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.

What changes mercury into methylmercury?

The problem begins in the sediment of waterways. The mercury settles into the sediment where sulfate-reducing bacteria (SRB) transform it into methylmercury (CH₃HgCl). From the many different genera of SRB, it is those that possess the ability to utilize acetate that methylate mercury faster than SRB that lack acetate utilization (A5). It then is released from the sediment into the water.

 

(Click on image to enlarge)

 

picture from  http://www.aboutseafood.com/

Where does bioaccumulation occur? Phytoplankton and other bacteria and small organisms become contaminated with mercury. As mercury travels throughout the food chain, it builds up in the tissues of fish that are predators of other, smaller fish.  This is the reason that public health warnings, regarding fish consumption, refer to larger fish, especially the sport-fish such as salmon, swordfish and tuna.

 

		How does it get into humans? By ingesting fish that contain mercury, we are exposing ourselves to potential health risks. The Food and Drug Administration FDA has issued warnings regarding the amount of fish that is safe to eat and the frequency of eating fish in relation to one’s body weight (AE14). Although mercury can harm to the environment and all species that live in it, the FDA has set those warning levels at extremely low amounts. Fish is still considered safe to eat in moderation. However, countries where fish is a staple diet food, people experience much higher health risks (AE7).

 

Once mercury is in the blood, it can be carried to the brain. Organic mercury easily crosses the blood brain barrier to truly reach the mass of neurons. To cross the blood brain barrier, it is hypothesized that it mimics methionine and is then carried across the barrier (J3,J4). There it can be demethylated (J2) into inorganic mercuric Hg. In the brain, the visual cortex and cerebral granular cells are particularly sensitive to MeHg (J5). Once there, it is unknown what truly causes the many symptoms of mercury toxicity, but many of mercury’s effects are suspected.