“What’s that?” her dad asks.
“Well, plain mercury is just that element in
old thermometers. It gets into the ocean from
air pollution caused by burning coal or from
volcanic eruptions. Plain mercury, in liquid
form, is not absorbed by the body and is
eliminated before it can do harm. But when it
is oxidized in the environment, it becomes an
ion, called inorganic mercury (Hg2+ ion).”
Nicole keeps reading. “Mercury usually
can’t pass through a cell membrane because
it is bound to large molecules. But when positively charged mercury ions (Hg2+) meet negatively charged sulfide ions (S2−), which are
produced by microbes in the water, the two
bond. The resulting compound, mercury(II)
sulfide (HgS), is small enough to pass
through microbial cells.
“Once inside, the mercury atoms bind to
methyl groups (–CH3), and then the cells
release methylmercury (CH3Hg+). Some of the
methylmercury diffuses into the open water
and is taken up by plankton before beginning
its journey up the food chain (Fig. 1).”
Nicole looks up to see if her dad is still listening, which he is, to her surprise.
“Inside a fish, a methylmercury molecule
has a high affinity for sulfur-containing anions,
(–SH) on the amino acid
cysteine (Fig. 2). The two
molecules combine to
form a compound called
Amino acids are the building blocks of proteins.
“The modified amino
acid has similar properties
to an amino acid called
methionine (Fig. 3), which
has a –CH2SCH3 group
(while the methylmercury-cysteine has a –
“Because methionine and methylmercury-cysteine act alike in chemical reactions, proteins that usually bind with methionine instead
bind with methylmercury-cysteine, carrying
methylmercury to the brain and other organs,
where it causes damage.
“So when the fish eat the plankton, they also
eat methylmercury, and it stays inside their bodies.” She scrolls down the page. “And that just
keeps happening.” She pauses again. “Until we
eat the fish. Since the fish we eat contains all the
mercury of all the fish they ever ate, and so on,
this is called ‘biomagnification.’ ”
Nicole looks at the label on the fish stick
box to check the list of ingredients. She won-
ders, “What fish are in these sticks anyway?”
Ingredient number one: pollock. “I remember
pollock from the big, color-coded list. It’s a
good fish, low mercury. And it looks like it
doesn’t hurt the environment to eat them.”
“Hah! High five! I told you they were safe.”
“Couldn’t we, at least, dress them up a little
next time? Something other than tartar sauce,
“Sure,” her dad says, as he points at a cook-
book sitting on the kitchen counter.
Choy, A. C. Mercury Levels in Pacific Fish Likely to
Rise in Coming Decades, Study Reports. Phys.
org (Science X Network), Aug 25 2013: http://
decades.html [accessed Jan 2016].
Groth, E. III. Ranking the Contributions of
Commercial Fish and Shellfish Varieties
to Mercury Exposure in the United States:
Implications for Risk Communication. Environ.
Res., April 2010, 110 ( 3), pp 226–236.
Mary Alexandra Agner is a science writer who lives
in Somerville, Mass. Her most recent ChemMatters
article, “Stuck on You,” appeared in the February/
March 2016 issue.
Figure 1. Volcanic eruptions, the burning of coal, and the mining of iron release mercury (Hg) in
the air, which can later contaminate water sources. There, some of the mercury is converted into
methylmercury (CH3Hg+) by small microorganisms called plankton and can be absorbed by fish.
Through a process called biomagnification, methylmercury levels in each successive stage of the
food chain increase.
Figure 3. Chemical structure of methionine
COOH H2N C H+
COOH CH3 Hg+ + +
Figure 2. When a cysteine molecule, which is common in many
proteins, reacts with methylmercury, it forms methylmercury-cysteine, which goes inside the brain, where it causes damage.