Researchers Struggle to Assess Responses to Ocean Acidification

Posted on SCIENCE News This Week: 05 October 2012 — By David Malakoff



Bubble bath. Carbon dioxide seeps off Italy give scientists a peek at what a more acidic ocean could mean for marine life.

MONTEREY, CALIFORNIA—Unlike many areas of global change, there’s no argument that rising CO2 emissions will make the world’s oceans more acidic. Average pH in surface waters is now 8.1—a 30% increase in acidity since the start of the industrial revolution—and forecasters say it could drop to 7.8 by 2100 if carbon emissions continue unabated. But how fisheries and other marine life will respond is far from clear.

Building a better crystal ball to gauge such effects emerged as a central challenge facing scientists attending the Third International Symposium on the Ocean in a High-CO2 World here last week. “Looking realistically into the future is hard,” says Jean-Pierre Gattuso, a biogeochemist at the French National Center for Scientific Research in Villefranche-sur-mer. “But we have to try if we want to understand what acidification will mean.”

The meeting was a coming-out party of sorts for scientists interested in the biological implications of the chemical changes occurring as the oceans absorb huge and growing amounts of atmospheric carbon dioxide. Just 8 years ago, an inaugural symposium on the topic in Paris drew only 125 researchers from 20 nations; this year, more than 550 scientists from 40 nations showed up. The field is getting “much bigger and more competitive,” Gattuso says.

Acidification researchers are also shifting their focus. To date, many experiments have involved simply plopping sea creatures into laboratory tanks full of acidified water for a few days or months to see how they respond. Many species suffer, researchers reported. Fish and shellfish larvae exposed to more acidic waters, for example, often fail to thrive: They don’t grow as big or live as long as those born in more alkaline waters. But some species show substantial resilience, reported biologist Sam Dupont of the University of Gothenburg, Kristineberg, in Sweden. After he used acidic water to completely dissolve the shells of developing sea urchins, for instance, the urchins were able to regrow them and live normally once they were returned to normal seawater.

Such limited studies, however, “can’t really tell you whether a species has the capacity to adapt to acidification, or how pH changes affect a larger ecosystem,” says marine scientist Gretchen Hofmann of the University of California, Santa Barbara.


Time machine. Coccolithophore fossils might help forecast how future pH changes will affect ocean phytoplankton.

One approach to leaping those limitations is to go back to the future, by looking for times in the fossil record when ocean ecosystems experienced similarly dramatic carbon dioxide–driven changes. One popular candidate, known as the Paleocene-Eocene Thermal Maximum (PETM), occurred 55 million years ago during rapid global warming (Science, 18 June 2010, p. 1500). Increasingly corrosive bottom waters appear to have helped drive many bottom-dwelling species extinct during the PETM, reported paleontologist Paul Bown of University College London. But what happened at the ocean’s surface is less clear. The fossil record suggests that many species of phytoplankton—the tiny plants at the base of the marine food chain—also disappeared but were replaced by other species, with little change in overall diversity.

But such coarse measures can’t tell you how ancient acidification might have affected reproduction or growth patterns in these marine communities, Bown says. To get that more detailed view, Bown and his colleagues have been analyzing some exquisitely preserved fossils of PETM phytoplankton called coccolithophores, which surround themselves with shieldlike plates of shell. By studying some closely related living species, the researchers found that they could estimate ancient coccolith growth and reproduction patterns by painstakingly counting the plates on individual fossils. (The number increases as the organisms grow.) So far, preliminary studies haven’t found deformed shells or other dramatic signs of lower pH, but Bown cautions against taking that as a sign that modern acidification won’t be a problem. Change in the PETM moved “much, much slower than today,” he says.

To evaluate acidification’s potential impact on modern ecosystems, researchers are also seeking out rare places where the future has already arrived, such as natural sea-floor seeps where bubbling carbon dioxide gas dramatically lowers the pH of surrounding seawater. Studies of seeps off Papua New Guinea have already shown that tropical coral reefs in acidified waters have fewer species and slower growth rates than nearby reefs (Science, 13 July, p. 146).

Similar changes are apparent on rocky bottoms at seeps in the more temperate Mediterranean waters, reported marine biologist Kristy Kroeker of Stanford University in Palo Alto, California. Off the coast of Italy, she’s been studying shallow-water seeps nestled beneath a towering Medieval castle. There, small, fast-growing invertebrates and weedy filamentous algae are displacing an array of larger, flashier shell-building species. That could be bad news for tourism if acidifying waters spark similar changes elsewhere, she says: “People are going to be much less likely to pay to go see a bunch of fleshy seaweeds.”

A third emerging approach is to study the evolutionary implications of acidification by pursuing longer and larger experiments. Several teams, for instance, have launched years-long “rapid evolution” experiments to evaluate how organisms adapt to acidified conditions. In one, a team led by David Hutchins of the University of Southern California in Los Angeles spent nearly 4 years raising about 600 generations of an important marine nitrogen-fixing bacterium, Trichodesmium, in either current or more acidic conditions. Bacteria in the acidified tanks greatly stepped up their nitrogen-fixing activity over time, he reported. And, surprisingly, they didn’t revert back to lower activity when moved back into the less acidic tanks, “suggesting true adaptive changes,” he says. Genetic studies backed up that idea, he reported: Acidification could have “major implications for the future of the marine nitrogen cycle” by altering bacterial populations.

Other researchers are taking to the field to find wild populations that have already adapted to acidic waters, which occur naturally in some parts of the ocean. “Basically, space is substituted for time, and we just go see if Mother Nature has already created populations that are fine-tuned,” says Hofmann, who is helping organize one such collaboration, the Ocean Margin Ecosystems Group for Acidification Studies (OMEGAS). It is studying sea urchins and mussels along North America’s West Coast, where waters tend to get more acidic along a south-to-north gradient from California to Canada. As a result of natural upwelling, the waters off Oregon “are like it is already 2100,” says Hofmann. She reported preliminary studies showing that sea urchin larvae there tolerate acidic water better than their California cousins. That early picture from OMEGAS’s crystal ball “is good news,” Hofmann says, because it suggests that the thorny urchins have “the resilience and genetic variation sufficient to tolerate” global acidification. Still, she says, that’s an experiment she’d rather not run.

Read this article online at Science AAAS