Warm Sea Urchins on Acid

Marine biologists break out in a cold sweat when they think about the impact of greenhouse gases on the oceans. It’s not just the fact that global warming raises the temperature of the sea. Scientists are also worried about acidity. The burning of fossil fuels pumps carbon dioxide into the atmosphere, and when it gets absorbed by seawater, it turns into carbonic acid and makes the oceans more acidic.

Warmer waters are stressful for marine life, making organisms like coral more vulnerable to disease. A lower ocean pH—i.e. a more acidic environment–makes it harder for marine invertebrates to construct their shells. But there has been little work looking at the combined effects of warmer waters and stronger acidity.

At a symposium here yesterday, physiologist Gretchen Hofmann of the University of California, Santa Barbara, reported that the combination can be deadly for the purple sea urchin, Strongylocentrotus purpuratus, that she works on. DNA studies are also revealing details about how the urchins battle the stress. “This is cutting edge,” says marine ecologist Jane Lubcencko of Oregon State University in Corvallis.

The first step in the research was to see what damage is caused by simply altering acidity alone. Hofmann has several tanks that contain water with varying acidity. Some are filled with normal seawater, while others have the stronger acidity that the Intergovernmental Panel on Climate Change predicts will plague ocean waters in 2100. In the more acidic tanks, it became harder for sea urchin larvae to build their skeletons, Hofmann reported. DNA microarrays by postdoc Anne Todgham showed that genes involved in constructing calcium carbonate skeletons were 3 times more active than normal. “The larva is desperately trying to make its body,” Hofmann said.

The effort takes a toll on the larvae. Those in the most acidic water grow “short and stumpy” skeletons, according to unpublished work by graduate student LaTisha Hammond and postdoc Mike O’Donnell. Other students in Hofmann’s lab are modeling how those deformities might affect the distance larvae travel before settling down. It’s not clear what the impact might be on adults, but Hofmann suspects they could end up smaller than usual. That could hurt the valuable fishery for urchins, which are harvested for their eggs.

In other experiments, Hoffmann’s team added the additional stress of heat to the acidic water. Postdoc Nann Fangue found that larvae survive brief stints in warmer water just fine if they live in normal temperature or high acidity. But subject them to water 9 degrees warmer and about 7% of the larvae in higher acidity water die, compared to 2% of those with normal acidity. Double the temperature and roughly 29% of larvae in acidic waters keel over, compared to 16% of controls.

Although average ocean temperatures aren’t expected to rise that high, they can rise about that much in tidepools, for example. And the results show that even greater mortality can result from the effort to cope with greater acidity. “Gretchen has the story dead on with the urchins,” says Andrew Baker of the University of Miami in Florida, who is studying the effects of temperature and acidity on corals. “Clearly the effects are worse together than separate.”

Hofmann is now working with Victoria Fabry of the California State University, San Marcos to study the impact of acidity and temperature on another organism, Limacina helicina. This pteropod, roughly the size of a peppercorn, is a key part of the food web in the southern ocean. Hofmann and Fabry did experiments in Antartica last month, and the frozen samples are being flown back to her lab for DNA analysis.

Erik Stokstad, Findings, 18 February 2008. Article.

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