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Published online 27 October 2008 | Nature | doi:10.1038/news.2008.1190
News
Urea pollution turns tides toxic
Kamikaze gulls that inspired Hitchcock's The Birds may have been
doomed by
leaky septic tanks.
Amy Coombs
The BirdsRun, Tippi, run!UNIVERSAL / THE KOBAL COLLECTION
Urea pollution can trigger ocean algae to produce a deadly toxin
called
domoic acid, scientists have discovered.
The research may help explain several mass animal deaths, including a
historic bird stranding event thought to have inspired Alfred
Hitchcock's
horror film The Birds.
Raphael Kudela, an ocean scientist at the University of California,
Santa
Cruz, and his team made the discovery after studying a form of sea
algae
called Pseudo-nitzschia australis. Although the algae's blooms are
normally
benign, they have long been known to sometimes begin making domoic
acid.
Much like the kamikaze gulls portrayed in the 1963 horror film,
animals
poisoned by domoic acid have erratic behaviour patterns. On 18 August,
1961,
residents in the town of Capitola, California, awoke to find sooty
shearwaters slamming into their rooftops, and their streets covered
with
dead birds. According to the local paper, Alfred Hitchcock ‹ who lived
a few
miles away ‹ requested news copy to use as "research material for his
latest
thriller", which was based on Daphne du Maurier's 1952 short story,
The
Birds.
Serial killer
Although researchers can only speculate that domoic acid caused this
historic event, modern toxicologists have conclusively linked the
toxin to
more recent cases. In 1987 contaminated shellfish poisoned 100 people
on
Prince Edward Island in Canada, killing three and causing many cases
of
amnesia. In 1998, 400 disoriented sea lions died along California's
central
coast ‹ domoic acid was traced back to contaminated fish that swam
through a
toxic bloom before being eaten by the sea lions. "Every few years
there is a
big outbreak that causes otters, pelicans or sea-lion deaths," says
Kudela.
"The acid binds very tightly to surface receptors on excitatory
neurons,
which makes it hard for the cells to switch off," says Melissa Miller,
a
veterinarian at the California Department of Fish and Game in Santa
Cruz.
While the toxin doesn't make animals homicidal, the brain damage it
causes
accounts for the strange behavior patterns that precede death, says
Miller.
Human pollution was thought to play a role, but researchers were never
able
to identify which contaminant causes P. australis to start producing
domoic
acid. "It's definitely a combination of factors, which makes it hard
to show
a cause and effect relationship," says Kudela.
So he and his colleagues tested a range of chemicals found in
fertilizers,
including nitrate, ammonium and urea, to determine their effects on
the
algae. Urea was the only chemical that increased domoic-acid
production. In
cases where the plankton mysteriously began making low levels of the
toxin
in clean water, adding urea nearly doubled production.
After taking water samples off the coast of California, they also
found that
urea concentrations in Monterey Bay and San Francisco Bay were high
enough
to account for some recent harmful algal bloom events.
Human cause
Urea isn't commonly found in agricultural fertilizers, but it is
present in
many garden products. Sewage-treatment plants tested as part of
Kudela's
study did not discharge much urea, but leaky septic tanks have been
known to
dump urea into the Chesapeake Bay and the Gulf of Mexico. "Marine
animals
release small amounts of urea, but the pollution problem is almost
entirely
human caused," says Kudela. The findings will appear in the November
issue
of Harmful Algae.1,2
"This work directly links human activities to the hundreds of marine
mammal
mortalities resulting from exposure to domoic acid," says Frances
Gulland,
director of veterinary science at the Marine Mammal Center in
Sausalito,
California.
Although there is no way to know for sure if urea caused the famous
incident
that inspired The Birds, Kudela says the pollutant was probably
leaching
into the ocean at the time. "There was a lot of new development back
then,
with many unregulated septic tanks going in," he says.
Kudela now plans to look for other chemical factors that may cause the
P.
australis tides to turn toxic.
*
References
1. Cochlan, W. P., Herndon, J. & Kudela, R. M., Harmful Algae
doi:10.1016/j.hal.2008.08.008 (2008).
2. Kudela, R. M., Lane, J. Q. & Cochlan, W. P., Harmful Algae
doi:10.1016/j.hal.2008.08.019 (2008).
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