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Heavy Metals Contamination Induces Oxygen Insufficiency Stress in OystersThis amplifies the stress induced by warming waters |
By Stefan Anitei, Science Editor
11th of October 2006, 10:40 GMT
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A recent study showed that oysters (Crassostrea virginica) exposed to high water temperatures and heavy metal pollutants suffer from stress induced by lack of oxygen.
Cadmium, a common heavy metal pollutant, decreased oyster's resistance to warmer waters, making it more vulnerable during the summer and to the rising ocean water temperatures.
"Half of the oysters exposed to the pollutant in 28°C (82°F) water died within 20 days", said Gisela Lannig of the Alfred Wegener Institute for Polar & Marine Research, Bremerhaven, Germany. "Oysters exposed to cadmium at lower temperatures showed much lower mortality rate, suggesting that the combination of high temperature and cadmium is more stressful than each of these conditions alone", she said.
"Oysters once thrived in the coastal regions of much of the eastern United States, but over-fishing, global warming, rising ocean water temperatures and increased pollution have combined to reduce the oyster population
to 5% of what it was 200 years ago", said Inna Sokolova of the University of North Carolina, Charlotte.
Oysters are a very important filtering species in the marine ecosystem. "We can't even imagine the expanse of oyster reefs 100-200 years ago and their impact on the ecosystem", Sokolova said. "For example, a century ago, the oyster population could completely filter the water in the Chesapeake Bay in three days.
Today, with the oyster population continuing to decline from poor water quality and disease, it would take a year to filter the same amount of water", she said.
Oysters also create a specific ecosystem of their own that other marine life depends upon. Many commercial fishes feed on these oyster beds. "Studies have shown that there are more than 300 species reliant on oyster reefs, including at least 12 species important for their commercial or recreational value, such as blue crabs, sheephead, croaker and stone crab", Sokolova said. "Some of these species, such as blue crabs, use oyster beds and reefs as nursery areas and as feeding grounds."
The researchers put oyster groups at 20°C (68°F), 24°C (75°F) and 28°C (82°F). "All three temperatures are within the range that oysters are exposed to in their natural environment", Lannig said.
"In fact, this summer, which was unusually hot, water temperatures reached 32°C", she said.
Some oysters were exposed to cadmium, others not. It was already understood that "warmer temperatures raise the oyster's metabolic rate, which raises its oxygen and energy requirements", as Lannig explained.
But oysters exposed to cadmium at 20°C and 24°C needed more oxygen than oysters that have not been exposed. Pollutants usually stress the oyster and the stress increases metabolism and oxygen demand. At 28°C there were no differences in oxygen intake between exposed and not exposed oysters.
The team presumes that the exposed oysters needed more oxygen, but they just couldn't obtain it. The researchers drew this conclusion - Lannig said - based on measurements of respiratory activity, the animal's blood oxygen content and mitochondrial function. "We found that mitochondria are very sensitive to cadmium at high temperatures", Lannig noted.
Mitichondria are structures inside the cell where organic substances are burned with oxygen and life energy is produced. In another experiment, oysters at 20°C were subjected to an acute temperature increase at 24°C overnight. "Here we saw that the cadmium-exposed oysters showed a decrease in blood oxygen content with the higher temperature, but not the control group that was not exposed to cadmium", Lannig said.
So, cadmium may be implied in a decrease on the oyster's blood capacity to carry oxygen. Understanding how pollutants affect the oyster's physiology is extremely important in protecting what remains from this ecologically important population.
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