Calcareous microalgae as indicators of ocean acidification

Calcareous phytoplankton, an important link in the oceanic carbon cycle, appears to be very sensitive to ocean acidification. An international study, notably involving researchers from the CNRS(1), shows that the secretion of the calcareous skeleton of the coccolithophores, a type of microalgae, decreases as marine waters become more acidic, although some super-calcified strains have managed to adapt to the most corrosive environments. These results are published in the journal Nature, August 4, 2011.

A third of all atmospheric carbon dioxide is absorbed by the oceans, where it is transformed into acidifying ions in seawater. This environment is also home to billions of microscopic cells that perform calcification at the ocean surface and thus regulate carbon fluxes in the biogeochemical cycles. Among the most abundant of these cells are the coccolithophores, which produce much of the marine sediments due to the accumulation of their plated calcareous micro-skeletons, called coccoliths, after their death. The response of coccolithophores to acidification had never been studied in their natural habitat on a global scale. Now a new approach has been taken, based on ocean observations, to understand how these organisms respond to ocean acidification.

For the first time, an international team of researchers studied this behavior in the wild, based on a large collection of coccolith samples and seawater from different oceans and their sediments. This allowed changes in calcification to be traced over the last 40,000 years. Using novel techniques for automatic recognition of species by microscopy and morphometry(2) developed at CEREGE(1), the weight (a few picograms, or a trillionth of a gram) of each of half a million calcareous plates measured was examined in relation to the water chemistry measured by the LOCEAN(1) team in Paris and the University of Perpignan. The results showed that, in general, coccolithophores are less calcified when the waters are low in carbonate (and therefore more acidic). This study therefore tends to show that, like corals, calcareous phytoplankton may suffer severe changes in the coming decades if ocean acidification accelerates.

An interesting and significant exception was found in coastal areas off Chile. Here, in the area of the ocean that is currently the most “acid” (pH 7.6 to 7.9), highly calcified coccoliths were observed that contrast with the general trend. Genetic analyses conducted at the Station Biologique de Roscoff(1) show that strains of coccolithophores in this region are different from those observed in other ocean areas. It seems that these coccoliths were able to adapt naturally to an environment that was unfavorable to their calcification, although the capacity for adaptation of this group to the acidified waters off Chile is still unknown. The contrasting result for this area, while a source of interest, does not mean that other groups of coccolithophores can be assumed capable of such adaptation to ocean acidification in the context of future increases in atmospheric and oceanic CO2.


(1)Laboratories involved are:
-The European Centre for Research and Teaching of Environmental Geosciences (CEREGE, CNRS / Université Paul Cézanne Aix-Marseille 3/Université Provence-Aix Marseille 1/IRD/Collège de France)
-The Laboratory of Oceanography and Climate: experiments and numerical approaches (LOCEAN, CNRS / UPMC / IRD / MNHN)
-Biological Station of Roscoff (CNRS / UPMC)
And the University of Perpignan
(2)Morphometry is the study and analysis of the geometry of objects

European Project on OCean Acidification (EPOCA):
The EU FP7 large-scale integrating project EPOCA (European Project on OCean Acidification) has the overall goal to fill the numerous gaps in our understanding of ocean acidification and its consequences. The EPOCA consortium brings together more than 160 scientists from 32 institutes and 10 European countries. The research of this four-year long project is partly funded by the European Commission.
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Sensitivity of coccolithophores to carbonate chemistry and ocean acidification, L. Beaufort, I. Probert, T. de Garidel-Thoron, E. M. Bendif, D. Ruiz-Pino, N. Metzl, C. Goyet, N. Buchet, P. Coupel, M. Grelaud, B. Rost, R. E. M. Rickaby et C. de Vargas, Nature, 04 august 2011.

Contact information:
Researcher CNRS l Luc Beaufort l T 06 10 96 93 95 l
Press CNRS l Laure Mégas l T 01 44 96 46 42 l

CNRS Press Releases, 4 August 2011. Article.

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