Paris, September 29, 2005

Ocean acidification threatens marine organisms

Within 50 to 100 years, the outer skeletons of some marine organisms may start to dissolve and no longer be capable of forming. This is due to the acidification of seawater, brought about by the absorption of carbon dioxide by the oceans. This research was carried out by an international team, made up in particular of researchers from three French laboratories (1), and their findings are published in the 29 September 2005 issue of the journal Nature.

Ocean acidification

© Victoria Fabry - California State University San Marcos

Scanning electron microscope images of the shells of two pteropods. Left: a pteropod after swimming in present day seawater, which is not corrosive. Right: a pteropod after swimming for 48 hours in seawater made corrosive by the absorption of CO2.


The burning of fossil fuels leads to an average daily per capita production of 11 kg of carbon dioxide, of which 4 kg is absorbed by the oceans. In total, over 25 million tons of CO2 dissolve in seawater every day. This reaction brings about the acidification of seawater, in other words an increase in the concentration of hydrogen ions (H+). The hydrogen ions then combine with the carbonate ions (CO32-) present in the water, thus lowering their concentration. This limits the production of calcium carbonate, which is the main constituent of the limestone which makes up the outer skeletons of marine organisms.


Using recent data and 13 numerical models, a team of European, Japanese, Australian and American oceanographers has simulated changes in carbonates on the basis of the future CO2 emission scenarios drawn up by the IPCC (2).



With the standard scenario (3) the prediction is that within about 50 years the coolest ocean surface water, such as in the Weddell Sea off Antarctica, will become corrosive for one type of limestone called aragonite. This means that pteropods are in danger, since the shells of these planktonic molluscs, which swim in the upper layers of the ocean, are made of aragonite. And if atmospheric CO2 levels continue to rise, it is highly probable that towards the end of the century seawater will become corrosive for aragonite throughout the whole of the Southern Ocean as well as in part of the North Pacific. These calcareous organisms, which are extremely abundant in those areas, would no longer be able to grow their shells. This kind of corrosive environment would have no precedent for perhaps the past several million years.


In addition to these predictions, experiments carried out at sea have shown that the shells of live pteropods do indeed dissolve when seawater reaches the corrosive levels forecast for the year 2100. A fall in the numbers of pteropods could cause a chain reaction since they make up the basic food for organisms from zooplankton to whales, as well as for species which are important commercially, such as North Pacific salmon.


Corals are also threatened by acidification, and this work shows that it is above all those which live in cold water which are the most likely to dissolve. Not only is their calcium carbonate skeleton essential for their own development, but it also provides a habitat for deep-sea fish, eels, crabs, sea urchins, etc, while the external skeleton of sea urchins is also directly threatened by acidification. Discovering exactly what impact these changes will have on ecosystems and on biodiversity is a challenge which future research must take up.







1) Laboratoire des sciences du climat et de l'environnement (Environmental and climate sciences laboratory) (LSCE: CEA – CNRS/IPSL)
Laboratoire d'océanographie et du climat (Oceanography and climate laboratory): experimental work and numerical analyses (LOCEAN: CNRS – IRD – MNHN – Université Paris 6)
Laboratoire d'études en géophysique et océanographie spatiales (laboratory for studies in spatial geophysics and oceanography)(LEGOS: CNRS – CNES – IRD – Université Toulouse 3)

2) Intergovernmental Panel on Climate Change

3) Scenario IS92a “business-as-usual”


Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying
organisms, Orr J. et al., Nature, 29 september 2005


Researcher contact information:
James Orr – LSCE – E-Mail:

Department of Earth Sciences and Astronomy at CNRS:
Guillaume Duveau
Tel: 01 44 96 43 13

Press contact information:
CNRS: Martine Hasler
Tel: 01 44 96 46 35

CEA: Célie Simeray
Tel: 01 40 56 14 88


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