Debate: Do Gobbled Algae Mean Carbon Fix Sunk?

Eli is a contributing correspondent for Science magazine.

According to the conventional wisdom in recent news accounts, the failure of a recent German-Indian expedition to grow and sink a massive algae bloom at sea is the death knell for a controversial method of reducing CO2 in the atmosphere.

But some prominent oceanographers say they are not sure the results should be interpreted that way. And ScienceInsider can report that politics may have played a larger role than has been previously reported in the unexpected results of the test of the experimental technique.

Previously, scientists had thought that the algae technique, known as iron fertilization, could contribute to the drawdown of up to 1 gigaton of carbon a year—more than 10% of current yearly emissions. Was that estimate far too optimistic?

Earlier this month, the controversial 10-week research expedition to the frigid Southern Ocean that rings Antarctica announced its surprising—and to some, disappointing—results. Scientists on the Indian-German expedition, known as LOHAFEX, grew a 300-km2 patch of algae in hopes that the plants' carbon would fall kilometers below, taking with it the atmospheric carbon it had pulled in during growth. But instead, the patch was devoured by crustaceans before it could bloom.

Victor Smetacek, the German oceanographer who led the expedition along with Victor  Wajih Naqvi, an Indian geochemist, says that result means that iron fertilization has a much lower sequestration potential for atmospheric CO2 and, thus, will play a smaller role in fighting climate change than previously expected. In a statement published after the experiment was completed, the Alfred Wegener Institute, where Smetacek works, said the results "dampened hopes on the potential of the Southern Ocean to sequester significant amounts of carbon dioxide and thus mitigate global warming."

Smetacek tells ScienceInsider that he based the 1-gigaton estimate on the availability of nitrogen nutrients found throughout the Southern Ocean. Little phytoplankton grows there despite plenty of available food. Scientists thought that by providing iron, a trace element required for growth, they could create large plankton blooms and draw down atmospheric carbon dioxide. (Iron fertilization enthusiasts focus on the Southern Ocean because other seas have much more natural algae, so growing blooms might just foster growth that would have happened anyway.)

The kind of algae Smetacek managed to grow were plankton without shells. Previous iron fertilization experiments had grown diatoms, which have glass shells that protect them from predators. Few diatoms grew in Smetacek's bloom because he fertilized waters that had low levels of silicon, which is required for their shells. So, not surprisingly, the plankton that Smetacek grew were devoured by the crustaceans in the area. "The system is more complex than people thought—including me," says Smetacek. "The amount of carbon that you can sink into the Southern Ocean is much less than I expected." Fifteen previous iron-fertilization experiments, though smaller, had shown generally more promising results. His experiment, he says, effectively rules out the northern half of the Southern Ocean as a potential site for iron fertilization.

But biogeochemist Kenneth Coale, director of Moss Landing Marine Laboratories in California, estimates that the silicon-rich southern part of the Southern Ocean would deliver up to twice as much potential carbon sequestration as the northern area Smetacek fertilized, in large part because of the diatoms and associated ecosystem dynamics. The predators that eat diatoms, it turns out, have large waste pellets that sink rapidly. Coale warns that calling iron fertilization a failed strategy on the basis of an experiment in low-silicon waters is just as unwise as declaring the technique a home run after a successful experiment would have been. "I would be reluctant to extrapolate from any one experiment anything having to do with the efficacy of iron fertilization as a carbon-sequestration strategy," says Coale.

Another scientist, Margaret Leinen, is the head of a company, Climos, that is hoping to commercialize iron fertilization to gain carbon credits at sea. The former head of geosciences at the National Science Foundation, she says the 1-gigaton-a-year figure for atmospheric CO2 was based on paleoclimate records. Chemical analyses of ocean cores show that the Southern Ocean drew down at least that much CO2 millions of years ago during glacial periods. "In the paleorecord, we find a lockstep correlation between the amount of [phytoplankton growing] and temperature," says Coale.

Smetacek had actually tried to find an area of ocean that would feature diatoms. Levels of silicon are generally higher south of 50° latitude. But Smetacek says the German government asked him to stay north of that line due to a treaty called CCAMLR designed to protect marine species in the Southern Ocean. Part of that restriction was no doubt connected to the fact that the LOHAFEX mission was controversial from the start, drawing criticism both from environmentalists and from the German environmental ministry. So Smetacek says he had to settle on a patch at 48° south latitude.

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