Mercury cycling in agricultural and managed wetlands, Yolo Bypass, California: spatial and seasonal variations in water quality

Management strategies to reduce microbial mercury methylation in constructed wetlands: Potential routes and future challenges

Constructed wetlands (CWs) are widely recognized as the potential hotspots for producing highly toxic methylmercury (MeHg). This presents an obstacle to the widespread application of CWs. A comprehensive discussion on strategies to control mercury methylation in CWs is currently lacking. This review highlighted the potential impacts of differences in oxygen supply and consumption in various CWs, the characteristics of influent quality, the interactions between different substrates and mercury (including mercury adsorption, reduction), and plants on microbial mercury methylation in CWs. We also proposed the potential strategies for human intervention in regulating or controlling microbial mercury methylation in CWs, including oxygenation, nitrate inhibition, selection of substrates with high adsorption capacity, weak reducibility and low organic matter release, and plant management. Knowledge summarized in this review would help achieve a comprehensive understanding of various research gaps in previous studies and point out future research directions by focusing on CWs types, influent quality, substrates selection and plants management, to reduce the mercury methylation in CWs.

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

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

Influence of irrigation water and soil on annual mercury dynamics in Sacramento Valley rice fields

Methylmercury (MeHg) is a human and environmental toxin produced in flooded soils. Little is known about MeHg in rice (Oryza Sativa L.) fields in Sacramento Valley, California. The objectives of this study were to quantify mercury fractions in irrigation water and within rice fields and to determine their mercury pools in surface water, soil, and grain. Soil, grain, and surface water (dissolved and particulate) MeHg and total mercury (THg) were monitored in six commercial rice fields throughout a winter fallow season and subsequent growing season. Both dissolved and particulate mercury fractions were higher in fallow season rice field water. Total suspended solids and particulate mercury concentrations were positively correlated (r = 0.99 and 0.98 for THg and MeHg, respectively), suggesting that soil MeHg was suspended in the water column and potentially exported. Dissolved THg and MeHg concentrations were positively correlated with absorbance at 254 nm (r = 0.47 and 0.58, respectively) in fallow season field water. In the growing season, fields with higher irrigation water MeHg concentrations (due to recycled water use) had elevated field-water MeHg (r = 0.86) and grain MeHg concentrations (r = 0.96). Based on a mass balance analysis, soil mercury pools were orders of magnitude larger than surface water or grain mercury pools; however, fallow season drainage and grain harvest were the primary pathways for MeHg export. Based on these findings, reducing (1) discharge when water is turbid, (2) straw inputs, and (3) use of recycled irrigation water could help reduce mercury exports in rice field drainage water.

Methylmercury photodegradation in paddy water: An overlooked process mitigating methylmercury risks

Photodegradation is critical to reduce the potent neurotoxic methylmercury (MeHg) in water and its subsequent accumulation along food chains. However, this process has been largely ignored in rice paddies, which are hotspots of MeHg production and receive about a quarter of the world's developed freshwater resources. Here, we reported that significant MeHg photodegradation, primarily mediated by hydroxyl radicals, occurs in the overlying water during rice growth. By incorporating field-measured light interception into a rice paddy biogeochemistry model, as well as photodegradation rates obtained from 42 paddy soils stretching ∼3500 km across China, we estimated that photodegradation reduced MeHg concentrations in paddy water and rice by 82 % and 11 %, respectively. Without photodegradation, paddy water could be a significant MeHg source for downstream ecosystems, with an annual export of 178 - 856 kg MeHg to downstream waters in China, the largest rice producer. These findings suggest that photodegradation in paddy water is critical for preventing greater quantities of MeHg entering human food webs.

Mechanisms and biological effects of organic amendments on mercury speciation in soil-rice systems: A review

Mercury (Hg) pollution is a well-recognized global environmental and health issue and exhibits distinctive persistence, neurotoxicity, bioaccumulation, and biomagnification effects. As the largest global Hg reservoir, the Hg cumulatively stored in soils has reached as high as 250-1000 Gg. Even more concerning is that global soil-rice systems distributed in many countries have become central to the global Hg cycle because they are both a major food source for more than 3 billion people worldwide and the central bridge linking atmospheric and soil Hg circulation. In this review, we discuss the form distribution, transformation, and bioavailability of Hg in soil-rice systems by focusing on the Hg methylation and demethylation pathways and distribution, uptake, and accumulation in rice plants and the effects of Hg on the community structure and ecological functions of microorganisms in soil-rice systems. In addition, we clarify the mechanisms through which commonly used humus and biochar organic amendments influence Hg and its environmental effects in soil-rice systems. The review also elaborates on the advantages of sulfur-modified biochars and their critical role in controlling Hg migration and bioavailability in soils. Finally, we provide key information about Hg pollution in soil-rice systems, which is of great significance for developing appropriate strategies and mitigation planning to limit Hg bioconcentration in rice crops and achieving key global sustainable development goals, such as the guarantee of food security and the promotion of sustainable agriculture.

Understanding the effects of sulfur input on mercury methylation in rice paddy soils

Sulfur could be introduced into paddy soils via dry or wet deposition, irrigation, and fertilization, which subsequently impacts the production of methylmercury (MeHg), a bioaccumulative neurotoxicant. However, effects of sulfur input on MeHg production are variable, possibly due to the multiple effects of sulfur on Hg mobility and/or microbial Hg methylators, leading to uncertainties in predicting MeHg risk. To address that, we explored the effects of different types and amounts of sulfur as well as concentrations of ambient sulfate on Hg methylation in paddy soils, and elucidated the mechanisms by quantifying changes in (1) Hg mobility and (2) microbial Hg methylators (e.g., sulfate-reducing bacteria, SRB). Our results indicated that MeHg levels increased by 40-86% and 30-96% in soils under various types (i.e., 200 mg kg^-1 elemental sulfur, ammonium sulfate, sulfur-coated urea and potassium sulfate (K₂SO₄)) and different amounts (i.e., 100, 200 and 400 mg kg^-1 K₂SO₄) of sulfur input. The enhanced MeHg production could be explained by increased Hg mobility but not changes in microbial Hg methylators. Besides, sulfate input increased MeHg levels (89-240%) in soils with low ambient sulfate levels (<100 mg kg^-1) but had no effect on high-sulfate soils (>380 mg kg^-1). These could be explained by the diverse responses of Hg mobility and microbial Hg methylators to sulfate input at different ambient sulfate levels. Our study opens the "black box" of Hg methylation under sulfur input, which would help reduce uncertainties in predicting MeHg risk in soils.

Trace metal(loid)s contamination in paddy rice (Oryza sativa L.) from wetlands near two gold mines in Côte d'Ivoire and health risk assessment

This study examined the concentrations of arsenic (As), cadmium (Cd), and mercury (Hg) in rice grains grown in wetlands associated with gold mining in central-southern of Côte d'Ivoire to evaluate potential health risks exposure via rice consumption. In total, 30 rice grains were sampled around Agbaou and Bonikro gold mines. Arsenic and cadmium concentrations were determined using an inductively coupled plasma-optical emission spectrometer (ICP-OES), while atomic absorption spectrometry (AAS) was used for mercury. Results showed that As and Hg average concentrations in rice were above the permissible limits, while Cd average concentrations were below the permissible limit established by FAO/WHO in both sites. Except for Hg at Agbaou, no significant (p < 0.05) difference was found between trace metal concentrations in the two sites. The average daily intake (ADI) of As via rice consumption exceeded the USEPA reference dose (RfD) of 0.0003 μg g^-1 day^-1, indicating that rice ingestion is a pathway of As exposure for adults and children in the area. The average values of non-carcinogen (HQ) for As and carcinogen (CR) for As and Cd risks index suggest that potential health risks exist for both adults and children due to rice consumption at Agbaou and Bonikro. The maximum safe weekly consumption (MSWC) of rice relative to As, Cd, and Hg was estimated for the study area. Overall, this study provides strong evidence that As could threaten local population health in Côte d'Ivoire regions where gold mine extraction is occurring through rice ingestion.

Geochemical cycle of mercury associated with wet deposition and inflows in a subalpine wetland

Subalpine wetland is a mercury (Hg) sensitive ecosystem, but there is poor understanding of Hg behavior in this typical wetland. Here, distribution and speciation of Hg in waters of a subalpine wetland (Dajiuhu) in China were investigated, and an initial model of the Hg geochemical cycle in the wetland was established based on Hg mass balance calculations. Concentrations of both total Hg (THg, 9.52 ± 6.61 ng L^-1) and total methyl mercury (TMeHg, 0.34 ± 0.44 ng L^-1) in the waters during the wet season were higher than in the dry season. The majority of THg was in dissolved form whereas most TMeHg was in particle form. The geochemical models suggested that, due to the wet deposition and surface runoff, the input of THg and TMeHg into the wetland in the wet season (222 and 2.74 g year^-1, respectively) was higher than that in the dry season (57.9 and 1.15 g year^-1, respectively). The output of THg and TMeHg from the wetland underground runoff in the wet season was estimated to be 154 and 2.51 g year^-1, respectively, and in the dry season was 15.9 and 0.43 g year^-1, respectively. Other losses of Hg were due to volatilization of Hg⁰ from the sediment water (30.5 and 12.5 g year^-1 in the wet and dry seasons, respectively). The flux of the settling of particulate Hg in the wet season was higher than that in the dry season. The fluxes of Hg diffusion from the porewater were relatively low in comparison to the fluxes of inflows and wet deposition. The flux of oxidation was higher than reduction, while the flux of methylation was higher than demethylation. These results indicated that the elevated levels of THg and MeHg in the Dajiuhu wetland are a consequence of rainfall and surface runoff inputs.

Mercury Methylation Genes Identified across Diverse Anaerobic Microbial Guilds in a Eutrophic Sulfate-Enriched Lake

Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into bioaccumulative, neurotoxic methylmercury (MeHg). The metabolic activity of methylating organisms is highly dependent on biogeochemical conditions, which subsequently influences MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. Here, we identified potential locations of MeHg production in the anoxic, sulfidic hypolimnion of a freshwater lake. At these sites, we used shotgun metagenomics to characterize microorganisms with the Hg-methylation gene hgcA. Putative methylators were dominated by hgcA sequences divergent from those in well-studied, confirmed methylators. Using genome-resolved metagenomics, we identified organisms with hgcA (hgcA+) within the Bacteroidetes and the recently described Kiritimatiellaeota phyla. We identified hgcA+ genomes derived from sulfate-reducing bacteria, but these accounted for only 22% of hgcA+ genome coverage. The most abundant hgcA+ genomes were from fermenters, accounting for over half of the hgcA gene coverage. Many of these organisms also mediate hydrolysis of polysaccharides, likely from cyanobacterial blooms. This work highlights the distribution of the Hg-methylation genes across microbial metabolic guilds and indicate that primary degradation of polysaccharides and fermentation may play an important but unrecognized role in MeHg production in the anoxic hypolimnion of freshwater lakes.

Mercury in rice paddy fields and how does some agricultural activities affect the translocation and transformation of mercury - A critical review

Human exposure to methylmercury (MeHg) through rice consumption is raising health concerns. It has long been recognized that MeHg found in rice grain predominately originated from paddy soil. Anaerobic conditions in paddy fields promote Hg methylation, potentially leading to high MeHg concentrations in rice grain. Understanding the transformation and migration of Hg in the rice paddy system, as well as the effects of farming activities, are keys to assessing risks and developing potential mitigation strategies. Therefore, this review examines the current state of knowledge on: 1) sources of Hg in paddy fields; 2) how MeHg and inorganic Hg (IHg) are transformed (including abiotic and biotic processes); 3) how IHg and MeHg enter and translocate in rice plants; and 4) how regular farming activities (including the application of fertilizer, cultivation methods, choice of cultivar), affect Hg cycling in the paddy field system. Current issues and controversies on Hg transformation and migration in the paddy field system are also discussed.

Recent advances in understanding and measurement of mercury in the environment: Terrestrial Hg cycling

This review documents recent advances in terrestrial mercury cycling. Terrestrial mercury (Hg) research has matured in some areas, and is developing rapidly in others. We summarize the state of the science circa 2010 as a starting point, and then present the advances during the last decade in three areas: land use, sulfate deposition, and climate change. The advances are presented in the framework of three Hg "gateways" to the terrestrial environment: inputs from the atmosphere, uptake in food, and runoff with surface water. Among the most notable advances: These and other advances reported here are of value in evaluating the effectiveness of the Minamata Convention on reducing environmental Hg exposure to humans and wildlife.

The Mercury-Tolerant Microbiota of the Zooplankton Daphnia Aids in Host Survival and Maintains Fecundity under Mercury Stress

Many aquatic organisms can thrive in polluted environments by having the genetic capability to withstand suboptimal conditions. However, the contributions of microbiomes under these stressful environments are poorly understood. We investigated whether a mercury-tolerant microbiota can extend its phenotype to its host by ameliorating host survival and fecundity under mercury-stress. We isolated microbiota members from various clones of Daphnia magna, screened for the mercury-biotransforming merA gene, and determined their mercury tolerance levels. We then introduced the mercury-tolerant microbiota, Pseudomonas-10, to axenic D. magna and quantified its merA gene expression, mercury reduction capability, and measured its impact on host survival and fecundity. The expression of the merA gene was up-regulated in Pseudomonas-10, both in isolation and in host-association with mercury exposure. Pseudomonas-10 is also capable of significantly reducing mercury concentration in the medium. Notably, mercury-exposed daphnids containing only Pseudomonas-10 exhibited higher survival and fecundity than mercury-exposed daphnids supplemented with parental microbiome. Our study showed that zooplankton, such as Daphnia, naturally harbor microbiome members that are eco-responsive and tolerant to mercury exposure and can aid in host survival and maintain host fecundity in a mercury-contaminated environment. This study further demonstrates that under stressful environmental conditions, the fitness of the host can depend on the genotype and the phenotype of its microbiome.

Rice life cycle-based global mercury biotransport and human methylmercury exposure

Protecting the environment and enhancing food security are among the world’s greatest challenges. Fish consumption is widely considered to be the single significant dietary source of methylmercury. Nevertheless, by synthesizing data from the past six decades and using a variety of models, we find that rice could be a significant global dietary source of human methylmercury exposure, especially in South and Southeast Asia. In 2013, globalization caused 9.9% of human methylmercury exposure via the international rice trade and significantly aggravated rice-derived exposure in Africa (62%), Central Asia (98%) and Europe (42%). In 2016, 180 metric tons of mercury were generated in rice plants, 14-fold greater than that exported from oceans via global fisheries. We suggest that future research should consider both the joint ingestion of rice with fish and the food trade in methylmercury exposure assessments, and anthropogenic biovectors such as crops should be considered in the global mercury cycle.

Fish consumption is considered to be the only significant dietary source of MeHg. Here the authors show that rice could also be a significant global dietary source, especially in South and Southeast Asia. International rice trade and joint ingestion of fish and rice could aggravate the MeHg exposure levels in many areas.

Wetland Management Strategy to Reduce Mercury in Water and Bioaccumulation in Fish

Wetland environments provide numerous ecosystem services but also facilitate methylmercury (MeHg) production and bioaccumulation. We developed a wetland-management technique to reduce MeHg concentrations in wetland fish and water. We physically modified seasonal wetlands by constructing open- and deep-water treatment cells at the downstream end of seasonal wetlands to promote naturally occurring MeHg-removal processes. We assessed the effectiveness of reducing mercury (Hg) concentrations in surface water and western mosquitofish that were caged at specific locations within 4 control and 4 treatment wetlands. Methylmercury concentrations in wetland water were successfully decreased within treatment cells during only the third year of study; however, treatment cells were not effective for reducing total Hg concentrations. Furthermore, treatment cells were not effective for reducing total Hg concentrations in wetland fish. Mercury concentrations in fish were not correlated with total Hg concentrations in filtered, particulate, or whole water; and the slope of the correlation with water MeHg concentrations differed between months. Fish total Hg concentrations were weakly correlated with water MeHg concentrations in April when fish were introduced into cages but were not correlated in May when fish were retrieved from cages. Fish total Hg concentrations were greater in treatment wetlands than in control wetlands the year after the treatment wetlands' construction but declined by the second year. During the third year, fish total Hg concentrations increased in both control and treatment wetlands after an unexpected regional flooding event. Overall, we found limited support for the use of open- and deep-water treatment cells at the downstream end of wetlands to reduce MeHg concentrations in water but not fish. We suggest that additional evaluation over a longer period of time is necessary. Environ Toxicol Chem 2019;38:2178-2196. Published 2019 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US government work, and as such, is in the public domain in the United States of America..

Mobilization of mercury species under dynamic laboratory redox conditions in a contaminated floodplain soil as affected by biochar and sugar beet factory lime

Mercury and its species are toxic and therefore strategies to immobilize them or to impede the formation of bioaccumulative MeHg are a hot topic of ongoing research. Biochar (BC) and sugar beet factory lime (SBFL) are suggested to have the potential to meet these goals. However, their ability to restrain the mobilization of total Hg (Hg_t), methylmercury (MeHg), and ethylmercury (EtHg) or the formation of MeHg and EtHg has not been examined to date. Moreover, the effect of systematically altered redox conditions on the release dynamics of Hg_t, MeHg, and EtHg in a contaminated floodplain soil as affected by these soil amendments has not been studied. Therefore, we investigated the impact of pre-defined redox conditions on the release dynamics of Hg_t, MeHg, and EtHg in a contaminated floodplain soil (CS) and the soil amended with either BC (CS+BC) or SBFL (CS+SBFL). The mobilization of Hg_t, MeHg, and EtHg was generally higher at low redox potential (E_H) and decreased with increasing E_H, irrespective of soil treatment. Both BC and SBFL diminished the release of Hg_t from soil but not the methylation and ethylation of Hg. In CS+SBFL approximately half of Hg_t was found in solution compared to CS. However, higher methylation efficiency (MeHg/Hg_t ratio) was found in CS+SBFL counterbalancing this benefit. Abundances of specific phospholipid fatty acids suggest the presence of sulfate-reducing bacteria, which are considered as primary Hg methylators. The results indicate that both BC and SBFL have the potential to curtail the release of Hg_t from inundated soils, while SBFL was more efficient. However, these amendments had no marked effect on the MeHg and EtHg concentrations. Therefore, further research should be conducted to identify soil additives that are capable to reduce the release and formation of these Hg species.

Impact of biochar on mobilization, methylation, and ethylation of mercury under dynamic redox conditions in a contaminated floodplain soil

Mercury (Hg) is a highly toxic element, which is frequently enriched in flooded soils due to its anthropogenic release. The mobilization of Hg and its species is of ultimate importance since it controls the transfer into the groundwater and plants and finally ends in the food chain, which has large implications on human health. Therefore, the remediation of those contaminated sites is an urgent need to protect humans and the environment. Often, the stabilization of Hg using amendments is a reliable option and biochar is considered a candidate to fulfill this purpose. We tested two different pine cone biochars pyrolyzed at 200 °C or 500 °C, respectively, with a view to decrease the mobilization of total Hg (Hg_t), methylmercury (MeHg), and ethylmercury (EtHg) and/or the formation of MeHg and EtHg in a contaminated floodplain soil (Hg_t: 41 mg/kg). We used a highly sophisticated automated biogeochemical microcosm setup to systematically alter the redox conditions from ~-150 to 300 mV. We continuously monitored the redox potential (E_H) along with pH and determined dissolved organic carbon (DOC), SUVA₂₅₄, chloride (Cl^-), sulfate (SO₄^2-), iron (Fe), and manganese (Mn) to be able to explain the mobilization of Hg and its species. However, the impact of biochar addition on Hg mobilization was limited. We did not observe a significant decrease of Hg_t, MeHg, and EtHg concentrations after treating the soil with the different biochars, presumably because potential binding sites for Hg were occupied by other ions and/or blocked by biofilm. Solubilization of Hg bound to DOC upon flooding of the soils might have occurred which could be an indirect impact of E_H on Hg mobilization. Nevertheless, Hg_t, MeHg, and EtHg in the slurry fluctuated between 0.9 and 52.0 μg/l, 11.1 to 406.0 ng/l, and 2.3 to 20.8 ng/l, respectively, under dynamic redox conditions. Total Hg concentrations were inversely related to the E_H; however, ethylation of Hg was favored at an E_H around 0 mV while methylation was enhanced between -50 and 100 mV. Phospholipid fatty acid profiles suggest that sulfate-reducing bacteria may have been the principal methylators in our experiment. In future, various biochars should be tested to evaluate their potential in decreasing the mobilization of Hg and to impede the formation of MeHg and EtHg under dynamic redox conditions in frequently flooded soils.

Understanding Enhanced Microbial MeHg Production in Mining-Contaminated Paddy Soils under Sulfate Amendment: Changes in Hg Mobility or Microbial Methylators?

Elevated methylmercury (MeHg) production in mining-contaminated paddy soils, despite the high fraction of refractory HgS(s), has been frequently reported, while the underlying mechanisms are not fully understood. Here, we hypothesized that sulfate input, via fertilization, rainfall, and irrigation, is critical in mobilizing refractory HgS(s) and thus enhancing Hg methylation in mining-contaminated paddy soils. To test this hypothesis, the effects of sulfate amendment on Hg methylation and MeHg bioaccumulation in mining-contaminated soil-rice systems were examined. The results indicated 28-61% higher net MeHg production in soils under sulfate amendment (50-1000 mg kg^-1), which in turn increased grain MeHg levels by 22-55%. The enhancement of Hg methylation by Hg mobilization in sulfate-amended soils was supported by two observations: (1) the increased Hg(aq) release from HgS(s), the dominant Hg species in the paddy soils, in the presence of sulfide produced following sulfate reduction and (2) the decreases of refractory HgS(s) in soils under sulfate amendment. By contrast, changes in the abundances/activities of potential microbial Hg methylators in different Hg-contaminated soils were not significant following sulfate amendment. Our results highlight the importance to consider enhanced Hg mobility and thus methylation in soils under sulfate amendment.

Rice root exudates affect microbial methylmercury production in paddy soils

Microbial methylmercury (MeHg) production in contaminated soil-rice systems and its accumulation in rice pose health risks to consumers, especially those in Asia. However, the mechanism responsible for microbial MeHg production in paddy soils is far from clear. While previous studies examined the effect of soil and microbial factors on soil MeHg levels, in this work we explored the impact of rice cultivation itself on microbial MeHg production, focusing on the root exudate organic matter as a potential source of electron donors for microbial methylators. Effects of the cultivation of two rice cultivars, Heigu246 (H-rice) and Neiwuyou8015 (N-rice), on MeHg production in soils were therefore investigated in pot and batch incubation experiments. Soil MeHg levels measured in H-rice treatment during the heading and harvest stages were 18-49% higher than in the control and 23-108% higher than in N-rice treatment. Consequently, MeHg levels in grain, straw, and root were 38%, 81%, and 40% higher in H-rice than those in N-rice, which was mainly attributed to cultivar-specific MeHg production in soils. Results of the batch experiments suggested that root exudate organic matter could be responsible for MeHg production in soils during rice cultivation, by increasing the abundances of potential microbial methylators. For instance, root exudate organic matter increased copy numbers of Hg methylation genes (hgcA) in soils 4.1-fold. Furthermore, the 211% higher concentration of acetate (a key electron donor for microbial methylators) in the root exudate of H-rice could account for the higher MeHg production under H-rice than N-rice cultivation. Our results suggest that root exudate organic matter, especially acetate, as its key component, contributes to the elevated soil MeHg concentrations during rice cultivation. The proposed mechanism provides new insights into the elevated risk of MeHg production in contaminated soil-rice systems, as well as cultivar-specific MeHg bioaccumulation.

Methylmercury Dynamics in Upper Sacramento Valley Rice Fields with Low Background Soil Mercury Levels

Few studies have considered how methylmercury (MeHg, a toxic form of Hg produced in anaerobic soils) production in rice ( L.) fields can affect water quality, and little is known about MeHg dynamics in rice fields. Surface water MeHg and total Hg (THg) imports, exports, and storage were studied in two commercial rice fields in the Sacramento Valley, California, where soil THg was low (25 and 57 ng g). The median concentration of MeHg in drainage water exiting the fields was 0.17 ng g (range: <0.007-2.1 ng g). Compared with irrigation water, drainage water had similar MeHg concentrations, and lower THg concentrations during the growing season. Significantly elevated drainage water MeHg and THg concentrations were observed in the fallow season compared with the growing season. An analysis of surface water loads indicates that fields were net importers of both MeHg (76-110 ng m) and THg (1947-7224 ng m) during the growing season, and net exporters of MeHg (35-200 ng m) and THg (248-6496 ng m) during the fallow season. At harvest, 190 to 700 ng MeHg m and 1400 to 1700 ng THg m were removed from fields in rice grain. Rice straw, which contained 120 to 180 ng MeHg m and 7000-10,500 ng m THg was incorporated into the soil. These results indicate that efforts to reduce MeHg and THg exports in rice drainage water should focus on the fallow season. Substantial amounts of MeHg and THg were stored in plants, and these pools should be considered in future studies.

Biochar and nitrate reduce risk of methylmercury in soils under straw amendment

There is growing evidence that incorporating crop straw into soils, which is being widely encouraged in many parts of the world, could increase net methylmercury (MeHg) production in soils and MeHg accumulation in crops. We explored the possibility of mitigating the risk of increased MeHg levels under straw amendment by transforming straw into biochar (BC). Greenhouse and batch experiments were conducted, in which soil MeHg concentrations, MeHg phytoavailability and accumulation in rice, dynamics of sulfate, nitrate and abundances of sulfate reducing bacteria (SRB) were compared in 'Control' (Hg contaminated soil), 'Straw' (soil with 1% rice straw), 'Straw+BC' (soil with 1% straw and 1% biochar), and 'Straw+BC+N' (soil with 1% straw, 1% biochar and 0.12% nitrate). Our results indicate that straw amendment increased MeHg concentrations in soils (28-136% higher) and rice plants (26% higher in grains, 'Straw' versus 'Control'), while co-application of biochar with straw reduced grain MeHg levels (60% lower, 'Straw+BC' versus 'Straw'). This could be mainly attributed to the reduced MeHg availability to rice plants (phytoavailability, extraction rates of MeHg by ammonium thiosulfate) under biochar amendment (64-99% lower, 'Straw+BC' versus 'Straw'). However, biochar amendment enhanced soil MeHg levels (5-75% higher, 'Straw+BC' versus 'Control'). Interestingly, nitrate addition helped reduce soil MeHg concentrations (11-41% lower, 'Straw+BC+N' versus 'Straw+BC') by facilitating nitrate reduction while inhibiting SRB activities. Subsequently, addition of nitrate with biochar, compared with biochar alone, further reduced grain MeHg levels by 34%. Therefore, straw biochar together with nitrate could possibly be effective in mitigating the risk of MeHg under straw amendment. Furthermore, the results evidence the impacts of straw management on the risk posed by MeHg in soils and emphasize the necessity to carefully consider the straw management policy in Hg-contaminated areas.

Challenges and opportunities for managing aquatic mercury pollution in altered landscapes

The environmental cycling of mercury (Hg) can be affected by natural and anthropogenic perturbations. Of particular concern is how these disruptions increase mobilization of Hg from sites and alter the formation of monomethylmercury (MeHg), a bioaccumulative form of Hg for humans and wildlife. The scientific community has made significant advances in recent years in understanding the processes contributing to the risk of MeHg in the environment. The objective of this paper is to synthesize the scientific understanding of how Hg cycling in the aquatic environment is influenced by landscape perturbations at the local scale, perturbations that include watershed loadings, deforestation, reservoir and wetland creation, rice production, urbanization, mining and industrial point source pollution, and remediation. We focus on the major challenges associated with each type of alteration, as well as management opportunities that could lessen both MeHg levels in biota and exposure to humans. For example, our understanding of approximate response times to changes in Hg inputs from various sources or landscape alterations could lead to policies that prioritize the avoidance of certain activities in the most vulnerable systems and sequestration of Hg in deep soil and sediment pools. The remediation of Hg pollution from historical mining and other industries is shifting towards in situ technologies that could be less disruptive and less costly than conventional approaches. Contemporary artisanal gold mining has well-documented impacts with respect to Hg; however, significant social and political challenges remain in implementing effective policies to minimize Hg use. Much remains to be learned as we strive towards the meaningful application of our understanding for stakeholders, including communities living near Hg-polluted sites, environmental policy makers, and scientists and engineers tasked with developing watershed management solutions. Site-specific assessments of MeHg exposure risk will require new methods to predict the impacts of anthropogenic perturbations and an understanding of the complexity of Hg cycling at the local scale.

Using sulfur stable isotopes to assess mercury bioaccumulation and biomagnification in temperate lake food webs

Nitrogen and carbon stable isotopes (δ¹⁵ N, δ¹³ C) are commonly used to understand mercury (Hg) bioaccumulation and biomagnification in freshwater food webs. Though sulfur isotopes (δ³⁴ S) can distinguish between energy sources from the water column (aqueous sulfate) and from sediments to freshwater organisms, little is known about whether δ³⁴ S can help interpret variable Hg concentrations in aquatic species or food webs. Seven acidic lakes in Kejimkujik National Park (Nova Scotia, Canada) were sampled for biota, water, and sediments in 2009 and 2010. Fishes, zooplankton, and macroinvertebrates were analyzed for δ³⁴ S, δ¹⁵ N, δ¹³ C, and Hg (methyl Hg in invertebrates, total Hg in fishes); aqueous sulfate and profundal sediments were analyzed for δ³⁴ S. Within lakes, mean δ³⁴ S values in sediments and sulfate differed between 0.53‰ and 1.98‰, limiting their use as tracers of energy sources to the food webs. However, log-Hg and δ³⁴ S values were negatively related (slopes -0.14 to -0.35, R² 0.20-0.39, p < 0.001-0.01) through each food web, and slopes were significantly different among lakes (analysis of covariance, lake × δ³⁴ S interaction term p = 0.04). Despite these relationships, multiple regression analyses within each taxon showed that biotic Hg concentrations were generally better predicted by δ¹⁵ N and/or δ¹³ C. The results indicate that δ³⁴ S values are predictive of Hg concentrations in these food webs, although the mechanisms underlying these relationships warrant further study. Environ Toxicol Chem 2017;36:661-670. © 2016 SETAC.

Mercury Bioaccumulation in Estuarine Fishes: Novel Insights from Sulfur Stable Isotopes

Estuaries are transitional habitats characterized by complex biogeochemical and ecological gradients that result in substantial variation in fish total mercury concentrations (THg). We leveraged these gradients and used carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S) stable isotopes to examine the ecological and biogeochemical processes underlying THg bioaccumulation in fishes from the San Francisco Bay Estuary. We employed a tiered approach that first examined processes influencing variation in fish THg among wetlands, and subsequently examined the roles of habitat and within-wetland processes in generating larger-scale patterns in fish THg. We found that δ³⁴S, an indicator of sulfate reduction and habitat specific-foraging, was correlated with fish THg at all three spatial scales. Over the observed ranges of δ³⁴S, THg concentrations in fish increased by up to 860% within wetlands, 560% among wetlands, and 291% within specific impounded wetland habitats. In contrast, δ¹³C and δ¹⁵N were not correlated with THg among wetlands and were only important in low salinity impounded wetlands, possibly reflecting more diverse food webs in this habitat. Together, our results highlight the key roles of sulfur biogeochemistry and ecology in influencing estuarine fish THg, as well as the importance of fish ecology and habitat in modulating the relationships between biogeochemical processes and Hg bioaccumulation.

The Contribution of Rice Agriculture to Methylmercury in Surface Waters: A Review of Data from the Sacramento Valley, California

Methylmercury (MeHg) is a bioaccumulative pollutant produced in and exported from flooded soils, including those used for rice ( L.) production. Using unfiltered aqueous MeHg data from MeHg monitoring programs in the Sacramento River watershed from 1996 to 2007, we assessed the MeHg contribution from rice systems to the Sacramento River. Using a mixed-effects regression analysis, we compared MeHg concentrations in agricultural drainage water from rice-dominated regions (AgDrain) to MeHg concentrations in the Sacramento and Feather Rivers, both upstream and downstream of AgDrain inputs. We also calculated MeHg loads from AgDrains and the Sacramento and Feather Rivers. Seasonally, MeHg concentrations were higher during November through May than during June through October, but the differences varied by location. Relative to upstream, November through May AgDrain least-squares mean MeHg concentration (0.18 ng L, range 0.15-0.23 ng L) was 2.3-fold higher, while June through October AgDrain mean concentration (0.097 ng L, range 0.6-1.6 ng L) was not significantly different from upstream. June through October AgDrain MeHg loads contributed 10.7 to 14.8% of the total Sacramento River MeHg load. Missing flow data prevented calculation of the percent contribution of AgDrains in November through May. At sites where calculation was possible, November through May loads made up 70 to 90% of the total annual load. Elevated flow and MeHg concentration in November through May both contribute to the majority of the AgDrain MeHg load occurring during this period. Methylmercury reduction efforts should target elevated November through May MeHg concentrations in AgDrains. However, our findings suggest that the contribution and environmental impact of rice is an order of magnitude lower than previous studies in the California Yolo Bypass.

Water management impacts rice methylmercury and the soil microbiome

Rice farmers are pressured to grow rice using less water. The impacts of water-saving rice cultivation methods on rice methylmercury concentrations are uncertain. Rice (Oryza sativa L. cv. Nipponbare) was cultivated in fields using four water management treatments, including flooded (no dry-downs), alternating wetting and drying (AWD) (with one or three dry-downs), and furrow-irrigated fields (nine dry-downs) (n=16 fields). Anoxic bulk soil was collected from rice roots during the rice maturation phase, and rice grain was harvested after fields were dried. Total mercury and methylmercury concentrations were determined in soil and polished rice samples, and the soil microbiome was analyzed using 16S (v4) rRNA gene profiling. Soil total mercury did not differ between fields. However, compared to continuously flooded fields, soil and rice methylmercury concentrations averaged 51% and 38% lower in the AWD fields, respectively, and 95% and 96% lower in the furrow-irrigated fields, respectively. Compared to flooded fields, grain yield was reduced on average by <1% in the AWD fields and 34% in the furrow-irrigated fields. Additionally, using 16S (v4) rRNA gene profiling, the relative abundance of genera (i.e., highest resolution via this method) known to contain mercury methylators averaged 2.9-fold higher in flooded and AWD fields compared to furrow-irrigated fields. These results reinforce the benefits of AWD in reducing rice methylmercury concentrations with minimal changes in rice production yields. In the furrow-irrigated fields, a lower relative abundance of genera known to contain mercury methylators suggests an association between lower concentrations of soil and rice methylmercury and specific soil microbiomes.

Eutrophication Increases Phytoplankton Methylmercury Concentrations in a Coastal Sea-A Baltic Sea Case Study

Eutrophication is expanding worldwide, but its implication for production and bioaccumulation of neurotoxic monomethylmercury (MeHg) is unknown. We developed a mercury (Hg) biogeochemical model for the Baltic Sea and used it to investigate the impact of eutrophication on phytoplankton MeHg concentrations. For model evaluation, we measured total methylated Hg (MeHg_T) in the Baltic Sea and found low concentrations (39 ± 16 fM) above the halocline and high concentrations in anoxic waters (1249 ± 369 fM). To close the Baltic Sea MeHg_T budget, we inferred an average normoxic water column Hg^II methylation rate constant of 2 × 10^-4 d^-1. We used the model to compare Baltic Sea's present-day (2005-2014) eutrophic state to an oligo/mesotrophic scenario. Eutrophication increases primary production and export of organic matter and associated Hg to the sediment effectively removing Hg from the active biogeochemical cycle; this results in a 27% lower present-day water column Hg reservoir. However, increase in organic matter production and remineralization stimulates microbial Hg methylation resulting in a seasonal increase in both water and phytoplankton MeHg reservoirs above the halocline. Previous studies of systems dominated by external MeHg sources or benthic production found eutrophication to decrease MeHg levels in plankton. This Baltic Sea study shows that in systems with MeHg production in the normoxic water column eutrophication can increase phytoplankton MeHg content.

High-Resolution Remote Sensing of Water Quality in the San Francisco Bay-Delta Estuary

The San Francisco Bay-Delta Estuary watershed is a major source of freshwater for California and a profoundly human-impacted environment. The water quality monitoring that is critical to the management of this important water resource and ecosystem relies primarily on a system of fixed water-quality monitoring stations, but the limited spatial coverage often hinders understanding. Here, we show how the latest technology in visible/near-infrared imaging spectroscopy can facilitate water quality monitoring in this highly dynamic and heterogeneous system by enabling simultaneous depictions of several water quality indicators at very high spatial resolution. The airborne portable remote imaging spectrometer (PRISM) was used to derive high-spatial-resolution (2.6 × 2.6 m) distributions of turbidity, and dissolved organic carbon (DOC) and chlorophyll-a concentrations in a wetland-influenced region of this estuary. A filter-passing methylmercury vs DOC relationship was also developed using in situ samples and enabled the high-spatial-resolution depiction of surface methylmercury concentrations in this area. The results illustrate how high-resolution imaging spectroscopy can inform management and policy development in important inland and estuarine water bodies by facilitating the detection of point- and nonpoint-source pollution, and by providing data to help assess the complex impacts of wetland restoration and climate change on water quality and ecosystem productivity.

Distribution and fractionation of mercury in the soils of a unique tropical agricultural wetland ecosystem, southwest coast of India

Mercury biogeochemistry is highly complex in the aquatic ecosystems and it is very difficult to predict. The speciation of mercury is the primary factor controlling its behavior, movement, and fate in these systems. The fluctuating water levels in wetlands could play a major role in the mercury transformations and transport. Hence, the agricultural wetlands may have a significant influence on the global mercury cycling. Kuttanad agricultural wetland ecosystem is a unique one as it is lying below the sea level and most of the time it is inundated with water. To understand the mobility and bioavailability of Hg in the soils of this agricultural wetland ecosystem, the present study analyzed the total mercury content as well as the different fractions of mercury. Mercury was detected using cold vapor atomic fluorescence spectrophotometer. The total mercury content varied from 0.002 to 0.683 mg/kg, and most of the samples are having concentrations below the background value. The percentage of mercury found in the initial three fractions F1, F2, and F3 are more available and it may enhance the methylation potential of the Kuttanad agroecosystem.

Mercury methylation in high and low-sulphate impacted wetland ponds within the prairie pothole region of North America

Using enriched stable (201)Hg injections into intact sediment cores, we provide the first reported Hg methylation potential rate constants (km) in prairie wetland ponds (0.016-0.17 d(-1)). Our km values were similar to other freshwater wetlands and did not differ in ponds categorized with high compared to low surface water concentrations of sulphate. Sites with high sulphate had higher proportions of methylmercury (MeHg) in sediment (2.9 ± 1.6% vs. 1.0 ± 0.3%) and higher surface water MeHg concentrations (1.96 ± 1.90 ng L(-1)vs. 0.56 ± 0.55 ng L(-1)). Sediment-porewater partitioning coefficients were small, and likely due to high ionic activity. Our work suggests while km measurements are useful for understanding mercury cycling processes, they are less important than surface water MeHg concentrations for assessing MeHg risks to biota. Significant differences in MeHg concentrations between sites with high and low sulphate concentrations may also inform management decisions concerning wetland remediation and creation.

Mercury in sediment, water, and fish in a managed tropical wetland-lake ecosystem

Mercury pollution has not been well documented in the inland lakes or fishes of Mexico, despite the importance of freshwater fish as a source of protein in local diets. Total mercury and methylmercury in waters, sediments, and the commercial fish catch were investigated in Lake Zapotlán, Mexico. Concentrations of total and methylmercury were very high in runoff and wastewater inputs, but very low in sediments and surface waters of the open water area of the lake. Concentrations of total mercury in tilapia and carp were very low, consistent with the low concentrations in lake water and sediments. Particle settling, sorption, the biogeochemical environment, and/or bloom dilution are all plausible explanations for the significant reductions in both total mercury and methylmercury. Despite very high loading of mercury, this shallow tropical lake was not a mercury-impaired ecosystem, and these findings may translate across other shallow, alkaline tropical lakes. Importantly, the ecosystem services that seemed to be provided by peripheral wetlands in reducing mercury inputs highlight the potential for wetland conservation or restoration in Mexico.

Mercury concentrations and distribution in soil, water, mine waste leachates, and air in and around mercury mines in the Big Bend region, Texas, USA

Samples of soil, water, mine waste leachates, soil gas, and air were collected from areas mined for mercury (Hg) and baseline sites in the Big Bend area, Texas, to evaluate potential Hg contamination in the region. Soil samples collected within 300 m of an inactive Hg mine contained elevated Hg concentrations (3.8-11 µg/g), which were considerably higher than Hg in soil collected from baseline sites (0.03-0.05 µg/g) distal (as much as 24 km) from mines. Only three soil samples collected within 300 m of the mine exceeded the probable effect concentration for Hg of 1.06 µg/g, above which harmful effects are likely to be observed in sediment-dwelling organisms. Concentrations of Hg in mine water runoff (7.9-14 ng/L) were generally higher than those found in springs and wells (0.05-3.1 ng/L), baseline streams (1.1-9.7 ng/L), and sources of drinking water (0.63-9.1 ng/L) collected in the Big Bend region. Concentrations of Hg in all water samples collected in this study were considerably below the 2,000 ng/L drinking water Hg guideline and the 770 ng/L guideline recommended by the U.S. Environmental Protection Agency (USEPA) to protect aquatic wildlife from chronic effects of Hg. Concentrations of Hg in water leachates obtained from leaching of mine wastes varied widely from <0.001 to 760 µg of Hg in leachate/g of sample leached, but only one leachate exceeded the USEPA Hg industrial soil screening level of 31 µg/g. Concentrations of Hg in soil gas collected at mined sites (690-82,000 ng/m(3)) were highly elevated compared to soil gas collected from baseline sites (1.2-77 ng/m(3)). However, air collected from mined areas at a height of 2 m above the ground surface contained concentrations of Hg (4.9-64 ng/m(3)) that were considerably lower than Hg in soil gas from the mined areas. Although concentrations of Hg emitted from mine-contaminated soils and mine wastes were elevated, persistent wind in southwest Texas disperses Hg in the air within a few meters of the ground surface.

Effects of early life exposure to methylmercury in Daphnia pulex on standard and reduced food ration

As a well-known eco-toxicological model organism, Daphnia pulex may also offer advantages in human health research for assessing long-term effects of early life exposures to coupled stressors. Here, we examine consequences of early life exposure to methylmercury (MeHg) under standard and reduced food ration. We exposed Daphnia for 24h in early life to varying concentrations of methylmercury(II) chloride (0, 200, 400, 800 and 1600ng/L) and thereafter kept Daphnia on either a standard or a reduced food ration. The data suggests an additive effect of MeHg concentration and food ration on decreasing lifespan, although MeHg concentration does not affect survival linearly. Food ration and MeHg concentration were predictive of reduced reproduction, and there is some evidence of an interaction (p=0.048). Multi-stressor work in alternative model systems may be useful for prioritizing research, taking into account potential antagonistic, additive or synergistic effects that nutritional status may have on chemical toxicity.

Rice methylmercury exposure and mitigation: a comprehensive review

Rice cultivation practices from field preparation to post-harvest transform rice paddies into hot spots for microbial mercury methylation, converting less-toxic inorganic mercury to more-toxic methylmercury, which is likely translocated to rice grain. This review includes 51 studies reporting rice total mercury and/or methylmercury concentrations, based on rice (Orzya sativa) cultivated or purchased in 15 countries. Not surprisingly, both rice total mercury and methylmercury levels were significantly higher in polluted sites compared to non-polluted sites (Wilcoxon rank sum, p<0.001). However, rice percent methylmercury (of total mercury) did not differ statistically between polluted and non-polluted sites (Wilcoxon rank sum, p=0.35), suggesting comparable mercury methylation rates in paddy soil across these sites and/or similar accumulation of mercury species for these rice cultivars. Studies characterizing the effects of rice cultivation under more aerobic conditions were reviewed to determine the mitigation potential of this practice. Rice management practices utilizing alternating wetting and drying (instead of continuous flooding) caused soil methylmercury levels to spike, resulting in a strong methylmercury pulse after fields were dried and reflooded; however, it is uncertain whether this led to increased translocation of methylmercury from paddy soil to rice grain. Due to the potential health risks, it is advisable to investigate this issue further, and to develop separate water management strategies for mercury polluted and non-polluted sites, in order to minimize methylmercury exposure through rice ingestion.

Mercury cycling in agricultural and managed wetlands: a synthesis of methylmercury production, hydrologic export, and bioaccumulation from an integrated field study

With seasonal wetting and drying, and high biological productivity, agricultural wetlands (rice paddies) may enhance the conversion of inorganic mercury (Hg(II)) to methylmercury (MeHg), the more toxic, organic form that biomagnifies through food webs. Yet, the net balance of MeHg sources and sinks in seasonal wetland environments is poorly understood because it requires an annual, integrated assessment across biota, sediment, and water components. We examined a suite of wetlands managed for rice crops or wildlife during 2007-2008 in California's Central Valley, in an area affected by Hg contamination from historic mining practices. Hydrologic management of agricultural wetlands for rice, wild rice, or fallowed - drying for field preparation and harvest, and flooding for crop growth and post-harvest rice straw decay - led to pronounced seasonality in sediment and aqueous MeHg concentrations that were up to 95-fold higher than those measured concurrently in adjacent, non-agricultural permanently-flooded and seasonally-flooded wetlands. Flooding promoted microbial MeHg production in surface sediment of all wetlands, but extended water residence time appeared to preferentially enhance MeHg degradation and storage. When incoming MeHg loads were elevated, individual fields often served as a MeHg sink, rather than a source. Slow, horizontal flow of shallow water in the agricultural wetlands led to increased importance of vertical hydrologic fluxes, including evapoconcentration of surface water MeHg and transpiration-driven advection into the root zone, promoting temporary soil storage of MeHg. Although this hydrology limited MeHg export from wetlands, it also increased MeHg exposure to resident fish via greater in situ aqueous MeHg concentrations. Our results suggest that the combined traits of agricultural wetlands - slow-moving shallow water, manipulated flooding and drying, abundant labile plant matter, and management for wildlife - may enhance microbial methylation of Hg(II) and MeHg exposure to local biota, as well as export to downstream habitats during uncontrolled winter-flow events.

Differentiating transpiration from evaporation in seasonal agricultural wetlands and the link to advective fluxes in the root zone

The current state of science and engineering related to analyzing wetlands overlooks the importance of transpiration and risks data misinterpretation. In response, we developed hydrologic and mass budgets for agricultural wetlands using electrical conductivity (EC) as a natural conservative tracer. We developed simple differential equations that quantify evaporation and transpiration rates using flow rates and tracer concentrations at wetland inflows and outflows. We used two ideal reactor model solutions, a continuous flow stirred tank reactor (CFSTR) and a plug flow reactor (PFR), to bracket real non-ideal systems. From those models, estimated transpiration ranged from 55% (CFSTR) to 74% (PFR) of total evapotranspiration (ET) rates, consistent with published values using standard methods and direct measurements. The PFR model more appropriately represents these non-ideal agricultural wetlands in which check ponds are in series. Using a flux model, we also developed an equation delineating the root zone depth at which diffusive dominated fluxes transition to advective dominated fluxes. This relationship is similar to the Peclet number that identifies the dominance of advective or diffusive fluxes in surface and groundwater transport. Using diffusion coefficients for inorganic mercury (Hg) and methylmercury (MeHg) we calculated that during high ET periods typical of summer, advective fluxes dominate root zone transport except in the top millimeters below the sediment-water interface. The transition depth has diel and seasonal trends, tracking those of ET. Neglecting this pathway has profound implications: misallocating loads along different hydrologic pathways; misinterpreting seasonal and diel water quality trends; confounding Fick's First Law calculations when determining diffusion fluxes using pore water concentration data; and misinterpreting biogeochemical mechanisms affecting dissolved constituent cycling in the root zone. In addition, our understanding of internal root zone cycling of Hg and other dissolved constituents, benthic fluxes, and biological irrigation may be greatly affected.

Mercury cycling in agricultural and managed wetlands of California, USA: seasonal influences of vegetation on mercury methylation, storage, and transport

Plants are a dominant biologic and physical component of many wetland capable of influencing the internal pools and fluxes of methylmercury (MeHg). To investigate their role with respect to the latter, we examined the changing seasonal roles of vegetation biomass and Hg, C and N composition from May 2007-February 2008 in 3 types of agricultural wetlands (domesticated or white rice, wild rice, and fallow fields), and in adjacent managed natural wetlands dominated by cattail and bulrush (tule). We also determined the impact of vegetation on seasonal microbial Hg methylation rates, and Hg and MeHg export via seasonal storage in vegetation, and biotic consumption of rice seed. Despite a compressed growing season of ~3months, annual net primary productivity (NPP) was greatest in white rice fields and carbon more labile (leaf median C:N ratio=27). Decay of senescent litter (residue) was correlated with microbial MeHg production in winter among all wetlands. As agricultural biomass accumulated from July to August, THg concentrations declined in leaves but MeHg concentrations remained consistent, such that MeHg pools generally increased with growth. Vegetation provided a small, temporary, but significant storage term for MeHg in agricultural fields when compared with hydrologic export. White rice and wild rice seeds reached mean MeHg concentrations of 4.1 and 6.2ng gdw(-1), respectively. In white rice and wild rice fields, seed MeHg concentrations were correlated with root MeHg concentrations (r=0.90, p<0.001), suggesting transport of MeHg to seeds from belowground tissues. Given the proportionally elevated concentrations of MeHg in rice seeds, white and wild rice crops may act as a conduit of MeHg into biota, especially waterfowl which forage heavily on rice seeds within the Central Valley of California, USA. Thus, while plant tissues and rhizosphere soils provide temporary storage for MeHg during the growing season, export of MeHg is enhanced post-harvest through increased hydrologic and biotic export.

Mercury cycling in agricultural and managed wetlands of California, USA: experimental evidence of vegetation-driven changes in sediment biogeochemistry and methylmercury production

The role of live vegetation in sediment methylmercury (MeHg) production and associated biogeochemistry was examined in three types of agricultural wetlands (domesticated or white rice, wild rice, and fallow fields) and adjacent managed natural wetlands (cattail- and bulrush or tule-dominated) in the Yolo Bypass region of California's Central Valley, USA. During the active growing season for each wetland, a vegetated:de-vegetated paired plot experiment demonstrated that the presence of live plants enhanced microbial rates of mercury methylation by 20 to 669% (median=280%) compared to de-vegetated plots. Labile carbon exudation by roots appeared to be the primary mechanism by which microbial methylation was enhanced in the presence of vegetation. Pore-water acetate (pw[Ac]) decreased significantly with de-vegetation (63 to 99%) among all wetland types, and within cropped fields, pw[Ac] was correlated with both root density (r=0.92) and microbial Hg(II) methylation (kmeth. r=0.65). Sediment biogeochemical responses to de-vegetation were inconsistent between treatments for "reactive Hg" (Hg(II)R), as were reduced sulfur and sulfate reduction rates. Sediment MeHg concentrations in vegetated plots were double those of de-vegetated plots (median=205%), due in part to enhanced microbial MeHg production in the rhizosphere, and in part to rhizoconcentration via transpiration-driven pore-water transport. Pore-water concentrations of chloride, a conservative tracer, were elevated (median=22%) in vegetated plots, suggesting that the higher concentrations of other constituents around roots may also be a function of rhizoconcentration rather than microbial activity alone. Elevated pools of amorphous iron (Fe) in vegetated plots indicate that downward redistribution of oxic surface waters through transpiration acts as a stimulant to Fe(III)-reduction through oxidation of Fe(II)pools. These data suggest that vegetation significantly affected rhizosphere biogeochemistry through organic exudation and transpiration-driven concentration of pore-water constituents and oxidation of reduced compounds. While the relative role of vegetation varied among wetland types, macrophyte activity enhanced MeHg production.

Reprint of "Methylmercury production in and export from agricultural wetlands in California, USA: the need to account for physical transport processes into and out of the root zone"

Concentration and mass balance analyses were used to quantify methylmercury (MeHg) loads from conventional (white) rice, wild rice, and fallowed fields in northern California's Yolo Bypass. These analyses were standardized against chloride to distinguish transport pathways and net ecosystem production (NEP). During summer, chloride loads were both exported with surface water and moved into the root zone at a 2:1 ratio. MeHg and dissolved organic carbon (DOC) behaved similarly with surface water and root zone exports at ~3:1 ratio. These trends reversed in winter with DOC, MeHg, and chloride moving from the root zone to surface waters at rates opposite and exceeding summertime root zone fluxes. These trends suggest that summer transpiration advectively moves constituents from surface water into the root zone, and winter diffusion, driven by concentration gradients, subsequently releases those constituents into surface waters. The results challenge a number of paradigms regarding MeHg. Specifically, biogeochemical conditions favoring microbial MeHg production do not necessarily translate to synchronous surface water exports; MeHg may be preserved in the soils allowing for release at a later time; and plants play a role in both biogeochemistry and transport. Our calculations show that NEP of MeHg occurred during both summer irrigation and winter flooding. Wild rice wet harvesting and winter flooding of white rice fields were specific practices that increased MeHg export, both presumably related to increased labile organic carbon and disturbance. Outflow management during these times could reduce MeHg exports. Standardizing MeHg outflow:inflow concentration ratios against natural tracers (e.g. chloride, EC) provides a simple tool to identify NEP periods. Summer MeHg exports averaged 0.2 to 1μgm(-2) for the different agricultural wetland fields, depending upon flood duration. Average winter MeHg exports were estimated at 0.3μgm(-2). These exports are within the range reported for other shallow aquatic systems.

Concurrent photolytic degradation of aqueous methylmercury and dissolved organic matter

Monomethyl mercury (MeHg) is a potent neurotoxin that threatens ecosystem viability and human health. In aquatic systems, the photolytic degradation of MeHg (photodemethylation) is an important component of the MeHg cycle. Dissolved organic matter (DOM) is also affected by exposure to solar radiation (light exposure) leading to changes in DOM composition that can affect its role in overall mercury (Hg) cycling. This study investigated changes in MeHg concentration, DOM concentration, and the optical signature of DOM caused by light exposure in a controlled field-based experiment using water samples collected from wetlands and rice fields. Filtered water from all sites showed a marked loss in MeHg concentration after light exposure. The rate of photodemethylation was 7.5×10(-3)m(2)mol(-1) (s.d. 3.5×10(-3)) across all sites despite marked differences in DOM concentration and composition. Light exposure also caused changes in the optical signature of the DOM despite there being no change in DOM concentration, indicating specific structures within the DOM were affected by light exposure at different rates. MeHg concentrations were related to optical signatures of labile DOM whereas the percent loss of MeHg was related to optical signatures of less labile, humic DOM. Relationships between the loss of MeHg and specific areas of the DOM optical signature indicated that aromatic and quinoid structures within the DOM were the likely contributors to MeHg degradation, perhaps within the sphere of the Hg-DOM bond. Because MeHg photodegradation rates are relatively constant across freshwater habitats with natural Hg-DOM ratios, physical characteristics such as shading and hydrologic residence time largely determine the relative importance of photolytic processes on the MeHg budget in these mixed vegetated and open-water systems.