Unit Overview

In the Introductory Unit we introduced students to mercury. They learned that it is toxic and they should, also, have at least a rough idea of the kinds of quantities that we might expect to find in human hair and in fish. They should also have a general familiarity with the way that mercury, in its different forms, moves through the environment; more specifically they should think about how it moves through their study site and organisms. Students using the activities in this unit will diagram the mercury cycle- as it pertains to their research site, and can develop a general understanding of toxicology and can play the Food Chain Game.

In the United States, thousands of chemicals are consumed and utilized in everyday items such as food, personal care products, prescription drugs, household cleaners, and garden/lawn care products. The effects of many of these chemicals on humans are unknown. Moreover, our use and disposal of these products ultimately contaminates our planet’s soil, water, and air. Safeguarding public health and the quality of the environment depends on identifying the effects of these chemicals and the levels of exposure at which they may become hazardous.

As high-school students study the flow of matter and energy in natural systems they develop an understanding that important molecules and elements are continually recycled. By studying toxicology, students add to this understanding by examining that hazardous chemicals in natural systems also are passed along the food chain. Thus, an herbicide used to kill weeds or a pesticide used to kill insects is not isolated to its target organism. We are, in fact, all connected on this planet to each other and the risk to our own health must always be weighed against the benefit attained by using hazardous chemicals.

Toxicology overlaps with many aspects of the sciences including biology, chemistry, physiology, microbiology, public health, and pharmacology. Overall it is a science that studies toxins and their effect on biological systems.

Mercury Toxicology 101

• Routes of exposure (i.e. how does mercury get into organisms?)

A human or other organism can be exposed to mercury from breathing in contaminated air, from ingesting contaminated water or food, or from having skin contact with mercury. Not all forms of mercury easily enter your body, even if they come in contact with it; so it is important to know which form of mercury you (or a creature you are studying) have been exposed to (inorganic mercury or methylmercury), and by which route (air, food, or skin). Once it is absorbed into the bloodstream, it travels rapidly to other parts of the body, including the brain and kidneys. Toxicologists use certain terms to talk about routes of entry for toxic substances into the body – you’ll see these terms if you ever look at a Materials Safety Data Sheet (MSDS) for any chemical or substance:

Ingestion: If you were to swallow small amounts of inorganic mercury metal–for example, from a broken oral thermometer–virtually none (less than 0.01%) of the mercury would enter your body through the stomach or intestines, unless these organs were diseased. Even when a larger amount of mercury (a half of a tablespoon) was swallowed by one person, very little entered the body. Methylmercury is the form of organic mercury that is most (about 95%) easily absorbed through the gastrointestinal tract. After eating fish or other foods that are contaminated, the methylmercury enters your bloodstream easily and travels rapidly to other parts of your body. Eating mercury-contaminated food is the primary pathway by which people (and other organisms) are exposed to mercury.

Inhalation: Inorganic mercury compounds may evaporate slowly at room temperature and could enter your body easily if you were to breathe in the vapors. When you breathe in mercury vapors, most (about 80%) of the mercury enters your bloodstream directly from your lungs. A small amount of the inorganic mercury can be changed in your body to metallic mercury and leave again as a mercury vapor when you exhale.

Skin contact: Only small amounts of methylmercury enter the bloodstream directly through the skin, but other forms of organic mercury (in particular dimethylmercury) can rapidly enter the body through the skin. Note: we won’t be working with any dimethylmercury, and the methylmercury in our study organisms will be tightly bound in those organisms, and won’t be able to move from those organisms into us by just skin contact.

• What happens once mercury is in the body?

Once in your body, inorganic metallic mercury can stay for weeks or months. When mercury enters the brain it can be "trapped" in the brain for a long time. Metallic mercury in the blood of a pregnant woman can enter her developing child and cause developmental disabilities. Most metallic mercury will accumulate in your kidneys, but some can also accumulate in the brain. Most of the metallic mercury absorbed into the body eventually leaves in the urine and feces, while smaller amounts leave the body in the exhaled breath.

Bioaccumulation and Biomagnification

Students now have an understanding of how mercury acts as a poison within an individual organism. Now we will show them how mercury “bioaccumulates” and “biomagnifies.”

    Bioaccumulation

Bioaccumulation is the accumulation of higher and higher concentrations of potentially toxic chemicals in individual organisms. It occurs in the case of chemicals that are ingested but cannot be broken down or excreted.

Every time an organism eats an organism that contains mercury, it takes in the mercury burden of that food item.

The methylmercury within the food item is a lipid soluble, not water soluble, chemical. This means it cannot be removed from the body in urine (which is water-based). Instead, methylmercury accumulates in different tissues within the body.

With every food item consumed more methylmercury is accumulated by the consumer and little of it can be excreted. The methylmercury is stored in the consumer– whether the consumer be an earthworm, owl, beetle, copepod, pickerel, or human. If any of these organisms is eaten, then their body burden is, in turn, accumulated by their consumer. If all you eat are small fish then you will accumulate all the mercury of every small fish you eat.

Bioaccumulation of mercury depends on what an organism eats and the longevity of the organism.

    Biomagnification

Biomagnification is the bioaccumulation occurring through several levels of a food chain.

Biomagnification is the increase in concentration of a substance from one trophic level to the next.

Biomagnification results from the fact that 1.) mercury bioaccumulates within an organism and 2.) organisms must consume a lot of other organisms to live.

Biomagnification depends on what trophic level you are observing, and how much mercury is present in the environment to begin with.

In summary, although the methylmercury form of mercury is a very small fraction of the global mercury cycle, it is, by far, the form most prevalent in the biosphere and therefore most important to us from a health perspective.

Key Ideas introduced in this Unit:

• Mercury bioaccumulates
• Mercury amounts in organisms increase at higher trophic levels
• Mercury is toxic to humans and other organisms
• Dragonflies are a key part of the aquatic food web and are exposed to mercury

Concepts and Relationships

• Mercury can enter organisms in several ways
• Mercury stays in the body for a long time
• Mercury primarily affects the brain, nervous system, and kidneys
• A fetus that is exposed to mercury can be born with severe nerve and/or kidney damage as well as developmental disabilities
• A study site contains a lot of components that might all have mercury: soil, water, organisms, plants, etc. (get acquainted with you field site and start thinking about how it may be affected by mercury)

Vocabulary

• Bioaccumulation—the collection of higher and higher concentrations of potentially toxic chemicals in individual organisms. It occurs in the case of chemicals that are ingested but cannot be broken down or excreted.
• Biomagnification—bioaccumulation occurring through several levels of a food chain.
• Food chain—a set of organisms that feed one upon another linearly, and thus pass energy along the chain. (An herbivore eats a plant, the herbivore is eaten by a carnivore, which is then eaten by another carnivore.)
• Food web—multiple, interconnected food chains. Some animals eat and are eaten by many other types of organisms in a complex web.
• Life history—the sequence and timing of events in an organism’s life cycle. For example, some life history traits include how long an organism spends as a larva, what it eats, and when it is ready to reproduce.
• Macroinvertebrate—an organism without a backbone that is large enough to be seen without a microscope
• Mercury cycle—the deposition and uptake of mercury from the emission source to the ingestion by organisms
• Methylmercury—the byproduct of metabolism by anaerobic bacteria, typically living in an acidic environment with access to carbon and sulfur
• Study organism—the organism that is the subject of a research project
• Toxicity—the degree to which something is poisonous
• Trophic level—as organisms eat other organisms, all the substances they contain are passed up the food chain, or up tropic levels. (Producers—green plants and algae. Primary consumers—herbivores. Secondary consumers—carnivores.

Student Prerequisites

How to measure volume and square inches
How to convert units such as µg/g to ppb or ppm (see Activities in Unit 1)
Proportions
Averages
Ranges
Computer skills
Library skills

Misconceptions about mercury

Mercury is not a problem in Maine
Quantities of mercury that we ingest are not sufficient to cause harm
There is nothing interesting to study at my site