution efficiency using a single elution step in the ultrasonic bath ranged from 20 ± 19% (for 1.5 mM TU) up to 64 ± 23% (for 10 mM TU). As expected, the results showed that increasing the concentration of TU in the elution solution improved the elution efficiency of MMHg. For single elution steps, MMHg recovery yields using 5 or 10 mM TU concentrations were not statistically different (ANOVA, F = 0.187,

<figure><img src="image_7.png" /></figure>

Fig. 4. Influence of TU concentrations (1.5, 5 and 10 mM, in 0.2 N HCl) on MMHg elution in APA binding gels spiked with 300 pg of MMHg, using an ultrasonic bath (US) versus agitation on a heated orbital shaker (A). Gel elutions were performed individually, or repeated sequentially on the same gel with renewing the TU solution (×3). Results correspond to Av. ± SD (n = 3).
$p = 0.687$). In the case of repeated, sequential elutions, MMHg recovery yields using either 1.5 mM TU and 5 mM TU were significantly higher than their single elution counterparts (Kruskal-Wallis, $H = 5.123$, $p = 0.024$, ANOVA, $F = 15.003$, $p = 0.012$, respectively), displaying the highest recovery yield ($94 \pm 3\%$) for 5 mM TU. In the case of 10 mM TU, no statistical difference could be detected among the two types of elutions (ANOVA, $F = 2.398$, $p = 0.196$), likely due to the less precise measurements obtained for the single elution step.

These results indicate that despite using the higher TU concentration, a single elution step is insufficient to quantitatively extract MMHg. The consideration of using three, successive and pooled elutions in the ultrasonic bath with renewing the TU solution at each step improved significantly the efficiency of MMHg elution. For the 1.5 mM TU concentration, the sequential elution almost quadrupled MMHg recovery relative to single elution conditions, yielding to a total elution efficiency of $89 \pm 6\%$ (with ultrasonic bath). The sequential elution with a 5 mM TU solution resulted in a cumulative recovery of $94 \pm 3\%$, overall doubling the MMHg recovery of the single elution. Finally, the sequential elution with 10 mM TU concentration improved the recovery of MMHg ($87 \pm 6\%$), compared to a single step elution ($64 \pm 23\%$), but was slightly less efficient than the 5 mM TU successive and pooled elutions. The triple, successive and pooled elution using the ultrasonic bath with a TU concentration of 5 mM represented the best, and more precise conditions to recover MMHg in APA binding gel by PT-GC-AFS ($94 \pm 3\%$, see Fig. 4) with almost no influence on MMHg derivatization efficiency ($93 \pm 3\%$, see Fig. 3). A Kruskal-Wallis test revealed no significant differences of MMHg recovery (%) from 3MFS-APA gels using the optimized procedure (3xUS - 5 Mm TU), for intra-day (mean $\pm$ SD, RSD, n: $94 \pm 3\%$, $3\%$, $7$) and inter-days experiments (mean $\pm$ SD, RSD, n: $94 \pm 3\%$, $3\%$, $5$), $H = 0.972$, $|\chi^2|_{\text{Critical}} = 3.841$, $p = 0.324$. This indicates that the optimized procedure provides high, precise and reliable MMHg recovery in APA binding gel. No further improvements could be obtained by using agitation using a heated orbital shaker, yielding to statically lower recoveries (ANOVA, $F = 25.023$, $p = 0.002$). This indicates that eluting APA binding gels using an ultrasonic bath is a more efficient procedure.

These optimized conditions were thus considered in the rest of the study to maximize instrument sensitivity and performance. Moreover, the elution procedure developed in this work requires only 3 h whereas an elution time of 24 h was mostly used in the majority of the previous studies.

### 3.3. Comparison of the performance of MMHg elution in APA and AG binding gels

The elution procedure optimized in this work was applied to spiked APA and AG binding gels manufactured using the same 3MFS resin by DGT Research for comparison, and employing the same procedure using Eppendorf tubes (Fig. 5, M&M Section 2.4). Although a precise and consistent MMHg recovery of $94 \pm 3\%$ ($n = 5$) was obtained for the 3MS-APA binding gel, MMHg recovery for its 3MFS-AG counterpart was significantly lower (ANOVA, $F = 151.744$, $p = 0.000$, F-critical = $5.318$) yielding to only $41 \pm 6\%$ ($n = 5$). The use of polyacrylamide gels instead of agarose for the binding gel increased MMHg recovery by more than 50%, with the optimized elution procedure. The precision of elution was also better for the 3MFS-APA binding gel ($3\%$, $n = 5$) compared to the 3MFS-AG binding gel ($14\%$, $n = 5$).

These results show that MMHg can be nearly quantitatively eluted from the APA binding gels, compared to the AG binding gels despite employing the same 3MFS resin. This indicates that the type of binding gel used to embed the chelating resin has a significant influence on MMHg elution when measuring samples by PT-GC-AFS. We found that this difference could be related to the mechanical resistance between the two types of gels during elution. Indeed, APA binding gels withstood the entire elution process well, remaining intact, easy to handle and to remove from the elution solution, which is free of 3MFS resin beads. In contrast, AG binding gels degraded into smaller fragments, forming a cloudy and colloidal suspension that retained 3MFS resin beads in the elution solution, even after centrifugation. The presence of residual 3MFS beads, which contain thiol-based chelating groups, is expected to reduce MMHg derivatization yields (Point et al., 2008) due to the re-complexation of a portion of MMHg in the elution solution when

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Influence of the nature of the binding gels (APA vs AG) employing the same 3MFS resin on MMHg elution efficiency. Binding gels were spiked with 300 pg of MMHg. Gel elutions were performed using ultrasonic elution, with 3 successive elutions with 5 mM TU solution. Measurements were performed on 5 replicates. Results correspond to $Av. \pm SD$ ($n = 3$).</figcaption></figure>
the pH is raised back to 4.9 prior to derivatization for PT-GC-AFS analysis (see Experimental Section). The optimal pH for MMHg derivatization (pH 4.9) falls within the effective complexation range of the 3MFS resin used in DGT binding gels (Gao et al., 2014).

Consequently, we recommend the use of 3MFS-APA binding gels for more accurate, quantitative, and efficient MMHg measurements when using PT-GC-AFS. In addition, polyacrylamide gels are preferable over agarose since the polysaccharide structure can be used as a carbon source by certain bacterial groups, thus potentially degrading the AG gel over time (Stroski et al., 2018).

# 4. Field application of DGTs to estimate MMHg concentrations in marine waters, comparison with discrete samples

## 4.1. Application of DGT (3MFS-APA) in coastal waters

The DGT-MMHg concentrations are expected to represent a time-integrated value at different depths. The DGT-MMHg concentration profiles in Peruvian coastal waters are reported in Fig. 6. Overall, DGT-MMHg concentrations ranged from 21 to 413 fM for Paracas Bay with an average value of 170 ± 97 fM (n = 26). The DGTs deployed at different depths revealed the occurrence of steep vertical MMHg gradient across the water column for the two consecutive deployments of 11 and 15 days, and for the two stations, respectively. At station E1 (inshore), DGT-MMHg concentrations were lower in surface water (0–2 m; 109 ± 16 fM; n = 3), compared to epibenthic waters (~5 m; 335 ± 95 fM; n = 4). A vertical MMHg gradient can also be observed at the more central station EM in Paracas Bay, with concentrations

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** DGT (APA-3MFS)-MMHg concentrations profiles at two stations (E1, EM) in Paracas Bay (Peru). Uncertainty for DGT-MMHg concentrations corresponds to the analytical uncertainty of PT-GC-AFS measurements, since only one DGT replicate was deployed at each depth. Dissolved MMHg concentrations (Av. + SD, n = 5) measured by SS-ID-GC-ICP-MS in near surface offshore waters and obtained during the MERTOX cruise are shown for comparison.</figcaption></figure>
ranging between 63 ± 42 fM (n = 4) at the surface (0–3 m) to 168 ± 22 fM (n = 4) just above the sediment interface (~10 m). Increasing MMHg concentration profiles with depth in the water column are similar to the trend obtained for sulfides in Paracas Bay over the same period of time (Diaz et al., 2025; Rioual et al., 2025). These findings suggest MMHg is mainly produced in benthic sediments under anoxic/suboxic conditions, by sulphate-reducing bacteria (King et al., 2000), and diffuse into the water column, similar to sulfides (Flores et al., 2023; Rioual et al., 2025).

The lower MMHg concentrations found at the central station EM relative to E1 are consistent with the circulation in the bay (Arellano et al., 2023; Chauca Vela, 2018; Flores et al., 2023), with a stronger inflow of oceanic upwelling water in the central part of the bay, reflecting the influence of outer water masses, relatively depleted in MMHg compared to enriched water masses influenced by coastal sediments. Dissolved MMHg concentrations available at 2 nearby stations, outside of Paracas Bay, measured on discrete surface water samples (102 ± 20 fM, av. ± SD, <30 m, n = 5) are consistent with DGT-MMHg concentrations at the surface at stations E1 and EM (80 ± 43 fM; av. ± SD, < 2 m, n = 5). Although these two datasets were collected on different dates and capture different periods of time, they provide consistent and reference MMHg concentration values in the low fM range in Peruvian coastal waters. However, this dataset remains limited because only a single DGT was deployed at each sampling point rather than triplicate samplers, and the comparison with discrete seawater samples relies solely on the few measurements available in the area. Despite these constraints, the results highlight the potential of combining easy-to-use DGTs with the accessibility of PT-GC-AFS analysis to study the coastal marine Hg cycle. DGTs display the potential to be used for the identification of MMHg sources and to provide indications of vertical MMHg gradients at a sub-meter scale.

## 4.2. Application of DGT (3MFS-AG) in the open ocean

The average DGT-MMHg (3MFS-AG) concentration in the SWATPO during the 11 months collection at 230 m depth was 50 ± 30 fM (Fig. 7, mean ± SD, n = 12). This unique data can also be considered as the first representative annual MMHg reference average value for the year 2020, at this depth, for this ocean region. This DGT-MMHg concentration is consistent with the average subsurface (0–500 m) dissolved MMHg concentration (55 ± 42 fM (Fig. 7, mean ± SD, n = 19)) obtained from water samples at station 12 (where the DGT mooring was installed) during the TONGA cruise in November 2019 (Torres-Rodriguez et al., 2025). In addition, the first DGT-MMHg concentration of the time series (December 2019, 106 fM) is in good agreement with the dissolved MMHg concentrations measured at the same depth at surrounding stations 4, 11 and 12 in November 2019 (Fig. 7, see Torres-Rodriguez et al., 2025 for more details). This result shows that DGTs deployed in the open ocean over short and long periods of time can provide consistent MMHg concentration estimates in the ultra-trace fM range in line with discrete measurements performed by SS-ID-GC-ICP-MS (Heimbürger et al., 2015) on water samples collected using standard GEOTRACES TMC procedures.

Fig. 8 shows the monthly time trend of DGT-MMHg concentrations, illustrating the occurrence of a 5-fold factor concentration difference over almost a year (18–106 fM). These results may suggest that seasonal changes in marine primary production in this region (Bonnet et al., 2023) could influence oceanic MMHg concentrations, as expected from modelling studies (Schartup et al., 2019), but other processes might be at play. Given that monthly Hg time-series measurements in the open ocean are practically unfeasible with conventional oceanographic cruises, the DGT time series presented here represent a preliminary but valuable step toward understanding MMHg dynamics. Nevertheless, these findings remain limited by the fact that the DGT autosampler prototype used at that time was only capable of

<figure><img src="image_10.png" /></figure>

Fig. 7. Comparison between MMHg concentrations (fM) estimated from DGT deployed at 230 m depth (red crosses, n = 12, from 01/12/ 2019–11/11/2020) with discrete measurements on seawater samples collected between 0 and 500 m depth during the TONGA cruise at stations 4 (purple dots), 11 (light blue dots), and 12 (green dots), near DGT mooring, in November 2019 (see Fig. 2). Each DGT was exposed sequentially for approximately 30 days using an automated in situ deployment device (THOE) during a year from 01/12/2019 to 11/11/2020. Each MMHg-DGT concentration is labeled with a number corresponding to the sampling month to illustrate the temporal variability of the measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

deploying one DGT at a time on monthly basis. This evaluation also relies on a small number of available discrete seawater measurements. Further improvements would consist of improving the DGT autosampler capacity to expose at least 3 DGT replicates at a time, and to collect more reference water samples to strengthen future intercomparisons.

DGTs equipped with 3MFS binding gels display a Hg binding capacity estimated to 2 mg (Clarisse and Hintelmann, 2006). In the case of a deployment in the open ocean, where MMHg is present at ultratrace level (fM), we assume that the use of 3MFS resin is suitable to offer sufficient binding capacity over a long period of time (see section 5). For instance, a recent study illustrated the application of DGTs for trace metals determination in marine abyssal environments for a deployment time between 1 month and 2 years (Schmidt et al., 2022; Schmidt and Paul, 2023). As indicated in the previous section, we found that the 3MFS-AG binding gels were more difficult to elute, with a need to apply a significant correction factor (41 ± 6 %) to estimate MMHg concentrations accumulated in the binding gel, although precise and reproducible elution results were obtained with the method developed in this work. New 3MFS-AG binding gels are not affected by this issue, but since they were manufactured after 2021, and not yet available in 2019, 3MFS-AG were used at that time in this work. Further work with the DGT-3MFS binding gels is required for studies in the open ocean, with an extended comparison with discrete water samples over time.
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Temporal evolution of DGT-MMHg concentrations (fM). Uncertainty for DGT-MMHg concentrations corresponds to the analytical uncertainty of PT-GC-AFS analysis, since only one DGT replicate was exposed every month. Horizontal bars represent the respective deployment time of each DGT by the autonomous automated deployment device.</figcaption></figure>

**5. Implications and recommendations for the use of DGT to study the marine MMHg cycle**

Our study suggests that DGT passive samplers can provide relatively precise and accurate MMHg estimates in marine waters in the low fM range, in the same range relative to reference measurements obtained in seawater samples using state-of-the-art procedures. We recommend using DGTs prepared with 3MFS-APA binding gels which provide quantitative and representative MMHg estimates, relative to 3MFS-AG binding gels. The latter require the application of a significant correction factor for the incomplete but still precise elution of MMHg from the binding gel, as well as a structural fragility/weakness that makes the handling of the binding gel difficult occasionally.

The simplicity, ease of use, in situ preconcentration capabilities, low blanks and low-cost consideration of DGT represent significant advantages in order to be further used to study the marine MMHg cycle, and possibly other trace elements. DGT provides an integrated value of MMHg concentrations during their time of deployment, over several weeks. Fig. 9 presents a theoretical illustration of the effect of DGT deployment time (days) on the accumulated MMHg amount (pg) using the recommended APA-3MFS DGT, based on dissolved MMHg concentrations measured in the SWTPO at stations 4, 11, and 12—namely surface waters (57 ± 28 fM, Av. ± SD, n = 16), intermediate waters (142 ± 40 fM, n = 18), and deep waters (240 ± 42 fM, n = 18) (See Torres-Rodriguez et al. (2025) for more details). Assuming that these concentrations are representative and stable over the entire deployment period, a minimum time of deployment of DGT of 1 day for deep waters, 2 days for intermediate waters and 5 days for surface waters is needed to reach the limit of quantification of MMHg in 3MFS-APA DGTs by PT-GC-AFS (1 pg). We thus recommend a deployment time of approximately 24 days for ocean surface waters, 10 days for intermediate waters and 6 days for deep waters to reach at least 5 pg of MMHg, representing 5 times the LOQ. This figure is intended primarily as an il-

lustrative tool for readers who may not be familiar with DGT applications in marine systems. It highlights that, because MMHg occurs at ultra-trace levels in seawater, long deployment times are essential to achieve quantifiable MMHg accumulation in the binding gel, particularly in surface waters where concentrations are lowest. Our study is among the very few to apply DGTs in open-ocean environments, and this example underscores the practical and operational constraints in terms of minimum and recommended time of deployment to achieve marine MMHg measurements by DGT.

Since longer deployment time is needed for DGTs in surface waters, problems associated with biofouling developing at the surface of DGT may be problematic. Biofouling could affect the diffusion of MMHg from seawater to the binding gel (Díez and Giaggio, 2018; Noh et al., 2019; Xue et al., 2024), and associated bacterial groups can damage the agarose gels that can be used as a food source (Stroski et al., 2018), thus influencing the results estimated from DGTs. In this work, we deployed DGTs below the photic layer, at a 230 m depth to minimize this issue. Further investigations including longer deployment time, and testing DGT in more contrasted types of marine water should be conducted in the future.

**6. Conclusion**

A revised and detailed step by step procedure for the elution of MMHg from 3MFS resin binding gels of DGT passive samplers has been developed in this work. This procedure represents an improved methodology relative to the early work by Clarisse and Hintelmann (2006). Our new procedure is based on successive pooled elution of DGT binding gels in an ultrasonic bath using 1.25 mL of 5 mM TU/0.2 N HCl solution, repeated 3 times. We found an average MMHg recovery of 94 % (RSD 3 %, n = 5) on spiked DGT Research's polyacrylamide APA-3MFS binding gels. DGT Research's older agarose 3MFS-AG binding gels were not eluted as well (MMHg recovery mean 41 %,
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Influence of deployment time (days) in the open ocean on theoretical MMHg mass (pg) accumulated in DGT in the SWTPO region for surface (0–200 m, yellow), intermediate (200–500 m, light blue) and deep waters (>500 m, dark blue). The MMHg concentration values used for the different ocean water layers correspond to the average MMHg concentration measured for the stations 4, 11 and 12 of the TONGA cruise for the different depth ranges. The LOQ for MMHg DGT blanks (dotted line, green – 1 pg) is also reported. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

RSD = 14 %, n = 5), although still providing precise and reproducible determinations.

This elution procedure was tested on DGTs deployed in the coastal ocean, in Paracas Bay (Peru), and from the SWTP open ocean. In both cases, DGT-MMHg concentrations were in good agreement relative to discrete seawater samples collected during oceanographic cruises, using established and referenced GEOTRACES procedures. The analysis of DGT elutions by PT-GC-AFS is easier and relatively more accessible to non-expert users than the analysis of water samples by SS-ID-GC-ICP-MS. DGTs deployed in Paracas bay (Peru) revealed the presence of vertical depth gradient in MMHg concentrations within the water column, highlighting the contribution of a benthic production of MMHg. Diffusive gradient in thin film time series in the SWTPO documented for the first time the existence of seasonal variations in MMHg concentrations, in subsurface waters of the open ocean.

The analytical development and applications conducted in this work show that DGT represents a promising, relatively simple, and powerful tool to study MMHg biogeochemistry at ultra-trace concentrations in the coastal and open ocean for the Hg research community and beyond. Regular discrete sampling is restrictive as it requires significant ship time, vessel infrastructure, field logistics and a higher cost. Passive DGT samplers provide a complementary time-integrated average concentration, which may be more representative over a defined exposure time relative to a discrete sample, that can capture short term processes. The use of an automated autonomous in situ system for deploying DGTs

over time makes it now possible to monitor changes in MMHg concentrations in the ocean, limiting the need for expensive oceanographic logistical costs. This innovative approach may be used to document the state and trend of MMHg contamination in marine waters in coastal and/or oceanic observational networks to help evaluate the implementation of the Minamata Convention.

### CRediT authorship contribution statement

**Isalyne Blondet:** Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization, Writing – review & editing, Writing – original draft. **Fanny Rioual:** Methodology, Investigation, Writing – review & editing, Writing – original draft. **Michelle I. Graco:** Project administration, Investigation, Writing – review & editing, Writing – original draft. **Jesús Ledesma:** Resources, Methodology, Writing – review & editing, Writing – original draft. **Kevin Diaz Ruiz:** Resources, Methodology, Investigation, Data curation, Writing – review & editing, Writing – original draft. **Arturo Aguirre-Velarde:** Resources, Methodology, Investigation, Writing – review & editing, Writing – original draft. **Jonathan Flye-Sainte-Marie:** Resources, Methodology, Investigation, Writing – review & editing, Writing – original draft. **Laure Laffont:** Methodology, Formal analysis, Writing – review & editing, Writing – original draft. **Lars-Eric Heimbürger-Boavida:** Validation, Resources, Investigation, Writing – review & editing, Writing – original draft. **Natalia Torres-Rodriguez:** Validation, Formal analysis, Writing
– original draft. **Sophie Bonnet:** Resources, Methodology, Investigation, Writing – review & editing, Writing – original draft. **Cécile Guieu:** Resources, Investigation, Writing – review & editing, Writing – original draft. **Vincent Raimbault:** Supervision, Writing – review & editing, Writing – original draft. **David Point:** Validation, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization, Writing – review & editing, Writing – original draft.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work is a contribution to the MERTOX project (PI DP) funded by French National Research Agency (ANR-17-CE34-0010) 'Unraveling the origin of methylMERcury TOXin in marine ecosystems. The TONGA ('shallow hydroThermal sources of trace elements: potential impacts on biological productivity and the bioloGicAl carbon pump') oceano-graphic project and campaign (PIs SB & CG) was funded by the TGIR Flotte Océanographique Française, the A-MIDEx of Aix-Marseille University, the LEFE-CYBER and GMMC program and the Agence Nationale de Recherche (ANR-18-CE01-0016) This work is a contribution to the INSU-LEFE project 'Exploration in situ du rôle des diazotrophes et des éléments traces métalliques dans la spéciation du mercure de l'Océan Tropical' (DIAZ-METALIS, PI DP). This work has been supported by the CNRS EC2CO HYPER project (PI JFSM). Lars-Eric Heimbürger-Boavida acknowledges funding via the French National Funding Agency project HydrOThermal Mercury (ANR-21-CE34-0026). I. Blondet thanks the University Paul Sabatier and the regional council of Midi-Pyrénées for funding her PhD grant. F. Rioual was supported by a scholarship of the Franco Peruvian Doctoral School of Life Sciences.

## Data availability

Data will be made available on request.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.marpolbul.2025.118813.

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