Nearly half of the photosynthesis on our planet is carried out in the oceans by unicellular organisms called phytoplankton. As a result, these cells figure importantly in controlling the amount of carbon dioxide, a greenhouse gas, in the atmosphere. Theoretically, increasing phytoplankton activity could reduce atmospheric CO2 and thereby rein in global warming. Earlier research had demonstrated that seeding a region of the Pacific Ocean with iron had triggered a phytoplankton bloom. Now three papers published in today's issue of the journal Nature describe a similar experiment in the Southern Ocean, providing new insight.
Phytoplankton activity is limited in most oceans by the availability of phosphorous, nitrogen and silicon. In the Pacific and in the Southern Ocean, however, these nutrients abound. Iron, on the other hand, is less plentiful there and thus restricts growth. In the Southern Ocean Iron RElease Experiment (SOIREE), researchers fertilized a sea plot eight kilometers in diameter with 8,663 kilograms of iron. The results were dramatic: by the end of the experiment, phytoplankton chlorophyll had tripled. A study of the long-term effects revealed that a month later, diffusion and "stirring" of the water had transformed the patch into a ribbon 150 kilometers long and four kilometers wide, which remained intact for at least two more weeks. Yet although the patch had accumulated somewhere between 600 and 3,000 tons of algal carbon, investigators found no evidence that any of that was exported from the surface to the deep ocean, as would be required for atmospheric CO2 reduction.
The third study concluded that, in principle, seeding the Southern Ocean with iron could lead to modest atmospheric CO2 sequestration. But in a commentary accompanying the three reports, Sallie W. Chisholm of M.I.T. notes that it could endanger the ocean ecosystem by deoxygenating the deep ocean or generating greenhouse gases more harmful than CO2. "We cannot expect to change the flux of carbon from the atmosphere to the oceans¿a single 'arrow' in this complex, self-organized system¿ without changing other features of the system in undesirable ways," she warns. "It is likely that we would not recognize these changes until it was too late to reverse them."