Iron surprise: algae absorb carbon dioxide.

In a politically charged experiment, an international team of researchers turned a patch of the Pacific Ocean green with plankton by adding trace amounts of iron to the water. The study demonstrated that fertilizing ocean plants with iron can spur them to absorb vast amounts of carbon dioxide from the air, but the scientists warned against using this technique to combat global warming.

"It's safe to say that there is nobody associated with this experiment who would advocate adding iron to the ocean," says chief scientist Kenneth Coale of Moss Landing (Calif.) Marine Laboratories.

Coale and his colleagues performed the experiment to test a hypothesis proposed by the late John Martin, an oceanographer at Moss Landing who sought to explain why plants fail to thrive in certain nutrient-rich areas of the sub-Arctic, Antarctic, and equatorial Pacific Oceans. Martin proposed that an iron deficiency in these waters prevented algae from making use of the abundant nitrates. According to the hypothesis, higher iron concentrations during the ice ages enabled algae to absorb carbon dioxide from the atmosphere, thereby helping to cool Earth.

The results of the experiment, completed last month, run counter to findings from a trial 2 years ago, when Coale and his coworkers first tried to seed the Pacific with iron. In the fall of 1993, the team added 480 kilograms of dissolved iron to a 64-square-kilometer region near the Galapagos Islands. The extremely dilute iron solution stimulated plankton growth enough to result in a fourfold increase in chlorophyll. But the iron had only an anemic impact on carbon dioxide concentrations in the water.

This year, the scientists produced a much stronger effect, even though they used less iron than last time. According to preliminary calculations, the experiment yielded a 30- to 40-fold increase in chlorophyll and caused enough plankton growth to absorb 350,000 kg of carbon dioxide from the seawater. "The ocean turned green for miles around," says Coale.

Instead of giving the ocean one large shot of iron, as they had in 1993, the scientists split the dose into three infusions administered over a week. This technique mimicked iron's natural means of getting to the open ocean, through wind-blown dust particles. Coale suspects that the smaller doses improved the plankton's access to the fertilizer by preventing dissolved iron from forming heavy particles and dropping out of the surface layer where plants live.

Natural conditions also helped boost the potency of the iron supplement. In the earlier experiment, a front of low-salinity water moved into the region, forcing the iron-doped water to sink. This time, the treated water stayed at the surface throughout the experiment. As the plants grew, they absorbed dissolved carbon dioxide from the water, thereby reducing the amount of gas released into the atmosphere by the ocean.

The experiment's success strongly supports the theory that iron controls the rates at which plants grow and absorb carbon dioxide in certain ocean areas, says Coale. "It further strengthens the hypothesis that iron may control the transition from glacial to interglacial climates," he adds.

Scientists involved in the experiment fear their results will encourage proponents of geoengineering, who have suggested seeding the ocean with iron to reduce global carbon dioxide concentrations. Large-scale additions of iron would alter the ocean's food web in unpredictable ways and might cause increased release of methane, an even more potent greenhouse gas, says oceanographer Sallie W. Chisholm of the Massachusetts Institute of Technology, who rejects iron as a quick fix.

Franaois M.M. Morel of Princeton University calls the study's results exciting and humbling. "This is a minute amount [of iron] and it had a tremendous effect. It says it is a little scary to be playing with nature."