Ocean CO2 studies look beyond coral

One million tons of atmospheric carbon dioxide (CO2) are dissolved into the oceans every hour, a process that helps maintain the Earth’s delicate carbon balance. But CO2 also makes seawater more acidic, and too much of it can wreak havoc on a marine species. Three sessions at the meeting described how marine scientists are trying to assess the effects of acidification.

The ocean’s average pH worldwide, now roughly 8.4, has dropped about 0.1 since preindustrial times. Scientists estimate that it could fall another 0.4 by 2100 if carbon emissions continue on their current trajectories. That could put nearly two-thirds of known cold water corals into corrosive waters, Ulf Riebesell of the Leibniz Institute of Marine Sciences in Germany told one colloquium. But although the risks to corals are well-known (Science, 4 May 2007, p. 678), the effects on other marine life are just beginning to be characterized.

Gretchen Hofmann of the University of California (UC), Santa Barbara, reported that a one-two punch of lower pH and higher temperature can be fatal for the purple sea urchin, Strongylocentrotus purports. Human’s lab studied urchins in tanks of seawater at normal pH and at the stronger acidity expected by 2100 under two possible atmospheres described by the Intergovernmental Panel on Climate Change. As the pH fell from 8.1 to 7.8, sea urchin larvae struggled to build their skeletons. DNA micro arrays showed that genes involved in biomineralization raised their activity threefold. “The larva is desperately trying to make its body,” Hofmann said. Unpublished results from the lab showed that larvae in the most acidic water grew “short and stumpy” skeletons. If the deformities carry over to adults, they could affect the valuable fishery for urchins, which are harvested for their eggs.

When Hofmann and colleagues warmed the acidified waters, mortality among the larvae skyrocketed. “Gretchen has the story dead on with the urchins,” comments 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 and Victoria Fabry of California State University, San Marcos, are now studying how acidity and temperature affect the pteropod Limacina helicina, a peppercorn-sized swimming snail that forms a key part of the food web in the Southern Ocean. In their evolutionary history, Riebesell says, many species of pteropods “have never seen an ocean as acidic as the one they’re going to see in the next 100 years.”

Paleoclimate researchers are also beginning to study how high CO2 levels might have impacted species in ancient seas. A team led by James Zachos of UC Santa Cruz is focusing on a 150,000-year period, 55 million years ago, when the amount of carbon released into the atmosphere–nearly 4 gigatons–is similar to the pulse researchers expect from current human emissions. Estimating ocean pH for this extreme event is tricky, Zachos said, because most of the standard indicators–calcium carbonate shells in sea-bottom cores–dissolved away. But he hopes computer modeling and isotope analysis of other shell samples will give his team a handle on the past–and possibly on our torrid future.

Kintisch E. & Stokstad E., 2008. Ocean CO2 studies look beyond coral. Science 319:10029. Article.

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