Plankton hold surprise for climate research

Amid concerns about the damage that rising levels of CO2 will do to the oceans, including to corals and other species, there seems to be good news for at least one group of creatures.

Biological oceanographer M. Debora Iglesias-Rodriguez of the National Oceanography Center at the University of Southampton, U.K., found that one species of single-celled phytoplankton, Emiliania huxleyi, increases its calcification rate as CO2 levels rise in the water.
Looking at ocean sediments, Iglesias-Rodriguez’ colleague, Paul Halloran of the University of Oxford, found that the average mass of coccolithophores, the class of phytoplankton to which Emiliania huxleyi belongs, has increased 40 percent over the last 220 years, as atmospheric CO2 levels have risen.

The question is, what effect will it have on the ocean’s CO2 uptake if these little guys continue to boom?

The ocean is full of phytoplankton, tiny plants that soak up carbon dioxide by photosynthesis. One group of these, the coccolithophores, also combine dissolved carbon dioxide and calcium to make chalky scales, called coccoliths. But the reaction that forms their calcium carbonate plates releases carbon dioxide in the process, offsetting the carbon gains from photosynthesis.

Previously, researchers have attempted to predict the effect of a more acidic ocean on coccolithophores by adding hydrochloric acid to laboratory cultures of the organisms. Such experiments have found that a more acidic environment hinders their calcification process.

But Iglesias-Rodriguez attempted a more realistic method of creating future ocean conditions by bubbling carbon dioxide through her cultures of the common coccolithophore, Emiliania huxleyi. Bubbling carbon dioxide dropped the pH and increased the amount of carbon dissolved in the water, as would happen in the open ocean.

Iglesias-Rodriguez’s team tested atmospheric CO2 levels up to 750 parts per million, which could be reached by 2100 according to one scenario, and found the opposite of what earlier experiments observed: The amount of calcium carbonate in the coccolithophores increased with increasing CO2 levels.

“We show for the first time that calcium in coccoliths could double by the end of the century,” she said.

The team also examined sediments from a site in the North Atlantic and found that the calcium carbonate mass in coccoliths increased by an average of 40 percent over the last 220 years, coinciding with the increase in atmospheric carbon dioxide. They report the findings today in Science.

“It’s quite significant,” said Scott Doney of Woods Hole Oceanographic Institution in Woods Hole, Mass., who was not part of the research. “I think it’s going to make a lot of people scramble around to try to figure out if this is a uniform finding in coccoliths.”

Iglesias-Rodriguez’s results suggest that carbon removal from increased photosynthesis and release from increased calcification will probably roughly cancel each other out. But booming coccolithophore populations could displace other phytoplankton, or coccolithophore predators could be affected by the bigger critters.

The increased calcium carbonate could also affect how much coccolithophore mass falls to the ocean floor, another carbon sink.

“Nobody’s thought through these new effects,” Doney said.

“At the moment, with the information we have, it’s just not possible to say what this means in terms of carbon,” Iglesias-Rodriguez agreed. “But it’s very important information for the models, because they are using the opposite information to what we find.”

Doney agreed. “A lot of conclusions have been drawn from a handful of studies, and the ocean has a lot of surprises for us.”

Jessica Marshall, Discovery News, 17 April 2008. Article.

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