by Jane Qiu
20 June 2016
High levels of
man-made pollutants have been found
in the shrimp-like
Hirondellea gigas (pictured),
which lives in the
deep-ocean Mariana Trench.
Daiju Azuma. CC-BY-SA
Crustaceans at depths of 10,000
contain higher concentrations
than do some animals in coastal
Toxic chemicals are accumulating in marine creatures in Earth's
deepest oceanic trenches, the first measurements of organic
pollutants in these regions have revealed.
"We often think deep-sea trenches
are remote and pristine, untouched by humans," says Alan
Jamieson, a deep-ocean researcher at the University of Aberdeen,
But Alan Jamieson and his
colleagues have found man-made organic pollutants at high levels in
shrimp-like crustaceans called amphipods that they collected from
two deep-ocean trenches, he told a conference on deep-ocean
exploration in Shanghai on 8 June.
"It's really surprising to find
pollutants so deep in the ocean at such high concentrations,"
says Jeffrey Drazen, a marine ecologist at the University of
Hawaii in Honolulu.
Before this work - which has not yet
been published - the study of pollutants in deep-sea organisms had
been limited to those that live at depths of 2,000 meters or less.
The latest research tested for levels of
organic chemicals in
amphipoda collected at 7,000-10,000
meters depth, from the
Mariana Trench in the western
Pacific Ocean - the world's deepest trench - and from the
near New Zealand.
The creatures were captured during two international expeditions in
2014, when researchers lowered unscrewed landers into the trenches
as part of a research program to study deep-ocean ecosystems,
sponsored by the US National Science Foundation.
In both trenches, the amphipoda contained polychlorinated
- used to make plastics and as anti-fouling agents to stop barnacles
growing on ships' hulls - and polybrominated diphenyl ethers
which are used as flame retardants.
Both chemicals are man-made and belong to a category of carbon-based
compounds called persistent organic pollutants (POPs)
because they are hard to break down.
Production of PCBs - which are
carcinogens - has been banned in many countries since the
PBDEs, which animal studies
suggest may disrupt hormone systems and interfere with
neural development, are only now being phased out.
The concentrations of PCBs in the
amphipods from the Mariana Trench were particularly high, and 15
times greater than those found in the Kermadec.
"It's even higher than in the
estuaries of two of the most polluted rivers - the Pearl River
and the Liao River - in China," says Jamieson.
By contrast, the Kermadec Trench
contains PBDEs at concentrations five times greater than the Mariana
- and at a level that is higher than in the coastal waters of New
Zealand's North Island, the study finds.
Douglas Bartlett, a deep-sea microbiologist at the Scripps
Institution of Oceanography (SIO)
in San Diego, California, says that the discovery is fascinating.
"It hits home very dramatically that
the trenches are not that remote after all, and the world is all
connected," he says.
"The take-home message is that when
you dump rubbish into the sea, it will ultimately sink. When
[pollutants] fall into the trenches, they have nowhere else to
So they're just going to keep
building up," says Jamieson.
Eventually, the trenches will have
higher levels of pollutants than in estuaries, where chemicals are
constantly flushed out to open waters, he says.
The researchers suspect that the proximity of the Mariana Trench to
large plastic manufacturers in Asia, as well as to a long-term US
military base on the island of Guam, may have contributed to its
high PCB levels.
The waters above the trench are also
part of the
North Pacific Gyre, a system of
strong swirling ocean currents that might be sucking materials on
the surface down into the deep sea.
Both the Mariana and Kermadec trenches
are around 11 kilometers deep.
"It sounds quite deep, but it's not
in terms of pollutant transport," says Jamieson.
The high POP levels are a cause for
concern, researchers say.
The deep-ocean canyons are "untapped
natural resources" - a potential supply of organisms that could
be valuable for a range of commercial applications, including
drug discovery, but which might be affected by the pollutants,
says Jeffrey Drazen.
Scientists think that deep-ocean
trenches could also be an important carbon sink, playing a key
part in regulating climate, Bartlett says. In part, that
is because in the trenches, carbon is pushed deep into Earth's
interior when one tectonic plate is thrust underneath another.
The trenches are also teeming with
microbes that may convert carbon-containing chemicals into forms
that aren't easily converted into carbon dioxide.
"If somehow microbial activities are
adversely affected because of all this pollution, I'd wonder
what that's doing to the carbon cycle in general," says