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

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.

Total mercury, methylmercury, and their possible controlling factors in soils of typical coastal wetlands in China

Coastal wetland soils play a critical role in the global mercury (Hg) cycle, serving as both an important repository for total mercury (THg) and a hotspot for methylmercury (MeHg) production. This study investigated Hg pollution in soils dominated by Phragmites australis (PA) and Spartina alterniflora (SA) across five representative China's coastal wetlands (Yellow River (YR), Linhong River (LHR), Yangtze River (CJR), Min River (MR), and Nanliu River (NLR)). The THg concentrations ranged from 16.7 to 446.0 (96.3 ± 59.3 ng g-1, dw), while MeHg concentrations varied from 0.01 to 0.81 (0.12 ± 0.12 ng g-1, dw). We further evaluated Hg risk in these wetlands using potential ecological risk index (Er) and geographical enrichment factor (Igeo). Most wetlands exhibited low to moderate ecological risk, except the PA habitat in the YR wetland, showing moderate to high risk. Soil organic matter significantly influenced THg and MeHg distribution, while MeHg% correlated well with soil salinity and pH. These findings highlight the importance of organic-rich coastal wetland soils in THg and MeHg accumulation, with the soil properties influencing net MeHg production. Furthermore, SA habitat generally exhibited higher MeHg%, suggesting its invasion elevates the ecological risk of MeHg in coastal wetlands. ENVIRONMENTAL IMPLICATION: Mercury (Hg), a global pollutant, poses great risks to wildlife and humans. Since industrialization, anthropogenic Hg release surpassed natural sources. Long-term exposure leads to biomagnification of Hg. This study assessed Hg and methylmercury pollution and risks in soils of five China's coastal wetlands dominated by Phragmites australis and Spartina alterniflora. Environmental factors (total carbon, total organic carbon, total nitrogen, salinity, pH) were analyzed to reveal key variables influencing Hg pollution and methylation. Essential for quantifying Hg pollution in coastal wetlands, the findings provide a scientific basis for effective wetland conservation policies and addressing environmental health in these regions.

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

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

The relative importance of mercury methylation and demethylation in rice paddy soil varies depending on the presence of rice plants

Neurotoxic methylmercury (MeHg) accumulates in rice grain from paddy soil, where its concentration is controlled by microbial mercury methylation and demethylation. Both up- and down-regulation of methylation is known to occur in the presence of rice plants in comparison to non-vegetated paddy soils; the influence of rice plant presence/absence on demethylation is unknown. To assess the concurrent influence of rice plant presence/absence on methylation and demethylation, and to determine which process was more dominant in controlling soil MeHg concentrations, we maintained six rhizoboxes of paddy soil with and without rice plants. At the peak of plant growth, we simultaneously measured ambient MeHg, ambient inorganic mercury (IHg), and potential rate constants of methylation and demethylation (Kmeth and Kdemeth) in soil using stable isotope tracers and ID-GC-ICPMS. We also measured organic matter content, elemental S, and water-extractable sulfate. We found MeHg concentrations were differentially controlled by MeHg production and degradation processes, depending on whether plants were present. In non-vegetated boxes, MeHg concentration was controlled by Kmeth, as evidenced by a strong and positive correlation, while Kdemeth had no relation to MeHg concentration. These results indicate methylation was the dominant driver of MeHg concentration in non-vegetated soil. In vegetated boxes, Kdemeth strongly and negatively predicted MeHg concentration, indicating that demethylation was the dominant control in soil with plants. MeHg concentration, Kmeth, and % MeHg all had significantly less variance in vegetated than in non-vegetated soils due to a consistent elimination of greater values. This pattern suggests that reduced MeHg production capacity was a secondary control on MeHg concentrations in vegetated soils. We observed no difference in the magnitude or variance of Kdemeth between treatments, suggesting that demethylation was robust to soil chemical conditions influenced by the plant, perhaps because of a wider taxonomic diversity of demethylators. Our results suggest that methylation and demethylation processes could both be leveraged to alter MeHg concentrations in rice paddy soil.

Concentration and speciation of mercury in atmospheric particulates in the Wuda coal fire area, Inner Mongolia, China

Coal-seam fire is a source of atmospheric mercury that is difficult to control. The Wuda Coalfield in Inner Mongolia, China, is one of the most severe coal fire disaster areas worldwide and has been burning for more than 50 years. To investigate atmospheric mercury pollution from the Wuda coal fire, gaseous elemental mercury (GEM) concentrations and atmospheric particulate mercury (PHg) speciation were measured using a RA-915+ mercury analyzer and the temperature-programmed desorption method. Near-surface GEM concentrations in the Wuda Coalfield and adjacent urban area were 80 ng m^-3 (65-90 ng m^-3) and 52 ng m^-3 (25-95 ng m^-3), respectively, which are far higher than the local background value (22 ng m^-3). PHg concentrations in the coalfield and urban area also reached significantly high levels, 33 ng m^-3 (25-45 ng m^-3) and 22 ng m^-3 (14-29 ng m^-3), respectively (p < 0.05). There is no clear evidence that PHg combines with organic carbon or elemental carbon, but PHg concentration appears to be controlled by air acidity. PHg mainly exists in inorganic forms, such as HgCl₂, HgS, HgO, and Hg(NO₃)₂·H₂O. This work can provide references for the speciation analysis of atmospheric PHg and the safety assessment of environmental mercury.

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..

Impacts of experimental alteration of water table regime and vascular plant community composition on peat mercury profiles and methylmercury production

Climate change is expected to alter the hydrology and vascular plant communities in peatland ecosystems. These changes may have as yet unexplored impacts on peat mercury (Hg) concentrations and net methylmercury (MeHg) production. In this study, peat was collected from PEATcosm, an outdoor, controlled mesocosm experiment where peatland water table regimes and vascular plant functional groups were manipulated over several years to simulate potential climate change effects. Potential Hg(II) methylation and MeHg demethylation rate constants were assessed using enriched stable isotope incubations at the end of the study in 2015, and ambient peat total Hg (THg) and MeHg concentration depth profiles were tracked annually from 2011 to 2014. Peat THg and MeHg concentrations and the proportion of THg methylated (%MeHg) increased significantly within the zone of water table fluctuation when water tables were lowered, but potential Hg(II) methylation rate constants were similar regardless of water table treatment. When sedges dominate over ericaceous shrubs, MeHg concentrations and %MeHg became significantly elevated within the sedge rooting zone. Increased desorption of Hg(II) and MeHg from the solid phase peat into pore water occurred with a lowered water table and predominant sedge cover, likely due to greater aerobic peat decomposition. Deeper, more variable water tables and a transition to sedge-dominated communities coincided with increased MeHg accumulation within the zone of water table fluctuation. Sustained high water tables promoted the net downward migration of Hg(II) and MeHg. The simultaneous decrease in Hg(II) and MeHg concentrations in the near-surface peat and accumulation deeper in the peat profile, combined with the trends in Hg(II) and MeHg partitioning to mobile pore waters, suggest that changes to peatland hydrology and vascular plant functional groups redistribute peat Hg(II) and MeHg via vertical hydrochemical transport mechanisms.

Mercury-Organic Matter Interactions in Soils and Sediments: Angel or Devil?

Many studies have suggested that organic matter (OM) substantially reduces the bioavailability and risks of mercury (Hg) in soils and sediments; however, recent reports have supported that OM greatly accelerates Hg methylation and increases the risks of Hg exposure. This study aims to summarize the interactions between Hg and OM in soils and sediments and improve our understanding of the effects of OM on Hg methylation. The results show that OM characteristics, promotion of the activity of Hg-methylating microbial communities, and the microbial availability of Hg accounted for the acceleration of Hg methylation which increases the risk of Hg exposure. These three key aspects were driven by multiple factors, including the types and content of OM, Hg speciation, desorption and dissolution kinetics and environmental conditions.

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.

Oligotrophic wetland sediments susceptible to shifts in microbiomes and mercury cycling with dissolved organic matter addition

Recent advances have allowed for greater investigation into microbial regulation of mercury toxicity in the environment. In wetlands in particular, dissolved organic matter (DOM) may influence methylmercury (MeHg) production both through chemical interactions and through substrate effects on microbiomes. We conducted microcosm experiments in two disparate wetland environments (oligotrophic unvegetated and high-C vegetated sediments) to examine the impacts of plant leachate and inorganic mercury loadings (20 mg/L HgCl₂) on microbiomes and MeHg production in the St. Louis River Estuary. Our research reveals the greater relative capacity for mercury methylation in vegetated over unvegetated sediments. Further, our work shows how mercury cycling in oligotrophic unvegetated sediments may be susceptible to DOM inputs in the St. Louis River Estuary: unvegetated microcosms receiving leachate produced substantially more MeHg than unamended microcosms. We also demonstrate (1) changes in microbiome structure towards Clostridia, (2) metagenomic shifts toward fermentation, and (3) degradation of complex DOM; all of which coincide with elevated net MeHg production in unvegetated microcosms receiving leachate. Together, our work shows the influence of wetland vegetation in controlling MeHg production in the Great Lakes region and provides evidence that this may be due to both enhanced microbial activity as well as differences in microbiome composition.

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.

Recent advances in the study of mercury methylation in aquatic systems

With increasing input of neurotoxic mercury to environments as a result of anthropogenic activity, it has become imperative to examine how mercury may enter biotic systems through its methylation to bioavailable forms in aquatic environments. Recent development of stable isotope-based methods in methylation studies has enabled a better understanding of the factors controlling methylation in aquatic systems. In addition, the identification and tracking of the hgcAB gene cluster, which is necessary for methylation, has broadened the range of known methylators and methylation-conducive environments. Study of abiotic factors in methylation with new molecular methods (the use of stable isotopes and genomic methods) has helped elucidate the confounding influences of many environmental factors, as these methods enable the examination of their direct effects instead of merely correlative observations. Such developments will be helpful in the finer characterization of mercury biogeochemical cycles, which will enable better predictions of the potential effects of climate change on mercury methylation in aquatic systems and, by extension, the threat this may pose to biota.

Accumulation and translocation of methylmercury and inorganic mercury in Oryza sativa: An enriched isotope tracer study

Methylmercury (MeHg) accumulation in rice is an emerging human health issue, but uptake pathways and translocation into the grain remain poorly understood. We grew Oryza sativa plants in pots of wetland soil amended with an enriched mercury isotope (94.3% ²⁰⁰Hg) tracer, alongside unvegetated control pots, and assessed both ambient and tracer MeHg and inorganic Hg (IHg) concentrations in soil and plant tissues at three growth stages. Based on similar ratios of ambient:tracer MeHg concentrations in soil and plant tissues, we provide the first direct evidence that MeHg is first synthesized in saturated soil and subsequently translocated to rice grains. There is no evidence of in planta methylation of IHg, but significant losses of MeHg from plant tissues between flowering and maturity indicates likely in planta demethylation. In this greenhouse experiment, lower percent of tracer MeHg in vegetated soils at late growth stages suggests that rice plants reduce the net MeHg accumulation capacity of soils, although the mechanism remains unclear. For IHg, roots accumulated Hg from the soil, straw from the soil and the atmosphere, and grain almost entirely from the atmosphere. Management strategies that aim to reduce MeHg accumulation in rice should focus on mercury methylation in paddy soils, but IHg reductions will depend on regional controls of atmospheric Hg.

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.

Investigation of biogeochemical controls on the formation, uptake and accumulation of methylmercury in rice paddies in the vicinity of a coal-fired power plant and a municipal solid waste incinerator in Taiwan

Recent studies have shown that rice consumption is another critical route of human exposure to methylmercury (MeHg), the most toxic and accumulative form of mercury (Hg) in the food web. Yet, the mechanisms that underlie the production and accumulation of MeHg in the paddy ecosystem are still poorly understood. In 2013 and 2014, we conducted field campaigns and laboratory experiments over a rice growing season to examine Hg and MeHg cycling, as well as associated biogeochemistry in a suite of paddies close to a municipal solid waste incinerator and a coal-fired power plant station in Taiwan. Concentrations of total Hg and MeHg in paddy soil and rice grain at both sites were low and found not to exceed the control standards for farmland soil and edible rice in Taiwan. However, seasonal variations of MeHg concentrations observed in pore water samples indicate that the in situ bioavailability of inorganic Hg and activity of Hg-methylating microbes in the rhizosphere increased from the early-season and peaked at the mid-season, presumably due to the anoxia created under flooded conditions and root exudation of organic compounds. The presence of Hg-methylators was also confirmed by the hgcA gene detected in all root soil samples. Subsequent methylation tests performed by incubating the root soil with inorganic Hg and an inhibitor or stimulant specific for certain microbes further revealed that sulfate-reducers might have been the principal Hg-methylting guild at the study sites. Interestingly, results of hydroponic experiments conducted by cultivating rice in a defined nutrient solution amended with fixed MeHg and varying levels of MeHg-binding ligands suggested that chemical speciation in soil pore water may play a key role in controlling MeHg accumulation in rice, and both passive and active transport pathways seem to take place in the uptake of MeHg in rice roots.

Barn owl feathers as biomonitors of mercury: sources of variation in sampling procedures

Given their central role in mercury (Hg) excretion and suitability as reservoirs, bird feathers are useful Hg biomonitors. Nevertheless, the interpretation of Hg concentrations is still questioned as a result of a poor knowledge of feather physiology and mechanisms affecting Hg deposition. Given the constraints of feather availability to ecotoxicological studies, we tested the effect of intra-individual differences in Hg concentrations according to feather type (body vs. flight feathers), position in the wing and size (mass and length) in order to understand how these factors could affect Hg estimates. We measured Hg concentration of 154 feathers from 28 un-moulted barn owls (Tyto alba), collected dead on roadsides. Median Hg concentration was 0.45 (0.076-4.5) mg kg(-1) in body feathers, 0.44 (0.040-4.9) mg kg(-1) in primary and 0.60 (0.042-4.7) mg kg(-1) in secondary feathers, and we found a poor effect of feather type on intra-individual Hg levels. We also found a negative effect of wing feather mass on Hg concentration but not of feather length and of its position in the wing. We hypothesize that differences in feather growth rate may be the main driver of between-feather differences in Hg concentrations, which can have implications in the interpretation of Hg concentrations in feathers. Finally, we recommend that, whenever possible, several feathers from the same individual should be analysed. The five innermost primaries have lowest mean deviations to both between-feather and intra-individual mean Hg concentration and thus should be selected under restrictive sampling scenarios.

Identification and prioritization of management practices to reduce methylmercury exports from wetlands and irrigated agricultural lands

The Sacramento-San Joaquin Delta's (Delta) beneficial uses for humans and wildlife are impaired by elevated methylmercury (MeHg) concentrations in fish. MeHg is a neurotoxin that bioaccumulates in aquatic food webs. The total maximum daily load (TMDL) implementation plan aimed at reducing MeHg in Delta fish obligates dischargers to conduct MeHg control studies. Over 150 stakeholders collaborated to identify 24 management practices (MPs) addressing MeHg nonpoint sources (NPS) in three categories: biogeochemistry (6), hydrology (14), and soil/vegetation (4). Land uses were divided into six categories: permanently and seasonally flooded wetlands, flooded and irrigated agricultural lands, floodplains, and brackish-fresh tidal marshes. Stakeholders scored MPs based on seven criteria: scientific certainty, costs, MeHg reduction potential, spatial applicability, technical capacity to implement, negative impacts to beneficial uses, and conflicting requirements. Semi-quantitative scoring for MPs applicable to each land use (totaling >400 individual scores) led to consensus-based prioritization. This process relied on practical experience from diverse and accomplished NPS stakeholders and synthesis of 17 previous studies. Results provide a comprehensive, stakeholder-driven prioritization of MPs for wetland and irrigated agricultural land managers. Final prioritization highlights the most promising MPs for practical application and control study, and a secondary set of MPs warranting further evaluation. MPs that address hydrology and soil/vegetation were prioritized because experiences were positive and implementation appeared more feasible. MeHg control studies will need to address the TMDL conundrum that MPs effective at reducing MeHg exports could both exacerbate MeHg exposure and contend with other management objectives on site.

Retrospective study of methylmercury and other metal(loid)s in Madagascar unpolished rice (Oryza sativa L.)

The rice ingestion rate in Madagascar is among the highest globally; however studies concerning metal(loid) concentrations in Madagascar rice are lacking. For Madagascar unpolished rice (n = 51 landraces), levels of toxic elements (e.g., total mercury, methylmercury, arsenic and cadmium) as well as essential micronutrients (e.g., zinc and selenium) were uniformly low, indicating potentially both positive and negative health effects. Aside from manganese (Wilcoxon rank sum, p < 0.01), no significant differences in concentrations for all trace elements were observed between rice with red bran (n = 20)and brown bran (n = 31) (Wilcoxon rank sum, p = 0.06-0.91). Compared to all elements in rice,rubidium (i.e., tracer for phloem transport) was most positively correlated with methylmercury (Pearson'sr = 0.33, p < 0.05) and total mercury (r = 0.44, p < 0.05), while strontium (i.e., tracer for xylemtransport) was least correlated with total mercury and methylmercury (r < 0.01 for both), suggesting inorganic mercury and methylmercury were possibly more mobile in phloem compared to xylem.

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.

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.