Recent advances in mercury research

During the last few decades considerable scientific knowledge has been developed on sources and emissions of mercury, its pathways and cycling through the environment, human exposure, and impacts on the environment and human health (e.g. summary of Global Mercury Assessment in UN Environment, 2019). Mercury is recognized as a toxic, persistent, and mobile contaminant. This contaminant does not degrade in the environment and it is mobile because of the volatility of the element and several of its compounds. Mercury has the ability to be transported within air masses over very long distances. High doses of organic compounds of mercury, particularly methylmercury can be fatal to humans and wildlife, and even relatively low doses can seriously affect the nervous system of organisms. Mercury has been also linked with harmful effects on the cardiovascular, immune and reproductive systems. Methylmercury passes through both the placenta and the blood-brain barrier, so exposure of women of child-bearing age and of children to methylmercury is of great concern.

An agreement has been reached between scientists, that mercury exposure today is of concern in many regions of the globe posing a threat to human and wildlife health. This scientific knowledge, obtained through studies carried out in various research centers in all continents, formed the basis for policy makers decision to develop an international agreement on reduction of emissions and exposure to mercury, known as the United Nations Minamata Convention on Mercury. While this Convention was signed in 2013 and ratified in 2017, work is ongoing assessing the completeness of our knowledge on mercury as a global environmental pollutant, and developing implementation of solutions to reduce the emissions and exposure to this pollutant. This was the focus of the International Conference on Mercury as a Global Pollutant (ICMGP). The 14th ICMGP occurred in Krakow, Poland in September 2019 where it was acknowledged that global anthropogenic emissions of mercury to the environment are still increasing. The estimated global anthropogenic emissions of mercury to the atmosphere for 2015 are approximately 20% higher than they were in 2010 (UN Environment, 2019). Human activities have increased total atmospheric mercury concentrations by about 450% above natural values. This increase is reflected in mercury loads of some aquatic food-webs exhibiting levels of concern for ecological and human health (UN Environment, 2019).

Major subject of discussions on the improvement of our knowledge on mercury sources, fate, impacts, and emission control options was defined at the 14th ICMGP Conference in 2019 as “Bridging knowledge on global mercury with environmental responsibility, human welfare and policy response”. Major questions that need to be addressed within this subject were defined as follows:

• How is mercury biogeochemical cycling changing on the global, regional, and local scales in response to perturbations caused by major anthropogenic drivers of the environmental change?

• What is the relative risk of mercury exposure to human health and wildlife in the context of human welfare?

• How can technological development contribute to the reduction of mercury exposure and improvement of environmental responsibility?

• How can scientific knowledge contribute to the implementation and effectiveness evaluation of the Minamata Conventions and other regulatory agreements? Importance of integration and implementation of emerging and future mercury research into the policy making.

The above questions are being addressed by new studies on mercury sources, fate, behaviour, and impacts on the environment and human health. Papers presented in this Special Issue of the journal of the Science and Total Environment report results from the latest studies on mercury in the environment. These results will contribute to more efficient implementation of targets of the Minamata Convention on Mercury.

Most of the papers presented in this Special Issue are related to the first question on perturbations of biogeochemical cycling of mercury in aquatic and terrestrial ecosystems. A review of research on atmospheric mercury is presented in three papers by Lyman et al., Cooke et al., and Sommar et al. The atmosphere is the beginning component of the mercury biogeochemical cycling and therefore it is quite important to accurately assess atmospheric sources of the contaminant, transport with air masses, and finally, atmospheric deposition to aquatic and terrestrial surfaces.

The paper by Lyman et al. addresses the chemistry of atmospheric mercury in gaseous forms, as well as solid and liquid phase aerosols. Then, a thorough review of processes important to understand wet and dry deposition of mercury is presented. An important aspect of spatial and temporal changes in atmospheric mercury concentrations is discussed. Key uncertainties and future research needs are identified in relation to mercury oxidation mechanisms, particle phase processes, cloud chemistry, dry deposition, and development of spatial and temporal trends. Finally, future measurement techniques are defined with the importance of routine calibration checks.

Atmospheric deposition of mercury is then further discussed by Cooke et al. with focus on how various natural archives, such as lake sediments and peat bogs, historical and more recent ice and tree rings, and measurements of stable Hg isotopes integrate signals of past atmospheric Hg deposition and air concentrations. The authors discuss what is known about preindustrial Hg cycling from natural archives, including the evidence for preindustrial Hg emissions to the atmosphere, and present key spatial patterns of industrial-era Hg deposition. They propose that instead of developing new Hg records one should focus on better assessment on what the existing records tell us about past atmospheric emissions, cycling, and deposition of mercury. The authors conclude that natural archives will play an important role in assessing the response of the atmospheric Hg pool to the Minamata Convention, and in the response of long-term Hg sinks to climate change.

Recent advances in understanding and measuring surface – atmosphere exchange of gaseous elemental mercury are discussed by Sommar et al. The authors provide a comprehensive literature synthesis, and methodological and instrumentation advances for terrestrial and marine elemental mercury flux studies in recent years. The authors outline the theory of a wide range of measurement techniques and various operational protocols. They point out that the most frequently used measurement techniques to determine the net elemental gaseous mercury fluxes are by far dynamic flux chambers for small-scale and micro-meteorological approaches for large-scale measurements. Furthermore, top-down approaches based on elemental gaseous mercury concentration measurements have been applied as tools to better constrain Hg emissions as an independent way to e.g. challenge emission inventories. During recent years new measurement methods with the use of Hg stable isotope ratios of samples involved in atmosphere-terrestrial interaction have been developed and provide in combination with concentration and/or flux measurements novel constraints to quantitative and qualitative assessment of the bi-directional gaseous elemental mercury flux. The authors conclude that recent efforts in the development of relaxed eddy accumulation and eddy covariance elemental mercury flux methods carry out the potential to facilitate long-term, ecosystem-scale flux measurements in order to reduce large uncertainties in gaseous elemental mercury flux estimates.

Recent advances in studying cycling of mercury in freshwater lakes are reviewed by Branfiruen et al. with focus on further development of conceptual model for this process. The authors provide the latest information on current state of knowledge on the mercury cycling, and our understanding of the complexity of MeHg formation in freshwater lakes. They also discuss the overlooked sources of MeHg to these ecosystems. The authors pointed out that focus in current cycling models on compartments and fluxes may be resulting in a failure when drawing conclusions on critical zones of MeHg production/degradation. They postulate that redirection of attention towards factors that truly regulate the supply of MeHg to the base of the food web will improve our ability to mechanistically model biotic exposure to MeHg in freshwater systems under current conditions. Primary producers, biofilms, and periphyton as key sources of MeHg to freshwater food webs are discussed in this context. A discussion on dissolved organic matter as a vector of delivery of MeHg to lakes and a driver of Hg speciation is also presented in the paper. The authors conclude that the improvement of current models along the above mentioned factors will provide a roadmap for better understanding of Hg cycling in freshwater lakes in the future climate.

A revised perspective on changes in global cycling of mercury in the ocean is presented by Bowman et al. The authors have discussed results of 30 years of Hg measurements in the ocean to discuss sources, sinks, and internal cycling of mercury. The analysis of over 200 high-resolution, full-depth profiles of total Hg, methylated Hg, and gaseous elemental Hg throughout the Atlantic, Pacific, Arctic, and Southern Oceans is presented. Vertical maxima of methylated Hg were found in surface waters, near the subsurface chlorophyll maximum, and in low-oxygen thermocline waters. The greatest concentration of Hg in deep water was measured in Antarctic Bottom Water, and in newly formed Labrador Sea Water. Mercury showed a decreasing trend over the past 20 years. Distribution of Hg in polar oceans was unique relative to lower latitudes with higher concentrations of total Hg near the surface and vertical trends of Hg speciation driven by water column stratification and seasonal ice cover. A very important conclusion was that global models of Hg in the ocean require a better understanding of biogeochemical controls on Hg speciation and improved accuracy of methylated Hg measurements within the international community.

A review of recent advances in studying mercury cycling in terrestrial ecosystems is presented by Bishop et al. Major foci of this paper were the assessment of mercury inputs to the terrestrial ecosystems from the atmosphere, uptake in food, and runoff with surface waters. In terms of impacts of atmospheric deposition, the authors concluded that decreasing atmospheric emissions of mercury can result in faster recovery of the terrestrial ecosystems, despite large ecosystem stores of legacy Hg. This is quite important message for policy makers proposing various options for reduction of mercury emissions to the atmosphere. However, the authors also conclude that the Arctic region has emerged as a hotbed of Hg cycling, with high stream fluxes and large stores of Hg poised for release from permafrost with rapid high latitude warming. This source of mercury cannot be controlled by humans. Regarding the issue of uptake of Hg by food, the authors confirm that freshly deposited Hg is more likely to be methylated and incorporated in rice than stored Hg. Thus, this is another message to policy makers on the necessity for mercury emission reductions. Concerning the runoff of Hg with surface waters, the authors conclude that much of the methylmercury carried by streams originates from the terrestrial ecosystems. Finally, it was pointed out that the impacts on the global Hg cycle depend to a large extent on how changing land cover, both intentional as in afforestation and deforestation, as well as unintentional factors related to climate change (e.g. greening and browning) will influence net ecosystem exchange of Hg with the atmosphere in the mid-latitudes and the tropics.

Microbial methylation of mercury in natural environment is one of the most important processes in the context of toxicological impacts of this pollutant on the health of ecosystems and humans. One of the major challenges for better understanding this process is to predict and assess the hotspots of methylmercury contamination of the aquatic ecosystems. Current understanding of how to effectively identify active mercury methylators and assess the bioavailability of different mercury species for methylation is presented by Tang et al. They pointed out that the prevalence and activity of mercury methylating microbes in the environment are affected by many biotic and abiotic variables. The changing environmental parameters, such as climatic conditions and geochemical indices are also important regulators of mercury methylating communities. The authors concluded that future research should focus on improvement of our understanding of microbial and geochemical factors affecting environmental mercury production. Methods to assess these factors should also be further developed.

An important issue of applications of stable mercury isotopes to better understanding of mercury biogeochemical cycling is presented by Tsui et al. The major focus of this paper is on how mercury isotopes can be used to address major ecological questions, including energy transfer across ecosystem surfaces and as tracer of animal movements. The authors discussed the main “signatures” of Hg isotope variation discovered to date and the associated biogeochemical processes that lead to them, the major ecosystem types that have been studied using Hg isotopes, and the ecological applications of Hg isotopes as demonstrated in previous and ongoing studies. The authors point out that ecological applications of stable Hg isotopes are growing and have great potential for use in the future. An interesting illustration of different ecological applications of mercury isotopes in large surface lakes and oceans (spatial and depth differences), and complexes forested watersheds was presented in the paper.

The second question about relative risk of mercury exposure to wildlife and human health is addressed by Chetelat et al. More specifically, the authors assess methylmercury exposure to wildlife by discussing the impact of ecological and physiological processes on methylmercury concentrations in variety of tissues. It was pointed out that wildlife tissues that represent current or near-term bioaccumulation, and in which methylmercury is the predominant mercury species, are most effective for biomonitoring ecosystems and understanding drivers of methylmercury exposure. The authors describe why physiological and ecological processes should be integrated when formulating biomonitoring strategies for assessment of methylmercury exposure to wildlife. An interesting summary of ecological, physiological, and biological influences on mercury concentrations in wildlife is presented in a simple table. The authors propose further research in order to evaluate the use of external tissues to measure methylmercury bioaccumulation, and methods that shall be used for this evaluation.

The third question on reduction of mercury contamination and exposure has been addressed in the paper on remediation of mercury contaminated sites by Eckley et al. Remediation is a complex process with careful assessment of site contamination needed for selection of methods to be used for remediation. The authors discuss the assessment of site contamination, focusing on the geographic distribution and speciation of the elevated mercury concentrations, the major pathways of release from the site, zones of methylation and the associated controlling variables, and human and ecological Hg exposure and risk. A major focus was identification of pathways of mercury release from contaminated sites to water and air. The authors provide a summary of remediation technologies, both established and emerging for contaminated soil, waste, sediment, and water. It was concluded that remediation solutions can be very site-specific due to complexity of mercury cycling and methylation process. It was also pointed out that post remediation monitoring is a very important action for the assessment of remediation efficiency and eventually for the assessment of further remediation, if necessary.

The fourth question on the contribution of science to improve effectiveness of implementation of the Minamata Convention is addressed already in the paper by Bank. The author reviews history of mercury policy related to the Minamata Convention and recent progress in science supporting this policy. A focus is made on various issues related to seafood safety and food security in general. The author concluded that new ways of thinking about mercury will be required in order to address these issues more holistically within the framework of the Minamata Convention. This is because Hg cycling and exposure dynamics are highly complex, and therefore, multitiered risk assessments that focus on environmental health and human health separately will need to be brought together. The author concludes that mercury science and policy will likely benefit from furthering the development of Hg research at the food-environment nexus and integrating seafood safety and food security themes and approaches into the framework of the Minamata Convention.

The paper by Gustin et al. synthesizes the information in each paper to point out recent scientific achievements in studying biogeochemical cycling of mercury in the context of linking these achievements to the improvement of implementation of UN Minamata Convention on Mercury targets. Major foci of this synthesis are placed on discussing recent advances on processes governing the fate and transport of mercury in the environment, as well as on measurement methods to study these processes. Finally, the authors discuss how these advances in knowledge fit in within the context of the Minamata Convention on Mercury.