Since the start of the Industrial Revolution, the calcium carapace of microscopic animals called foraminifera living in the Southern Ocean have fallen in weight by a third, the study found.

The amoeba-like organisms, about the size of a grain of sand, live in the surface waters of oceans around the world.

They are an important part of the ecological chain and also provide a bulwark against global warming.

They transform carbon dioxide (CO2) from the air into calcium-based shells. When they die, their carbon-rich shells sink to the ocean floor, effectively storing the atmospheric CO2 forever.

Previous studies have shown that other marine animals, notably corals, are losing their ability to form exoskeletons from calcium.

However, the potential causes for this are tangled, and include rising temperatures and nutrient runoff from coastal agriculture as well as acidification.

This is the first study to look specifically at acidification and pin it to greenhouse-gas pollution, which is driven especially by the invisible product of burning oil, gas and coal.

"It is the invasion of anthropogenic (man-made) CO2 that is causing this particular source of acidification," said co-author William Howard of the Antarctic Climate and Ecosystems Cooperative Research Center in Hobart, Tasmania.

Howard's team collected shells of one foraminifera species, Globigerina bulloides, as they drifted toward the sea floor, and compared them to older specimens that had sunk several hundred years earlier.

The newer shells had 30-to-35 percent less mass, they reported in the online edition of the journal Nature Geoscience.

"If forams and other shell makers are not making shells, that might change the transfer of carbon from the surface ocean into the deep ocean," said Howard.

"It changes the efficiency of the biological pump, and would tend to lessen the degree to which the ocean takes up carbon. That's a feedback that we have to be concerned about," he said in a phone interview.

Only in the last five years have scientists become aware of the extent of ocean acidification and its potential to disrupt Earth's carbon cycle, which balances the absorption and release of CO2 into the atmospshere.

"The problem with this impact is that it is so persistent and so long-lived, unlike other pollution carbons. We will have a harder time turning this impact around," Howard said.

The geo-chemical mechanisms that buffer acidification work very slowly, he explained.

"It is like taking an antacid tablet for an upset stomach and then having to wait hundreds -- or thousands -- of years for it to work."

If the loss of shell mass threatens the survival of the amoeba-like creatures, it could also disrupt a food chain reaching from the plankton they eat, all the way up to large sea mammal such as whales.

"We don't know yet what those impacts might be," Howard said.

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