Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs

Sources and Transformation of Methylmercury in Paddy Water: Insights from Mercury Isotopes Collected by Diffusive Gradients in Thin Films

Despite concerns about methylmercury (MeHg) contamination in rice, the sources and transformation mechanisms of MeHg within paddy field water, the primary source of MeHg in rice, remain unclear. Determination of the isotopic composition of MeHg in paddy water is crucial to clarify these processes. However, there is a lack of sampling and analytical methods for quantifying MeHg isotopes in water samples. In this study, we use diffusive gradients in thin films (DGT) in situ to collect MeHg from paddy water to determine the concentration of MeHg and the associated isotopic composition. This technique enables high collection efficiency of aqueous MeHg with limited Hg isotope mass-dependent fractionation (∼- 0.2‰ δ202Hg) and mass-independent fractionation (<0.1‰ Δ199Hg). Field applications using the developed DGT method suggest that in situ methylation of soluble Hg(II) drives the generation of MeHg in paddy water. MeHg in overlying water exhibits a Δ199Hg/Δ201Hg ratio of 1.07 ± 0.09, indicating significant photoreduction of aqueous Hg(II) before methylation. The absence of photodemethylation Δ199Hg/Δ201Hg ratio (∼1.36) suggests limited MeHg demethylation in the overlying water. This study provides insights into the sources and transformation of MeHg in rice paddies and helps develop mitigation strategies to reduce MeHg exposure through rice consumption.

Methylmercury exposure of the sponge O. lobularis induces strong tissue and cell defects

Mediterranean marine biota suffers from various anthropogenic threats. Among them, pollutants such as mercury (Hg) represent important environmental issues that are exacerbated by bioaccumulation and bioamplification along food webs via its organic form, monomethylmercury (MMHg). To date, very little is known regarding the impact of mercury on Porifera and the few available studies have been exclusively focused on Demospongiae. This work studies the effect of MMHgCl at different biological levels of Oscarella lobularis (Porifera, Homoscleromorpha). Bioaccumulation assays show that MMHgCl significantly accumulated in sponge tissues after a 96-h exposure to 0.1 μg L-1. Toxicity assays (LC5096h) show a sensibility that depends on life-stage (adult vs bud). Additionally, we show that the exposure to 1 μg L-1 MMHgCl negatively impacts the epithelial integrity and the regeneration process in buds, as shown by the loss of cell-cell contacts and the alteration of osculum morphogenesis. For the first time in a sponge, a whole set of genes classically involved in metal detoxification and in antioxidant response were identified. Significant changes in catalase, superoxide dismutase and nuclear factor (erythroid-derived 2)-like 2 expressions in exposed juveniles were measured. Such an integrative approach from the physiological to the molecular scales on a non-model organism expands our knowledge concerning sensitivity and toxicity mechanisms induced by MMHg in Porifera, raising new questions regarding the possible defences used by marine sponges.

Exceptionally high levels of total mercury in deep-sea sharks of the Southeastern Mediterranean sea over the last ∼ 40 years

Deep-sea habitats are currently recognized as a hot spot for mercury (Hg) accumulation from anthropogenic sources, resulting in elevated concentrations of total mercury (THg) in deep-sea megafauna. Among them, deep-sea sharks (Class Chondrichthyes) are characterized by high trophic position and extended longevity and are, therefore, at high risk for mercury contamination. Despite this, sharks are overexploited by fishing activity in increasingly deeper water, worldwide, imposing health risks to human consumption. While it is imperative to better understand long-term mercury contamination in deep-sea megafauna, few historical data sets exist to capture this process. Here we explore four decades (1985-2022) of THg accumulation in five species of deep-sea sharks (G. melastomus, E. spinax, S. rostratus, C. granulosus, and D. licha) of the ultra-oligotrophic Southeastern Mediterranean Sea (SEMS) sampled during 19 research cruises. We exhibited exceptionally high THg levels (per length/weight), the highest as 16.6 μg g-1 (wet wt.), almost entirely (98.9 %; n = 298 specimens) exceeding the limit for safe consumption (0.3-0.5 μg THg g-1 wet wt.). The maximal THg levels of the long-lived species D. licha and C. granulosus in the SEMS were enriched by a factor of ∼ 7 and >10 compared to counterpart species from other oceanic areas, respectively. We attribute this to the ultra-oligotrophic conditions of the SEMS, which cause slower growth rates and dwarfism in deep-sea sharks, resulting in an extended exposure time to mercury contamination. In the long-lived species, C. granulosus and D. licha, a temporal increase of average THg levels of ∼ 80 % was recorded between 1987-1999 and 2021-2022. This likely reflects the long-term accumulation of historical anthropogenic Hg in deep-sea environments, which is further amplified in marginal seas such as the Mediterranean, impacted by global air pollution crossroads and surrounded by land-based pollution sources. Future consumption of products from deep-sea sharks is potentially high risk to human health.

Deglacial volcanism and reoxygenation in the aftermath of the Sturtian Snowball Earth

The Cryogenian Sturtian and Marinoan Snowball Earth glaciations bracket a nonglacial interval during which Demosponge and green-algal biomarkers first appear. To understand the relationships between environmental perturbations and early animal evolution, we measured sulfur and mercury isotopes from the Datangpo Formation from South China. Hg enrichment with positive Δ199Hg excursion suggests enhanced volcanism, potentially due to depressurization of terrestrial magma chambers during deglaciation. A thick stratigraphic interval of negative Δ33Spy indicates that the nonglacial interlude was characterized by low but rising sulfate levels. Model results reveal a mechanism to produce the Δ33S anomalies down to -0.284‰ through Rayleigh distillation. We propose that extreme temperatures and anoxia contributed to the apparent delay in green algal production in the aftermath of the Sturtian glaciation and the subsequent reoxygenation of the iron-rich and sulfate-depleted ocean paved the way for evolution of animals.