Blooms of algae created by pumping nutrients into the ocean can suck up at least ten times more carbon dioxide (CO2) from the atmosphere than was previously thought. But the findings lend no support to controversial schemes to encourage such blooms in order to reduce global warming, the authors warn.

The conclusion comes from a survey of a large, annually recurring natural algal bloom near Kerguelen, an archipelago halfway between South Africa and Australia. Natural vertical mixing of the ocean there steadily supplies iron and other nutrients from sediments at the ocean floor to the surface waters, encouraging phytoplankton growth. This growth converts carbon in the air (as CO2) to organic matter, thus reducing the amount of greenhouse gas in the atmosphere and alleviating climate warming.

Stephane Blain, a chemical oceanographer at the Oceanography and Biogeochemistry Laboratory in Marseille, France, led a 47-strong international team to investigate this bloom in detail in early 2005. Their results show that the ability of such blooms to suck up CO2, and to carry that carbon to the ocean floor when the organisms die, is much greater than previous field studies had suggested.