Seafood And Mercury Contamination – A Deep Dive

Sep 15, 2022 | Nutrition, General, Natural World | 0 comments

A bluefin tuna fish and his friend swimming in the ocean.

Seafood, Eat Food

Fish and seafoods have been eaten by humans for around 40,000 years. They are great dietary sources of omega 3 fatty acids, and oily fish constitute the majority of food sources rich in vitamin D. These factors mean that seafood provides significant health benefits, including a reduced risk of cardiovascular disease, and improved developmental outcomes in children. [1]

These many plus sides to seafood are unfortunately mitigated by the fact that ocean pollution has huge implications in relation to the food we consume. One of the most widely discussed and well known contaminants in seafood is mercury, a heavy metal element that is a liquid at room temperature. Fish and seafood constitutes the most significant dietary source of mercury exposure for most people. [2]

As a natural element, mercury has a long record in history of human interaction. In ancient Greece, the element was given the name ‘hydrargyros’, directly translating to ‘water-silver’. The Roman’s latinised this name to ‘Hydrargyrum’, and the legacy of this can still be seen today in mercury’s elemental symbol ‘Hg‘.

A photo of liquid elemental mercury being poured in to a glass dish.
Ingestion of Mercury Is Associated With A Wide Range Of Damaging Effects

Whilst the mesmerising shimmer of liquid mercury has long captivated the human imagination, it remains nonetheless, toxic to humans. This toxicity has been revealed in events such as hat makers becoming as ‘mad as a hatter‘ in 18th and 19th century England, due to the felt manufacturing process exposing the workers to high volumes of mercury. In 1956, the town of Minamata in Japan saw outbreaks of so-called ‘Minamata disease‘ – a horrifying large scale case of mercury poisoning caused by a chemical plant releasing large amounts of mercury in to Minamata Bay.

It is clear that as humans, we ideally want to keep mercury out of our bodies to maintain our health. Why is mercury so toxic? Where does the mercury in fish come from? In this article, we will answer the questions and discuss potential solutions that can help us eat seafood safely. Firstly, let’s take a look at which fish and seafood species contain the most mercury.

Mercury Levels In Commonly Eaten Seafoods

Using FDA data acquired over the years of 1990 to 2012, we can chart the average mercury levels found in many commonly eaten seafoods. These values are in parts per million (PPM).

Seafood SpeciesAverage Mercury Concentration (PPM)
Swordfish0.995
Bigeye Tuna0.689
Marlin0.485
Mackerel (South Atlantic)0.182
Common Tuna (Fresh)0.144
Common Tuna (Canned)0.126
Cod0.111
Crab0.065
Haddock (Atlantic)0.055
Mackerel (North Atlantic)0.05
Squid0.024
Salmon (Fresh)0.022
Anchovies0.016
Sardines0.013
Oysters0.012
Clams0.009

Source: [3]

These values can fluctuate in reality, but these averages provide a useful guide. The sea that the fish are caught in also plays a large role in determining the final levels of mercury in seafood, and fish caught in the Atlantic have been found to have higher mercury levels than fish caught in closed seas such as the Mediterranean and Black Sea. [4]

Cooking also tends to decrease bioaccessibility of mercury from seafood. More intense cooking processes such as grilling and frying result in mercury becoming less bioaccessible than in raw or steamed seafood. Depending on the seafood in question, cooking methods can reduce mercury bioaccessibility by up to 96%. [5] Indeed, cooking seafood well is one of the key points on our Seafood Safety Checklist that we provide later in this article.

Whilst not commonly eaten in many countries, whale and dolphin tissues have been found to have extremely high levels of mercury, and consumption of pilot whales has been linked to numerous health and developmental issues for some people in the Faroe Islands. [6]

Samples taken in Japan by the Environmental Investigation Agency revealed that pilot whale meat can exhibit staggeringly high mercury levels of 19 ppm. Dolphin meat was also found to have extremely high mercury levels of 11ppm. [7] It is not advisable to consume any species of whale, dolphin, or shark for this reason.

How Does Mercury Get In To Seafood?

Mercury is a highly reactive heavy metal that is rarely found as a free element in the natural world. Estimates are that mercury is found at a low quantity of 0.08 parts per million in the Earth’s crust, with coal, cinnabar, and the mineral sphalerite all being sources of elemental mercury. Plant growth, erosion of certain minerals, and geological activity such as volcanic eruptions and underwater geothermal vents are all natural sources of oceanic mercury. [8]

A photo of an industrial plant near a water source.
Heavy Industry Including Coal Burning Power Stations Release Mercury In To The Atmosphere

In the modern world, elemental mercury is used for chlorine production, dental amalgams, thermometers, and batteries. Due to it’s natural presence in coal, mercury is emitted from coal burning electric power plants, whilst mining industries such as gold mining also release mercury vapours. Coal burning and gold mining contribute approximately 2220 tonnes of mercury in to the atmosphere each year, and these mercury vapours eventually fall in rain. This mercury laden rainfall allows mercury to travel vast distances from it’s initial source of origin. [9] [10]

Unsurprisingly, mercury has been steadily increasing in volume in the oceans since the start of heavy industrial practices. Indeed, current ocean mercury concentrations are around 250% higher than pre-industrial levels, with around 60,000 to 80,000 tonnes of mercury thought to be in the world’s oceans today. The marine research team GEOTRACES found that two thirds of oceanic mercury is concentrated in the upper 100 meters of water. It is in these shallow waters that most fish live. [11]

Despite this, mercury concentrations in samples of seawater are actually relatively low when compared with the organs and tissues of fish and marine life. A gulp of seawater during an ocean swim is not going provide a toxic dose of mercury. Different oceans have different levels of mercury, but an average of volume of 1.5 picomoles (0.6 nanograms per millilitre) was noted across oceans in a 2002 survey. [12] Despite this seemingly low volume, many marine species exhibit very large quantities of mercury in their systems.

This of course raises an important question; If the seawater the fish live in only contains diffuse volumes of mercury, how does this lead to large and toxic amounts accumulating in the species that live in it? To answer this question, we have to look at how mercury accumulates in organisms in a process called biomagnification.

Methylmercury And Biomagnification

Mercury’s high reactivity means it readily forms compounds when it reacts with other substances, creating methylmercury or inorganic mercury salts. Methylmercury is the most commonly found organic mercury compound, and is formed when bacteria react with elemental mercury in water or soil. In the oceans, aquatic bacteria are responsible for this conversion via a biochemical process called methylation. Methylmercury is then able to be hosted in plankton, the mix of free floating organisms that drift in currents. Plankton species include bacteria, archaea, algae, protozoa, and some micro-organisms. Plankton forms much of the basis of ocean food webs, and is eaten by fish, whales, and even bivalvia species such as clams and mussels. [13]

A digital microscope photo of an oceanic plankton species capable of hosting methylmercury.
Oceanic Plankton Species Introduce Methylmercury To Biological Food Webs

Methylmercury takes a long time to be removed from the body, and as such, remains in tissues and organs of marine species for considerable lengths of time. As methylmercury works it’s way up the food chain from plankton to large predatory species, it becomes more and more concentrated in the larger, longer lived animals.

This process is called biomagnification, and is the reason that large predatory species can exhibit levels of mercury up to ten times higher than their prey species. Ocean predators high up in the food chain such as swordfish, sharks, and marlins contain the highest mercury levels of all seafood. Even non-aquatic species that regularly eat fish such as walruses and predatory birds also exhibit high levels of mercury in their tissues upon analysis.

This process of biomagnification makes methylmercury a particularly dangerous pollutant, as even small amounts of it in an ecosystem can develop in to highly toxic concentrations in living organisms. This is also the reason why pilot whales, with their large size and a lifespan of 45-60 years, can exhibit dangerously high levels of methylmercury. Indeed, consumption of whale and dolphin has been linked to many instances of methylmercury poisoning.

Human Ingestion of Mercury And Methylmercury

If elemental mercury is ingested by a human, it is oxidised by the enzyme catalase. This oxidation creates a compound called mercuric ion, which has been shown to damage DNA functions in human cells, as well as causing kidney damage. [14] [15]

Fortunately, elemental mercury in liquid form is poorly absorbed in the gastrointestinal tract, with less than 0.1% being absorbed by humans. Furthermore, mercuric ion does not readily cross some tissue barriers, which helps mitigate the extent of cellular damage. The half-life of elemental mercury in tissues is approximately 60 days. The kidneys play a key role in dealing with mercury, and help to remove it from the body through urine. It is thought that around 1% of the body’s mercury load can be excreted in urine each day. [9] [16] [17]

A photo of two fishing boats at sea catching seafood in large nets.
Seafood Constitutes The Most Significant Dietary Source Of Mercury Exposure For Most People

As we discussed earlier in this article, aquatic bacteria species enable the methylation of elemental mercury, converting it in in to methylmercury. Roughly half of the total mercury content found in fish and whales is in the form of methylmercury. [18] [19]

Methylmercury is significantly more toxic than elemental mercury. This is because methylmercury is readily absorbed in the gastrointestinal tract, with measured absorption rates as high as 90%. As such, methylmercury in fish constitutes the most significant dietary source of mercury exposure for most people. [2] [17]

Once methylmercury is absorbed, it enters the bloodstream and can travel around where it is able to deposit itself in many of the organs. Significant exposure to methylmercury can cause widespread damage in the body, including to the brain, heart, kidneys, lungs, and immune system. This process also occurs in aquatic species too, and the major organs of fish display higher levels of mercury than other bodily tissues such as the muscles.

Methylmercury then begins to create water-soluble complexes in the body, with a special affinity for the amino acid cysteine. Due to the presence of a thiol group in cysteine’s chemical structure, cysteine has a high affinity for the heavy metals copper, zinc, and iron that the body needs and uses for everyday biological processes.

Cysteine’s affinity for heavy metals also applies to the ones we don’t want in the body, including lead, cadmium, and mercury. This affinity results in cysteine being able to fuse with methylmercury, creating a complex appropriately named methylmercury-L-cysteine.

The 2D molecular diagrams of Methylmercury-L-cysteine and L-methionine, showing the similar structure.
Methylmercury-L-cysteine Molecular Structure Compared To L-methionine

It is thought that methylmercury-L-cysteine deceives our natural protein transport pathways, which mistake it for L-methionine due to it’s similar chemical structure. L-methionine is an essential amino acid that is a precursor to cysteine and taurine, and so is regularly transported in to tissues.

This tragic case of molecular mimicry causes the body to actively transport methylmercury-L-cysteine into human tissues. It is thought that the L-type amino acid transporter, abbreviated to LAT1, may be a key mechanism in this process, acting as a vehicle that facilitates transport. Methylmercury is therefore able to bypass many cellular defences, and due to it’s lipophilic, short-chain alkyl structure, it can also cross the blood-brain barrier with relative ease. [17] [20]

Methylmercury is a potent neurotoxin, and can lead to the loss of nerve cells, with particular damage being observable in pockets of the cerebrum and cerebellum. [21] This damage can lead to numbness, tremors, and can cause impairments to vision and memory. Methylmercury takes a long time to excrete, and has a half life in the body of around 70 days. Approximately 90% of bodily methylmercury is excreted in the bile, with the remainder being excreted in urine. [17]

Due to it’s ability to readily cross the placental barrier, pregnant women with high levels of methylmercury circulating in their bloodstream can even expose their child to this neurotoxin in the womb, which can result in nerve damage and developmental problems for the child. Methylmercury can also be transmitted to children through breast milk, and so mothers who breastfeed their children are advised to try and reduce mercury intake during this time. [22] [23] [24]

Is It Safe To Eat Seafood?

It is known that nearly all fish and seafood species contain at least some methylmercury, and it is certain that methylmercury causes significant health problems when ingested in large quantities. What is less well known however, is if we actually absorb all the methylmercury we ingest when eating seafood.

A salmon seafood meal with lime, pesto, and bread.
Back On The Menu? Seafood Can Offer Many Nutritional Benefits

Those who eat fish and seafood regularly may be pleased to know that modern research is suggesting that there are steps we can take to reduce the amount of mercury we absorb through seafood.

“Recent research suggests that assuming all of the ingested Methylmercury is absorbed into systemic circulation may be in error. This may result in inaccurate estimations of exposure and risk, and may hamper our ability to properly balance the potential risks and benefits of seafood consumption.”

Mark A. Bradley, School of Dietetics and Human Nutrition, McGill University. (2017) [5]

Tea drinkers may also be in luck! Black and green teas contains a rich variety of polyphenols (often called tannins), and some of these compounds have shown the potential to reduce mercury absorption when consumed with a meal. In a 2009 study, a green tea extract showed the ability to reduce absorption of mercury from a meal by 82–92%, whilst a black tea extract showed a reduction of 88–91%. [25] It has been established that drinking tea with meals can reduce the absorption of iron, and these results indicate that the polyphenols in tea can play a similar role with mercury and methylmercury.

If tea isn’t your thing, black coffee also has been shown to reduce mercury absorption by 50-60% when consumed alongside seafood. [26]

Regular eaters of canned fish will also be relieved to know that canned tuna has been found to have reasonably low mercury bioaccesibility, even when compared to some cooked fish. It is theorised that the heat from the canning process may play a role in breaking down certain proteins, thus disrupting the reactivity of protein bound mercury. Whilst the exact mechanism is not yet fully understood, it has nonetheless been found that one can of tuna per week represents a very low mercury risk, and is not likely to cause issues. [27]

a photo of sardines in an open can.
The Process Of Canning Fish Seems To Reduce The Bioaccessibility Of Protein Bound Mercury

This is welcome news, as seafood has many health benefits, and eliminating fish from the diet completely in an attempt to avoid mercury may do more harm than good to the average person. Regular fish consumption has shown the ability to support brain health, reduce bone fracture frequency, and improve outcomes in cases of colon cancer. [28]

These health benefits of fish are the reason that both the United States Food and Drug Administration, and the United Kingdom’s National Health Service both recommend a varied diet that includes at least one or two portions of fish each week. In particular, at least one portion of oily fish such as salmon, mackerel, herring, and sardines is advised due to the high values of omega-3 fatty acids and vitamin D in these fish. [29]

Using the information we have provided in this article, we can summarise the key points to develop some helpful guidelines to ensure we consume seafood and fish in a safe manner. Here are our top tips to make sure you are eating seafood intelligently and safely.

Seafood Mercury Safety Checklist

● Avoid regular consumption of marine predators high up in food webs such as marlins, sharks, and swordfish. Fish lower down on marine food webs such as sardines, salmon, and cod are better options if seafood is to be eaten multiple times per week.

● Cooking methods such as grilling and frying can significantly reduce the bioaccessibility of mercury in seafood. The methylmercury in raw or lightly steamed seafood remains much more bioaccessible in comparison.

● Omega-3 fatty acids found in oily fish have been shown to benefit adults and the development of children, so consumption of low mercury seafood species is advisable. Supplementation with vegan omega-3 fatty acids derived from algae may be a suitable alternative if oily fish is not a regular feature in the diet. [29]

● Drinking green tea, black tea, or coffee with a seafood meal has been shown in studies to decrease absorption of mercury.

● Do not take the food supplement L-Cysteine on days when you consume seafood or fish, as this could further facilitate the formation of methylmercury-L-cysteine.

● Pregnant women should limit their seafood and fish ingestion, as methylmercury can readily cross the placental barrier and cause developmental problems for the child. The NHS recomend that expectant mothers do not consume more than 4 cans of Tuna per week. Mothers who breastfeed their children should also reduce seafood intake to lower the risk to the child.

● Children have a lower body weight and size than adults, and so their mercury tolerance is lower than fully grown adults.

Conclusion

Some amount of mercury will always find it’s way in to fish, as it is emitted by natural geological processes, including from underwater hydrothermal vents. Human heavy industry however, also contributes significantly to ocean mercury levels, as rainfall and run-off result in elemental mercury entering in to oceans, rivers, and lakes.

A photo of a radiant setting sun, glowing as it sinks beneath ocean waves.
Reducing Anthropogenic Mercury Emissions Has A Measurable Impact On Ocean Mercury Levels

Whilst we can’t control the amount of mercury that the natural sources produce, we can reduce the human contribution to this issue. Recent research from 2021 indicates that mercury may get ‘stored away’ in deep ocean sediments over time, and this process could eventually lead to mercury levels lowering in the areas of ocean where fish live. Whilst this is possible, it of course relies on a reduction in mercury producing industries such as coal power stations and small scale gold mining. [30]

For now, it is worth remembering that not all sources of seafood contain the same levels of mercury. By selecting species that are known to have lower mercury levels and cooking them well, seafood can form part of a nutritious and balanced diet. A cup of tea or coffee alongside the meal may also help reduce mercury absorption.

Due to the risk of methylmercury ingestion, some individuals may decide to reduce or completely avoid seafood intake. Those on a vegan diet are of course, already avoiding mercury ingestion from seafood.

For both these groups, vegan supplemental omega-3 grown from algae can play a role in providing the beneficial DHA and EPA fatty acids that oily fish would usually offer in the diet without any risk of mercury ingestion. Vitamin D supplementation may also prove useful for those who are not getting a regular intake of dietary vitamin D, as oily fish are among the most vitamin D rich food sources.

By using the table we provided earlier in the article, and following our seafood mercury safety checklist, we hope this information gives you the chance to make more informed decisions about the type and amount of seafood you consume.


[References]

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Written by Keymer Health

September 15, 2022

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