SMaReF - Sino-Swedish Mercury Management Research Framework

Last changed: 14 November 2022
The SMaREF logotype. Illustration.

Sino-Swedish Mercury Management Research Framework (SMaReF) was a research cooperation going on between 2014-2019. SMaReF aimed at creating collaboration between Sweden and China research about processes about mercury in the environment.


Transformation processes in soil and water converts emissions of Hg to the extremely toxic form known as methylmercury (MeHg), which accumulate in organisms and damaging the mental development of children. Globally, the most important source of exposure for humans is fish, but recent studies show that rice is the main source for uptake among large sections of the population in China.

The purpose of SMaReF was to improve the fundamental understanding of the processes underlying the formation of MeHg in soil and water, as well as introduction of MeHg in food webs. 

More information about the background of the project can be found further down on this page.

Work packages

The project was divided in four work packages (WP).

  • WP A: Land-Air Exchange – In and out of the landscape
  • WP B: Methylation: Synergistic effects
  • WP C. Entry into the food chain – When to intervene
  • WP-D Remediation -When it is too late?

More information about the work packages can be found further down on this page.


Workshop 17-27 May 2014

On the 17-27 May 2014 forty researchers from China, Sweden, USA, Finland, Switzerland and Norway met to discuss the issue on mercury as an environmental pollutant. The workshop was held in Uppsala, Vindeln and ended in Abisko.

More information about the workshop with link to presentations can be found further down on this page.

PhD-course autumn 2015

A PhD-course organized by SMaReF called Hg biogeochemical cycling was held Tuesday 25 August to Tuesday 1 September 2015. The course was a collaboration between SLU and Uppsala University. 


Lists of the project participants and their affiliation during the project.

  • Swedish participants
    • Kevin Bishop, Swedish University of Agricultural Sciences
    • Staffan Åkerblom, Swedish University of Agricultural Sciences
    • Pianpian Wu, Swedish University of Agricultural Sciences
    • Ulf Skyllberg, Swedish University of Agricultural Sciences
    • Mats Nilsson, Swedish University of Agricultural Sciences
    • Markus Meili, Stockholm University
    • Stefan Bertilsson, Uppsala University
    • Jingying Xu, Uppsala University
    • Andrea Garcia Bravo, Uppsala University
    • Erik Björn,  Umeå University
  • Chinese participants
    • Xinbin Feng, State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences
    • Jiubin Chen, State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences
    • Guangle Qiu, State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences
    • Haiyu Yan, State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences
    • Xuewu Fu
    • Jonas Sommar, State Key Laboratory of Environmental Geochemistry, Chinese Academy of Sciences
    • Zhonggen Li
    • Jiaxu Wang
    • Lihai Shang
    • Wei Zhu
    • Adlane Bayou
    • Ze Wu
    • Chao Zhang
    • Hongming Cai
    • Tao Jiang, Southwest University (SWU), Chongqing, China
  • Other countries
    • Jeffra Schaefer, Rutgers University, USA
    • Stefan Osterwalder, University of Basel, Switzerland
    • Martin Jiskra, CNRS, Observatoire Midi Pyrénées, France


Transformation processes in soil and water converts emissions of Hg to the extremely toxic form known as methylmercury (MeHg), which accumulate in organisms and damaging the mental development of children. Globally, the most important source of exposure for humans is fish, but recent studies show that rice is the main source for uptake among large sections of the population in China.

The purpose of this project is to improve the fundamental understanding of the processes underlying the formation of MeHg in soil and water, as well as introduction of MeHg in food webs. The project adopts a land use perspective, where the effect of the main industries, forestry and rice in focus. Both of these operations handle environments where formation of MeHg, and the subsequent introduction of the food web (fish and rice), reach maximum levels. This is troublesome, because none of these activities is the ultimate cause of the mercury problem, but the land use aspect also provides opportunities for modification of the methods that can counteract the net formation of MeHg and its introduction into the food web.

The project focuses on four steps in mercury biogeochemical cycles that we consider particularly important: 1) gas exchange of Hg0 between soil and atmosphere, 2) the factors controlling MeHg formation and degradation, 3) introduction of MeHg in rice and aquatic food webs, as well as 4 ) risk assessment and remediation of heavily mercury contaminated environments. Through an increased fundamental understanding of the processes and factors that control these, the project provides a scientific basis for a strategy, concerning both the identification of particularly critical environments such as the development of methods to prevent human exposure to MeHg in China and Sweden.

The project combines strong competencies from State Key Laboratory of Environmental Geochemistry at the Institute of Geochemistry, the Chinese Science Academy (SKLEG), where over 30 researchers active in the high silver-related projects, with Swedish research groups from the Swedish University of Agricultural Sciences, Uppsala, Stockholm and Umeå University (SLU + ), who leads the research field in Sweden and Europe.

Capabilities of the two countries complement each other, where SKLEG represents a unique expertise in the development of Relaxed Eddy Accumulation and analysis of natural isotopic fractionation of Hg. SLU + is leading regarding experimental studies of Hg and MeHg transformation processes using stable isotopes, chemical speciation of Hg using synchrotron X-ray spectroscopy and molecular microbial characterization. The project combines the experience of different types of pollution picture, climate, habitats, and land use. Sweden's forest and wetland habitats and associated land use (forestry) in a northern climate, combined with China's nature-given gradients: inland-coastal and north-south, with other types of environments and land use (rice) provides a wide range of properties that we will able to use to test the generalizability of our hypotheses and conclusions.

The Chinese Ministry of Environment (Ministry of Environmental Protection, MEP) is the single most important stakeholder of the project.

Det kinesiska miljöministeriet (Ministry of Environmental Protection, MEP) är den enskilt viktigaste sakägaren för projektet.

Results from the project will be reported annually. A final report to MEP will include recommendations regarding reduction of MeHg in rice, as well as suggestions on how risk assessment and remediation of highly contaminated environments is to be achieved.

Also Swedish authorities will be able to benefit greatly from the project. In the first instance, by the generalization of the SLU + newly presented concepts for the identification of wetland habitats (including the effects of wetland restoration) and heavily contaminated land that poses a significant risk for MeHg formation, will be tested for a larger number of different types of environments, both within and outside of Sweden. Likewise, the ongoing studies at SLU + to similarly identify forest land and agricultural practices that lead to MeHg formation, to be given a much greater safety.

WP A: Land-Air Exchange – In and out of the landscape

The possibility that mercury evades from wetlands back to the atmosphere is particularly important since wetlands are where MeHg enters rice and are hotspots of net MeHg production in Sweden. A more rapid decline in wetland Hg as deposition declines could thus have a relatively large effect on Hg in boreal aquatic ecosystems, and rice cultivation might be managed to promote Hg evasion.

Knowing how quickly changes in anthropogenic Hg emission can translate into improvements in the Hg contamination of landscapes is essential for the difficult decisions ahead that aim to further control the release of Hg to the atmosphere – especially in China where Hg emissions are driving global increases. If the only process that removes Hg from the soils were runoff, it would take centuries without any Hg deposition to significantly reduce the amount of anthropogenic Hg available for methylation.

But Hg can evade as a gas and reliable quantification of surface-atmosphere exchange processes for Hg0 is crucial for understanding the role of the biosphere in the Hg biogeochemical cycle. Flux sampling tools- already implemented for a broad range of trace gases should be utilized for Hg0. As a result, both SKLEG and SLU+ have been pushing the limits of measurement technology by developing REA- Hg. Cooperation will facilitate rapid advances in the understanding of how costly global investments in Hg emission controls influence Hg in the environment.

Land-atmosphere exchange over wetlands could be particularly significant for reducing the present Hg load to both rice and fish since the same processes that promote methylation in wetlands also create volatile species that could evade.

In a global perspective, wetlands belong to the least studied types of ecosystem for Hg0 land atmosphere gas exchange even though emergent macrophytes in such systems have been shown to emit significant amount of Hg0. In northern peatlands (350 *106 ha globally) there is a complete lack of studies, albeit theoretical and empirical data suggest that the evasion term could be important here. In China, 2/3 of the total wetland area of 66*106 ha is made up of artificial systems (paddy crop fields etc.). Hg0 land-atmosphere gas exchange studies over such ecosystems are very scarce and have used older, less reliable approaches.

Since Hg binds strongly to organic matter, atmospheric deposition has created a store of Hg in mires that slowly leaches in both forms, as Hg and MeHg, into the nearby surface waters. The current understanding is that reductions in atmospheric Hg deposition will have little immediate impact on the terrestrial loading of Hg and MeHg to the aquatic food chain since there is already so much Hg in mires and soils. That view, however, is largely based on mass balances, where evasion of Hg0 back to the atmosphere is assumed small but has been hard to quantify. It appears justified to ignore this term in the forest environment, based on a few pioneering studies of Hg exchange in the soil-forest canopy-atmosphere system. However there are several reasons to believe that mires may have a different Hg mass balance than forests. These include the absence of a forest canopy and low redox that can make more Hg available for evasion to the atmosphere. Elevated Hg availability, low pH, sufficient sulfate availability and low iron concentration can also make mires sites of Hg2+ reduction (with Hg2+ signifying all inorganic forms of Hg) and subsequently allow volatile loss of Hg0 to the atmosphere. This loss may constitute an important pathway to reduce the pool of Hg available for methylation. If this is the case, then reduced Hg deposition could deplete the pool faster than currently estimated. It has also proven difficult to explain the store of Hg in peats from mass balance estimates that only include wet deposition and runoff.

WP B: Methylation: Synergistic effects

New information will be provided that will help to identify management practices in rice and forest production that minimize MeHg formation and subsequent bioaccumulation (WP C). Thus, we anticipate WP B to provide new information on Hg biogeochemistry that could be relevant for major land use in a wide range of northern hemisphere environments.

Long-range, atmospheric transport of Hg released by combustion of fossil fuels has increased the Hg in soils and sediments by a factor of 3 to 10 times. Of most concern is the transformation of a fraction of this inorganic Hg to MeHg, which bio-accumulates in organisms. Anoxic environments, i.e. wetlands, lake sediments and bottom waters are “hotspots” for methylation in the landscape. Forestry harvesting can also increase the transport of Hg and MeHg to surface waters, resulting in up to 20% increases in bio-accumulation of Hg in fish. Rice accumulates harmful levels of MeHg because cultivation of rice in stagnant waters with redox gradients can promote net methylation (6). The association of rooted plants with microbial communities creates a microenvironment favourable for Hg methylation. The intensity of the Hg methylation might vary with the activity of SRB and IRB bacteria associated to rice roots as well as selenium content. Identifying the main factors controlling Hg methylation in roots and soils of rice cultures could help us to reduce MeHg in rice.

The concentration dynamics of MeHg is the net result of three major processes: 1) formation by Hg methylation 2) degradation (demethylation) and Hg2+ reduction to Hg0 and evasion to the atmosphere. The latter process (quantified in WP A) indirectly controls the availability of Hg2+ for methylating bacteria. Since the methylation of Hg is taking place inside bacterial cells, the process is dependent on bioavailable forms of Hg with sufficient concentrations to enter cells. Both passive uptake of neutral Hg(HS)2 0(aq) and facilitated uptake of Hg complexed by low molecular mass organic acids and thiols occur. The major process of MeHg degradation is driven by UV light in surface waters. Biotic demethylation processes are less well understood, but in several groups of bacteria detoxification process acting via the merB operon has been identified.

Land use influences the formation of MeHg in many ways, including drainage-flooding cycles in rice paddies and alteration of water table levels after forest harvest or wetland restoration. Field and controlled mesocosm experiments have shown that nutrient status, sulfur and climate interact to yield very different net methylation rates and aquatic bioaccumulation. In boreal wetlands, intermediate nutrient status has shown to promote the highest net production of MeHg.

Since both methylation and demethylation processes occurs in parallel, understanding the factors and conditions in control of both processes, as well as the Hg2+ reduction process, can influence management alternatives to minimize or possibly mitigate the net formation of MeHg and its introduction into the food-web. It is clear that the availability of energy (high quality organic matter), electron acceptors [Fe3+, SO4 2- and CO2] and common nutrients are important drivers for the formation of MeHg. Sulfur is particularly important since it is an electron acceptor as well as a major regulator of the redox-potential in soils/sediments, but also indirectly affects MeHg by regulating the bioavailability of Hg for methylating bacteria. Because of this, there is a complex interplay between different factors influencing net methylation. To understand this interplay requires a combined chemical and microbiological approach where processes are addressed at a molecular level.

In WP B we will improve the scientific understanding of the formation and degradation of MeHg in relation to wetland and soil management for the production of rice and forest in China and in Sweden. Controlled experiments and environmental gradients that take advantage of the wide spectrum of different soils and wetland environments studied by the groups in Sweden and China will be utilized to develop a molecular conceptualization of the major environmental factors controlling MeHg formation in soils and waters.

WP C. Entry into the food chain – When to intervene

Knowledge of the variation in susceptibility to MeHg uptake will open up a new field of Hg research on “smarter” land management methods that account for where and when MeHg moves from water/soil into the aquatic plants and animals [food chain and rice] of commercial significance.

While WP-B focuses on ways to adapt land management to reduce MeHg, that management will be more effective if we knew more about where and when MeHg moves into biota. For instance in boreal regions most Hg enters aquatic ecosystems during spring flood when the MeHg concentration is at a low level. The highest MeHg concentrations in water are however seen in summer when flows are low. Which is more important to control - concentration peaks in summer or the mass flux in spring? The answer depends on the timing and efficiency of Hg biouptake. The enormous spatial variability that can be seen in the Hg content of some small fish has been explained by differences in their local environment. This suggests a large variability in space and time of bioaccumulation, much of which can be linked to the topography and hydrology of watersheds.

In a dynamic environment affected by seasonality in hydrology, ecophysiology, and land use, monitoring of Hg and MeHg over time and space in phytoplankton, zooplankton and macroinvertebrates can tell us much about the transfer of Hg from water into the food chain, because these organisms occupy low trophic positions and respond rapidly to changes. Algal blooms are of particular interest in this regard. The influence of ancillary factors such as pH, sulfur and dissolved organic carbon (DOC) on the biouptake of MeHg is also controversial.

Apart from field studies, a promising way of addressing the natural complexity of Hg biotransfer is the (novel) experimental work in aquatic mesocosms by SLU+ based on isotope spikes into different environmental compartments to simultaneously quantify the transfer of Hg between different forms and compartments in the food web.

Of particular interest in the proposed framework is the striking difference between low biotic Hg levels in comparatively polluted reservoirs and lakes in China, but high biotic Hg levels in comparatively pristine lakes and reservoirs in Scandinavia. This may result from differences in the connectivity of aquatic food web to high-MeHg zones, food web structures, and biodilution effects at the base of the food web. Related to all this, an underlying key factor may be whether the input of organic matter is dominated by terrestrial production (humic matter) as in much of Scandinavia, or by aquatic production (algal blooms) as in much of China. Combining parallel field studies using natural stable isotopes of C and N with experimental studies using stable isotopes of Hg is proposed.

The discovery of problematic levels of MeHg in rice highlights a new problem of human exposure to Hg. MeHg concentrations in tissues of rice plants followed the trend root > hull > stalk > leaf, which indicates a key role of the root in the uptake of MeHg accumulated in rice seeds. While other forms of Hg are kept out of the root by phytochelators, the MeHg in paddies is hypothesized to move easily across root barriers and to the upper parts of rice tissues. Understanding what controls the uptake, including selenium concentrations, and if it is passive or facilitated, is of great interest and might ultimately help reduce MeHg accumulation in rice. The discovery of MeHg in rice is so recent that much remains to be learned in this area including the efficiency of uptake in rice during different stages in the cultivation, as well as between different rice strains.

WP-D Remediation -When it is too late?

The costs associated with Hg contaminated sites and their remediation are immense. Better identification of high risk sites and lowering of their MeHg production will make more effective use of available remediation resources.

In addition to large-scale contamination by diffusive sources of Hg, the Swedish EPA has recognized 41 000 local sites highly contaminated by metals and/or organic pollutants by point sources. Many of the sites with greatest potential risk are sediments, soils, estuaries and lakes contaminated with Hg released mainly from chlor-alkali plants and the pulp and paper industry. In China, even if there are no complete inventories, an overview by SKLEG has identified a large number of sites contaminated by Hg. Sites include Hg and gold mining areas, metal smelters and a range of different types of industry (including paper and pulp). The size of these activities in China suggests that the number of sites contaminated by Hg would be immense.

In the first stage, knowledge and experiences from studies by SLU+ would help to identify sites in China that would be first priority remediation objects. A useful basis for risk assessments are incubation experiments to determine rates of methylation and demethylation in order to identify and quantify factors and parameters useful in risk assessment in a previous study on a small number of sites in Sweden, the production of labile organic matter was identified as the most important factor behind net MeHg formation.

The next step is to develop methods for in situ Hg remediation that can minimize the formation of MeHg. Given the local skills and situation, SKLEG has investigated phytoremediation to remediate Hg contaminated sites. Greenhouse and field experiments indicated that thiosulphate assisted phytoextraction could enhance the phytoremediation efficiency decrease the concentration of Hg associated with Fe/Mn oxides and organic matter in soils. SLU+ has worked with stabilization of inorganic Hg2+ to making it less available for methylation and shifting environmental conditions in favor of MeHg degradation. Theoretically this can be obtained by shifting the bacterial activity towards those mainly involved in demethylation, or by stimulation of the abiotic MeHg  degradation processes. In wetlands, amendments of Fe have shown to significantly decrease the net formation of MeHg. Similarly, studies of highly contaminated sediments in Sweden suggest certain forms of Fe indeed may lower the formation of MeHg. It is still unclear whether the Fe amendment shifts the bacterial activity from methylating (SRB) to less methylating, and possibly MeHg degrading, IRB, or if Fe indirectly inhibited methylation by lowering soluble Hg-sulfides during formation of FeS(s).

Modern tools in environmental genomics and environmental microbiology available within SLU+ can help address this. Bacteria detoxify Hg2+ into Hg0 by enzymatic reduction using the mer operon. Recently the Hg methylating genes have been described for SRB and IRB. This makes it possible to detect demethylation and methylation activities using metagenomics techniques. The use of stable isotopes to follow chemical Hg transformations and the expression of genes involved in Hg transformations will help us to elucidate the insights of Hg geochemistry and biological processes of the Hg cycle in soils, sediments, rice and freshwater systems. Metagenomics will allow us to understand the main driver of Hg transformations and the differences between wetlands and lakes. Enhanced demethylation activity in response to experimental treatments can in a first instance be linked to changes in the composition of the active microbial community but by sequencing the combined expressed genes of the microbiota (metatranscriptomics), specific metabolic traits leading to enhanced demethylation can be also identified and used for obtaining a detailed mechanistic understanding of the mechanisms underlying changes in demethylation rates.

At certain sites, phytoremediation, where plants prone to accumulation of Hg and/or MeHg are used to concentrate and harvest the contaminant, is an alternative. A third option, employed at a few sites in Sweden, is to excavate sediments contaminated by Hg from a larger part of the contaminated area, and to concentrate and encapsulate the excavated sediment in a confinement covering a smaller part of the contaminated area.

Workshop 17-27 May 2014 (programme and presentations)

Monday 19 May

08:00-09:00 Introduction

08:00-08:15 Kevin Bishop Opening talk

08:15-08:30 Xinbin Feng Hg in China

08:30-08:45 Lars Hylander Hg in Sweden

09:00-11:30 WP C: Entry into the food chain – When to intervene  

Chair: Erik Björn and Lihai Shang

09:00-09:15 Markus Meili Collaborating on Hg in ubiquity: some options derived from some experiences.

09:15-09:30 Haiyu Yan

09:30-09:45 Pianpian Wu Understanding methylmercury bioavailability at the base of aquatic food chain.

10:15-10:30 Guangle Qiu Methylmercury accumulation in rice plant (Oryza stiva L.) grown at mercury contaminated sites in China.

10:30-10:45 Hua Zhang

13:00-15:00 WP A: Land-Air Exchange – in and out of the landscape   

Chair: Quangle Qiu and Mats Nilsson

13:00-13:15 Jonas Sommar Hg0 air-surface exchange over terrestrial ecosystems in China - Past years studies and future directions.

13:15-13:30 Stefan Osterwalder Half a decade of TGM-flux measurements over a boreal mire. SMAREF - key to the perfect REA?

13:30-13:45 Xuewu Fu Monitoring of atmospheric mercury and deposition in China.

13:45-14:00 Ingvar Wängberg (Special talk): On-going mercury research at IVL.

15:30-16:30 WP D: Remediation – When it is to late 

Chair: Jonas Sommar and Sofi Jonsson

15:30-15:45 Jianxu Wang

15:45-16:00 Ulf Skyllberg

16:00-16:15 Guangle Qiu Antagonistic effect of selenium on mercury accumulation in rice plant (Oryza sativa L.).

16:15-16:30 Zhonggen Li Hg emissions from a coal-fired power plant in North China and its environmental impacts. 

Tuesday 20 May

09:00-13:45 WP B: Methylation and demethylation: Net methylation in Scandinavia and China

Chair: Hua Zhang and Ulf Skyllberg

09:00-09:15 Mats Nilsson

09:15-09:30 Lihai Shang The influence factors on mercury methylation/demethylation rate in reservoirs.

10:00-10:15 Xinbin Feng

10:15-10:30 Erik Björn

10:30-10:45 Ulf Skyllberg

11:00-11:15 Andrea Garcia Bravo Mercury methylation in lakes: electrons matter.

11:15-11:30 Staffan Åkerblom Boreal peat lands being potential sources of MeHg in changing environments.

12:45-13:00 Kevin Bishop Beyond forestry and mercury.

13:00-13:15 Tao Jiang Role of natural organic mattert (NOM) in the geochemical process of mercury under changing redox environment.

13:50-14:30 International outlook

Chair: Kevin Bishop and Xinbin Feng

13:50-14:10 Matti Verta Minamata treaty.

14:10-14:30 Anders Bignert CHEMSTRESS

Wednesday 21 May

09:00-12:00 Genomics workshop

Chair: Stefan Bertilsson

09.00-09.30 Stefan Bertilsson How can genomic tools help us understand biogeochemical processes? An introduction.

09.30-10.00 Andrea Garcia Bravo Microbial community structure in lake sediments: from sample to phylogenetic tree.

10.00-10.15 Jingying Xu From phylogenetic trees to metabolic traits: the case of mercury methylation.

10:45-11:10 Sarahi Garcia Towards an understanding of the abundant and uncultivated: Freshwater Actinobacteria.

11:10-11:50 Jeffra Schaefer A molecular approach for identifying the microbial guilds responsible for methylmercury cycling in wetlands.

12:00-14:30 Lunch and Uppsala sight seeing

14:30-16:00 Hg isotope workshop (Room J, Almas allé 10, SLU)

Chair: Jiubin Chen

14:30-15:10 Jiubin Chen Hg isotopes in aqueous environment.

15:30-15:45 Sofi Jonsson

15:45-16:00 Christian Zdanowicz Hg isotope ratios in Arctic precipitation, modern and ancient.

16:00-16:15 Zhonggen Li Total Hg and Hg isotopes in soils, street dust, and lake sediment around a large scale Zinc smelter in Hunan, China.

16:30-16:45 Martin Jiskra Overview of stable Hg isotope measurements in Sweden: Tracing sources and processes.

16:45-17:30 Discussions about the opportunities to utilize natural abundant and enriched stable Hg isotope in SMaReF. Leader: Jiubin Chen and Ulf Skyllberg

Thursday 22 May

08:00-09:30 Parallel discussions: Council conclave/younger generation, Council conclave: Kevin Bishop and Xinbin Feng, Younger generation: Andrea Garcia Bravo and Lihai Shang 

10:00-11:00 Conclusions from discussions council and younger generation

12:00-22:00 Travel to Vindeln

Friday 23 May

Morning Excursion Vindeln research park (Svartberget and Flakaliden)

14:00-15:00 WP group discussions

15:30-16:00 Report from WP group discussions, Discussion leader: Kevin Bishop and Xinbin Feng  

16:00-17:00 Conclusions of SMaReF workshop: How to go further?

Saturday 24 May

Morning Continue on WP group discussions

Afternoon Visit to Umeå University campus

Sunday 25 May

07:00-07:30 Bus to Vindelns station

07:42-16:00 Train to Kiruna and Abisko. Breakfast on train to Abisko

18:00- Dinner in Abisko

Monday 26 May

All day Abisko excursion

Tuesday 27 May

08:00-09:00 Breakfast Abisko

11:00-13:00 Bus to Kiruna Airport

14:00 Flight back to Arlanda, Stockholm/Uppsala