1416 27 MARCH 2020 • VOL 367 ISSUE 6485 SCIENCEPHOTO:THE ASAHI SHIMBUN/GETTY IMAGESmethylmercury and dumped it directly
into the bay.
The dead cat now challenging that pic-
ture dates to 1959, when the then-mysterious
neurological disease was sweeping through
the city. A doctor working for the Chisso fac-
tory mixed wastewater with cat food and fed
it to cats, which started to convulse and were
paralyzed before dying. He autopsied two of
them, known only as 400 and 717. Their be-
havior and lesions in their brains suggested
the same disease as the one raging outside.
Factory supervisors hushed up the finding.
The Chisso cats were lost until 2001,
when Komyo Eto, a pathologist at the Na-
tional Institute for Minamata Disease, stud-
ied samples of the cats and the wastewater
that had been found in storage at nearby
Kumamoto University—along with lab note-
books lent by the factory doctor’s wife. His
measurements showed that less than half
of the mercury in the cats’ brain samples
occurred as methylmercury; the rest was
inorganic. Only a minuscule fraction of the
wastewater’s mercury was methylmercury,
but Eto thought that was probably because
the compound had broken down over the
4 decades since the samples were taken.
Now, Pickering and her colleagues have
reanalyzed samples of cat 717’s cerebel-
lum at the Stanford Synchrotron Radiation
Lightsource, blasting them with x-rays and
analyzing the resulting spectrum for the
fingerprints of specific molecules. The mole-
cules that best fit the spectrum don’t contain
any methylmercury at all, they found. In-
stead, three-quarters of the sample’s mercury
appears to be an obscure organic compound
called alpha-mercuri-acetaldehyde that likely
came straight from the wastewater, they ar-
gue. The rest was inorganic mercury.
The finding suggests the Minamata
disaster—and methylmercury poisoning
more generally—is due for a rethink, the
researchers argue in their study, published
in Environmental Science & Technology
in January. Methylmercury played no sig-
nificant role in the poisoning, says Graham
George, Pickering’s husband and co-leader of
the experiment, who is also at the University
of Saskatchewan. Because previous studies
used less sensitive techniques, they missed
the major form of mercury in Minamata
samples, he argues. “Were there other more
prevalent forms of mercury present that
were not detected? Yes, we think so.”
But to other researchers, the team may be
overstating its conclusions in the service of
a larger agenda. Many of the authors have
downplayed the toxicity of methylmercury
for years, says Philippe Grandjean, an en-
vironmental toxicologist at Harvard Uni-
versity. The work doesn’t do anything more
than identify an unusual chemical in one
preserved cat brain, he says. “They did an ad-
mirable piece of chemistry, but it should not
be interpreted beyond what it really shows.”
Charles Driscoll, an environmental sci-
entist at Syracuse University, says the new
mercury compound might be a product ofthe cat’s metabolism or an artifact of the
sample’s long preservation. And even if it
did spew directly from the factory, the resi-
dents of Minamata were exposed to mer-
cury from seafood they ate, not from factory
wastewater, he says. “Quite a few things in
[the study] give me pause,” he says. “I was,
frankly, surprised it would get published.”
The dispute reflects a schism dating to
the 1980s, when dueling research teams
looked at the neurological consequences of
methylmercury from seafood. One team,
at the University of Rochester , studied the
brain development of children in the Sey-
chelles islands, where the diet is heavy in fish
that can acquire methylmercury in the open
ocean from natural and human sources of
mercury. “We have not been able to confirm
any adverse effects of methylmercury from
fish,” said Gary Myers, a University of Roch-
ester neurologist who also participated in
the study on cat 717. But a competing study
by Grandjean and others in the Faroe Islands
concluded methylmercury from seafood was
harming children.
In 2000, when the U.S. Environmental
Protection Agency (EPA) defined the maxi-
mum daily amount of mercury thought to
be safe to ingest, the agency found the Faroe
Islands evidence more convincing, and set a
low limit. In 2019, EPA began to reassess this
limit; Grandjean fears the new study could
be used to support relaxing the standard. At
the same time, the Trump administration is
working to weaken a separate rule called the
Mercury and Air Toxics Standards, which
limits mercury emissions from power plants.
Last year, Grandjean, Driscoll, and other
mercury researchers submitted a comment
to EPA as part of the agency’s reassessment.
They pointed out that Minamata is not the
only example of methylmercury toxicity; the
compound caused another industrial poison-
ing in Iraq in 1971, and studies of babies and
children around the world have found that
even low-level exposure can harm brain de-
velopment, Grandjean says. Compared with
past decades, “We know better now.”
Eto, who loaned the samples and was cred-
ited as a study co-author, said in an email that
he still believes methylmercury was the most
important cause of the tragedy. But Pickering
and George plan to move beyond a single cat
sample to bolster their controversial claim.
They have already borrowed preserved sam-
ples from human victims from the National
Institute for Minamata Disease, and plan to
test for the same obscure compound. jJoshua Sokol is a journalist in Cambridge,
Massachusetts.In 1986, decades after the Minamata disaster, workers
still discarded mercury-tainted fish from the bay.NEWS | IN DEPTH