MeHg Accumulation Research Articles

Kinetic characteristics of and critical stages for mercury accumulation in rice (Oryza sativa L.)

By studying the dynamic characteristics of and key growth stages for mercury (Hg) enrichment in rice, the Hg migration and translocation processes in this species can be better understood. In this study, a pot experiment was conducted, wherein two rice cultivars, Tianyouhuazhan (TYHZ, indica) and Zhendao 18 (ZD18, japonica), were selected and planted for analysing the Hg accumulation kinetic characteristics in rice plants. The plants were sampled at each growth stage, and the biomass and total Hg (THg) and methylmercury (MeHg) concentrations of each tissue were measured. The relative Hg contribution rates (CRs) in whole rice plants and rice grains were calculated, and the growth stage with the highest relative contribution was identified as the key growth stage for Hg accumulation. The results indicated that in rice, the MeHg translocation capability was stronger than the THg translocation capability. Significant differences in the kinetic characteristics of Hg accumulation were found between the two rice cultivars, and the TYHZ rice grains had a stronger Hg accumulation ability than the ZD18 rice grains. The key growth stages for THg accumulation in whole rice plants of both cultivars were the tillering and booting stages, while that for MeHg accumulation was the tillering stage. The key period for Hg accumulation in rice grains was the grain filling stage for both cultivars. The insights from this study could provide scientific guidance for the safe production of rice in Hg-contaminated soil.

Response of methylmercury in paddy soil and paddy rice to pristine biochar: A meta-analysis and environmental implications

Biochar has received increased research attention due to its effectiveness in mitigating the potential risks of mercury (Hg) in agricultural soils. However, there is a lack of consensus on the effect of pristine biochar on the net production, availability, and accumulation of methylmercury (MeHg) in the paddy rice-soil system. As such, a meta-analysis with 189 observations was performed to quantitatively assess the effects of biochar on Hg methylation, MeHg availability in paddy soil, and the accumulation of MeHg in paddy rice. Results suggested that biochar application could significantly increase the production of MeHg in paddy soil by 19.01%; biochar could also decrease the dissolved and available MeHg in paddy soil by 88.64% and 75.69%, respectively. More importantly, biochar application significantly inhibited the MeHg accumulation in paddy rice by 61.10%. These results highlight that biochar could decrease the availability of MeHg in paddy soil and thus inhibit MeHg accumulation in paddy rice, although it might facilitate the net production of MeHg in paddy soil. Additionally, results also indicated that the biochar feedstock and its elementary composition significantly impacted the net MeHg production in paddy soil. Generally, biochar with a low carbon content, high sulfur content, and low application rate might be beneficial for inhibiting Hg methylation in paddy soil, meaning that Hg methylation depends on biochar feedstock. These findings suggested that biochar has great potential to inhibit MeHg accumulation in paddy rice, and further research should focus on selecting biochar feedstock to control Hg methylation potential and determine its long-term effects.

Elevated methylmercury production in mercury-contaminated paddy soil resulted from the favorable dissolved organic matter variation created by algal decomposition

Algae-derived organic matter (AOM) may considerably regulate methylmercury (MeHg) production and accumulation in the paddy fields by changing the soil-dissolved OM (SDOM) properties. In this study, a 25-day microcosm experiment was performed to compare the responding mechanisms of MeHg production in the Hg-contaminated paddy soil-water system to the input of algae-, rice-, and rape-derived OMs. Results showed that algal decomposition could release much more cysteine and sulfate than crop straws. Compared with crop straw-derived OMs, AOM input greatly increased the dissolved organic carbon concentrations in soil but resulted in a greater decrease in tryptophan-like fractions while accelerated the formation of high-molecular-weight fractions in soil DOM. Moreover, AOM input significantly increased MeHg concentrations in the pore water by 19.43%–3427.66% and 52.81%–5846.57% compared to rape- and rice-derived OMs, respectively (P < 0.05). And, a similar MeHg changing pattern was also observed in the overlying water (10–25 d) and the soil solid-phase particles (15–25 d) (P < 0.05). Correlation analysis revealed that MeHg concentrations in the AOM-added soil-water system had significantly negative and positive relationships with the tryptophan-like C4 fraction and molecular weight (E2/E3 ratio) of soil DOM, respectively (P < 0.01). These findings suggest that AOM has a higher capacity than crop straw-derived OMs to promote MeHg production and accumulation in the Hg-contaminated paddy soils by creating a favorable soil DOM variation and providing more microbial electron donors and receptors.

Effects of varying amounts of different biochars on mercury methylation in paddy soils and methylmercury accumulation in rice (Oryza sativa L.)

There is growing evidence for the potential of biochars (BCs) in remediating mercury-contaminated paddy soils, but the high doses commonly used in laboratory studies discourage BC application in practice. To address these difficulties, we compared the effects of varying amounts of BCs from different sources on the formation of methylmercury (MeHg) in soil and its accumulation in rice through microcosm and pot experiments. The addition of a wide range of added doses (0.3, 0.6, 1, 2, 4 and 5 %, w/w) of BCs derived from different biomass feedstocks (i.e., corn stalk, wheat straw, bamboo, oak and poplar) significantly decreased the fraction of ammonium thiosulfate ((NH4)2S2O3)-extractable MeHg in the soil, although the MeHg contents varied with BC types and doses during soil incubation. However, the extractable MeHg in the soil did not continuously decrease with increasing BC doses, especially at doses of >1 %, resulting in limited further reductions. Moreover, a relatively low application rate (0.3–0.6 %, w/w) of BCs (i.e., corn stalk, wheat straw and bamboo-derived BC), especially of bamboo-derived BCs, significantly decreased the MeHg levels (42–76 %) in rice grains (brown rice). Meanwhile, the extractable soil MeHg decreased (57–85 %), although the MeHg in the soil varied under BC amendment during rice cultivation. These results provide further evidence that applying BC produced from different raw carbon materials (e.g., lignocellulosic biomass) could effectively reduce MeHg accumulation in rice grains, possibly due to MeHg bioavailability reduction in the soil. Our results suggest the possibility of mitigating MeHg accumulation in rice with a low dose of BCs, with great potential for use in remediating moderately contaminated paddy soils.

Mercury stable isotopes reveal sources of methylmercury and prey in giant Pacific bluefin tuna from the western North Pacific Ocean

AbstractSources of methylmercury (MeHg) in adult Pacific bluefin tuna (Thunnus orientalis, PBT) from the western North Pacific Ocean (WPO) were examined using mercury stable isotopes. Significant increases in δ202Hg and Δ199Hg values with PBT size and age, along with those of potential prey, indicate a shift in the source of MeHg accumulated by PBT as they age. Among adults from the WPO, this shift likely involves greater accumulation of MeHg from epipelagic prey in the Kuroshio extension in large vs. small and medium‐sized PBT. For all adults, little MeHg is accumulated in the spawning grounds near Taiwan. Significantly lower Δ199Hg/Δ201Hg ratios in adult PBT and their prey from the WPO than from the central and eastern North Pacific indicate different sources or transformations of MeHg prior to accumulation in the WPO food web than further east. Our results show that MeHg sources to oceanic food webs vary across the North Pacific Ocean and regionally within the WPO.

Monitoring Hg2+ and MeHg+ poisoning in living body with an activatable near-infrared II fluorescence probe

Noninvasively imaging mercury poisoning in living organisms is critical to understanding its toxicity and treatments. Especially, simultaneous fluorescence imaging of Hg2+ and MeHg+in vivo is helpful to disclose the mysteries of mercury poisoning. The key limitation for mercury imaging in vivo is the low imaging signal-to-background ratio (SBR) and limited imaging depth, which may result in unreliable detection results. Here, we designed and prepared a near-infrared II (NIR II) emissive probe, NIR-Rh-MS, leveraging the “spirolactam ring-open” tactic of xanthene dyes for in situ visualization of mercury toxicity in mice. The probe produces a marked fluorescence signal at 1015 nm and displays good linear responses to Hg2+ and MeHg+ with excellent sensitivity, respectively. The penetration experiments elucidate that the activated NIR-II fluorescence signal of the probe penetrates to a depth of up to 7 mm in simulated tissues. Impressively, the probe can monitor the toxicity of Hg2+ in mouse livers and the accumulation of MeHg+ in mouse brains via intravital NIR-II imaging for the first time. Thus, we believe that detecting Hg2+ and MeHg+ in different organs with a single NIR-II fluorescence probe in mice would assuredly advance the toxicologic study of mercury poisoning in vivo.

Diverse Methylmercury (MeHg) Producers and Degraders Inhabit Acid Mine Drainage Sediments, but Few Taxa Correlate with MeHg Accumulation.

Methylmercury (MeHg) is a notorious neurotoxin, and its production and degradation in the environment are mainly driven by microorganisms. A variety of microbial MeHg producers carrying the gene pair hgcAB and degraders carrying the merB gene have been separately reported in recent studies. However, surprisingly little attention has been paid to the simultaneous investigation of the diversities of microbial MeHg producers and degraders in a given habitat, and no studies have been performed to explore to what extent these two contrasting microbial groups correlate with MeHg accumulation in the habitat of interest. Here, we collected 86 acid mine drainage (AMD) sediments from an area spanning approximately 500,000 km2 in southern China and profiled the sediment-borne putative MeHg producers and degraders using genome-resolved metagenomics. 46 metagenome-assembled genomes (MAGs) containing hgcAB and 93 MAGs containing merB were obtained, including those from various taxa without previously known MeHg-metabolizing microorganisms. These diverse MeHg-metabolizing MAGs were formed largely via multiple independent horizontal gene transfer (HGT) events. The putative MeHg producers from Deltaproteobacteria and Firmicutes as well as MeHg degraders from Acidithiobacillia were closely correlated with MeHg accumulation in the sediments. Furthermore, these three taxa, in combination with two abiotic factors, explained over 60% of the variance in MeHg accumulation. Most of the members of these taxa were characterized by their metabolic potential for nitrogen fixation and copper tolerance. Overall, these findings improve our understanding of the ecology of MeHg-metabolizing microorganisms and likely have implications for the development of management strategies for the reduction of MeHg accumulation in the AMD sediments. IMPORTANCE Microorganisms are the main drivers of MeHg production and degradation in the environment. However, little attention has been paid to the simultaneous investigation of the diversities of microbial MeHg producers and degraders in a given habitat. We used genome-resolved metagenomics to reveal the vast phylogenetic and metabolic diversities of putative MeHg producers and degraders in AMD sediments. Our results show that the diversity of MeHg-metabolizing microorganisms (particularly MeHg degraders) in AMD sediments is much higher than was previously recognized. Via multiple linear regression analysis, we identified both microbial and abiotic factors affecting MeHg accumulation in AMD sediments. Despite their great diversity, only a few taxa of MeHg-metabolizing microorganisms were closely correlated with MeHg accumulation. This work underscores the importance of using genome-resolved metagenomics to survey MeHg-metabolizing microorganisms and provides a framework for the illumination of the microbial basis of MeHg accumulation via the characterization of physicochemical properties, MeHg-metabolizing microorganisms, and the correlations between them.

Methylmercury bioaccumulation in water flea Daphnia carinata by AIEgen

Mercury ion (Hg2+) is a toxic heavy metal ion and Hg2+ is convertible to methylmercury (MeHg) by many aquatic microorganisms, leading to bioaccumulation and biomagnification in aquatic organisms, which can interfere with brain development and function in humans. This study employs a newly developed AIEgen (Aggregation-induced emission fluorogen) to quantify and visualise the process of MeHg bioaccumulation in vivo on the species of water flea Daphnia carinata. Two approaches to MeHg absorption were taken, either by direct incubation in a MeHg solution or by indirect consumption of algae contaminated with MeHg. We analysed the relationship between the ratio of photoluminescence (PL) intensities (I585/I480) and MeHg concentration (CMeHg) and generated a master curve for determining MeHg concentration based on the measurement of PL intensities. Fluorescent image analysis showed the occurrence of MeHg in D. carinata to be mainly in the compound eyes, optic nerve and carapace. This study indicates that MeHg absorption can be quantified and visualised in the body of zooplankton, and the MeHg transfer to zooplankton is more likely through direct exposure than via indirect food intake. The accumulation of MeHg in the eye and the nervous system could be the cause of the high mortality of D. carinata exposed to MeHg in water.

Inorganic versus organic fertilizers: How do they lead to methylmercury accumulation in rice grains

Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18–41.69%) was significantly (p < 0.05) lower than that induced by organic fertilizers (80.49–106.86%). Inorganic fertilizers led to a larger increase (50.39–99.28%) in thousand-kernel weight than MeHg concentrations (5.18–41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.

Sulfur-driven methylmercury production in paddies continues following soil oxidation

Methylmercury (MeHg) production in paddy soils and its accumulation in rice raise global concerns since rice consumption has been identified as an important pathway of human exposure to MeHg. Sulfur (S) amendment via fertilization has been reported to facilitate Hg methylation in paddy soils under anaerobic conditions, while the dynamic of S-amendment induced MeHg production in soils with increasing redox potential remains unclear. This critical gap hinders a comprehensive understanding of Hg biogeochemistry in rice paddy system which is characterized by the fluctuation of redox potential. Here, we conducted soil incubation experiments to explore MeHg production in slow-oxidizing paddy soils amended with different species of S and doses of sulfate. Results show that the elevated redox potential (1) increased MeHg concentrations by 10.9%−35.2%, which were mainly attributed to the re-oxidation of other S species to sulfate and thus the elevated abundance of sulfate-reducing bacteria, and (2) increased MeHg phytoavailability by up to 75% due to the reductions in acid volatile sulfide (AVS) that strongly binds MeHg in soils. Results obtained from this study call for attention to the increased MeHg production and phytoavailability in paddy soils under elevated redox potentials due to water management, which might aggravate the MeHg production induced by S fertilization and thus enhance MeHg accumulation in rice.

Geochemical and Dietary Drivers of Mercury Bioaccumulation in Estuarine Benthic Invertebrates

Sediments represent the main reservoir of mercury (Hg) in aquatic environments and may act as a source of Hg to aquatic food webs. Yet, accumulation routes of Hg from the sediment to benthic organisms are poorly constrained. We studied the bioaccumulation of inorganic and methylmercury (HgII and MeHg, respectively) from different geochemical pools of Hg into four groups of benthic invertebrates (amphipods, polychaetes, chironomids, and bivalves). The study was conducted using mesocosm experiments entailing the use of multiple isotopically enriched Hg tracers and simulation of estuarine systems with brackish water and sediment. We applied different loading regimes of nutrients and terrestrial organic matter and showed that the vertical localization and the chemical speciation of HgII and MeHg in the sediment, in combination with the diet composition of the invertebrates, consistently controlled the bioaccumulation of HgII and MeHg into the benthic organisms. Our results suggest a direct link between the concentration of MeHg in the pelagic planktonic food web and the concentration of MeHg in benthic amphipods and, to some extent, in bivalves. In contrast, the quantity of MeHg in benthic chironomids and polychaetes seems to be driven by MeHg accumulation via the benthic food web. Accounting for these geochemical and dietary drivers of Hg bioaccumulation in benthic invertebrates will be important to understand and predict Hg transfer between the benthic and the pelagic food web, under current and future environmental scenarios.

Grass Shrimp (Palaemonetes pugio) as a Trophic Link for Methylmercury Accumulation in Urban Salt Marshes.

Grass shrimp (Palaemonetes pugio) represent a potential link in the transfer of methylmercury (MeHg) from salt marsh sediments to transient young-of-the-year (YOY) fish. Across six salt marshes subject to varying degrees of Hg contamination, MeHg concentration in grass shrimp was significantly correlated with MeHg in sediment (p < 0.05, R2 = 0.81). Bioenergetic models show that grass shrimp alone account for 12-90% of MeHg observed in YOY striped bass and 6-22% of MeHg in YOY summer flounder. Direct accumulation of MeHg from grass shrimp to YOY fish increased with MeHg levels in grass shrimp and sediment. However, in the most contaminated salt marshes with the highest levels of MeHg in grass shrimp and sediment, indirect accumulation of MeHg from grass shrimp by YOY summer flounder, whose diet is dominated by benthic forage fish (mummichog), is predicted to plateau because higher concentrations of MeHg in grass shrimp are offset by a lower proportion of grass shrimp in the mummichog diet. Our results demonstrate that grass shrimp are an important trophic link in the bioaccumulation of MeHg in salt marsh food webs and that MeHg accumulation in YOY fish varies with both the concentration of MeHg in salt marsh sediments and benthic food web structure.

Important Roles of Thiols in Methylmercury Uptake and Translocation by Rice Plants.

The bioaccumulation of the neurotoxin methylmercury (MeHg) in rice is a significant concern due to its potential risk to humans. Thiols have been known to affect MeHg bioavailability in microorganisms, but how thiols influence MeHg accumulation in rice plants remains unknown. Here, we investigated effects of common low-molecular-weight thiols, including cysteine (Cys), glutathione (GSH), and penicillamine (PEN), on MeHg uptake and translocation by rice plants. Results show that rice roots can rapidly take up MeHg, and this process is influenced by the types and concentrations of thiols in the system. The presence of Cys facilitated MeHg uptake by roots and translocation to shoots, while GSH could only promote MeHg uptake, but not translocation, by roots. Conversely, PEN significantly inhibited MeHg uptake and translocation to shoots. Using labeled 13Cys assays, we also found that MeHg uptake was coupled with Cys accumulation in rice roots. Moreover, analyses of comparative transcriptomics revealed that key genes associated with metallothionein and SULTR transporter families may be involved in MeHg uptake. These findings provide new insights into the uptake and translocation of MeHg in rice plants and suggest potential roles of thiol attributes in affecting MeHg bioavailability and bioaccumulation in rice.

Effects of microplastics on the accumulation and neurotoxicity of methylmercury in zebrafish larvae

Microplastics (MPs) and methylmercury (MeHg) have attracted increasing attention due to ubiquitous occurrence and toxicity. This study aimed to investigate whether MPs could absorb MeHg and thus modify its bioconcentration and neurotoxicity in the zebrafish larvae (Danio rerio). The pseudo-second-order model (R2 = 0.989) was found to be suitable for describing the adsorption kinetics of MeHg onto MPs. Compared with Freundlich and Temkin models, the Langmuir isotherm model provided a better fit with the experimental data exhibiting a maximum monolayer adsorption capacity of 54.945 mg/g. These results suggested that adsorption occurs mainly by a chemical process dominated by monolayer adsorption. MPs adsorbed MeHg to form MPs/MeHg complex, which was ingested by zebrafish larvae, and promoted accumulation of MeHg. Thus, the presence of MPs aggravated the reduction of locomotor activity induced by MeHg, and downregulation of neurotransmitters related genes, such as ache, gfap and scl1A3b. Metabolome analysis also revealed disrupted glutathione (GSH) metabolism upon exposure of MeHg alone and in combination with MPs, as reflected by the increased in the ratio of GSH and oxidized glutathione. These effects were also confirmed by upregulation of oxidative stress-related genes, such as sod, sod mt and gpx4a. Collectively, these results indicated that MPs could act as a carrier of MeHg and enhance its accumulation in zebrafish, thereby disrupting locomotor activity by excessive oxidative stress. This study provides a scientific basis for improving health risk assessment of environmental pollutants, particularly those potentially able to adsorb to MPs by virtue of their chemical nature.

Mobilization and methylation of mercury with sulfur addition in paddy soil: Implications for integrated water-sulfur management in controlling Hg accumulation in rice

Sulfur-fertilizer is commonly applied in croplands and in immobilizing Hg in contaminated soil. However, there is still great uncertainty and controversy concerning Hg transportability and transformation when supplying sulfur in paddies with complex conditions. Herein, we explored the effect of adding sulfate in paddy soil at different rice growth stages on soil Hg release and MeHg accumulation in rice and uncovered the correlation between sulfur induced MeHg production and the dynamically changed soil Eh, dissolved Fe, and dissolved organic carbon (DOC). In specific, sulfate addition at early stages (flooding period) triggered the decrease of Eh and increase of DOC and dissolved Fe, which in turn promoted Hg release and favored MeHg generation (increased by 235.19–555.07% vs control). Interestingly, adding sulfate at late stages (drainage condition), as compared with that at early stages, alleviated Hg release and MeHg production accompanied by the increase of Eh and decrease of dissolved Fe and DOC. The microcosmic experiment further confirmed the reduction of sulfate to sulfide promoted the change of Eh, thereby stimulating HgS dissolution in soil extract. The results give clues on the rational application of sulfur-fertilizer and through the water-sulfur fertilizer management considering the correspondingly changed soil conditions to diminish Hg bioavailability and MeHg production in paddies and paddy-like environments.

Antibiotic application may raise the potential of methylmercury accumulation in fish

Mercury (Hg) biotransformation can significantly affect the Hg speciation and bioaccumulation in fish, where gut microbiota play an important role in this process. Antibiotics have been extensively used in aquaculture and can affect gut microbial structure. However, the influence of antibiotics on Hg biotransformation in fish has not been thoroughly understood. The present study investigated the effects of antibiotic (florfenicol) application on gut microbiota and subsequent impacts on Hg biotransformation and bioaccumulation in tilapia (Oreochromis mossambicus). The results showed that the florfenicol treatment did not affect IHg accumulation in the IHg-exposed fish or the MeHg accumulation in the MeHg-exposed fish. However, methylation was significantly weakened (from 0.015% d−1 to 0.005% d−1) and demethylation was completely terminated (from 0.046% d−1 to non-observable level) in the florfenicol-treated fish as compared to the control fish. This can be ascribed to the major shift in the richness of microbial methylators/demethylators in fish gut. Furthermore, florfenicol disturbed the homeostasis of gut microbiome and enhanced the growth of opportunistic pathogens. Our results strongly suggested that antibiotic application significantly altered the gut microbial community, thereby increasing the potential of MeHg accumulation by fish. This study highlights the importance of appropriate use of antibiotics in aquaculture as well as decreasing the environmental risks of Hg contamination in fish.

Importance of hydraulic residence time for methylmercury accumulation in sediment and fish from artificial reservoirs

Excessive methylmercury (MeHg) accumulation in dietary fish is a global concern due to its harmful effects on human health, however, environmental factors affecting MeHg accumulation in reservoir ecosystems are not clearly known. In this study, we aim to identify the main sources of MeHg in the water column and the critical factors related to MeHg concentration and methylation rate constant (km) in sediment and total Hg concentration in fish using five-year (2016–2020) monitoring data of the five artificial reservoirs. The preliminary mass budgets constructed using the measurement and online data showed that sediment transport dominated over runoff in the long residence time reservoirs (400–475 days), while runoff dominated over sediment transport in the short residence time reservoirs (10 days). Whereas the sediment km showed a comparable variation with the algal biomass, the sediment MeHg concentration and the length-normalized Hg concentration in the barbel steed and bluegill increased in the longer residence time reservoirs with lower algal biomass. As MeHg accumulation in sediment and fish tends to increase in the slowly overturning reservoirs, the hydraulic residence time should be carefully managed to meet the best protection of human health from chronic Hg exposure by fish consumption.

Dual role of titanium dioxide nanoparticles in the accumulation of inorganic and methyl mercury by crustacean Daphnia magna through waterborne and dietary exposure

Titanium dioxide nanoparticles (nTiO2) are widely used in numerous products, yet their role in the accumulation and transfer of other contaminants in the aquatic food webs is not well understood. The influence of nTiO2 on inorganic (IHg) and monomethyl mercury (MeHg) accumulation in invertebrate Daphnia magna through waterborne and dietary exposure was thus thoroughly investigated. The results showed that nTiO2 led to a substantial decrease of the total mercury body burden (THg) in D. magna in direct waterborne exposure to IHg/MeHg. However, exposure to nTiO2 pre-treated with IHg/MeHg resulted in an increase of the THg body burden in daphnids. The presence of nTiO2 led to a substantial decrease of the THg in D. magna when exposed to IHg/MeHg via algal food. These effects were more pronounced for IHg than that for MeHg due to the higher adsorption capabilities of nTiO2 for IHg. In addition, high concentrations of nTiO2 favored the trophic transfer of IHg/MeHg through feeding on nTiO2 pre-treated with Hg, however lessened it when D. magna were fed on alga pre-treated with IHg/MeHg. Comparable assimilation efficiency (AE), determined as Hg retained in daphnids after depuration, was observed in D. magna when exposed to IHg/MeHg via algal food regardless the absence or presence of 20 mgL−1 nTiO2. By contrast, an increase of the AE of MeHg through feeding on nTiO2 and alga was found in the presence of higher concentration of 200 mgL−1 nTiO2. The present results will help to better understand the role of nTiO2 on bioavailability and trophic transfer of global contaminants, such as mercury, known to bioaccumulate and biomagnify in the aquatic environment.

Bioavailability and methylation of bulk mercury sulfide in paddy soils: New insights into mercury risks in rice paddies

Mercury sulfide (HgS) constitutes the largest Hg reservoir in the lithosphere but has long been considered to be not bioavailable and a minor participant in Hg biogeochemical cycling. Herein, we report that bulk α-HgS can be dissolved and methylated in paddy soils, especially with rice culture. Bulk α-HgS exposure did not affect rice growth compared to the control group but significantly increased methylmercury (MeHg) contents in the rhizospheric soils (e.g., 80.15% and 108.71% higher for bulk α-HgS treatment vs. control at the seedling and maturation stages, respectively). Moreover, bulk α-HgS exposure induced an apparent accumulation of MeHg (50% higher for bulk α-HgS treatment vs. control) and markedly elevated total Hg (THg) in rice grains. The presence of DOM and reduced sulfide in paddy soils was further evidenced to drive the mobilization and dissolution of bulk α-HgS, thereby resulting in THg and MeHg accumulation in rice grains. These findings highlight the bioavailability of HgS in rice paddies and suggest that bulk HgS should be considered when assessing Hg health risks and developing efficient remediation approaches in Hg-contaminated croplands.

Selenium-amended biochar mitigates inorganic mercury and methylmercury accumulation in rice (Oryza sativa L.)

Rice, as a dominant crop in China and Asia, can be a major route of methylmercury (MeHg) exposure for humans in inland China, especially in those living in mercury (Hg) polluted areas. Soil is the most prominent MeHg accumulation source for rice grains. The development of management practices to reduce MeHg in rice grains is crucial. This study explored the mitigation effect of biochar (BC) and sodium selenite-amended biochar (BC + Se) on MeHg production in paddy soil and accumulation in rice. Mercury-contaminated soil was treated with 1% and 5% of both BC and BC + Se. Soil MeHg concentration slightly increased under 1% BC/BC + Se compared to control soil but decreased at the rate of 5%. Moreover, soil phytoavailable MeHg (P–MeHg) diminished as the amount of Se-amended BC increased. BC + Se effectively mitigated MeHg accumulation in rice grains. The highest average contents of MeHg and inorganic Hg (IHg) in rice seeds were found in the control samples, followed by the 1%-BC, 5%-BC, 1%-BC + Se, and 5%-BC + Se samples. Under the 5%-BC + Se treatment, rice MeHg levels were reduced significantly (94%) compared to the control, and P–MeHg concentrations in soil were lower than all the other experimental groups throughout the rice-growing season. These results demonstrate the effectiveness of BC + Se in reducing MeHg and IHg accumulation in rice and could be employed for remediation of Hg polluted paddies.