Introduction Mercury, a heavy metal element which is mostly only thought to be in thermometers, has many other uses. Mercury has very beneficial properties which make it versatile in the manufacturing of products and, therefore valuable to our economy. It is found in thermometers, in light bulbs, and even used by dentists. However, the toxicity of mercury in humans and other mammals can be quite serious. Every year thousands of people unknowingly exposed to mercury through inahlation of particulates as well as the consumption of fish and seafood. But how does mercury get into the fish in the first place? The main source of atmospheric mercury comes from the burning of coal for energy. There are many stages in the transfer of mercury, from combustion or other power plants, to the bioaccumulation in fish and finally ingested by wildlife and humans. The focus of this project is on methylmercury, a structurally and chemically different species of mercury which is poorly understood, but thought to be the most neurotoxic form of mercury. The transformation process from inorganic mercury to methylmercury occurs from sulfate-reducing bacteria (SRB) in the sediments of waterways. Once in the water, the cycle of bioaccumulation in fish begins. These series of events are described here as they relate to risks of fish consumption. The effects of mercury toxins on the brain, and other areas of the central nervous system, are discussed in detail. As well as one of the first reports of mercury poisoning which occured in Minamata Bay, Japan. |
Mercury in Fish: What, When, Why, Where and How?
By Amy Hanna Mercury, what is it? Mercury, we have all heard of it; in high school chemistry, they show you some in a small bottle and tell you really bad things will happen if you touch it. This is especially horrific in the event that a thermometer breaks in the house (depending on your age, medical thermometers once contained mercury...AND, they were glass!!). Thermometer breakage would cause my mom to act as if there were a nuclear meltdown in the bathroom. "You kids stay out of here until I get this cleaned up, and don’t touch it”. I can still hear the panic in her voice. It always made me wonder, why is mercury so dangerous? Mercury, abbreviated Hg on the Periodic Table of Elements, is the heaviest element in liquid state. Moreover, it is the only metal element that exists in liquid phase at ordinary temperatures. Interestingly, possessing a property comparable energy, mercury can neither be created nor destroyed (AE1). When does Hg enter the atmosphere and waterways? Hg is an element that is found naturally in the Earth's crust. Natural causes permit some mercury to enter into the environment, £ 25% (AE5), "e.g. volcanic eruptions, fires, surface emissions" (AE1). However, the majority, ≥75%, is from industries; much of which is due to man’s seemingly endless need for mass consumption. Sadly, it is this surrender to a "one-more-mall-won't-hurt” mentality that is leading the way down an environmental path of uncertainty. "Over a decade, mercury pollution has ballooned from an obscure concern into a pressing environmental problem” (AE4).
There are many uses for mercury in the manufacturing of products, from electronic parts to anti-fungal wood preservatives (AE5). Today, industrial wastes are the largest contributor of inorganic mercury that is released into the atmosphere. Mercury then returns to earth via precipitation, but is also deposited during dry conditions (AE1). Much of the mercury that returns to the Earth finds its’ way into the rivers, lakes, streams and the oceans. Mercury contamination levels of marine ecosystems are rising by 4.8% per year; and, fish would need half a century to recover (e.g. for Hg levels in the fish to drop) if the problem were annihilated today (AE5). Why is it toxic? Mercury exists in several different chemical forms. These different forms have a major impact on the levels of toxicity, which occur in various species. It is still unclear how mercury affects fish and animal tissue (A3). By identifying the form of mercury that exists in fish, we may advance our understanding of its toxic reactions. The species of mercury, which arises in fish, is aqueous methylmercury chloride, or CH3HgCl, is frequently represented incorrectly as CH3Hg+ (A3). Harris, et. al., indicates that since the H-Cl bond is extremely covalent, it does not disassociate in aqueous solution; thus, bioaccumulation occurs in “top-predator” fish. The toxicity of CH3HgCl seems to be linked to its hydrophobic characteristics that increase its affinity to cross the cell membrane, more than other forms of Hg (A3). However, more studies are required to determine conclusive evidence of the effects of mercury bioaccumulation in fish. |