What is the biggest carbon sink in the world? It’s not our virgin forests but our oceans, which represent over two thirds of the carbon dioxide transfers with the atmosphere. This balance is just as fragile as that of our climate. Since the start of the industrial revolution, our oceans have absorbed 120 billion metric tons of CO₂. The excess carbon dioxide dissolves in our seas and oceans, raising acidity levels. And then what happens? The EPOCA project launched by the EU last June in Nice is setting out to answer that question. With a 16.5 million euro budget over four years, the 27 project partners – including the CNRS and the CEA – will be studying the impact of acidification on marine ecosystems and microorganisms, particularly plankton and mollusks, whose calcareous skeletons are very sensitive to water acidity.