Published online 17 April 2008 | Nature | doi:10.1038/news.2008.760

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But the ocean organisms may not remove more carbon dioxide from the air.

The microscopic marine organisms called coccolithophores, one of nature’s most prolific consumers of atmospheric carbon dioxide, may continue to absorb carbon at today's rates — even as greenhouse-gas concentrations continue to rise.

Coccolithophores are phytoplankton that live in the upper layers of the world's oceans. The creatures use calcium carbonate, the material that makes up chalk and seashells, to build tiny plates on their exterior.

The tiny phytoplankton fashion their protective shells from calcium carbonate. M. Debora Iglesias-Rodriguez

Although building these plates produces carbon dioxide, coccolithophores also consume the gas while undergoing photosynthesis. On balance, the phytoplankton absorb more carbon dioxide than they produce.

Previous research has suggested that changing ocean chemistry may prod the plankton to become more efficient carbon sinks, eliminating more and more carbon dioxide from the atmosphere. But a study appearing today in Science suggests they may be no help at all1.

"It is unlikely that coccolithophores are going to be mitigating or exacerbating the rise of carbon dioxide in the atmosphere," says author M. Debora Iglesias-Rodriguez of the University of Southampton, UK.

Testing trends

To simulate the effect of rising atmospheric carbon-dioxide levels, Iglesias-Rodriguez and her colleagues bubbled air enriched with carbon dioxide through cultures of Emiliania huxleyi, a coccolithophore species found throughout the world’s oceans.

When the team increased the carbon-dioxide concentration to the levels projected for the year 2100, they found that E. huxleyi made twice as much plate material and thus produced more carbon dioxide. But this increase was balanced by a parallel increase in photosynthesis.

All told, Iglesias-Rodriguez says, the results suggest that coccolithophores may not alter carbon dioxide levels, even as the ocean absorbs increasing amounts of the gas. While coccolithophores may be unaffected in the short term, eventually the ocean may grow too acidic from rising carbon dioxide levels. Large quantities of the calcium carbonate the coccolithophores make will dissolve, with unknown effects on the organisms.

Although the team found that carbon dioxide boosted plate production or 'calcification', other experiments with coccolithophores have shown the opposite trend — a reduction in calcium carbonate production2.

“This is the first time where we actually see increased calcification in response to high CO 2 ,” says Victoria Fabry, a biologist at California State University, San Marcos. "The results here are not consistent, and we need to find out why they're not."

Fabry notes that the work was on only one of roughly 250 species of coccolithophores, and others might respond differently.

Record of change

Still, Iglesias-Rodriguez says she is encouraged by her team's field measurements, which examined layers of E. huxleyi that had settled at the bottom of the North Atlantic Ocean. The sediment contained a record of coccolithophore growth since the dawn of the industrial revolution. From 1780 to 2004, the average weight of calcium carbonate in the phytoplankton increased by more than 50%, indicating atmospheric changes are having an effect.

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In December, the team plans to travel to Antarctica to conduct more experiments using coccolithophores in the seawater samples. Such experiments could help further clarify how they respond to increasing concentrations of carbon dioxide under real-world conditions, as laboratory tests often vary from experiment to experiment.