Counting carbon. Scientists lower sediment traps to measure the amount of carbon transferred to the depths of the ocean from iron-fed algal blooms.

Raymond T. Pollard

Tucking Carbon Deep Into the Ocean

Can phytoplankton save us from global warming? Large swaths of the floating algae suck in CO2 from the atmosphere, although whether they trap this greenhouse gas for good has been unclear. Now, researchers report that iron-fed phytoplankton do indeed sequester CO2 over the long term. However, the authors caution against seeding the sea with iron--a scheme some scientists have proposed to mitigate climate change--until more is known about its effects on ocean ecosystems.

A patch of sea near the Crozet Islands in the southwestern Indian Ocean is home to nature's own iron-fertilization experiment. Regional water currents funnel iron-rich runoff north of the islands, leaving iron-poor waters to the south. Biological oceanographer Richard Sanders of the National Oceanography Centre, Southampton, in the United Kingdom, and colleagues looked at phytoplankton in both areas. Iron acts like a fertilizer in high-nutrient waters, and indeed the team found many more phytoplankton growing north of the islands. Because of the extra biomass, northern waters sequestered two to three times more carbon than those to the south of the islands--even at depths of 3000 meters--the team reports in today's issue of Nature. That's important, Sanders says, because surface phytoplankton can release CO2 back into the atmosphere when it dies or is eaten, whereas phytoplankton in the deep sea traps the gas much longer.

Despite the positive news, Sanders cautions against using the findings to buttress so-called geoengineering schemes that would involve seeding the ocean with iron. Additionally, their research shows that that only 2% to 7% as much carbon is sequestered as compared to some geoengineering predictions. Scientists may be able to boost that number by adding more and more iron to the seas, but such levels might harm ocean ecosystems.

Biogeochemist Kenneth Coale of Moss Landing Marine Laboratories in California says the work "strengthens the connection between iron supply and low atmospheric CO2," a trend that scientists have seen in ice-core records but hadn't tested in the open ocean. But biogeochemist Jorge Sarmiento of Princeton University says that because carbon measurements in the study varied so widely, the data are "weaker proof [of iron-enhanced carbon sequestration] than I think we would have liked." Still, he says, "it is a significant step in the right direction."