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The tree ring has been regarded as an emerging archive to reconstruct historical atmospheric mercury (Hg) trends, but with the large knowledge gaps in the reliability. In this study, we comprehensively evaluated the Hg source, radial translocation and age effect of Masson pine (Pinus massoniana) tree ring at Mt. Jinyun in Chongqing, to assess the suitability of such tree ring as the archive of atmospheric Hg. Results showed that distinct variabilities among Masson pine tree-ring Hg concentration profiles. The Hg concentration significantly increased along with stem height (P < 0.05), indicating the Hg in tree rings mainly derived from foliage uptake atmospheric Hg. We found a distinct age effect that the tree ring of young trees had the higher Hg concentration. Besides, we used the advection-diffusion model to demonstrate how Hg concentration shifted by the advection or/and diffusion in tree rings. The modeling results showed that the advection induced radial translocation during the young growth period of tree was a plausible mechanism to result in the tree-ring Hg record largely different from the trend of anthropogenic Hg emissions in Chongqing. We finally suggest that in further Hg dendrochemistry, better discarding the tree-ring Hg profile of the young growth period to reduce impacts of the radial translocation and age effect. [Display omitted] [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is a crucial driver of various biogeochemical processes in aquatic systems. Thus, many lakes and streams have been investigated in the past several decades. However, fewer studies have sought to understand the changes in DOM characteristics in the waters of the Three Gorges Reservoir (TGR) areas, which are the largest artificial reservoir areas in the world. Thus, a field investigation of dissolved organic carbon (DOC) concentrations and of chromophoric dissolved organic matter (CDOM) properties was conducted from 2013 to 2015 to track the spatial-temporal variability of DOM properties in the TGR areas. The results showed that the alternations of wet and dry periods due to hydrological management have a substantial effect on the quantity and quality of aquatic DOM in TGR areas. Increases in DOC concentrations in the wet period show an apparent “dilution effect” that decreases CDOM compounds with relatively lower aromaticity (i.e., SUVA 254 ) and molecular weight (i.e., S R ). In contrast to the obvious temporal variations of DOM, significant spatial variability was not observed in this study. Additionally, DOM showed more terrigenous characteristics in the dry period but weak terrigenous characteristics in the wet period. Furthermore, the positive correlation between SUVA 254 and CDOM suggests that the aromatic component controls the CDOM dynamics in TGR areas. The first attempt to investigate the DOM dynamics in TGR areas since the Three Gorges Dam was conducted in 2012, and the unique patterns of spatial-temporal variations in DOM that are highlighted in this study might provide a new insight for understanding the role of DOM in the fates of contaminants and may help in the further management of flow loads and water quality in the TGR area. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) plays an important environmental and ecological role in inland aquatic systems, including lakes. In this study, using fluorescence analysis, we investigated the seasonal dynamics of DOM characteristics in Changshou Lake, which is a typical inland lake in the Three Gorges Reservoir (TGR) area. We also discuss the environmental implications of DOM for mercury (Hg) dynamics. Based on the origins of two end-members, the variations in DOM observed in this study in Changshou Lake suggest that hydrological processes (e.g., terrestrial inputs resulting from runoff and humic-like component residences) and biological activities (e.g., microbial and algae growth) are the two main principal components controlling the seasonal dynamics of DOM characteristics. Furthermore, the dynamics of dissolved Hg co-varied with variations in DOM properties, rather than with dissolved organic carbon (DOC) concentrations. This indicates that the previously reported simple correlations between DOC and Hg were not comprehensive and may lead to misunderstanding the interactions between DOM and Hg. Therefore, we recommend that when using DOM-Hg correlations to evaluate the role of DOM in the environmental fate of Hg, especially in field investigations of the spatial and temporal distribution of Hg, the properties of DOM must be taken into account. [ABSTRACT FROM AUTHOR]

Laboratory incubation experiments were performed to identify diurnal characteristics of migration and transformation of mercury (Hg) and effects of nitrate (NO 3 − , a hydroxyl radical donor by photolysis) in Jialing River, Chongqing, China. It is found that there are strong diurnal signals of [monomethylmercury (MMHg)] and [reactive Hg (RHg)] in sediment, pore water and overlying water, which suggest that solar radiation may be an important variable that involved in aquatic Hg cycling. Photo-degradation (PD) of MMHg plays a key role in Hg cycling in water systems, and the rates are measured to be 38.22% in March, 2012. The presence of NO 3 − has a marked effect on MMHg PD under solar radiation, and affects inorganic species reducting to Hg 0 , resulting in more Hg species available for methylation. So NO 3 − is an important factor for Hg cycling in water systems. Diffusive flux of MMHg, RHg and dissolved Hg (DHg) are 1.92–2.34, 3.43–3.64 and 3.04–5.71 ng m −2 d −1 at daytime, and 6.04–6.92, 3.22–3.25 and 7.79–8.37 ng m −2 d −1 at nighttime, respectively, implying that sediment is a major Hg source for shallow-water area in Jialing River at springtime. These results show great importance for understanding Hg biogeochemical processes in clear, oligotrophic, shallow, sluggish, and agriculturally-impacted waters. [ABSTRACT FROM AUTHOR]

Photo-reduction of divalent mercury (Hg(II)) in aquatic systems plays a key role in global biogeochemistry cycling of mercury (Hg) in the Earth's surface environment. The mechanisms of this process with various Cl concentrations ([Cl]) under different pH values and irradiation wavelength ranges are still unclear. In this work, photo-reduction and photo-oxidation experiments of Hg with different [Cl] and pH values under various light conditions were conducted. The results show that photo-reduction rate constants of Hg(II) decrease with the increasing of [Cl] in neutral solution under full light spectrum. Photo-reduction rate constants of Hg(II) with Cl is highly dependent on Hg(II) species, which is determined by [Cl] and pH value. Irradiation wavelength ranges have significant effects on reaction processes of photo-reduction of Hg(II) and photo-oxidation of Hg(0) in the presence of Cl. When cut off ultraviolet (UV) radiation (280-400 nm), the reduction rate constants decrease without Cl, and increase with higher [Cl], and the photo-oxidation rates of Hg(0) decrease with or without Cl. Except Cl complexation stabilize the reducible Hg ions in solutions, photo-oxidation is an important reason for Cl lowering photo-reduction rate of Hg(II). The results are of great importance for understanding the photo-redox characters of Hg(II) with Cl. [ABSTRACT FROM AUTHOR]

Abstract: Photo-degradation (PD) of monomethylmercury (MMHg) is recognized as the most important process in mercury cycling that maintains MMHg at low concentration in water systems. Chloride ion (Cl−) is present widely in ecosystems and it may have an effect on MMHg PD. In this investigation, MMHg PD with various Cl− concentrations under different light radiation intensity and wavelength were performed in a quartz cylinder reactor. Various light radiations were obtained from xenon and ultraviolet (UV) bulbs. The results showed that the first order reaction coefficient decreased (1.35–0.37h−1) with Cl− concentrations, and increased (0.14–0.76h−1) with UV-B radiation intensity. The wavelength mediated PD rates varied from 0.29h−1 to 0.96h−1. The peak emission of Hg0 was excited by UV-B radiation and Hg0 flux of the reactions without Cl− (94.06ng) was higher than that with Cl− by 1–4 times. All the results suggest that Cl− could block MMHg PD and subsequently influence the end products from PD. The wavelength and light intensity have a significant effect on MMHg PD and Hg0 emission. Calculation of MMHg species (%) at initial and terminal time with various Cl− concentrations show that MMHg species changed by Cl− complexation and thus result in different PD mechanisms and pathways. [Copyright &y& Elsevier]

The exchange of mercury (Hg) across the air-water interface is an important part of Hg biogeochemical cycle. Mercury fluxes across the air-water interface in paddy fields were measured by a Dynamic Flux Chamber (DFC) coupled with a Lumex® multifunctional mercury analyzer RA-915 at two sites (Chengjiang (CJ) and Caoshang (CS)) in Beibei, Chongqing, China in 2008. The results showed that mercury emission followed a power-law relationship with solar radiation and air temperature, and it increased exponentially with water temperature at both sites. Mercury emission was mainly influenced by the solubility of gaseous elemental Hg, photo-thermal effect, electron activity (Eh) and air Hg concentrations. Solar radiation made the greatest direct contribution to mercury emission during the daytime (0.80), with an 83.60% contribution, whereas at nighttime the water temperature (0.72) contributed to 71.65% of emissions. The temperature gradient between water and air might also influenced mercury emission across the air/water interface at nighttime. These findings suggest that paddy fields could act as a significant source of atmospheric mercury, and it can contribute significantly to the atmospheric mercury in a local region. [ABSTRACT FROM AUTHOR]

Abstract: Total gaseous mercury flux measurements of forested and open field soils in the subtropical forest zones, in Chongqing, Southwestern China were monitored from April 2011 to March 2012 to provide insight into the characteristics of gaseous mercury flux with and without vegetation covers. Samples were collected from surfaces of forest and open field as the most representative terrestrial surfaces in Jinyun Mountain. Simultaneously meteorological parameters at the soil level relating GEM fluxes to soil temperature, air humidity, solar radiation were analyzed and variations of atmospheric GEM concentration were examined. The results showed that annual averaged fluxes from soil in the forest and open-air site were 14.23 ng m2 h−1 and 20.74 ng m2 h−1 respectively, with a clear seasonal variation for each surface. The soil surface displayed the largest difference between evening and daytime flux, particularly in spring and summer in open field surface (i.e., with low flux in evening (1.93 ng m2 h−1) to high flux in daytime (97.86 ng m2 h−1) in summer). GEM fluxes in the open-air site were positively correlated with solar irradiation and soil temperature, and inversely correlated with air humidity. In the forest site, GEM fluxes showed weaker correlation with meteorological variables. Based on the whole year experiments and field tests, it can be found that in some season (especially fall), the mercury emission fluxes were more highly dependent on solar radiation and less dependent on temperature. The regression results demonstrated that each surface displayed similar responses to time series change in meteorological parameters. A comparison between one year periods in the two sites showed much larger emission from the open field with forest conversion to bare soil. [Copyright &y& Elsevier]

Abstract: In order to provide insight into the characteristics of Hg exchange in soil/water–air surface from cropland (including paddy field and dry land), Hg fluxes were measured in Chengjiang. Mercury fluxes were measured using the dynamic flux chamber method, coupled with a Lumex® multifunctional Hg analyzer RA-915+ (Lumex Ltd., Russia). The Hg fluxes from paddy field and dry land were alternatively measured every 30min. Data were collected for 24–48h once per month for 5months. Mercury fluxes in both fields were synchronously measured under the same conditions to compare Hg emissions between paddy field and dry land over diurnal and seasonal periods and find out what factors affect Hg emission on each surface. These results indicated that air Hg concentrations at the monitoring site was double the value observed at the global background sites in Europe and North America. The Hg release fluxes were 46.5±22.8ngm−2 h−1 in the warm season, 15.5±18.8ngm−2 h−1 in the cold season for dry land, and 23.8±15.6ngm−2 h−1 in the warm season, 6.3±11.9ngm−2 h−1 in the cold season for paddy field. Solar radiation is important in the emission of Hg over both sites. Hg exchange at the soil/air and water/air interfaces showed temporal variations. The amount of Hg emission from dry land was higher than that from the paddy field, and the emission in daytime was higher than that at night. Moreover, Hg emissions from land covered by crops, was lower than that for bare land. [ABSTRACT FROM AUTHOR]

Abstract: An investigation was conducted to estimate mercury emission to the atmosphere from different environmental surfaces and to assess its contribution to the local mercury budget in Chongqing, China. Mercury flux was measured using dynamic flux chamber (DFC) at six soil sites of three different areas (mercury polluted area, farmland and woodland) and four water surfaces from August 2003 to April 2004. The mercury emission fluxes were 3.5±1.2–8.4±2.5ngm−2 h−1 for three shaded forest sites, 85.8±32.4ngm−2 h−1 for farming field, 12.3±9.8–733.8±255ngm−2 h−1 for grassland sites, and 5.9±12.6–618.6±339ngm−2 h−1 for water surfaces. Mercury exchange fluxes were generally higher from air/water surfaces than from air/soil surfaces. The mercury negative fluxes were found in tow soil sites at overcast days (mean=−6.4±1.5ngm−2 h−1). The diurnal and seasonal variations of mercury flux were observed in all sites. The mercury emission responded positively to the solar radiation, but negatively to the relative humidity. The mercury flux from air/soil surfaces was significantly correlated with soil temperature, which was well described by an Arrhenius-type expression with activation energy of 31.1kcalmol−1. The annual mercury emission to the atmosphere from land surface is about 1.787t of mercury in Chongqing. [Copyright &y& Elsevier]

Abstract: This investigation presents a first assessment of the contribution to the regional mercury budget from anthropogenic and natural sources in Chongqing, an important industrial region in southwest China. The emissions of mercury to atmosphere from anthropogenic sources in the region were estimated through indirect approaches, i.e. using commonly acceptable emission factors method, which based on annual process throughputs or consumption for these sources. The natural mercury emissions were estimated from selected natural sources by the dynamic flux chamber technique. The results indicated that the anthropogenic mercury emissions totaled approximately 8.85 tons (t), more than 50% of this total originated in coal combustion and 23.7% of this total emission in the industrial process (include cement production, metal smelting and chemical industry). The natural emissions represented approximately 17% of total emissions (1.78 t yr−1). The total mercury emission to atmosphere in Chongqing in 2001 was 10.63 t. [Copyright &y& Elsevier]

Field investigation and simulating experiments were carried out for understanding the accumulation and transformation of mercury in soil in relation to the deposition of atmospheric mercury. A positive correlation between the atmospheric mercury concentration and the content of mercury in soil was observed in the field investigation, with the correlation coefficient being 0.741** (n=52). The mercury content in soil decreased with the increasing distance from the mercury emission source. Simulated experiment demonstrated that the higher the mercury content in air was, the higher was the amount of mercury accumulated in soil, which was in accordance with the results found from the field investigation. Transformation process occurred once mercury deposited into the soil. Analyses of soil samples exposed to air with mercury contents of 796.4±186.3 ng/m3 for 2 months indicated that 24.58–26.86% of total mercury deposited into the soil existed in Hg0 form, 0.10–0.12% in active form, 14.56–18.75% in HCl-dissoluble form, 0.86–5.84% in organic-bound form and 52.64–55.29% in residual form. [Copyright &y& Elsevier]

Biochar, an effective and low-cost amendment for immobilizing heavy metals, has been extensively studied. However, the simultaneous inhibition effects of biochar on the plant uptake for arsenic (As) and cadmium (Cd) in co-contaminated soils are still ambiguous due to their distinct environmental behaviors. A meta-analysis was conducted to quantitatively assess the effects using 1030 individual observations from 52 articles. On average, biochar application significantly decreased the bioavailability of Cd in soils by 50.12%, while slightly increased the bioavailability of As in soils by 2.39%. The more instructive result is that biochar application could also simultaneously reduce the concentration of As and Cd in plants by 25.48% and 38.66%, respectively. The orders of the decreased percentage of As and Cd in various tissues were root < stem< leaf < grain, and root < leaf < stem < grain, respectively. According to the analysis of critical factors, manure biochar, low pyrolysis temperature (at <400 °C), low application rate (<2%), and high SOC (>30 g/kg) were more conducive to reduce the bioaccumulation of As and Cd simultaneously in co-contaminated soils. Pristine and modified biochar could inhibit As and Cd accumulation in crops, but their efficiencies need to be further improved to ensure the safety of crop productions. Overall, the meta-analysis suggests that biochar has the potential to remedy the As and Cd co-contaminated soils. Unlabelled Image • Biochar decreased the available Cd (50.12%) while increased the available As (2.39%) in soils. • Biochar reduced the accumulation of As (25.48%) and Cd (38.66%) in plants. • Biochar feedstock determined the effect size and direction on As and Cd accumulation in plants. • Biochar efficiency should be further improved to remedy As and Cd co-contaminated soils. [ABSTRACT FROM AUTHOR]

The reducing capacity (RC) of natural organic matter plays an important role in the carbon cycle and biogeochemical fates of environmental contaminants in the aquatic system. However, the electron donation potentials of dissolved organic matter (DOM) from high-latitude lakes are still uncertain. In this study, we collected DOM samples from high-latitude lakes across the Arctic and boreal regions in Sweden and Norway to investigate the effects of the DOM concentration and characteristics on its ambient reducing capacity (ARC). Mercury (Hg(II)) abiotic reduction in darkness was used to determine the ARC. The results showed that the DOM in Arctic lakes is less terrestrial-dominant than in reference sites (i.e., forest lakes). Between the two categories of Arctic lakes, tundra lakes are more terrestrial-influenced compared to mountain lakes. Additionally, terrestrial-originated DOM is a main controlling factor for enhancing the ambient reducing capacity, whereas the DOM concentration, i.e., dissolved organic carbon (DOC), resulted in variations in the Hg/DOC ratios that also cause the variations of the observed ARC values. Thus, comparisons of the ARC values can be conducted while oxidant/DOC ratios are kept the same and reported through the method using heavy metals as a chemical probe. After correction for Hg/DOC ratio interference, the ambient reducing capacity of DOM followed the order: boreal forest lakes > Arctic tundra lakes > Arctic mountain lakes. This study highlights that the DOM concentration should also be considered when estimating the ARC as compared to the previous that mainly focusing on the properties of DOM such as its origins. As climate change is projected to be severe in high latitudes, this study demonstrates a significant connection between aquatic DOM geochemical reactivity and terrestrial inputs, which is crucial for a better prediction of the role of DOM in high-latitude lakes in the context of climate change. Image 1 • Reducing capacities of DOM is ubiquitous in lakes systems regardless of latitudes. • Terrestrial-dominant dissolved organic matter shows higher electron donation reactivity. • Only characteristics of DOM cannot explain the differences of reducing capacities observed. • Comparisons of ambient reducing capacities should be conducted within a caution. [ABSTRACT FROM AUTHOR]

Foliage uptake of atmospheric elemental mercury (Hg0) and subsequent translocation by the phloem is the main pathway for Hg accumulation in tree rings. Tree rings have been used as the emerging natural archive to directly reconstruct centennial trends of atmospheric Hg0 level. The tree-ring Hg records in remote regions have successfully reconstructed the peak of anthropogenic Hg emissions in Europe and North America in 1960s − 1970s and the distinct increase of Hg emissions in Asia since 1980s. Combining the Hg concentrations and isotopic signatures would provide historical atmospheric Hg trends and Hg emission source shifts. The mechanisms for Hg translocation, specifically the radial translocation and impacts of environmental and tree physiological factors, are yet to be clarified to explain the nonlinear relation between atmospheric Hg0 concentration and Hg signals in tree rings. Thus, we recommend to trace Hg accumulation and translocation processes and their Hg isotopic fractionation in tree rings, and examine the relationship between tree-ring Hg profile and atmospheric pollution level in specific tree species. Finally, we suggest to develop more statistical models to quantify environmental and tree physiological impacts on Hg accumulation and translocation in tree rings. [ABSTRACT FROM AUTHOR]

Although mercury (Hg) is toxic at all valence states, organic Hg, especially monomethylmercury (MeHg, CH3Hg+, methylmercury), is more toxic than inorganic Hg (Hg(II), Hg2+). As Hg is predominately methylated via bio-methylation, studies on the mechanism of Hg bio-methylation by microorganisms are particularly important. It is widely accepted that MeHg is mainly produced by anaerobic microorganisms in the environment, and the Hg-methylating process is a cellular reaction. Hence, the production rate of MeHg is not only related to the presence and activities of Hg-methylating anaerobic microorganisms, but also the transport of Hg(II) into microbial cells and the biochemical reactions for forming MeHg in the cells. Therefore, this review described the current state of knowledge on: 1) what kind of microorganisms can methylate Hg; 2) what pathways are the Hg(II) take to enter into the microbial cells; 3) what kind of biochemical reactions happen in the cells; and 4) biogeochemical factors affecting Hg methylation, which are critical to elucidate the regulating mechanisms of Hg methylation by microorganisms. Simultaneously, the existing issues or controversies on biotic Hg methylation were also discussed. [ABSTRACT FROM AUTHOR]

Forests play a significant role in the biogeochemical cycling of mercury (Hg). Litterfall represent a dominant pathway for Hg to reach the ground surface under a forest canopy. In order to understand Hg accumulation in the litterfall, the dynamics of Hg, carbon (C), nitrogen (N), microbial C and N, as well as bacterial community in the decaying litterfall of two typical subtropical forest stands in southwest China were investigated for one year. THg levels in the litterfall after one-year increased to 124.64% and 135.90%, and MeHg levels in the litterfall increased up to 295.65% and 209.38% of the initial values in the mixed broadleaf-conifer forest and evergreen broadleaf forest respectively. Differently, the concentrations of THg and MeHg in the organic layer of the underlying soil were quite stable. The concentrations of THg in the decaying litterfall corresponded negatively with C/N ratios. Bacterial community analysis found that the bacteria previously being confirmed as Hg methylators did not occur in the genera of the decomposing litterfall in the two forest stands, which might imply that the increase of MeHg during decomposing did not mainly due to the contribution of confirmed Hg methylators, and other sources of MeHg might exert certain roles. [ABSTRACT FROM AUTHOR]

The water-level fluctuation zone (WLFZ) has been considered as a hotspot for mercury (Hg) methylation. Flooding-tolerant herbs are gradually acclimated to this water-land ecotone, tending to form substantial root systems for improving erosion resistance. Accompanying rhizosphere microzone plays crucial but unclear roles in methylmercury (MeHg) formation in the WLFZ. Thus, we conducted this study in the WLFZ of the Three Gorges Reservoir, to explore effects of the rhizosphere of a dominant flooding-tolerant herb (bermudagrass) on MeHg production. The elevated Hg and MeHg in rhizosphere soils suggest that the rhizosphere environment provides favorable conditions for Hg accumulation and methylation. The increased bioavailable Hg and microbial activity in the rhizosphere probably serve as important factors driving MeHg formation in the presence of bermudagrass. Simultaneously, the rhizosphere environments changed the richness, diversity, and distribution of hgcA -containing microorganisms. Here, a typical iron-reducing bacterium (Geobacteraceae) has been screened, however, the majority of hgcA genes detected in rhizosphere, near-, and non-rhizosphere soils of the WLFZ were unclassified. Collectively, these results provide new insights into the elevated MeHg production as related to microbial processes in the rhizosphere of perennial herbs in the WLFZ, with general implications for Hg cycling in other ecosystems with water-level fluctuations. [Display omitted] [ABSTRACT FROM AUTHOR]

• Biochar significantly changes the nature of dissolved organic matter (DOM) in soil. • Biochar addition increases the aromaticity and molecular weight of soil DOM. • Modified biochar can effectively reduce the methylation rate in paddy soil. • Biochar significantly decreases MeHg bioaccumulation in rice plants, especially in grain. • Soil DOM modified by biochar is vital to the fate of mercury in rice field. Biochar has been used increasingly as a soil additive to control mercury (Hg) pollution in paddy rice fields. As the most active component of soil organic matter, soil dissolved organic matter (DOM) plays a vital role in the environmental fate of contaminants. However, there are very few studies to determine the impact of biochar on the Hg cycle in rice paddies using insights from DOM. This study used original and modified biochar to investigate their effect on DOM dynamics and their potential impact on methylmercury (MeHg) production and bioaccumulation in rice plants. Porewater DOM was collected to analyze the variations in soil-derived DOM in paddy soils. The results showed that the addition of biochar, whether in original or modified form, significantly reduced the bioaccumulation of MeHg in rice plants, especially in hulls and grains (p <0.05). However, MeHg production in soils was only inhibited by the modified biochar. Biochar addition induced a significant increase in DOM's aromaticity and molecular weight (p <0.05), which decreased Hg bioavailability. Furthermore, enhanced microbial activity was also observed in DOM (p <0.05), further increasing MeHg production in the soil. Thus, the effect of biochar on the fate of Hg cycle involves competition between the two different roles of DOM. This study identified a specific mechanism by which biochar affects Hg behavior in rice paddy soil and contributes to understanding the more general influence of biochar in agriculture and contaminant remediation. [Display omitted] [ABSTRACT FROM AUTHOR]

• Mercury is readily released into the environment during the Manganese mining and manganese ore smelting factories. • The concentrations of Hg and MeHg showed obvious seasonal variation in river water, the bed retention sediments and the fluvial bank sediments. • The content of Hg mainly associated with crystalline Fe–Mn oxyhydroxides in sediments. Manganese (Mn) ores contain substantial concentrations of mercury (Hg), and mining and smelting of Mn ores can bring Hg into the surrounding aquatic environment through atmospheric deposition, leaching of electrolytic Mn residue and Mn gangue dump. However, limited is known that how these processes influence the environmental behaviors of Hg in waterbody. Therefore, the seasonal distribution and existing form of Hg in water and sediment in one Mn ore area in Xiushan County, Chongqing were investigated. Our results showed that the mean Hg and methylmercury (MeHg) concentrations in water (n =35) were 5.8 ± 4.6 ng/L and 0.22 ± 0.14 ng/L, respectively. The mean Hg concentrations in retained riverbed and fluvial bank sediment (n =35) were 0.74 ± 0.26 mg/kg and 0.63 ± 0.27 mg/kg, respectively (the mean MeHg concentrations were 0.64 ± 0.40 µg/kg and 0.51 ± 0.30 µg/kg, respectively). It indicated that the mining and smelting of Mn ores were the main sources of anthropogenic Hg, and Mn may inhibit Hg methylation in rivers in Mn ore areas. Mercury in the bound to iron/ Mn (Fe/Mn) oxides of low crystallinity fraction (Hg-OX) accounted for 4.01% and 5.25% of the total Hg concentrations in the retained riverbed and fluvial bank sediment, respectively. The amount of Hg bound to Fe/Mn oxides in sediment increased significantly due to the manganese mining activities in the investigated area. Therefore, it could be hypothesized that high Hg concentrations in river sediment in Mn mining areas are closely related to high Mn concentration in sediment. [Display omitted] [ABSTRACT FROM AUTHOR]

• The mixed forest presented the highest organic soil mercury accumulation. • Litterfall mercury input is the important source for mercury accumulation in soils. • Vegetation and climate play an important role in influencing mercury distribution. • Harsh climate and sparse plants lead to low soil mercury levels in the alpine forest. Understanding atmospheric mercury (Hg) accumulation in remote montane forests is critical to assess the Hg ecological risk to wildlife and human health. To quantify impacts of vegetation, climatic and topographic factors on Hg accumulation in montane forests, we assessed the Hg distribution and stoichiometric relations among Hg, carbon (C), and nitrogen (N) in four forest types along the elevation of Mt. Gongga. Our results show that Hg concentration in plant tissues follows the descending order of litter > leaf, bark > root > branch > bole wood, indicating the importance of atmospheric Hg uptake by foliage for Hg accumulation in plants. The foliar Hg/C (from 237.0 ± 171.4 to 56.8 ± 27.7 µg/kg) and Hg/N (from 7.5 ± 3.9 to 2.5 ± 1.2 mg/kg) both decrease along the elevation. These elevation gradients are caused by the heterogeneity of vegetation uptake of atmospheric Hg and the variation of atmospheric Hg° concentrations at different altitudes. Organic soil Hg accumulation is controlled by forest types, topographic and climatic factors, with the highest concentration in the mixed forest (244.9 ± 55.7 µg/kg) and the lowest value in the alpine forest (151.9 ± 44.5 µg/kg). Further analysis suggests that soil Hg is positively correlated to C (r 2 = 0.66) and N (r 2 = 0.57), and Hg/C and Hg/N both increase with the soil depth. These stoichiometric relations highlight the combined effects from environmental and climatic factors which mediating legacy Hg accumulation and selective Hg absorption during processes of organic soil mineralization. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Wet deposition fluxes of mercury (Hg) species were estimated during the period from June 2010 to July 2011 at 3 sites in core urban areas in Chongqing, China. Rain water was collected by automatic deposition sampler for total Hg (THg) and methylmercury (MeHg) analyses. Our results showed that annual wet deposition fluxes of THg and MeHg were 28.7 ± 5.1 and 0.28 ± 0.09 μg m−2 yr−1, respectively. We estimate that MeHg wet deposition contributes 9.2–19.6% of the total MeHg flux to a local reservoir. Cumulative THg fluxes collected from nine principal rainfall events (>1.5 μg m−2 yr−1) at three sampling sites accounted for 20% of annual overall THg wet deposition. Statistical analyses yielded significant relationship between rainfall amounts and Hg wet deposition fluxes (r 2 = 0.76, p < 0.01 for THg; r 2 = 0.65, p < 0.01 for MeHg), suggesting that rainfall is one of the most important factors controlling the Hg fluxes. Obvious seasonal variations of THg fluxes were observed, with the maximum value occurring in summer (accounting for 40% of total annual THg deposition) while minimum values occurred in winter in all the three sampling sites. 72-h backward trajectory analysis on nine large deposition events at the three sites, using HYSPLIT model indicated that most of air masses which contain a relatively high precipitation amount and pass through major Hg sources would have an elevated Hg load. [Copyright &y& Elsevier]

Nitrogen (N 2)-fixing legumes can be used for phytoremediation of toxic heavy metal Mercury (Hg) contaminated soil, but N 2 -fixation highly relies on phosphorus (P) availability for nodule formation and functioning. Here, we characterized the significance of P deficiency for Hg accumulation and toxicity in woody legume plants. Consequences for foliar and root traits of rhizobia inoculation, Hg exposure (+Hg) and low P (-P) supply, individually and in combination were characterized at both the metabolite and transcriptome levels in seedlings of two Robinia pseudoacacia L. provenances originating from contrasting climate and soil backgrounds, i.e., GS in northwest and the DB in northeast China. Our results reveal that depleted P mitigates the toxicity of Hg at the transcriptional level. In leaves of Robinia depleted P reduced oxidative stress and improved the utilization strategy of C, N and P nutrition; in roots depleted P regulated the expression of genes scavenging oxidative stress and promoting cell membrane synthesis. Rhizobia inoculation significantly improved the performance of both Robinia provenances under individual and combined +Hg and -P by promoting photosynthesis, increasing foliar N and P content and reducing H 2 O 2 and MDA accumulation despite enhanced Hg uptake. DB plants developed more nodules, had higher biomass and accumulated higher Hg amounts than GS plants and thus are suggested as the high potential Robinia provenance for future phytoremediation of Hg contaminated soils with P deficiency. Soil pollution and nutrient depletion constitute serious threats for human and environmental health. However, information on the responses of plants to combined Hg pollution and P depletion has so far not been reported, at both, the metabolite and transcriptome levels. In the present study, we characterized the interaction of Hg exposure with P depletion in Robinia -rhizobia associations of two Robinia provenances originating from different environments at the metabolic and transcriptional levels. Provenance-specific differences in Robinia performance upon rhizobia inoculation at Hg and low P exposure were identified. From these results we recommend one of the two Robinia provenances for future remediation of Hg contaminated soils with low P content. [Display omitted] • P depletion showed antagonistic effects when combined with Hg exposure on Robinia-rhizobia symbiosis. • Rhizobia inoculation improved plant performance under combined Hg and depleted P. • Provenance-specific responses of Robinia plants. • Mechanisms in physiological and transcriptional levels were reported. • The DB provenance is feasible than GS for phytoremediation of Hg exposure in P-depleted soil. [ABSTRACT FROM AUTHOR]

Biodegradation of soil organic matter (SOM), which involves greenhouse gas (GHG) emissions, plays an essential role in the global carbon cycle. Over the past few decades, this has become an important research focus, particularly in natural ecosystems. SOM biodegradation significantly affects contaminants in the environment, such as mercury (Hg) methylation, producing highly toxic methylmercury (MeHg). However, the potential link between GHG production from SOM turnover in contaminated soils and biogeochemical processes involving contaminants remains unclear. In this study, we investigated the dynamics of GHG, MeHg production, and the relationship between biogeochemical processes in soils from two typical Hg mining sites. The two contaminated soils have different pathways, explaining the significant variations in GHG and MeHg production. The divergence of the microbial communities in these two biogeochemical processes is essential. In addition to the microbial role, abiotic factors such as Hg species can significantly affect MeHg production. On the other hand, we found an inverse relationship between CH 4 and MeHg, suggesting that carbon emission reduction policies and management could inadvertently increase the MeHg levels. This highlights the need for an eclectic approach to organic carbon sequestration and contaminant containment. These findings suggest that it is difficult to establish a general pattern to describe and explain the SOM degradation and MeHg production in contaminated soils within the specific scenarios. However, this study provides a case study and helpful insights for further understanding the links between environmental risks and carbon turnover in Hg mining areas. [Display omitted] • The relationship between GHG and MeHg production in two Hg-contaminated soils was investigated in this study. • Because of the soil dependence, a general pattern cannot depict the SOM degradation and MeHg production. • The relationship between GHG and MeHg could be attributed to organic matter, microbial community, and Hg speciation. • An inverse relationship was observed between the production of CH 4 and MeHg in Hg-contaminated soils. [ABSTRACT FROM AUTHOR]

Multifunctional lakes are highly susceptible to anthropogenic influences, potentially introducing exogenous pollutants or nutrients into aquatic sediments. This, in turn, affects the mercury (Hg) methylation in the sediments. This study was conducted in the Changshou Lake, a representative multifunctional lake in southwestern China, with a specific focus on investigating the Hg variations, the potential of Hg methylation, and the influential factors affecting the methylation process within sediments across different functional areas. The results revealed significant variations in total Hg concentrations between the ecological culture area (area I), the ecological tourism area (area II), and the wetland protection area (area III), suggesting the possibility of exogenous Hg introduction associated with human activities. Furthermore, sediments from areas I and II displayed a greater potential for Hg methylation. This was ascribed to the enhanced diversity and relative abundance of Hg-methylating microorganisms, especially Geobacteraceae , induced by elevated levels of dissolved organic carbon in these two areas from human activities like historical cage culture. This study provides evidence that anthropogenic activities enhance the process of Hg methylation in the sediments of multifunctional lakes, highlighting the necessity of implementing comprehensive scientific water quality management practices to mitigate the negative impacts of human influences on these unique ecosystems. [Display omitted] • Human activities increased sediment Hg levels in multifunctional lakes. • Human activities can promote sediment MeHg production in multifunctional lakes. • DOM concentration and Hg-methylating microorganisms drive sediment Hg methylation. [ABSTRACT FROM AUTHOR]

Biochar could reshape microbial communities, thereby altering methylmercury (MeHg) concentrations in rice rhizosphere and seeds. However, it remains unclear whether and how biochar amendment perturbs microbe-mediated MeHg production in mercury (Hg) contaminated paddy soil. Here, we used pinecone-derived biochar and its six modified biochars to reveal the disturbance. Results showed that selenium- and chitosan-modified biochar significantly reduced MeHg concentrations in the rhizosphere by 85.83% and 63.90%, thereby decreasing MeHg contents in seeds by 86.37% and 75.50%. The two modified bicohars increased the abundance of putative Hg-resistant microorganisms Bacillus , the dominant microbe in rhizosphere. These reductions about MeHg could be facilitated by biochar sensitive microbes such as Oxalobacteraceae and Subgroup_7. Pinecone-derived biochar increased MeHg concentration in rhizosphere but unimpacted MeHg content in seeds was observed. This biochar decreased the abundance in Bacillus but enhanced in putative Hg methylator Desulfovibrio. The increasing MeHg concentration in rhizosphere could be improved by biochar sensitive microbes such as Saccharimonadales and Clostridia. Network analysis showed that Saccharimonadales and Clostridia were the most prominent keystone taxa in rhizosphere, and the three biochars manipulated abundances of the microbes related to MeHg production in rhizosphere by those biochar sensitive microbes. Therefore, selenium- and chitosan-modified biochar could reduce soil MeHg production by these microorganisms, and is helpful in controlling MeHg contamination in rice. [Display omitted] • Effects of six modified biochars on the enrichment of MeHg in rice were compared • Selenium and chitosan modified biochars greatly reduced MeHg contents in rice. • Bacillus altered soil MeHg production by MeHg degradation under biochar amendment. • Saccharimonadales and Clostridia could increase MeHg contents in rhizosphere. [ABSTRACT FROM AUTHOR]

Previous studies have suggested that growth dilution may be an important factor contributing to the low fish Hg levels in China. To evaluate the impact of growth rate to MeHg bioaccumulation in fish in the Three Gorges Reservoir (TGR), this study used two fish species, Aristichthys nobilis (A. nobilis) and Coilia nasus (C. nasus), which differ significantly in their growth rates. A combined bioenergetic-toxicokinetic model was used to simulate methylmercury (MeHg) concentrations in these two species. The model simulations were compared with the field data and showed good fits. It explained 44.0% and 46.5% of the variation in MeHg concentrations in A. nobilis and C. nasus , respectively. Sensitivity analysis revealed that growth rate accounted for 50.9% and 16.0% of MeHg concentrations in A. nobilis and C. nasus , respectively. This indicated that growth rate was the most critical factor affecting MeHg concentrations in fast-growing fish, such as A. nobilis. However, in species with low growth rate, such as C. nasus , the effect of growth rate was not as prominent as that in fast-growing fish. As a result, MeHg elimination rates and diet MeHg levels could offset the effect of growth, and become the decisive factors for MeHg concentrations in slow-growing fish. [Display omitted] ● A bioenergetics-bioaccumulation model accurately predicts Hg bioaccumulation in fish from the TGR area. ● Growth rate s was the most critical factor regulating MeHg concentrations in fast-growing fish. ● Hg elimination rates and diet Hg levels determine MeHg concentrations in slow-growing fish. [ABSTRACT FROM AUTHOR]

Three novel friedelane triterpenes, monospermonal, monospermonol and monospermondiol, along with six known friedelanes, 3-oxofriedelane, 3-oxofriedelan-28-al, 3,12-dioxofriedelane, 3-oxo-28-hydroxyfriedelane, 3-oxo-11β-hydroxyfriedelane, 3-oxo-12α-hydroxyfriedelane and an oleanane triterpenoid, 3β-hydroxyolean-12-ene, were isolated from the stems of Celastrus monospermus Roxb. The structures of novel compounds, monospermonal, monospermonol and monospermondiol, were identified as 3,12-dioxofriedelan-28-al, 3,12-dioxo-28-hydroxyfriedelane and 3-oxo-12α,28-dihydroxyfriedelane, respectively by spectroscopies including 2D NMR methods. These nine friedelane-type triterpenes were tested for antimicrobial activity against luminescent bacterium Photobacterium leiognathi N1 that had been isolated from 'glowing' fish meat. Monospermonal and monospermonol showed moderate activity. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) in rice is presumed to be derived from MeHg formed in the soil, although it is still controversial. Moderate soil mercury (Hg) concentration can affect the diversity of soil microorganisms and may also impact the physiological changes and MeHg absorption of rice. In this study, the pot experiment was conducted to explore the effects of Hg concentration gradients (0, 0.3, 3, and 30 mg kg−1) stress on Hg transformation in the rhizosphere, Hg translocation in rice, and physiological changes in rice leaves during the whole rice growing season. Moderate soil Hg concentration (3 mg kg−1) greatly increased the MeHg/THg (1.69%) of rhizosphere, while 30 mg kg−1 soil Hg concentration sharply reduced the MeHg/THg (0.29%) of rhizosphere. Highest MeHg/THg of the four groups all appeared at the blooming or filling stage. There was a significant positive correlation between Fe2+ in rhizosphere and MeHg/THg, but no significant correlation between SO 4 2− and MeHg/THg was observed. Although the 3 mg kg−1 soil Hg concentration significantly enhanced MeHg concentrations in seeds, it considerably reduced the bioaccumulation factors of MeHg in roots, stalks, old leaves and young leaves. Soil Hg concentration of 30 mg kg−1, to a certain extent, curtailed MeHg concentrations in seeds, while MeHg concentrations in the husk were significantly increased. Consistent with the result that there was no significant difference for THg concentrations in old and young leaves among the four Hg treatment groups, the content of chlorophyll, H 2 O 2 , malondialdehyde and antioxidant substances, and the activities of antioxidant enzyme in old and young leaves varied indistinctly among groups. Moderate soil mercury concentration (3 mg kg−1) could extremely enhance MeHg production in the rhizosphere soil and its accumulation in rice; MeHg production in the rhizosphere soil increased greatly at the blooming or filling stage, whereas little effect on antioxidant systems in leaves was observed. Unlabelled Image • Highest MeHg/THg occurred with soil mercury concentration of 3 mg kg−1. • Highest MeHg/THg appeared at the blooming or filling stage. • MeHg concentrations in seeds reduced to some extent under Hg stress of 30 mg kg−1. • Little effect on the antioxidant system in leaves with mercury gradient was observed. [ABSTRACT FROM AUTHOR]

Mercury (Hg) methylation is mainly a microbial process mediated by anaerobes. The continued study of Pseudomonas putida (P. putida) strain TGRB4 genome was inspired by the fact that it can transform Hg into the highly toxic methylmercury (MeHg) under aerobic conditions. P. putida strain TGRB4 is a Gram-negative rod-shaped Gamma-proteobacterium (γ-proteobacterium), isolated from the soil in a typical water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR), which suffered from seasonally water level alternations every year. Draft genome assembly of P. putida strain TGRB4 is presented here, which was sequenced using Illumina Hiseq and PacBio single-molecule real-time (SMRT) platforms. Its genome harbors a total of 5504 genes and a G + C content of 62.6%. We further identified the enzymes related to Hg methylation, and found two well-known methyltransferase, including 5-methyl-tetrahydrofolate (5-methyl-THF) and S-adenosylmethionine (AdoMet), were annotated in the genome of P. putida strain TGRB4. This genome information could be treated as a research material to further study the Hg methylation mechanisms under aerobic environment. [ABSTRACT FROM AUTHOR]

Soil dissolved organic matter (DOM) plays an essential role in the Three Gorges Reservoir (TGR) as a linkage between terrestrial and aquatic systems. In particular, the reducing capacities of soil DOM influence the geochemistry of contaminants such as mercury (Hg). However, few studies have investigated the molecular information of soil DOM and its relationship with relevant geochemical reactivities, including redox properties. We collected samples from eight sites in the TGR areas and studied the link between the molecular characteristics of DOM and their electron donation capacities (EDCs) toward Hg(II). The average kinetic rate and EDC of soil DOM in TGR areas were (0.004 ± 0.001) hr−1 and (2.88 ± 1.39) nmol e−/mg DOM bulk , respectively. Results suggest that higher EDCs and relatively rapid kinetics were related to the greater electron donating components of lignin-derived and perhaps pyrogenic DOM, which are the aromatic constituents that influenced the reducing capacities of DOM in the present study. Molecular details revealed that even the typical autochthonous markers are important for the EDCs of DOM as well, in contrast to what is generally assumed. More studies identifying specific DOM molecular components involved in the abiotic reduction of Hg(II) are required to further understand the relations between DOM sources and their redox roles in the environmental fate of contaminants. Image 1 [ABSTRACT FROM AUTHOR]

Understanding heavy metals (HMs) accumulation and transportation is the foundation to assess the ecological risks caused by the pollution of HMs in terrestrial ecosystems. There are large knowledge gaps regarding impacts of vegetation succession on shaping the HMs accumulation, transportation and allocation in the remote alpine regions. Herein, we comprehensively investigated the distribution and source contribution of mercury (Hg), cadmium (Cd) and chromium (Cr) along with vegetation succession in a deglaciated forest chronosequence of Qinghai-Tibet Plateau. Results showed that Hg and Cd were highly enriched in organic soils, while Cr concentrations and pool sizes decreased significantly with the vegetation succession. Atmospheric Hg deposition contributed to the dominant Hg sources in topsoil (74 − 87%), whereas moraine weathering was the main source of Cr (73 − 76%). Both moraine (18 − 48%) and atmospheric deposition inputs (52 − 82%) affected Cd accumulation in topsoil. Over the last century, the accumulation rate of Hg and Cd showed the distinctly decreasing trends due to the vegetation leading to the elevated atmospheric depositions at the earlier deglacial sites. The negative accumulation rate of Cr along with the vegetation succession reflected the formation of organic soil diluting the geogenic inputs of Cr. [Display omitted] • Atmospheric deposition likely dominants the major source of Hg in topsoil. • Moraine weathering likely as the main source of Cr in topsoil. • The main source of Cd shifted from moraine weathering to atmospheric depositions. • Vegetation succession shapes the accumulation of Hg, Cd and Cr in the glacial retreat area. [ABSTRACT FROM AUTHOR]

The Three Gorges Reservoir (TGR) holds the distinction of being China's largest reservoir, and the presence of pollutants in the fish from the reservoir have a direct impact on the health of local residents. Thus, 349 fish specimens of 21 species and 1 benthos (Bellamya aeruginosas) were collected from four typical tributaries of the TGR from 2019 to 2020. These specimens were analyzed for the concentrations of total mercury (THg) and methylmercury (MeHg), and some representative samples were tested for δ13C and δ15N values to reveal the characteristics of bioaccumulation and biomagnification. The maximum safe daily consumption was estimated based on the oral reference dose (0.1 μg kg−1 bw/day according to US-EPA, 2017). The results showed that the mean THg and MeHg concentrations in fish from the TGR tributaries were 73.18 ± 49.21 ng g−1 and 48.42 ± 40.66 ng g−1, respectively, with the trophic magnification factors (TMFs) of THg and MeHg being 0.066 and 0.060, respectively. Among all the fish species in the tributaries, the highest daily maximum safe consumption amount was 1253.89 g for S. asotus consumed by adults, while the lowest was 62.88 g for C. nasus consumed by children. [Display omitted] • Hg levels in fish from the tributaries of Three Gorges Reservoir were generally low. • The risks associated with consuming fish for the general population were low. • Public awareness is needed regarding the potential health risk of Coilia nasu to children. • Chinese government's Yangtze River fishing ban policy could reduce Hg exposure risk. [ABSTRACT FROM AUTHOR]

[Display omitted] • Methylators were dominating in rice rhizosphere, rather than the bulk soil. • Hg-promoted methylators are key taxa in the rhizosphere at the flowering stage. • In planta demethylation and methylation in rice roots are possible. • Desulfovibrionaceae may play an important role in Hg detoxification of rice. The plant microbiota can affect plant health and fitness by promoting methylmercury (MeHg) production in paddy soil. Although most well-known mercury (Hg) methylators are observed in the soil, it remains unclear how rice rhizosphere assemblages alter MeHg production. Here, we used network analyses of microbial diversity to identify bulk soil (BS), rhizosphere (RS) and root bacterial networks during rice development at Hg gradients. Hg gradients greatly impacted the niche-sharing of taxa significantly relating to MeHg/THg, while plant development had little effect. In RS networks, Hg gradients increased the proportion of MeHg-related nodes in total nodes from 37.88% to 45.76%, but plant development enhanced from 48.59% to 50.41%. The module hub and connector in RS networks included taxa positively (Nitrososphaeracea, Vicinamibacteraceae and Oxalobacteraceae) and negatively (Gracilibacteraceae) correlating with MeHg/THg at the blooming stage. In BS networks, Deinococcaceae and Paludibacteraceae were positively related to MeHg/THg, and constituted the connector at the reviving stage and the module hub at the blooming stage. Soil with an Hg concentration of 30 mg kg−1 increased the complexity and connectivity of root microbial networks, although microbial community structure in roots was less affected by Hg gradients and plant development. As most frequent connector in root microbial networks, Desulfovibrionaceae did not significantly correlate with MeHg/THg, but was likely to play an important role in the response to Hg stress. [ABSTRACT FROM AUTHOR]

Abstract As a global toxic pollutant, mercury (Hg) bioaccumulation within food chain could be influenced by human disturbance. Ten typical fish species were collected from Changshou Lake, an artificial lake used to carry out cage fish culture, to investigate the C/N isotopic compositions and Hg bioaccumulation in fish. The results showed that the total Hg (THg) and methylmercury (MeHg) levels in fish muscles ((56.03 ± 43.96) and (32.35 ± 29.57) ng/g, wet weight), comparable with those in most studies in China, were significantly lower than the international marketing limit (0.5 mg/kg). Past human input for cage culture in this lake led to abnormal 15N enrichment in food chain, as the quantitative trophic levels based on δ15N were different with that classified by feeding behaviors. This phenomenon subsequently demonstrated that it should be considered thoughtfully with respect to the application of the traditional method for understanding Hg bioaccumulation power by the slope of log 10 [Hg] with δ15N regression in specific water body (i.e., Changshou Lake). In addition, no significant linear correlation between Hg and body weight or length of some fish species was observed, suggesting that the fish growth in the eutrophic environment was disproportionate with Hg bioaccumulation, and fish length or weight was not the main factor affecting Hg transfer with food web. The occurrence of human disturbance in aquatic system presents a challenge to a better understanding of the Hg bioaccumulation and biomagnification within the food chain. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract Several strains of archaea have the ability to methylate or resist mercury (Hg), and the paddy field is regarded to be conducive to Hg methylation. However, our knowledge of Hg-methylating or Hg-resistant archaea in paddy soils is very limited so far. Therefore, the distribution of archaea and bacteria in the rhizosphere (RS) and bulk soil (BS) of the rice growing in Xiushan Hg-mining area of southwest China was investigated. Bacterial and archaeal 16S rRNA gene amplicon sequencing of the rice rhizosphere along the Hg gradient was conducted. THg concentrations in RS were significantly higher than that in BS at site S1 and S2, while MeHg concentrations in RS was always higher than that in BS, except S6. Bacterial species richness estimates were much higher than that in archaea. The bacterial α-diversity in high-Hg sites was significant higher than that in low-Hg sites based on ACE and Shannon indices. At the genus level, Thiobacillus , Xanthomonas , Defluviicoccus and Candidatus Nitrosoarchaeum were significantly more abundant in the rhizosphere of high-Hg sites, which meant that strains in these genera might play important roles in response to Hg stress. Hg-methylating archaea in the paddy field could potentially be affiliated to strains in Methanosarcina , but further evidence need to be found. The results provide reference to understand archaeal rhizosphere community along an Hg gradient paddy soils. Graphical abstract Unlabelled Image Highlights • Communities of bacteria and archaea in Hg-polluted rice rhizosphere were studied firstly. • Bacteria between high and low-Hg sites clustered together respectively, so did archaea. • merA abundance was significant higher in high-Hg sites than low-Hg sites. • Methanogen Hg-methylators could potentially be affiliated to Methanosarcina. [ABSTRACT FROM AUTHOR]

Abstract The origin and composition of dissolved organic matter (DOM) in porewater of lake sediments is intricate and decisive for fate of pollutants including mercury (Hg). While there are many reports on the relationship between dissolved organic carbon concentration (DOC) and mercury (Hg) concentrations in aquatic systems, there are few in which DOM compositional properties, that may better explain the fate of Hg, have been the focus. In this study, porewaters from sediments of three lakes, Caihai Lake (CH), Hongfeng Lake (HF) and Wujiangdu Lake (WJD), all located in southwest China, were selected to test the hypothesis that DOM optical properties control the fate of Hg in aquatic ecosystems. Porewater DOM was extracted and characterized by UV–Vis absorption and fluorescence spectroscopy. A two end-member (autochthonous and allochthonous DOM) mixing model was used to unveil the origin of DOM in porewaters of the three lakes. Our results show a higher input of terrestrial DOM in the pristine lake CH, as compared to lakes HF and WJD lakes, which were both influenced by urban environments and enriched in autochthonous DOM. While the relationships between the concentrations of DOC and the different chemical forms of Hg forms were quite inconsistent, we found important links between specific DOM components and the fate of Hg in the three lakes. In particular, our results suggest that allochthonous, terrestrial DOM inhibits Hg(II) availability for Hg methylating micro-organisms. In contrast, autochthonous DOM seems to have been stimulated MeHg formation, likely by enhancing the activity of microbial communities. Indeed, DOM biodegradation experiments revealed that differences in the microbial activity could explain the variation in the concentration of MeHg. While relationships between concentrations of DOC and Hg vary among different sites and provide little information about Hg cycling, we conclude that the transport and transformation of Hg (e.g. the methylation process) are more strongly linked to DOM chemical composition and reactivity. Graphical abstract Image 1 Highlights • Porewaters contain DOM from both autochtonous and allochtonous (terrestrial) origins. • Higher contribution of terrestrial DOM in pristine lakes hampered net MeHg formation. • Autochthonous DOM enhanced microbial activity and net MeHg formation. • Total concentrations of Hg were linked mainly to terrestrial DOM. • DOM properties besides DOC should be routinely incorporated in Hg field study. [ABSTRACT FROM AUTHOR]

Reservoirs tend to have enhanced methylmercury (MeHg) concentrations compared to natural lakes and rivers, and water level fluctuations can promote MeHg production. Until now, little research has been conducted on the effects of microorganisms in soils for the formation of MeHg during different drying and flooding alternating conditions in the Three Gorges Reservoir (TGR). This study aimed to understand how water level fluctuations affect soil microbial composition and mercury concentrations, and if such microbial variations are related to Hg methylation. The results showed that MeHg concentrations and the ratios of MeHg to THg (MeHg%) in soils were higher in the seasonally drying and flooding alternating areas (DFAs, 175–155 m) than those in the non-inundated (NIAs, > 175 m) and inundated areas (IAs, < 145 m). However, MeHg% in all samples was less than 1%, indicating that the Hg methylation activity in the soils of the TGR was under a low level. 454 high-throughput sequencing of 16S rRNA gene amplicons showed that soil bacterial abundance and diversity were relatively higher in DFA compared to those in NIA and IA, and microbial community composition varied in these three areas. At the family level, those groups in Deltaproteobacteria and Methanomicrobia that might have many Hg methylators were also showed a higher relative abundance in DFA, which might be the reason for the higher MeHg production in these areas. Overall, our results suggested that seasonally water level fluctuations can enhance the microbial abundance and diversity, as well as MeHg production in the TGR. [ABSTRACT FROM AUTHOR]

In order to reveal the mercury (Hg) emission and exchange characteristics at the soil-air interface under different vegetation cover types, the evergreen broad-leaf forest, shrub forest, grass, and bare lands of Simian Mountain National Nature Reserve were selected as the sampling sites. The gaseous elementary mercury (GEM) fluxes at the soil-air interface under the four vegetation covers were continuously monitored for two years, and the effect of temperature and solar radiation on GEM fluxes were also investigated. Results showed that the GEM fluxes at the soil-air interface under different vegetation cover types had significant difference (p < 0.05). The bare land had the maximum GEM flux (15.32 ± 10.44 ng.m-2.h-1), followed by grass land (14.73 ± 18.84 ng.m-2.h-1), and shrub forest (12.83 ± 10.22 ng.m-2.h-1), and the evergreen broad-leaf forest had the lowest value (11.23 ± 11.13 ng.m-2.h-1). The GEM fluxes at the soil-air interface under different vegetation cover types showed similar regularity in seasonal variation, which mean that the GEM fluxes in summer were higher than that in winter. In addition, the GEM fluxes at the soil-air interface under the four vegetation covers in Mt. Simian had obvious diurnal variations. [ABSTRACT FROM AUTHOR]

Soil-derived dissolved organic matter (DOM) has a major influence in biogeochemical processes related to contaminant dynamics and greenhouse gas emissions, due to its reactivity and its bridging role between the soil and aquatic systems. Within the Three Gorges Reservoir (TGR, China) area, an extensive water-fluctuation zone periodically submerges the surrounding soils. Here we report a characterization study of soil-derived DOM across the TGR areas, using elemental and optical analysis, infrared spectroscopy (FTIR), pyrolysis-GC–MS (Py-GC–MS) and thermally assisted hydrolysis and methylation (THM-GC–MS). The results showed that the soil DOM from the TGR area is a mixture of “allochthonous” (i.e., plant-derived/terrigenous) and “autochthonous” (i.e., microbial) origins. The terrigenous DOM is composed primarily of phenolic and aliphatic structures from lignin and aliphatic biopolymers (i.e. cutin, suberin), respectively. Multivariate statistics differentiated between two fractions of the microbial DOM, i.e. chitin-derived, perhaps from fungi and arthropods in soil, and protein-derived, partially sourced from algal or aquatic organisms. Molecular proxies of source and degradation state were in good agreement with optical parameters such as SUVA 254 , the fluorescence index (FI) and the humification index (HIX). The combined use of elemental analysis, fluorescence spectroscopy, and Py-GC–MS provides rigorous and detailed DOM characterization, whereas THM-GC–MS is useful for more precise but qualitative identification of the different phenolic (cinnamyl, p -hydroxyphenyl, guaiacyl, syringyl and tannin-derived) and aliphatic materials. With the multi-methodological approach used in this study, FTIR was the least informative, in part, because of the interference of inorganic matter in the soil DOM samples. The soil DOM from the TGR's water fluctuation zone exhibited considerable compositional diversity, mainly related to the balance between DOM source (microbial- or plant-derived), local vegetation and anthropogenic activities (e.g., agriculture). Finally, the relationship between DOM composition and its potential reactivity with substances of environmental concerns in the TGR area are also discussed. [ABSTRACT FROM AUTHOR]

• DOM characteristic/reactivity behavior not always conservative in end-member mixing. • Bio/photodegradation behavior not conservative owing to two end-member interaction. • Higher proportion of terrestrial DOM caused higher ·OH production and redox capacity. • Peak C and SUVA 254 are appropriate indicator to predict DOM reactivities in end-member mixing. Dissolved organic matter (DOM) is an essential component of environmental systems. It usually originates from two end-members, including allochthonous and autochthonous sources. Previously, links have been established between DOM origins/sources and its biogeochemical reactivities. However, the influence of changes in DOM characteristics driven by end-member mixing on DOM biogeochemical reactivities has not been clarified. In this study, we investigated variations of DOM reactivities responding to the dynamics of DOM characteristics induced by different mixing ratios of two DOM end-members derived from humic acid (HA) and algae, respectively. Four biogeochemical reactivities of DOM were evaluated, including biodegradation, ·OH production, photodegradation, and redox capacity. Results showed that the variations of DOM characteristics due to the two end-members mixing significantly impact its biogeochemical reactivities. However, not all spectral parameters and reactivities followed the conservative mixing behavior. In contrast to reactivities of ·OH production and redox capacity, mixed samples showed apparent deviations from conservative linear relationships in biodegradation and photodegradation due to the interaction between the two end-members. Regarding the role of DOM properties influencing reactivity changes, peak A and M were recognized as the most stable parameters. However, peak C and SUVA 254 were identified as the most vital contributors for explaining DOM reactivity variations. These findings suggest that a general model for describing the dynamic relationship between DOM source and reactivity cannot be proposed. Thus, the dynamics of DOM reactivity in diverse ecosystems cannot be estimated simply by the "plus or minus" of the reactivity from individual end-member. The effect of end-member mixing should be evaluated in a given reactivity instead of generalization. This study provides important insights for further understanding the dynamics of DOM's environmental role in different ecosystems influenced by variations of source inputs. In future, more field investigations are needed to further verify our findings in this study, especially in the scenario of end-member mixing. [Display omitted] [ABSTRACT FROM AUTHOR]

Over 3000 mercury (Hg)-contaminated sites worldwide contain liquid metallic Hg [Hg(0) l ] representing a continuous source of elemental Hg(0) in the environment through volatilization and solubilization in water. Currently, there are few effective treatment technologies available to remove or sequester Hg(0) l in situ. We investigated sonochemical treatments coupled with complexing agents, polysulfide and sulfide, in oxidizing Hg(0) l and stabilizing Hg in water, soil and quartz sand. Results indicate that sonication is highly effective in breaking up and oxidizing liquid Hg(0) l beads via acoustic cavitation, particularly in the presence of polysulfide. Without complexing agents, sonication caused only minor oxidation of Hg(0) l but increased headspace gaseous Hg(0) g and dissolved Hg(0) aq in water. However, the presence of polysulfide essentially stopped Hg(0) volatilization and solubilization. As a charged polymer, polysulfide was more effective than sulfide in oxidizing Hg(0) l and subsequently stabilizing the precipitated metacinnabar (β-HgS) nanocrystals. Sonochemical treatments with sulfide yielded incomplete oxidation of Hg(0) l , likely resulting from the formation of HgS coatings on the dispersed µm-size Hg(0) l bead surfaces. Sonication with polysulfide also resulted in rapid oxidation of Hg(0) l and precipitation of HgS in quartz sand and in the Hg(0) l -contaminated soil. This research indicates that sonochemical treatment with polysulfide could be an effective means in rapidly converting Hg(0) l to insoluble HgS precipitates in water and sediments, thereby preventing its further emission and release to the environment. We suggest that future studies are performed to confirm its technical feasibility and treatment efficacy for remediation applications. [Display omitted] • Sonochemical oxidation and stabilization of liquid elemental mercury [Hg(0) l ] are evaluated in water and soil. • Sonication promotes the breakup and oxidation of Hg(0) l beads via acoustic cavitation. • Sonication coupled with polysulfide prevents Hg(0) l solubilization and volatilization loss. • Polysulfide is more effective than sulfide in sonochemical oxidation of Hg(0) l and formation of HgS nanocrystals. • Sonochemical treatment offers a potentially effective means for rapidly converting Hg(0) l to HgS in water and soil. [ABSTRACT FROM AUTHOR]

The water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in China experiences a drying and wetting rotation every year, and the water level induced redox variation may influence inorganic sulfur speciation and mercury methylation. In this work, a simulative flooding and drying experiment and a sulfate added flooding experiment were conducted to study this topic. The results showed that sulfate was reduced from the 10th d during the flooding period based on the detected sulfide in water and the increased elemental sulfur (S 0 ) in sediment. Sulfate reduction and sulfide re-oxidation led to the increase of S 0 contents with the maximal values of 1.86 and 0.46 mg kg −1 during the flooding and drying period, respectively. Methylmercury (MeHg) content in sediment displayed a rising trend (0.16–0.28 μg kg −1 ) in the first 40 d during the flooding period, and then declined from 0.28 to 0.15 μg kg −1 . A positive correlation between MeHg content and S 0 content in soil ( r = 0.53 , p < 0.05) was found during the flooding period, and a positive but not significant correlation between the percent of MeHg in THg (%MeHg) and S 0 content ( r = 0.85 , p = 0.08). In sulfate added flooding simulation, MeHg content in sediment did not increase with the sulfate concentration increasing. The increased pyrite in high-sulfate treatment may fix mercury through adsorption process. This study demonstrated that inorganic sulfur species especially S 0 and chromium reducible sulfur (CRS) play an important role on mercury methylation in the WLFZ of the TGR. [ABSTRACT FROM AUTHOR]

Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg2+) or further to elemental Hg (Hg0). Together, this work elucidates variations of N-fixing and MeHg-degrading microbes in decaying litterfall and their relationships with Hg accumulation, providing novel insights into understanding the biogeochemical cycle of Hg in the forest ecosystem. [Display omitted] • A gradually-complex network evolved for the decaying litterfall of MBC forest. • A relatively static network existed for the decomposing EB forest litterfall. • N-fixing bacteria may mediate the increase of THg levels in the decomposing litterfall. • MeHg enhancement is likely regulated by aerobic MeHg-degrading microbes. • Underlying soil is the source of litterfall THg, while MeHg is transported from litterfall to soil. [ABSTRACT FROM AUTHOR]

Rhizosphere, rich in roots exudates, is the essential zone for pollutants behaviors and plant nutrition. Cynodon dactylon (C. dactylon), dominating in the water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR), China, has extensive root systems, probably deeply influencing the mercury (Hg) migration and nutrients assimilation. However, it is unclear whether Hg migration was influenced by the combination of nutrient uptake and the secretion of low molecular weight organic acids (LMWOAs). Therefore, we investigated nutrients and Hg migration trends as well as the distribution of LMWOAs secreted by C. dactylon roots in different distance to rhizosphere through laboratory simulation. The results showed that nutrients diffusion, caused by plants uptake, from bulk soil to the rhizosphere increased Hg mobility. LMWOAs could not only benefit the produce and migration of available nitrogen (AN) and available potassium (AK) but accelerate Hg movement. Meanwhile, the lack supplement of AN and AK for C. dactylon assimilation might provoke Hg translocation from soil to roots. Fortunately, Hg preferred to remain in the roots rather than transferring to shoots during deficient nutrients supplement periods. The study supplemented the research on Hg fate in the WLFZ of the TGR, suggesting that nutrients uptake is also essential in Hg migration. [Display omitted] • Nutrients uptake by Cynodon dactylon accelerates mercury migration. • Low molecular weight organic acids activate nutrients and mercury in rhizosphere. • Rhizosphere nutrients supplement effects Cynodon dactylon resistance to mercury. [ABSTRACT FROM AUTHOR]