Global Patterns of Mercury Speciation and Biomagnification in

Overview

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Key numbers

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MeHg strongly correlated (R2 = 0.99), but MeHg-THg ranged
65−101%, demonstrating substantial interspecific variability and challenging the assumption of near-complete methylation.
total Hg (THg) increases with trophic level, from ∼5% in
time, several of these species are threatened with extinction, IUCN Red List (Supporting Information, Table S1).
assemblage of shark species (N = 18), from the Mediterranean mL of high-purity HNO3 and 0.5 mL of high-purity HCl, both
By identifying species that are both highly contaminated and Belgium) was included, yielding a mean recovery of 109 ± 9% (n = 7;
monitoring, fisheries management, and public health, and Reference), which produced an average recovery of 102 ± 6% across
Figure 1. PCA biplot based on THg, MeHg, trophic position (TP), and MeHg/Hg in elasmobranch individuals (n = 63). PC1 (50% variance)
reflects Hg bioaccumulation mainly driven by THg and MeHg loadings, while PC2 (37% variance) represents trophic-related gradients in %MeHg.
deleterious effects on human health: Information, Table S2). Only 18.7% (N = 26) of these studies
established as 0.227 kg for adults.36 The US EPA recommends a imately 8% of shark biodiversity (N = 536 species43). While
in noncarcinogenic risk. since 1980s (1975−2025; R2 = 0.56, n = 139, p < 0.0001)
0.14, n = 29, p = 0.0355). Elevated THg concentration were also observed in large
MeHg data to nearly 10% of known shark biodiversity. Given indicative of biomagnification processes at higher trophic
Information, Tables S1 and S3). Principal Component Analysis Habitat and diet are major drivers of THg variability in
(50% variance) was driven by THg and MeHg, representing Mediterranean deep-sea sharks, including H. griseus, H. perlo,
bioaccumulation, while PC2 (37% variance) captured trophic- G. melastomus, E. spinax, and D. licha, exhibited the highest
related variation, including MeHg-THg ratios, reflecting THg (Table S3), consistent with prior reports.17,46,48 Among
predators, consistent with increasing MeHg proportion at (Supporting Information, Table S1 and S3).
reproductive mode was observed, suggesting that Hg variability in C. maximus to 17.7 mg kg−1 in H. griseus, Table S3), with a
THg ranged from 0.05−0.11 mg kg−1 in C. macloti to a Species-level correlations were significant in C. sorrah (R2 =
microstoma (0.44−1.10 mg kg−1), M. mosis (0.26−4.60 mg MeHg-THg ratios ranged from 64.8% (C. maximus) to
(0.31−1.02 mg kg−1) (Supporting Information, Table S3). The MeHg dominates THg in shark muscle,37,44,45,57 but this
instance, C. maximus ranged from 390 to 437 cm TL, C. higher MeHg proportions than invertebrate feeders, with
melanopterus from 59 to 61 cm TL, R. acutus from 78 to 85 cm Squalus mitsukurii and Galeorhinus australis showing ∼90%
TL, and C. sorrah from 68 to 90 cm TL. MeHg compared to ∼75% in Mustelus canis and Mustelus
loads, associated with the wide size range of the sampled p = 0.0008) (Figure 3). Expanding MeHg analyses across
Figure 2. Robust linear regression of MeHg on THg (mg kg−1) in individuals (13−16 cm) exhibited mean THg concentrations
Figure 3. Comparison of MeHg-THg ratios among shark species, with colored bars indicating sampling regions: Mediterranean Sea (orange),
MeHg (Kruskal−Wallis test; THg: H = 9.02, d.f. = 2, n = 32, p Ocean species, THQs were >1 for both P. glauca and I.
= 0.011; MeHg: H = 9.26, d.f. = 2, n = 32, p = 0.0098), likely oxyrinchus, a finding of particular concern since fillets of these
while E. spinax and G. melastomus only subadult and adult requiring <2 meals/month (Supporting Information, Table
risks for 83.3% of species, while realistic weekly consumption ecosystems as well, measured concentrations in sharks were
led to THQ > 1 in 50% of species, primarily Mediterranean checked against the Environmental Quality Standards EQSbiota.
deep-sea sharks (Supporting Information, Table S4), indicating All shark species exceeded EQSbiota thresholds (0.02 mg kg−1),
ature data collection and validation process. Table S1:
Global temporal distribution of these studies. Table S3:
reproductive mode. Tables S4 and S5: Human health mercury based on water column measurements. Nature 2014, 512,
Ochoa-Herrera, V. Concentrations and stable isotopes of mercury in 4773.

Methods (brief)

ocean basins is lacking. spectrometry system (iCAP Q ICP-MS, Thermo Scientific) after
Thus, the present study provides a multiocean assessment of microwave-assisted digestion carried out with a Milestone ETHOS
One (dissolved mass approximately 20 mg). For digestion, a
Sea, Indian and Atlantic Oceans, including nine species for obtained through sub-boiling distillation). The digestion protocol
ratios were determined to assess inter- and intraspecific mg of each sample was combined with 5 mL of an extracting solution
role in toxicity and risk assessment. The reliability of THg as a between the peristaltic pump and the ICP-MS nebulizer.35
2.1. Sample Collection
Muscle samples of Carcharhinus macloti, C. melanopterus, C. sorrah, calculated as 10b, with “b” being the slope of the following linear
markets in Djibouti City (2017−2021).26 Cetorhinus maximus samples log10(C ) = a + b × TP
previously reported by Boldrocchi et al. (2022).30 Additional samples indicates decreasing concentration with trophic level. TMF was
melastomus, Heptranchias perlo, Hexanchus griseus) were collected in 2.4. Human Health Risk Assessment
commercial samples, species identification was based on labeling
All samples consisted exclusively of post-mortem tissues obtained and 48 days year−1 considering fish consumption once a week); Ed is
sample sizes ranging from 1 to 10 individuals (Supporting 21.632, p < 0.0001).
homogeneous life stages of the sampled individuals. For variation.28,47,58 For instance, piscivorous sharks exhibited
loads, associated with the wide size range of the sampled p = 0.0008) (Figure 3). Expanding MeHg analyses across
shark samples from the Mediterranean Sea, Indian and Atlantic of approximately 1.7 mg kg−1, whereas larger specimens (42
for global consumer risk (Figure 5). sampled in Portugal exceeded the legal Hg limit, potentially

Implications

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Update history

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Commit	Date	Change	Description
b01ec52c	2026-08-04	major	2 sections added
d49e450f	2026-08-03	major	5 sections added; narrative text revised