BOSTON–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 (ScienceNOW, 17 February 2007).
Warmer waters are stressful for marine life, making organisms such as 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 at the annual meeting of the American Association for the Advancement of Science (ScienceNOW's publisher), 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 Lubchenco 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, whereas 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 three 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 Those in the most acidic water grow "short and stumpy" skeletons, according to unpublished work by graduate student LaTisha Hammond and postdoc Mike O'Donnell.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 that 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°C warmer, and about 7% of the larvae in higher acidity water die, compared with 2% of those in water with normal acidity. Double the temperature and roughly 29% of larvae in acidic waters keel over, compared with 16% of controls.
Although average ocean temperatures aren't expected to rise that high, they can rise about that much in tide pools, 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 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 Antarctica last month, and the frozen samples are being flown back to her lab for DNA analysis.