Tiny ocean plankton can reduce global warming by soaking up unexpectedly large amounts of carbon dioxide but their carbon-bloated cells might damage marine food chains, scientists said on Sunday.

Experiments in a Norwegian fjord showed that plankton -- small drifting plants or creatures -- could absorb up to 39 percent more carbon dioxide, the main greenhouse gas, in seawater pens mimicking projected climate conditions to 2150.

"This is a massive and surprising change in the carbon content of these organisms," said Ulf Riebesell, a marine biologist at the Leibnitz Institute of Marine Sciences in Kiel, Germany, who led the German and Norwegian experiments.

Other studies have shown that the oceans have soaked up almost half of the carbon dioxide emitted by burning fossil fuels since the Industrial Revolution, but few have looked at how greenhouse gases may affect life in the oceans in future.

The study was good news for the climate by indicating plankton could help absorb ever more carbon, helping brake rising temperatures that the U.N. climate panel says will bring more heatwaves, storms, droughts and floods.

But Riebesell said there was bad news for marine life from the study, published in the journal Nature.

OCEAN CURSE

"What appears to be a blessing for the atmospheric greenhouse effect may prove to be a curse for deep ocean ecosystems," the Leibnitz Institute said in a statement.

When plankton die and sink, their decomposition consumes oxygen vital to marine animals living in the depths. "This will enlarge the parts of the oceans that have very little oxygen," Reibesell told Reuters.

Dissolved carbon dioxide is a weak acid so a quicker transport of carbon to the seabed could accelerate acidification that makes it harder for creatures such as shellfish or crabs to build protective shells.

In addition, plankton are food for other marine life and may be less nutritious with a higher carbon content.

"When tiny crustaceans were fed on the carbon-rich plankton, their growth rate and their reproduction rate decreased," Riebesell said. He likened the shift to that from a healthy salad to a greasy hamburger.

The study was based on plankton grown in 27 cubic metre (950 cu ft) seawater containers reaching 10 metres deep in a west Norwegian fjord. The scientists hope to expand their research to other places, such as the Arctic or the U.S. Great Lakes.