Study finds oysters can take heat and heavy metals, but not both

Pollution is bad for the sea life and so is global warming, but aquatic organisms can be resilient. However, even organisms tough enough to survive one major onslaught may find that a double whammy is more than their molecular biology can take.

A new study has found that even relatively low levels of heavy metal pollution can interfere with the metabolic processes of oysters, and that the effects of the pollution become particularly notable when oyster metabolism is also affected by high seasonal temperatures. The combined effect is strong enough to lead to fatal weakness and disease, adding a fundamental explanation for documented oyster declines in the wild. The effect also reveals an additional impact that warming coastal waters may have on cold-blooded organisms.

Investigating the mechanisms by which the heavy metal cadmium and temperature can each affect metabolic processes in oysters, a new report by a team headed by University of North Carolina at Charlotte ecophysiologist Inna Sokolova finds that both cadmium and temperature independently decrease the efficiency of metabolic processes in the oysters’ mitochondria – the place where stored food is turned into the energy living cells run on.

The study also finds that cadmium can cause an increase in the production of reactive oxygen species – dangerous metabolic by-products – while higher temperatures hamper the cellular processes that normally prevent the compounds from causing damage. The findings will appear in the December issue of the Journal of Experimental Biology.

“We are studying a combination of factors,” said Sokolova. “Essentially what we are trying to look at is how oysters that live in metal-polluted environments respond to an increase in temperature, including normal seasonal increases in the summer, and global climate change, which will add to the problems they are already having in warm periods.”

Coming from human sources as diverse as cadmium-nickel batteries, yellow paint and mining, heavy metals like cadmium are common pollutants in estuaries where oysters live, Sokolova notes, but their concentrations occur at only few parts per billion – quantities long assumed to be too low to threaten marine life.

“We have looked at oysters’ metabolism, and have found out that their respiration rate increases when they are exposed to cadmium at environmentally relevant levels, as the organism spends more energy on basal maintenance,” Sokolova said.

“Metabolism also increases when they are exposed to higher temperatures. At some point, when they are exposed to both cadmium and higher temperatures, their metabolism can not go up any more and they start dying because they have hit the maximum level.”

In its native habitat on the east coast of North America, the eastern oyster lives in estuaries where the temperature ranges fluctuate seasonally - from 0 to 4 degrees centigrade in the winter to temperatures as high as 35 degrees centigrade in the summer. According to Sokolova, past studies have shown that oysters stop growing at about 28 degrees centigrade, a temperature that can persist under normal summer conditions for several months in a row.