One type of algae, haptophytes, reproduced rapidly in the ocean after a nutrient-rich eddy of water, 1,530 kilometers northeast of the island of South Georgia, was fertilized with iron by Indian and German scientists.
Ulrich Bathmann, head of bioscience at the Alfred Wegener Polar and Oceanography Institute (AWI) in Bremerhaven, Germany, said the growth had been different from previous, smaller-scale experiments.
He told the DPA news agency, "This would be an argument against large-scale commercial iron fertilization, since the effects are not calculable in advance."
The AWI experiment has met bitter opposition in Germany from some environmentalists who fear that using algae to sequester carbon dioxide in the ocean might wreck the ecosystem. AWI says the debate requires facts, which the scientists are obtaining now.
Bathmann added it was still not certain if algae could capture vast amounts of carbon dioxide from the atmosphere and keep it out of circulation long term.
The experiment, codenamed Lohafex, is continuing aboard a German ship, the Polarstern, until mid-March. The ship mixed 10 tons of ferrous sulphate (FeSO4) with seawater and pumped it out into the South Atlantic.
"After just four days, the team observed a significant increase in algae," said Bathmann. "It was a surprise that they reproduced so quickly."
In the previous experiments, it took 10 to 14 days for this to happen. The haptophytes had responded to fertilization most.
"Other algae types barely increased or did not increase at all," he said.
Haptophytes are common in coastal waters and blooms sometimes lead to them washing up on beaches as a foam. Tiny animals, the zooplankton, feed on them.
"Those organisms will eat a large part of the algae," said the scientist, explaining that the zooplankton would breathe the carbon dioxide back into the surface water, instead of the gas sinking as dead algae to the ocean bottom.
"At the moment, the algae are reproducing faster that than the zooplankton can eat them," he said. "The exciting question in the next few weeks will be how much of the algae will be left over.
"That is why we cannot make any predictions about how much carbon dioxide the algae are able to capture."