Acidic oceans may be water of life for plankton

Most life in the ocean will suffer as carbon dioxide levels increase and the water becomes more acidic. Some plankton will buck the trend, however, thriving and putting on weight as carbon dioxide levels rise – but it remains to be seen how this will affect the global carbon balance.

Débora Iglesias-Rodríguez, from the National Oceanography Centre at the University of Southampton, UK, and her colleagues, simulated the increase in dissolved carbon dioxide in surface ocean waters by bubbling carbon dioxide through cultures of coccolithophores, a type of single-celled photosynthesising plankton.

In previous experiments water acidity had been regulated by simply adding acid or base, but this method has been criticised for being too artificial. Iglesias-Rodríguez’s method found that higher carbon dioxide concentrations increased calcification, speeding up growth of the tiny calcite plates on the plankton cell.

Carbon storage

Coccolithophores appear to benefit in two ways. The extra carbon dioxide aids photosynthesis, while the more acidic waters increase the concentration of bicarbonate – the main ingredient for coccolith plates, known as liths. Making the liths results in the release of carbon dioxide, but when dead plankton fall to the ocean floor the carbon in the shells is locked away in deep ocean chalk deposits.

“Increased bicarbonate appears to stimulate an increase in mass of calcium carbonate produced by each coccolithophore cell,” says Paul Halloran, a co-author from the University of Oxford.

The team’s result is not confined to the lab. By studying fossil coccolithophores from a deep ocean core, they found that there has been a 40% increase in average coccolith mass over the last 220 years, mirroring the rise in carbon dioxide levels.

Other scientists think the results make sense and help to explain how coccolithophores survived the last rapid global warming event – the Palaeocene-Eocene thermal maximum 56 million years ago.

Cocco and insensitive

“Coccolithophores seemed to sail through the surface water acidification then, so perhaps they are quite insensitive to this kind of change,” says Paul Bown from University College London.

As yet it isn’t clear how these super-sized coccolithophores will affect the global carbon balance. If the oceans become too acidic, the shells of dead plankton will dissolve and release their carbon before they fall to the ocean floor.

“We can’t yet be confident that stimulating plankton production will draw down carbon dioxide,” says Bown.

Kate Ravilious, New Scientist, 18 April 2008. Article.

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