Published online 6 June 2008 | Nature | doi:10.1038/news.2008.877

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Bleak future for acid oceans

Volcanic vents give a glimpse of a world without corals and other creatures.

healthy coralHealthy coral in water of normal acidity.Riccardo Rodolfo-Metalpa

If climate change continues apace, the oceans of the future may be empty of corals, sea urchins and other organisms that are vital to the marine ecosystem. A survey of life around oceanic volcanic vents has found that these animals cannot survive in environments rich in carbon dioxide that mimic the future seas.

The world's oceans act as a giant CO2 sponge, soaking up an estimated 2 billion tonnes of the gas each year. Researchers credit that absorption with reducing the impact of human emissions on climate change, but there is an unintended consequence. The rise in CO2 concentration lowers the pH levels of the sea, making it more acidic. That rise in acidity, in turn, removes calcite and aragonite from the marine environment, two minerals commonly used for building shells. Marine biologists fear that the acidity rise could hurt organisms such as coral and destabilize ocean ecosystems. The results of the survey are published online today by Nature1.

acidic coralCoral kept in slightly acidic water for two months shows extensive wear and tear.Riccardo Rodolfo-Metalpa

Until now, those concerns have been based on experiments conducted in large aquariums rather than in open water, according to Jason Hall-Spencer, a marine biologist at the University of Plymouth, UK. To find out how acidification might change an ocean ecosystem, Hall-Spencer and his colleagues studied a series of undersea volcanoes off the coast of Italy. The volcanic vents have been spewing CO2 into the surrounding Mediterranean for millennia, and it seemed a good place to observe how acidification might affect the oceans. "As you snorkel and dive through the zones, it's like looking into the future," he says.

'Worrying results'

The results of the survey are "quite worrying," Hall-Spencer says. As researchers travelled towards the vents, they found that organisms such as coral disappeared. Microbial organisms that had calcite shells also vanished. "They cannot form their skeletons," he says.

The results could have far-reaching implications, Hall-Spencer warns. For example, coral reefs prevent coastal erosion in many areas, and some microbes are key to marine ecosystems. The survey also found that, in the absence of these organisms, alien species of algae overran the surrounding environment. "I'm quite worried that [the algae] are going to radically change the marine shallow habitats around the planet," he says.

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"It's a really exciting result," says Scott Doney, an oceanographer at Woods Hole Oceanographic Institution in Massachusetts. Doney says that the findings confirm many of the earlier suspicions of oceanographers, and believes that more work must now be done to understand how acidification might ripple through marine sea webs. "The trick is extrapolate from one small 100-metre stretch of coast to entire ecosystems," he says.

Hall-Spencer says he is now looking for other vent systems in the Pacific and Atlantic oceans that might provide clues to what would happen in other regions. 

  • References

    1. Hall-Spencer, J. M. et al. Nature advance online publication doi:10.1038/nature07051 (2008).
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