Study sees an advantage for algae species in changing oceans

Contrary to expectations, a microscopic plant that lives in oceans around the world may thrive in the changing ocean conditions of the coming decades, a team of scientists reported Thursday.
The main threat to many marine organisms is not global warming but ocean acidification, as carbon dioxide from the air dissolves into the water and turns into carbonic acid. Acid dissolves calcium carbonate in the skeletons of corals, for example; many scientists fear that acidification of the oceans will kill many, if not most, coral reefs by the end of the century.

Similar concerns have been raised about coccolithophores, single-cell, carbonate-encased algae that are a major link in the ocean food chain. Earlier experiments with a species of coccolithophore, Emiliania huxleyi, had found that lower pH levels (more acidic) hindered the algae’s ability to build the disks of carbonate that form its shell.

In Friday’s issue of the journal Science, however, scientists led by M. Debora Iglesias-Rodríguez of the National Oceanography Center at the University of Southampton in England and Paul Halloran, a graduate student at the University of Oxford, report that they found the exact opposite. The algae grew bigger in the more acidic water.

Dr. Iglesias-Rodríguez said the conflicting findings probably arose from differences between how the experiments were conducted. In the earlier work, the researchers lowered the pH by directly adding acid to the water.

In the work reported in Science, the scientists added the acid indirectly by bubbling carbon dioxide into the water, which more closely mimicked the chemical reactions that are occurring in the oceans. As a consequence, in addition to the lowered pH, levels of carbon dioxide in the water also rose — speeding up the algae’s photosynthesis machinery — as did the levels of bicarbonate ions, the building material for the carbonate disks.

“It’s a really complex problem,” Dr. Iglesias-Rodríguez said. “You cannot look at calcification in isolation. You have to look at photosynthesis as well.”

The pH scale, which measures the concentration of hydrogen ions, runs from zero, the most acidic, with the highest concentration of ions, to 14, the most alkaline, with almost no ions. Ocean water today is somewhat alkaline, at 8.1, down from 8.2 at the start of the Industrial Revolution two centuries ago.

The laboratory findings agree with what has been observed in the oceans. Over the past 220 years, the average mass of a coccolithophore increased 40 percent as ocean pH levels dropped.

The hopeful news for coccolithophores, however, does not overturn the gloomy predictions for corals or negate ocean acidification as an impending ecological disruption, Dr. Iglesias-Rodríguez said. Rather, she said, it points to how little data biologists currently have.

Chang K., The New York Times, 18 April 2008. Article.

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