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The mercury cycle

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The mercury cycle is a biogeochemical cycle influenced by natural and anthropogenic processes that transform mercury (Hg) through multiple chemical forms and environments.

Mercury is present in the Earth's crust and is expressed in various inorganic and organic forms in Earth’s land, ocean, and atmosphere. Mercury exists in three oxidation states: Hg(0) (elemental mercury), Hg(I) (mercurous mercury), and Hg(II) (mercuric mercury). Mercury emissions to the ocean and atmosphere can be primary sources, which release mercury from the lithosphere, or secondary sources, which exchange mercury between surface reservoirs.[1] Annually, over 5,000 Mg of mercury is released to the atmosphere by primary emissions and secondary re-emissions.[2]

Microbes in aquatic environments convert inorganic mercury into methylmercury, a potent neurotoxin. The biomagnification of Methylmercury in aquatic species creates negative effects in higher trophic mammals, especially humans, and has generated a complex human health issue.[2][3]

Forms of mercury

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Mercury is introduced to the environment through volcanism, geothermic processes, and anthropogenic sources. Unlike nutrient cycling, mercury does not follow cyclical distribution patterns, meaning it transforms between each species with no set order.[4] Mercury can be found in organic and inorganic forms.[5] It can also be found in mineral form, as both cinnabar and metacinnabar.[6] The distribution of mercury throughout the water column suggests that mercury is scavenged by fish and primary producers.[7]

Elemental mercury (Hg(0)) is found in the atmosphere and surface waters.[4] Mercury is relatively insoluble in water but is capable of dissolution and can be found in supersaturated concentrations under ice in surface water, but near saturation in open water.[8] Atmospheric deposition is the main form of mercury input into the ocean.[9]

Hg(II) enters the ocean as a gas from the atmosphere. Methylation reactions convert Hg(II) into Methylmercury (MeHg) and dimethylmercury (DMHg), and demethylation reactions convert these forms back to Hg(II). Not commonly found in under surface waters, Hg(II) is found in surface waters and in ocean sediment. The most predominant route of mercury input into the Arctic Ocean is long-range transport, as opposed to point source introduction which can be acute.[9] This is an inorganic form of mercury.[5]

MeHg is one of three forms of mercury that stem from elemental mercury and is more common than its other methylated form, DMHg,[4] although both forms are considered organic mercury.[5] It can be found as any of three alklyl forms known as aryl, short chain, and long chain alkyl compounds.[5] It is the most toxic form, acting as a neurotoxin in organisms due to its ability to combine with lipids and fats.[4] Dimethylmercury, the lesser product of methylation, is created through the reduction of Hg(II). Inorganic methylation has been documented as a result of anaerobic and aerobic microbes, as well as reduction reactions.[4] This is the form that unicellular organisms most commonly ingest where it is slowly eliminated relative to its uptake rate.[10]

Mercury reservoirs

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Terrestrial reservoir

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Most mercury resides in the lithosphere and occurs in the form of the mercury sulfide mineral cinnabar, which is one of the only significant ores of mercury.[11] Mercury accumulates in deep sediments, the largest mercury reservoir, from burial of deep-ocean mercury.[1] The burial of deep-ocean mercury in ocean sediments is the largest sink of mercury from surface reservoirs.[1] The residence time of mercury in the deep mineral reservoir is around a billion years.[12] Mercury is also accumulated in surface soils due to sequestration by vegetation or deposition of atmospheric mercury. Surface soils are a relatively small mercury reservoir (∼10–15 Gg) with a residence time of around 100 years, while slower-decaying soils and organic matter like peat act as a larger mercury reservoir (∼300 Gg) with a residence time of ~400 years.[13][14] Organic-rich sedimentary rocks can also be enriched in mercury.[15]

Oceanic reservoir

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In the ocean, mercury exists primarily in elemental (Hg(0)), reactive (Hg(II)), and particulate forms. In its various forms, the distribution of mercury throughout the water column suggests that mercury is scavenged by fish and primary producers.[7] Marine organisms can also produce organic species of mercury such as Methylmercury (MeHg). It is estimated that ~200 Gg mercury exists in the deep ocean and ~70 Gg mercury exists in the surface (< 500 m) ocean.[16] Despite the smaller pool of mercury in the ocean's surface waters relative to the deep ocean, the effects of anthropogenic mercury emissions is most notable at the ocean's surface, where mercury concentrations are estimated to have increased by 200% over the past century.[13] Changes in deep ocean mercury concentrations are less significant and are primarily limited to regions of deep water formation in the North Atlantic and Southern Oceans where sinking surface waters have more recently been in contact with the atmosphere.[13] The residence time of mercury in the ocean's surface waters is ~0.6 years, and the residence time for mercury on continental shelves in the ocean is ~0.3 years.[17][18]

Atmospheric reservoir

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Approximately 95% of atmospheric mercury is in the form of gaseous elemental mercury (Hg(0)).[19] The residence time of Hg(0) in the atmosphere is around half a year, allowing Hg(0) to be transported long distances from its emission source and widely distributed across the planet.[17] Over time, Hg(0) is oxidized to form reactive gaseous mercury (Hg(II)), which is more readily deposited to the Earth's surface.[19][20] The Hg(II) form of mercury has a residence time in the atmosphere of only a few weeks, and therefore cannot be transported far from its formation or emission source.[20][21] Some deposited Hg(II) is rapidly converted to Hg(0) and re-emitted to the atmosphere. When the recycling of mercury between the surface and atmosphere is considered, the overall residence time of mercury in the atmosphere increases from 0.5 to 1.6 years.[17] Mercury concentrations in the atmosphere vary due to local sources and depositional processes. In the northern hemisphere troposphere, an average background concentration of 2 nanograms per cubic meter has been estimated.[21]

Physical processes

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Major processes in the mercury cycle. Thick arrows represent fluxes of mercury between ocean, land, and atmosphere. Thin arrows represent transport within a reservoir or transformation between different forms of mercury, including elemental (Hg(0)), reactive (Hg(II)), organic methylated (MeHg), and mineral (HgS) mercury.

Terrestrial emissions

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Terrestrial mercury is released to the atmosphere and hydrosphere by both geologic and anthropogenic processes. Primary sources of mercury emissions include natural processes such as volcanic activity, mineral weathering, and release from mercury-rich surface soils, as well as anthropogenic processes such as gold mining, burning coal, and production of non-iron metals such as copper or lead.[22][23][24][25][26] Secondary natural sources, which re-emit previously deposited mercury, include vegetation, evasion from oceans and lakes, and biomass burning, including forest fires.[27] From 1997-2006, around 8% of all mercury emissions to the atmosphere was from biomass burning.[28] Anthropogenic mercury emissions from primary sources are leading to increased concentrations of mercury in surface reservoirs.[29]

Atmospheric transport

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Mercury is primarily transported and distributed across the Earth's surface by atmospheric circulation. Atmospheric mercury is transferred back to land and water surfaces by wet deposition and dry deposition.[17]

Elemental mercury (Hg(0)), which makes up around 95% of atmospheric mercury, is the most stable form of mercury found in the atmosphere.[9][20][13] Hg(0) remains in the atmosphere for six months to a year and is therefore well-distributed throughout the globe despite localized emission sources.[9][17] Hg(0) does not readily dissolve in water and leaves the atmosphere primarily through dry deposition.[30]

The other primary form of atmospheric mercury, reactive gaseous mercury (Hg(II)), is returned to the Earth's surface within a few weeks of formation by both dry and wet deposition.[30][20][31] Wet deposition of Hg(II) is the primary pathway for return of atmospheric mercury to the Earth's surface.[30] Due to this short residence time of Hg(II), Hg(II) is either deposited near its emission source or can be deposited in regions where local atmospheric reactions oxidize Hg(0) to form Hg(II). Because photooxidation is very slow, Hg(0) can circulate over the entire globe before being oxidized and deposited.[30] Around 60% of atmospheric mercury is deposited to land while 40% is deposited to water.[30] A fraction of deposited mercury instantaneously re-volatilizes back to the atmosphere.[1]

Oceanic deposition and evasion

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Wet and dry deposition of atmospheric mercury are responsible for 90% of the mercury found in surface waters, including in the open ocean.[32][33] Once in the ocean, Hg(II) is rapidly converted into Hg(0). The ocean is generally supersaturated in Hg(0) with respect to the atmosphere, resulting in evasion of Hg(0) from the ocean and a net flux of Hg(0) to the atmosphere from the ocean on short time scales.[34] Over decadal time scales, mercury concentrations in the surface waters of the ocean respond to atmospheric changes.[13] Over long time scales, the ocean is the primary sink for atmospheric mercury from anthropogenic sources.[35] Deep-ocean mercury is buried in ocean sediments and returned to the terrestrial mercury reservoir, closing the mercury cycle.

Anthropogenic influence

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Due to anthropogenic emissions, release of mercury from land into the global atmosphere-ocean-land cycle is estimated to have increased by a factor of three to five from natural mercury mobilization rates.[1] The increased rate of mercury cycling is primarily due to mining, fossil fuel combustion, and production of metals and other industrial materials by humans.[1] Fossil fuel combustion, primary that of coal, is the largest anthropogenic mercury source, contributing around 60% of all human-released mercury.[1] From measurements beginning in the late 1970s, atmospheric mercury levels were observed to peak in the late 1980s, decrease in the late 1990s, and remain relatively constant into the 21st century.[36] It is estimated that human activities have released more than 1540 Gg of mercury since 1850.[37]

Mercury cycle in the Arctic

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The dynamics and implications of the mercury cycle are of particular concern in the Arctic. Vast stores of mercury in thawing permafrost and rapid drawdowns of atmospheric mercury during polar springtime depletion events have drawn attention to the Arctic in recent decades as a key region for the cycling of Mercury between the Earth’s reservoirs. Microbes unique to the Arctic are able to convert some harmful mercury from Hg(II) to Hg(0) using cellular processes, however a great deal of harmful mercury still manages to travel through the food web. In addition, as MeHg bioaccumulates up the food web, 90% of which is derived from anthropogenic sources, it has created negative consequences for Arctic communities that rely on a subsistence lifestyle as means for survival.

Sources and physical processes

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Atmospheric transport

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There are no anthropogenic point sources of mercury in the high Arctic.[13] Due to low levels of mercury emissions in the Arctic, accumulation and deposition of mercury in the Arctic is primarily due to atmospheric transport from lower latitudes. Approximately 2/3 of mercury deposition in the Arctic is from natural primary and secondary sources.[13] Around 28% of mercury deposited in the high Arctic is released from anthropogenic sources in other regions across the globe, primarily in East Asia.[13] Of secondary (re-emitted) mercury sources, 40–45% originate from land while 30–34% originate from oceans.[38] Around 10% (15 Mg per year) of atmospheric mercury transport to the Arctic is from wildfire emissions, the majority of which originate from Eurasian boreal forests.[39] Global wildfires act as a source of Arctic mercury year-round, but transport peaks in the northern hemisphere fire season (summer and fall).[39] Background concentrations of Hg(0) in the Arctic atmosphere generally range from less than 0.05 to 0.3 nanograms per cubic meter.[9] It has been suggested that some Hg(0) in the Arctic atmosphere originates from the reduction of Hg(II) in snow.[9]

Deposition in the ocean

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Permafrost soils store more mercury than the ocean, atmosphere, and all other soils combined.[40] It is estimated that 793 ± 461 Gg mercury is frozen in northern hemisphere permafrost, the majority of which lies within the Arctic circle.[40] As permafrost thaws in a warming climate, mercury is released at increasing rates from the large permafrost reservoir, likely increasing the riverine flux of mercury to the Arctic ocean. Riverine flux of mercury to the Arctic Ocean is estimated at 80 Mg per year.[40] Around 15 Mg mercury is released into the ocean each year from coastal erosion.[40] ~25 Mg mercury is deposited directly into the ocean from the atmosphere annually, with an additional 20 Mg entering the ocean from melting of snow on sea ice.[40] Of all the mercury deposited into the Arctic ocean, ~90 Mg is evaded from the ocean back into the atmosphere annually.[40] Exchanges of mercury between the ocean surface and deep waters are about equal. Atmospheric mercury can be oxidized into more reactive forms, such as Hg(II) and particulate elemental mercury, which are readily deposited.[9] This is most common in the spring when favorable conditions occur in the Arctic (e.g. cold temperatures, adequate sunlight) and reactive, catalyzing halogens in sea ice (e.g. Bromine, Chlorine, Bromine Oxide, Chlorine Oxide) are present. There are uncertainties about the specific roles played by major atmospheric oxidants in this process.

Large volumes of atmospheric mercury are rapidly deposited in the Arctic during atmospheric mercury depletion events (AMDEs).[9] AMDEs are a recurrent polar springtime event that result in a large drawdown of atmospheric mercury into the ocean.[20] During AMDEs, halogens released from freezing sea ice cause photochemical reactions that convert Hg(0) to the reactive mercury species Hg(II), which rapidly deposits onto frozen surfaces.[20] AMDEs were first identified in the late 1990s after continuous measurements of atmospheric mercury concentrations began in the Arctic. Episodic decreases of Hg(0) concentrations in March and June were soon detected, prompting further study of this phenomenon. During AMDEs, atmospheric mercury concentrations have been found to drop from 1.7 to less than 0.1 nanograms per cubic meter in less than 24 hours.[20] AMDEs are estimated to result in the deposition of 325 tonnes of mercury annually to the Arctic.[41] In recent years, AMDEs have been occurring earlier in Spring than in previous records.[42] Atmospheric circulation controls on deposition of mercury in the Arctic have also been indicated by correlations between AMDEs and the Polar/Eurasian Teleconnection Pattern.[42]

Distribution in the ocean

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Hg(0) levels tend to be low in Arctic ocean waters. Due to there being no strong distribution patterns in total mercury, there are also no strong patterns for Hg(II), since it is a product of mercury methylation and Hg(0) oxidation.[43] There appears to be a correlation between higher Hg(II) levels and distance from river inflows, suggesting rivers to be a source of mercury in the ocean. Euphotic zones in the Arctic show both biological and photochemical Hg(II) reduction as well as methylmercury (MeHg) photodegradation.[43] In euphotic zones, wind speed and ice cover are shown to dictate elemental mercury concentration. Low concentration has been observed with high Chlorophyll levels, while dissolved gaseous mercury concentration has been observed highest in ice covered areas. This relationship indicates mercury evasion caused by ship passages. The aphotic zones of the Arctic show signs of biotic reduction of Hg(II), which is considered to be the primary mechanism for Hg(0) production. Reductive demethylation is another possible mechanism, however not as prominent.[43]

MeHg is shown to be in low concentrations in open surface waters, and this is thought to be the result of photodemethylation decreasing surface water concentrations. Mixing throughout the water column is thought to be the primary transport system for MeHg and dimethylmercury (DMHg), and also play a role in the deposition of MeHg to land (DMHg is evaded to the atmosphere, where it undergoes photodegradation and becomes MeHg).[43]

The sedimentary layer in coastal and estuarine systems hold a prominent reserve of mercury transported from rivers, and due to a combination of small particle size, burrowing macrofauna, and high organic matter concentrations.[44] Mercury is then recycled, however, the rates of methylation are dependent on the physical composition, chemistry, and movement of the sediment, thus physical and biological aspects such as hydrodynamics, bioturbation, and organic matter content influence the biogeochemical potential for methylation.[44] It is possible that mercury is transferred from the sedimentary layer into the water column through physical processes such as passive diffusion, advection, and particulate resuspension.[44]

Microbes that cycle mercury

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Mercury-converting genes

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Microbial activities play an important role in alleviating toxicity and mobility of pollutants such as mercury, but few studies have shown their roles outside of temperate regions.[45] [46] However, recent studies have shown that microbes play an important role in Hg cycling in the Arctic. Specific microbes have the ability to demethylate methylmercury (MeHg) to ionic mercury (Hg(II)), then reduce Hg(II) to the volatile (easily moveable) elemental form (Hg(0)).[47] [48] To study these unique microbes, scientists took samples from microbial mats in an Arctic coastal lagoon and from surface marine algae. Within the samples, most of the microbes were psychrophiles. Further results from their experiment indicate that microbes do this by using a process that is utilized by microbes throughout the world, however the genes grouped into an operon that encodes this process - mer - is unique to Arctic microbes.[47] Only approximately 5% of surface microbes were metabolically active in using their mer genes, and converted about 68% of bioavailable mercury to Hg(0). Microbes deeper in depth have less access to light so Mer-mediated activity increased to transform about 90% of bioavailable Hg. Following conversion of dangerous levels of harmful Hg by these microbes, normal microbial growth can resume because the environment is no longer toxic.[47]

merA gene specifics
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The mer operon contains a number of genes that respond to and use mercury to contribute to the mercury cycle. For example, an activator/repressor gene on the operon tells the present periplasmic scavenging gene when there is mercury nearby to collect. A number of shuttling genes on this operon move the cell closer to the Hg then allow it to engulf the Hg. merB is the gene on the mer operon that demethylates MeHg to Hg(II), and merA reduces Hg(II) to Hg(0).[49] MerA is the gene studied most often in scientific experiments as it has the most direct consequences for the rest of the food web.

The merA gene requires mercury concentrations in the nanomolar/liter range to activate and begin converting Hg.[50][51] The typical range of mercury concentration for a 'pristine' environment that is considered not contaminated with Hg may still have concentrations in the picomolar/liter range.[52][53] 17 unique phylotypes of this gene have been identified to encompass known MerA diversity; these phylotypes are closely related to others found in the Antarctic bacteria Polaromonas vacuolata and psychrophilic Alaskan bacteria Sphingopyxis alaskensis.[47] Though merA is unique to the Arctic, the similar hgcA gene is associated with microbes in anoxic environments in other parts of the world. HgcA however methylates Hg to produce MeHg - the opposite process that MerA conducts.[54]

Snowpack chemistry influence

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Snowpack chemistry affects microbial community structure in the Arctic.[48] This means that how the snow and ice are positioned helps determine which microbes live there - including the microbes that have the merA gene. Snow is a strong reservoir to entrap many small molecules, including everything from nutrients to potentially harmful chemicals such as Hg.[55] [56] Chemical parameters with the most influence microbial community structure are (1) bioavailable mercury, (2)  MeHg, (3) pH and organic acids, and (4) ions. This indicates that the way snow and ice particles are arranged influences the amount of bioavailable Hg there is for microbes with merA to convert, therefore decreasing the amount MeHg in the system, which allows other microbes lacking merA to flourish. This diverse microbial community may then conduct processes that produce chemicals to affect factors (3) and (4). The majority of processes conducted by communities following this trend are aerobic and facultative, followed by anaerobic. The most common microbes seen in these communities are Alphaproteobacteria, Gammaproteobacteria, Bacteroidetes, Actinobacteria, and Firmicutes.[57]

Biomagnification and transport of methylmercury (MeHg) across the Arctic marine food web. Concentrations of MeHg increase with trophic level.

Bioaccumulation and biomagnification

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Methylmercury (MeHg) in the Arctic marine food web

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Of all the different mercury compounds that are transported into the Arctic ecosystem from both organic and anthropogenic sources, MeHg is an especially concerning high-profile metal.[2][9] It is considered to be a potent neurotoxin that bioaccumulates as it travels through each increasing trophic level of the Arctic marine food web resulting in adverse biological effects.[2][58][59] Unlike other toxins or contaminants that enter the biosphere, MeHg is not processed through the body and released back into the ecosystem. It biomagnifies and increases in concentration over time, commonly by ingestion through predator-prey relationships, where top apex predators face the highest risk of exposure.[9][58]

Species in higher trophic levels tend have greater dietary and energetic requirements as well as longer lifespans therefore, facilitating an increase in MeHg trophic transfer.[60] Once an organism is exposed, the body stores MeHg at a faster rate than it can be processed or excreted where it is then absorbed into the gastrointestinal tract, liver, tissues, and crosses the blood-brain barrier.[58][3] MeHg is so readily taken up and stored, in orders of magnitude, because it binds to the sulfhydryl groups of amino acids, which are key components of protein formation.[58] While fauna in upper trophic levels exhibit the highest concentrations of MeHg, the initial exposure starts with its uptake by phytoplankton, algae, and bacteria in the water column, usually through diffusion or active transport, to which it can then be transmitted across marine food webs.[59][3]  

Biomagnification across trophic biota

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The Arctic marine food web is composed of seven functional groups spanning through five distinct trophic levels including primary producers, invertebrates, bony fishes, seabirds, marine mammals, and humans.[9] To understand how MeHg biomagnifies as it transfers up the food web, it is important to take into account seasonal variation, environmental and physical factors, as well as ecological constraints such as trophic dynamics, community structure, planktonic growth rates, feeding behavior, and organismal life-history characteristics.[2][9][60] Similarly, biogeochemical factors will strongly influence the MeHg reservoir available for both passive and active uptake in lower trophic levels.[9][59] For example, dissolved inorganic carbon (DIC) will strongly influence transmission such that when DIC levels are low, there is an increase in the bioavailability of MeHg which will allow for easier diffusion and transport through the cell membrane of grazing phytoplankton.[3][60]

While phytoplankton introduce and initiate transfer into the marine food web, trophic models evaluating MeHg transfer from both primary consumers, including smaller herbivorous zooplankton and secondary consumers such as larger carnivorous zooplankton, have suggested that biomagnification is most prominent with the latter of the two size categories.[3][60] Trophic MeHg dilution occurs with small herbivorous zooplankton, given that their low grazing fluxes and higher elimination rates inhibit them from accumulating more MeHg than they are receiving from their prey. Conversely, larger zooplankton show elevated magnification rates due to increased grazing which leads to increased lifespan and greater predation potential.[60]

The introduction of MeHg at the lowest levels of the food web creates a gateway for biological uptake into higher trophic species.[2][9] Research pertaining to foraging behavior of Arctic beluga whale (Delphinapterus leucas) populations has further outlined the distribution of MeHg among different species of bony fishes as direct impact of dietary exposure.[3]  During the summer the Beaufort Sea beluga whales sexually segregate into three distinct habitat groups, each with different ranges and variation in foraging preferences, which yields different energetic requirements per group[9][3]. Belugas that reside in shallow estuarine habitats feeding on species of coastal fishes such as Pacific herring, cisco, Saffron cod, and Arctic cod exhibited the lowest concentration of MeHg, followed by belugas who foraged in offshore habitats near the ice edge[3][9][61]. Lastly, belugas foraging on benthic amphipods, flounder, and sculpin in deep water surrounded by heavy ice cover exhibited the highest concentration[3][61].

Through the sampling of eggs and muscle tissue biomagnification of MeHg has also been exhibited in multiple species of marine seabirds including kittiwakes, murres, eiders, fulmars, and gulls where the concentration varies between species as a result of migration patterns, dietary preference, and habitat distribution.[9][58] Seabird eggs can often display high enough concentrations of MeHg that results in declining reproductive success as well as reduced hatchability and clutch size.[3] Furthermore, numerous studies analyzing both current and archived historic tissues (hair, teeth, and feathers) of Artic species and humans, some of which date back as far as 800 years, indicates there has been a ten-fold increase in mercury concentration since the mid-to-late 19th century; which suggests an average rate of increase of approximately 1% to 4% per year.[2][3][9]

While increasing MeHg concentration in Arctic marine biota has been established to be occurring over long time scales, approximately 90% of current contamination in higher trophic Arctic animals is of anthropogenic origin.[9][3] Current data shows that multiple species of seabirds and marine mammals such as polar bears (Ursus maritimus), pilot whales (Globicephala melas), hooded seals (Cystophora cristata), ringed seals (Pusa hispida), and belugas have concentrations of MeHg in their tissues that exceed the toxicological threshold which contribute to neurochemical effects, kidney, and liver damage.[3] Conversely, one of the only Arctic species to not exhibit significant increasing levels of MeHg compared to historic trends are walruses (Odobenus rosmarus) given the fact their diet primarily consists of clams and bivalves, which tend to exhibit low mercury levels.[2]

Measuring toxin transport

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In the ocean, the concentration of MeHg is in the femtomolar range, which makes detection and measurement of the toxin in seawater quite taxing because concentrations are almost undetectable, and yet are still high enough to ignite bioaccumulation through consumption.[62][63] In order to accurately measure MeHg in seawater, contamination through collection, analysis, and storage procedures must be avoided. Water samples are often stored in either glass or teflon bottles, which undergo a rigorous cleaning, commonly with a hot acid bath, before they are used.[64][65] Alongside specific handling of samples, techniques used to measure aqueous MeHg includes cold vapor atomic fluorescence spectrometry, inductively coupled plasma mass spectrometry, and direct ethylation derivitization.[64][63]

Evaluating the concentration of MeHg across trophic levels in the Arctic poses further challenges due to other factors such as remoteness, accessibility, sample sizes, and lack of resources.[2] Research has identified relationships between stable isotope ratios and signatures in relation to mercury concentration in animal tissues.[2][66] An animal’s relative trophic status and feeding patterns over short time scales can be defined by measuring the abundance of stable isotopes of elements found in the food web, especially carbon (δ13C) and nitrogen (δ13N), which can infer contaminant transfer between predator and prey.[2][66] Since trophic interactions among species are a key defining factor influencing the degree of bioaccumulation and biomagnification of MeHg, stable isotope measurements are considered an integrative way to interpret contaminant trends of marine food webs.[2][66]

Another way mercury is identified across the food chain is through measuring its molar ratio to corresponding selenium (Se) concentrations among seabirds and mammals.[58] Se is a homeostatically regulated trace element that exhibits degrees of variability and is also considered highly toxic.[67] While not conclusive and no exact protective molar ratio between the two elements has been established,[67] it is proposed that Se counteracts the toxic effects of mercury by synthesizing metal binding proteins where mercury is then bound as an insoluble Se compound.[58]  In order to be used as a viable risk management and communication tool for the general public, more research is needed to identify if a protective linear relationship of Se:Hg exists as well as how this ratio reaches different thresholds across multiple species and populations.[67] Identifying global sources and sinks of MeHg is not only an environmental issue, but more importantly it is a human health issue.[9] Adverse effects of MeHg biomagnification among higher tropic mammals include changes to central nervous, reproductive, cardiovascular, and immune system functions.[58]  This is especially important in regards to indigenous cultures that make up Arctic communities.[3] Their native reliance on fish as well as large terrestrial and marine mammals as part of a cultural subsistence lifestyle adds a human health dimension such that these food sources are pathways for MeHg exposure.[9][59]

Impacts on Arctic Indigenous peoples

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Methylmercury exposure through traditional diet

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Since the mid 1990’s, the concern of health side effects from methylmercury consumption in the Arctic has been rising. [68][69] The populations most at risk are Arctic Indigenous peoples inhabiting the Arctic from the following locations: Canada, Russia, Denmark, and the United States.[70] Mercury exposure affects Canada’s Arctic Indigenous peoples through their cultural subsistence lifestyle that is highly reliant on marine animals[71]. The traditional diet of the Northern Inuit tribes include top marine mammal predators like beluga, narwhal, and seals, all of which are major sources of methylmercury because of bioaccumulation. [72]

In the ocean, inorganic mercury converts to MeHg, which is its toxic organic state. It then enters the food web, biomagnifies, and is transferred up the food chain, successively increasing in concentration through each trophic level. [73] The Biomagnification of methylmercury affects subsistence items including: fish, mammals, seabirds, and vegetation.

Western development has increased the contamination of mercury, and it is seen as a lack of respect by Indigenous peoples.  In northern Quebec, 40% of the fish, 32% of ocean mammals, and 64% of terrestrial mammals exceeded the guidelines mercury level of 0.5μ/g. [74]

Effects of methylmercury consumption

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Once ingested, 90% of MeHg is absorbed throughout the gastrointestinal tract, which can induce damage to the central nervous system that can be severe and irreversible.[75] Side effects can include: muscle weakness, vision changes, numbness in extremities, birth defects, and in severe cases, death.[75]

Methylmercury exposure in early childhood presents risk to their development[76] For children born in these Arctic tribes, 95% of their methylmercury intake comes from their dependence of marine mammals such as beluga muktuk, narwhal muktuk, ringed seal liver, fish, caribou meat and ringed seal meat.[76]Boucher et al.[71] studied a cohort of Native Arctic children since birth for 20 years and found above the Canadian guidance value in 17% of children. Of those 17% of children's, there were adverse effects on recognition memory.

Diet transitions

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Mercury levels are rising in high protein foods that were once a traditional part of Arctic Indigenous peoples culture. Lye et al.[77], found that 53.3% of Indigenous women aged 18-39 in northern Quebec had values above the guidance value. Arctic Indigenous peoples are now replacing their high protein foods with high carb foods. [76] The process that occurs when the traditional and local diet of Native Americans is replaced with a ‘western’ store bought diet is called the nutrition transition. The store bought food often holds high contents of refined carbohydrates and saturated fats, but low in the nutrients, vitamins and essential unsaturated fats that Native American bodies need and are used to. This can lead to obesity and related diseases among native communities[78][79] as the traditional nutrient-rich organ meats are not available at grocery stores.[80]

Science communication

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As methylmercury consumption is a fairly new discovery, scientists at first thought that the spike in mercury was due to Minamata disease. An example of this can be seen from the 1970’s when the Salluit Tribe in Salluit, Quebec, residents reported high levels of mercury in their blood. The media misinformed the public and referred to their high mercury levels as Minamata disease. As a result of fear from the Arctic Indigenous community, the Salluit Tribe cut out their country diet, and they saw a drop in mercury levels but an increase in obesity and related diseases. While mercury levels dropped, this incident showed how misleading information can make it harder for scientists to pinpoint where the high mercury levels were coming from and how transitioning to an new diet a detrimental option for Arctic Indigenous peoples[76].  

See also

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References

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  2. ^ a b c d e f g h i j k l Dietz, Rune; Outridge, Peter M.; Hobson, Keith A. (2009-12-01). "Anthropogenic contributions to mercury levels in present-day Arctic animals--a review". The Science of the Total Environment. 407 (24): 6120–6131. doi:10.1016/j.scitotenv.2009.08.036. ISSN 1879-1026. PMID 19781740.
  3. ^ a b c d e f g h i j k l m n Lehnherr, Igor (2014). "Methylmercury biogeochemistry: a review with special reference to Arctic aquatic ecosystems". Environmental Reviews. 22(3): 1–15. doi:10.1139/er-2013-0059.
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  5. ^ a b c d Broussard, Larry A.; Hammett-Stabler, Catherine A.; Winecker, Ruth E.; Ropero-Miller, Jeri D. (2002-08-01). "The Toxicology of Mercury". Laboratory Medicine. 33 (8): 614–625. doi:10.1309/5HY1-V3NE-2LFL-P9MT. ISSN 0007-5027.
  6. ^ "Basic Information about Mercury". EPA. United States Environmental Protection Agency.
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