Nearly 60 years ago a mysterious illness swept through families in fishing villages along Japan’s Minamata Bay. Those affected presented symptoms of a neurological disease: loss of feeling in the limbs, impaired vision, difficulty walking, and trouble speaking. Many with severe symptoms became paralyzed, and dozens died within weeks. Then, newborns in the region began to exhibit neurological disorders and crippling birth defects. The culprit of this public health disaster was methylmercury, a toxic form of the heavy metal mercury that made its way from a nearby fertilizer plant into the community’s fish-based food supply [1].

According to two new reports, mercury levels have risen greatly in the environment – they have tripled in surface ocean waters since the industrial revolution, and human activities such as mining, coal burning and other industrial processes are to blame [2,3]. Do the increasing mercury levels pose an immediate threat to human health, or do they simply foreshadow the possibility of a public health disaster if we don’t take action to curb mercury pollution? To understand the implications of the marine mercury increase, it is important to know exactly how mercury affects human health and how it moves through the environment.

Mercury is a toxic metal that exists naturally in multiple forms, each having different implications on human health. The major form of mercury involved in current human exposure and toxicity (through consumption of certain seafood) is methylmercury. Upon entering the body, methylmercury binds to the amino acid cysteine and is subsequently absorbed by the intestine where it enters the bloodstream and is dispersed to various organs. Once inside a cell, the compound elicits its toxic effect by interacting with sulfur containing chemical groups, many of which are involved in a cell’s antioxidant response. Methylmercury is therefore thought to interfere with a cell’s oxidative balance, ultimately causing oxidative stress and leading to reduced cell function and cell death. Methylmercury even has the ability to cross the blood-brain barrier where it can elicit neurotoxic effects, or enter the placenta where it can cause a variety of developmental defects in the fetus. [4,5].

Figure 1 ~ Estimated percentage contributions to global mercury emissions. The majority of mercury found at the ocean’s surface (newly deposited) is hypothesized to be mainly caused by humans since the industrial revolution, whereas deep ocean mercury (older deposits) is not as effected by humans. Adapted and based on modeling work from Amos et al 2013 [10]. 

Mercury that makes its way into the ocean has both natural and anthropogenic (human-caused) sources. Natural mercury emissions arise from things like volcanoes, forest fires, and erosion of the Earth’s crust [6]. These sources contribute significantly to global levels, but based on numerous modeling studies and experimental evidence it is thought that the anthropogenic sources are the leading cause in the rapid increase of oceanic mercury since the industrial revolution (Figure 1). Currently, gold mining is considered to be the greatest source of anthropogenic mercury. Small-scale mining operations around the world use mercury, which acts like a sponge, to gather up small, hard-to-collect gold particles. Once the gold has been collected, the mercury is burned off and released into the atmosphere, leaving the gold behind [7]. A second major source of mercury emissions is coal-burning power plants, which release mercury as a byproduct that is contained naturally in coal [5]. The mercury from these and other industrial sources can remain in the atmosphere for months until it is oxidized and deposited into the ocean. Because atmospheric mercury is stable for such long periods of time, emitted mercury can travel many miles from a particular location and pollute regions far from the original source – leaving no part of the land or ocean untouched.

But how do oceanic mercury levels pose a public health threat to humans? It’s all about who eats who in the ocean’s food chain. After atmospheric mercury is deposited in the ocean it accumulates and is captured by organic particles, eventually sinking to the ocean’s depths. Marine bacteria can process the accumulated mercury into methylmercury, the highly toxic form of mercury which created the disaster in Minamata Bay. Methylmercury enters the food chain when it is consumed by small marine creatures like plankton. In turn, these are eaten by other marine organisms which are then consumed by larger organisms that then are consumed by even larger creatures…you get the point. As the organic mercury is passed along in each round of predation, it becomes concentrated at higher and higher levels in a process called bioaccumulation. Eventually, mercury concentrations reach their peak amounts in the animals at the top of the marine food chain such as tuna, swordfish, and sharks [5]. These animals can have concentrations of mercury that are millions of times greater than that of the ocean waters in which they live. It is by consuming these top predators that humans gain exposure to mercury [1].

Because oceanic mercury levels are so much lower than those found in the top marine predators, the quest for scientists to monitor how levels have changed over time and how humans have contributed to the process has proven time-consuming and painstaking. A new comprehensive study now provides the first global measurements of oceanic mercury content. The scientists behind the study didn’t just collect samples from the surface of the ocean but at various depths and locations in the Pacific, Atlantic, Southern and Arctic Oceans. This strategy allowed the authors to get a sense of how mercury levels have changed over time in the ocean since the water near the surface had more recently been exposed to pollution by humans compared to the deeper waters [3,8].

By comparing the concentrations of mercury with those of the well-studied anthropogenic emissions of the greenhouse gas carbon dioxide (CO2), the team was able to estimate the amount of anthropogenic mercury. The results suggest that mercury levels in the shallow parts of the ocean have more than tripled since the industrial revolution, and that this increase was directly caused by humans. Additionally, the researchers found that more mercury has concentrated in the Arctic and North Atlantic than in the other regions, meaning that the marine life and human populations living in this region could be more susceptible to its adverse effects [3].

So are the effects of this mercurial increase greatly impacting us right now? Probably not. But even though there may be no immediate threat to humans, it is important to consider the consequences of what may happen within the next century. The authors involved in the oceanic study warn that if anthropogenic mercury emissions remain apace, the deep waters may lose their ability to contain excess mercury. This could lead to a more rapid accumulation in the shallow waters and an increase in mercury within the marine food sources, ultimately resulting in an increased human exposure and negative health consequences.

But though this threat should be taken seriously, the results of the new studies do give hope. The fact that Earth’s oceans aren’t uniformly contaminated indicates the anthropogenic effects may be reversible or curbed if proper precautions are taken [8]. Furthermore, although the total amount of anthropogenic mercury emissions since the industrial revolution is more than double what was previously believed, the total emissions have slowed since the 1970s, suggesting that regulating emissions can have an effect (Figure 2, [2]).

Figure 2 ~ Estimated global mercury release. A new study suggests that anthropogenic mercury emissions since the industrial revolution are more than twice as much as previously calculated. Adapted from Horowitz et al 2014 [2].

While it is still unclear how exactly the levels of inorganic mercury (those measured in the study) will affect the levels of methylmercury, steps are already being made to prevent public health disasters like Minamata from occurring. In October of 2013, the Minamata Convention on Mercury was drafted and has now been signed by over 100 nations. The document not only aims to curb global emissions of mercury, but also calls for the signing nations to invest in researching mercury release and movement through the environment [9]. The treaty doesn’t necessarily pave a clear path as to how to accomplish such tasks, but it is a critical step in depolluting the oceans.

In the near future, it will be important to increase our understanding of the mechanisms by which the increased levels of mercury correlate with levels of methylmercury – the type that bioaccumulates in top marine predators. This information will allow us to determine how profound an impact the increase in marine mercury will have on human health. Along these same lines, we will need to develop a better understanding of the mechanisms by which mercury elicits its toxic effects in humans and other animals.

So for now, don’t stop eating your favorite ocean fish, but be aware that if marine mercury levels continue to rise a tuna salad may no longer be on the menu.

Tyler Huycke is a PhD candidate in the Biological and Biomedical Sciences Program at Harvard Medical School.

[1] Normile, D. (2013). “In Minamata, Mercury Still Divides.” Science 341(6153): 1446-1447.\

[2] Horowitz, H. M., et al. (2014). “Historical mercury releases from commercial products: global environmental implications.” Environ Sci Technol 48(17): 10242-10250.

[3] Lamborg, C. H., et al. (2014). “A global ocean inventory of anthropogenic mercury based on water column measurements.” Nature 512(7512): 65-68.

[4] Fretham, S. J. B., et al. (2012). “Mechanisms and modifiers of methylmercury-induced neurotoxicity.” Toxicology Research 1(1): 32-38.

[5] Bernhoft, R. A. (2012). “Mercury toxicity and treatment: a review of the literature.” J Environ Public Health 2012: 460508.

[6] Mercury Emissions: The Global Context. Environmental Protection Agency.

[7] Wade, L. (2013). “Gold’s Dark Side.” Science 341(6153): 1448-1449.

[8] Casselman, A. (2014). Humans have tripled mercury levels in upper ocean. Nature News.

[9] Lubick, N. and D. Malakoff (2013). “With Pact’s Completion, the Real Work Begins.” Science 341(6153): 1443-1445.

[10] Amos, H. M., et al. (2013). “Legacy impacts of all-time anthropogenic emissions on the global mercury cycle.” Global Biogeochemical Cycles 27(2): 410-421.

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