New insights into the biomineralization of mercury selenide nanoparticles through stable isotope analysis in giant petrel tissues

Organ-specific mercury stable isotopes, speciation and particle measurements reveal methylmercury detoxification processes in Atlantic Bluefin Tuna

Identifying metabolism and detoxification mechanisms of Hg in biota has important implications for biomonitoring, ecotoxicology, and food safety. Compared to marine mammals and waterbirds, detoxification of MeHg in fish is understudied. Here, we investigated Hg detoxification in Atlantic bluefin tuna Thunnus thynnus using organ-specific Hg and Se speciation data, stable Hg isotope signatures, and Hg and Se particle measurements in multiple tissues. Our results provide evidence for in vivo demethylation and biomineralization of HgSe particles, particularly in spleen and kidney. We observed a maximum range of 1.83‰ for δ202Hg between spleen and lean muscle, whereas Δ199Hg values were similar across all tissues. Mean percent methylmercury ranged from 8% in spleen to 90% in lean muscle. The particulate masses of Hg and Se were higher in spleen and kidney (Hg: 61% and 59%, Se: 12% and 6%, respectively) compared to muscle (Hg: 2%, Se: 0.05%). Our data supports the hypothesis of an organ-specific, two-step detoxification of methylmercury in wild marine fish, consisting of demethylation and biomineralization, like reported for waterbirds. While mass dependent fractionation signatures were highly organ specific, stable mass independent fractionation signatures across all tissues make them potential candidates for source apportionment studies of Hg using ABFT.

Nanoparticles as a younger member of the trace element species family - a food perspective

Speciation analysis plays a key role in understanding the biological activity and toxicity of an element. So far, classical speciation analysis focused only on the dissolved fraction of an elemental species, whereas nanoparticle forms of analytes are being widely found in consumer and industrial products. A significant contributor to human exposure to nanoparticles is through food into which nanoparticles can be incorporated from endogenous sources or they may be formed naturally in the living organisms. Nanoparticles often undergo changes in the food matrices and upon consumption, in the gastrointestinal tract, which present a significant challenge to their characterisation. Therefore, a combination of both classical and nanoparticle speciation analytical techniques is needed for the characterisation of both dissolved and particulate forms of the chemical species. This article presents and discusses the current trends in analysis of nanoparticle behaviour in the gastrointestinal tract and formation and characterisation of biogenic nanoparticles.

Simple and rapid formic acid sample treatment for the isolation of HgSe nanoparticles from animal tissues

The present work explores for the first time the potential of formic acid on the extraction of tiemannite (HgSe) nanoparticles from seabird tissues, in particular giant petrels. Mercury (Hg) is considered one of the top ten chemicals of major public health concern. However, the fate and metabolic pathways of Hg in living organisms remain unknown. Methylmercury (MeHg), largely produced by microbial activity in the aquatic ecosystems is biomagnified in the trophic web. HgSe is considered the end-product of MeHg demethylation in biota and an increasing number of studies focuses on the characterization of this solid compound to understand its biomineralization. In this study, a conventional enzymatic treatment is compared with a simpler and environmentally friendly extraction by using formic acid (5 mL of = 50 % formic acid) as exclusive reagent. The analyses by spICP-MS of the resulting extracts from a variety of seabird biological tissues (liver, kidneys, brain, muscle) reveal comparable results by both extraction approaches in terms of nanoparticles stability and extraction efficiency. Therefore, the results included in this work demonstrate the good performance of employing organic acid as simple, cost effective and green procedure to extract HgSe nanoparticles from animal tissues. Moreover, an alternative consisting of a classical enzymatic procedure but with ultrasonic assistance reducing the extraction time from 12 h to 2 min is also described for the first time. The sample processing methodologies developed, combined with spICP-MS, have emerged as powerful tools for the rapid screening and quantification of HgSe nanoparticles in animal tissues. Finally, this combination allowed us to identify the possible occurrence of Cd particles and As particles associated with HgSe NPs in seabirds.

Basic and advanced spectrometric methods for complete nanoparticles characterization in bio/eco systems: current status and future prospects

The use of engineered nanoparticles in the environment and human life has increased in the last 20 years. The risk assessment concerning application of nanomaterials in biological systems requires their thorough characterization. Understanding the correlations between physicochemical properties of nanoparticles concerning not only the size, particle size distribution, number concentration, degree of aggregation, or agglomeration but also solubility, stability, binding affinity, surface activity, chemical composition, and nanoparticle synthesis yield allows their reliable characterization. Thus, to find the structure-function/property relationship of nanoparticles, multifaceted characterization approach based on more than one analytical technique is required. On the other hand, the increasing demand for identification and characterization of nanomaterials has contributed to the continuous development of spectrometric techniques which enables for their qualitative and quantitative analysis in complex matrices giving reproducible and reliable results. This review is aimed at providing a discussion concerning four main aspects of nanoparticle characterization: nanoparticle synthesis yield, particle size and number concentration, elemental and isotopic composition of nanoparticles, and their surface properties. The conventional and non-conventional spectrometric techniques such as spectrophotometry UV-Vis, mass spectrometric techniques working in conventional and single-particle mode, or those based on optical emission detection systems are described with special emphasis paid on their advantages and drawbacks. The application and recent advances of these methods are also comprehensively reviewed and critically discussed.

Presence of nano-sized mercury-containing particles in seafoods, and an estimate of dietary exposure

The toxicity of nano-sized particles of mercury (NP-Hg), which are thought to be generated during the detoxification of methyl mercury (MeHg), may differ from that of MeHg, elemental Hg (Hg⁰), and inorganic Hg (I-Hg). From a human health perspective, it is important to evaluate the presence of NP-Hg in seafoods. We investigated the in vivo formation of NP-Hg in fish and shellfish, which are the main sources of Hg exposure in humans. NP-Hg was measured in 90 fish samples with single-particle inductively coupled plasma mass spectrometry (spICP-MS) after enzyme degradation with pancreatin and lipase. In addition to NP-Hg, total Hg (T-Hg), MeHg, and selenium (Se) concentrations were evaluated. Transient Hg signals were detected as nanoparticles from almost all samples by using spICP-MS. Higher particle number concentrations (C_PN) were observed in the tuna-swordfish group than in the shellfish group (17.7 × 10⁷ vs. 1.2 × 10⁶ particles/g, respectively). Although the C_PN and maximum particle mass increased significantly with increasing T-Hg concentration, the increase in C_PN was greater than those in maximum particle mass. Assuming that the NP-Hg detected was HgSe (tiemannite) and spherical based on previous reports, the maximum particle diameter was estimated to be 89 nm. The mean dietary exposures to NP-Hg, T-Hg, and MeHg were estimated to be 0.067, 5.75, and 5.32 μg/person per day, respectively. Generation of NP-Hg was inferred to be widespread in marine animals, with a preferential increase in the number of particles rather than an increase in particle size. The mean dietary exposure to NP-Hg in Japanese people was estimated to be 1.2 ng/kg body weight (BW) per day. Compared to PTWI of 4 μg/kg BW per week (0.57 μg/kg BW per day) derived by JECFA (2011), the health risk from redissolved I-Hg from NP-Hg is small.

Internal Dynamics and Metabolism of Mercury in Biota: A Review of Insights from Mercury Stable Isotopes

Monitoring mercury (Hg) levels in biota is considered an important objective for the effectiveness evaluation of the Minamata Convention. While many studies have characterized Hg levels in organisms at multiple spatiotemporal scales, concentration analyses alone often cannot provide sufficient information on the Hg exposure sources and internal processes occurring within biota. Here, we review the decadal scientific progress of using Hg isotopes to understand internal processes that modify the speciation, transport, and fate of Hg within biota. Mercury stable isotopes have emerged as a powerful tool for assessing Hg sources and biogeochemical processes in natural environments. A better understanding of the tissue location and internal mechanisms leading to Hg isotope change is key to assessing its use for biomonitoring. We synthesize the current understanding and uncertainties of internal processes leading to Hg isotope fractionation in a variety of biota, in a sequence of better to less studied organisms (i.e., birds, marine mammals, humans, fish, plankton, and invertebrates). This review discusses the opportunities and challenges of using certain forms of biota for Hg source monitoring and the need to further elucidate the physiological mechanisms that control the accumulation, distribution, and toxicity of Hg in biota by coupling new techniques with Hg stable isotopes.

Comment on "New insights into the biomineralization of mercury selenide nanoparticles through stable isotope analysis in giant petrel tissues"

Some birds and cetaceans can demethylate the toxic methylmercury cysteinate (MeHgCys) complex into inert mercury sulfide (HgSe) through the formation of an intermediate tetrahedral selenolate complex with selenocysteine (Sec) residues (Hg(Sec)₄). The nucleation of the HgSe biominerals involves the substitution of the Se ligand for the Sec residues, which is considered to occur in the form of multinuclear Hg_x(Se,Sec)_y clusters mediated by proteins. Queipo-Abad et al. (2022) isolated HgSe nanoparticles from the biological tissues of giant petrels and measured the mass-dependent fractionation of the ²⁰²Hg isotope (δ²⁰²Hg). They concluded that the δ²⁰²Hg values of the HgSe nanoparticles from each tissue of individual petrels are specific to the HgSe species alone and that the Hg(Sec)₄ → HgSe reaction occurs without fractionation of the ²⁰²Hg isotope. We show (1) that the HgSe nanoparticles are likely mixtures of MeHgCys, Hg(Sec)₄, and HgSe, and therefore that the δ²⁰²Hg values are not species-specific, and (2) that the ²⁰²Hg isotope is actually fractionated during the Hg(Sec)₄ → HgSe reaction, and therefore that this isotope can be used to trace the Hg metabolic pathways between tissues in a single individual and in different animals.