A Look At Methyl-Mercury Contamination From My Guest, Rachel Lund

With all the talk about Mercury contamination of fish, I thought you might love to hear from my beautiful and talented niece, college student, Rachel Lund. She was involved in a research project at her college to look at methyl-mercury contamination in rivers. Here is she is presenting as a guest Blogger.

Mercury contamination in fish and a brief look into methyl-mercury research

Mercury: the only element besides bromine that is a liquid at STP, it looks like liquid silver, and it is 13.5 times denser than water. The Romans named it after their god of speed and mobility. It is second only to lead as a leading cause of heavy metal poisoning. It has historically been used in mining, industry, and medicine. However, because of its toxicity, it has been widely taken out of manufactured products. Mercury is found naturally in the environment in cinnabar ore and as a product of volcanic explosions.⁶ But it is also a very dangerous by-product of burning fossil fuels. So despite the phasing out of mercury in our products, mercury is still being released into our environment. Mercury also does not easily leave the environment once it is there – some isotopes of mercury have a half-life of 444 years!⁷ Mercury has incredibly severe effects on health if exposed to significant amounts of it, including skin rash, fatigue, headaches, disruption of the nervous system, brain damage, miscarriages, birth defects, damage to DNA, damage to chromosomes, and damage to sperm.⁸

Mercury takes several different forms, including elemental mercury, methylmercury, and ethylmercury. Ethylmercury is used as a preservative in some vaccines and poses no health risks. Elemental mercury, the topic of our previous discussion, is very dangerous, as already explained. Methylmercury is one of the most dangerous forms of mercury and is the type of mercury found in fish. Methylmercury is comprised of a mercury atom bonded to a methyl group.⁹ Sulfate-reducing bacteria in the sediment of marine environments “eat” elemental mercury that is released into the environment and produces methylmercury as a by-product.¹⁰ Methylmercury poses the same type of health risks as mercury, but it is more dangerous in that it is bioaccumulative, it undergoes biomagnification, and it mimics cysteine, an essential amino acid in the human body.¹¹ Methylmercury binds to the sulfur in cysteine and due to the strength of the bond, it is impossible to remove from the amino acid once it has been bound. The human body, however, does not recognize any difference and allows the “contaminated” amino acid to cross the blood-brain barrier – as well as cross through the placenta in pregnant women. Once in the brain, methylmercury can wreak havoc on the nervous system and cause serious, permanent damage, as well as death in the most extreme cases. Due to the bioaccumulation and biomagnification of methylmercury, the methylmercury concentrations we ingest when we eat fish become even more magnified in our bodies.

Despite the risk of methylmercury poisoning, fish are still an important part of a healthy, balanced diet. Fish are major sources of long-chain omega-3 fats, vitamin D, and selenium, they are high in protein, and they are low in saturated fat. In fact, studies have shown that eating approximately one to two 3-ounce servings of fatty fish a week reduces the risk of death by heart disease by 36%.¹² However, it is important to keep in mind which fish are safest to eat. Fish on the lower end of the food chain contain the lowest levels of methylmercury and are safe to eat 2-3 times a week – this includes fish such as tilapia, trout, and salmon. Fish in the middle of the food chain are safe to eat once a week – this includes fish such as bass, tuna, and halibut. Fish at the top of the food chain should be eaten very rarely or completely avoided – this includes fish such as shark, swordfish, and marlin. A more complete list of the methylmercury concentrations in fish can be found on the FDA website by following this link:

https://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm115644.htm

Over the summer of 2016, I had the opportunity to conduct environmental chemistry research with my advisor, Dr. Jeff Jeremiason, Associate Professor of Chemistry and Environmental Studies at Gustavus Adolphus College. My research focused on determining whether riparian areas are a significant source of methylmercury to rivers. Riparian areas are simply put, the area immediately surrounding a river. We analyzed water and soil samples from different depths in and around the river to determine the location of the “hotspots” for methylation – the transformation of elemental mercury into the methylmercury. More information on this project and Dr. Jeremiason’s other projects can be found on his website: https://jeffjeremiason.com, which I designed as a subset of my work with him. I spent the summer of 2016 in the laboratory preparing samples for analysis, in the field collecting samples around Minnesota and helping to interpret data.

Above: A picture of me after collecting soil samples from the bottom of a lake. We fit three people and lots of expensive equipment into a canoe and went “fishing” for dirt.

Above right: A picture of me collecting soil samples from the bottom of a river. It was too deep to be able to bend over to collect the samples without full submerging, so I got to go swimming!

Above: Some of our instrumentation for analyzing samples for mercury and methylmercury.

Above: A picture of me presenting my research project at the Midstates Consortium for Math and Science Undergraduate Research Symposium in the Physical Sciences, Math, and Computer Science at Washington University in St. Louis.

Notes:

¹ Mercury’s atomic symbol comes from its Latin name hydrargyrum, which translates to “liquid silver.”

² Atomic numbers indicate how many protons (positively-charged sub-atomic particles responsible for the identities of the elements) are in the nuclei of an atom of an element.

³ Atomic mass is given in the units g/mol, or grams per mole. A mole is defined as 6.022x10²³ units of something. In this case, it would be 6.022x10²³ atoms. So 200.59 g/mol means 6.022x10²³ atoms of mercury has a mass of 200.59 grams. 6.022x10²³ is also known as Avogadro’s number, after Italian scientist Amedeo Avogadro.

⁴ STP is short-hand for Standard Temperature and Pressure. STP is defined as 20°C, or about room temperature, and 1 atm, or about 14.7 Psi, by the NIST, the National Institute of Standards and Technology. 1 atm is short for 1 atmosphere, a unit of pressure commonly used among scientists. 1 atm is equal to the amount of pressure the atmosphere exerts at sea-level.

⁵ Density is typically given in g/mL (grams per milliliter) or g/cm³ (grams per cubic centimeter.)

⁶ Cinnabar is also known by its formal name: mercuric sulfide, or HgS – a mercury atom bound to a sulfur atom.

⁷ Isotopes are variants of elements that differ in the number of neutrons (neutrally charged sub-atomic particles) in their nucleus (the core of an atom that is comprised of protons and neutrons). Half-life refers to the amount of time a quantity of something to decay to half of its initial amount. Elements decay by means of exponential decay, meaning they will never fully disappear. Exponential decay can be described by the formula:

N(t) is the amount of something left after a certain time. N₀ is the initial amount of something. t is the time passed. t_1/2 is the value of the half-life. This means that if something has a half-life of 444 years as an isotope of mercury does, it would take about 4,425 years for 1 atom of that isotope to decay to one one-thousandth of itself.

⁸ http://www.who.int/mediacentre/factsheets/fs361/en/

⁹ A methyl group is comprised of one carbon atom bonded to three hydrogen atoms. Carbon can, and prefers, to make four bonds, so methyl groups have a positive charge. Methyl is abbreviated as Me, or CH₃⁺. Methylmercury is abbreviated as MeHg, or CH₃Hg⁺.

¹⁰ Sulfate-reducing bacteria gain their energy by turning sulfate (SO4-) into hydrogen sulfide (H2S). This is essentially how they “breathe.” The bacteria oxidize molecular hydrogen while reducing sulfate to hydrogen sulfide. Oxidation is the loss of electrons or an increase in oxidation state. A reduction is the gain of electrons or a decrease in oxidation state. Oxidation states are an indication of oxidation degree. Essentially, it is the hypothetical charge on an atom or chemical compound if all of the bonds in the compound were 100% ionic. Ionic bonds are when two things are bound together by transferring electrons (one thing gains electrons while the other loses them). Electrons are negatively-charged sub-atomic particles and are found in a cloud surrounding an atom’s nucleus.

¹¹ Bioaccumulation means an organism contains higher concentrations of mercury than their surroundings do. Biomagnification means organisms at the top of the food chain have higher concentrations of mercury than organisms at the bottom of the food chain. Amino acids are the “building blocks” of proteins.

¹² https://www.hsph.harvard.edu/nutritionsource/fish/

Authored by:

Rachel Lund

Chemistry ACS Major

Gustavus Adolphus College